Sweet potato : chemistry, processing and nutrition 9780128136379, 0128136375

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Sweet Potato

Sweet Potato Chemistry, Processing, and Nutrition

Edited by

TAI-HUA MU Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, P.R. China

JASPREET SINGH School of Food and Advanced Technology and Riddet Institute, Massey University, Palmerston North, New Zealand

Academic Press is an imprint of Elsevier 125 London Wall, London EC2Y 5AS, United Kingdom 525 B Street, Suite 1650, San Diego, CA 92101, United States 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom Copyright © 2019 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-813637-9 For Information on all Academic Press publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Charlotte Cockle Acquisition Editor: Nancy Maragioglio Editorial Project Manager: Susan Ikeda Production Project Manager: Vignesh Tamil Cover Designer: Christian Bilbow Typeset by MPS Limited, Chennai, India

List of Contributors Lawrence Akinola Arogundade Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China; Chemistry Department, College of Physical Sciences, Federal University of Agriculture, Abeokuta, Nigeria Koji Ishiguro Division of Field Crop Research and Development, Hokkaido Agricultural Research Center, NARO, Hokkaido, Japan Lovedeep Kaur School of Food and Advanced Technology and Riddet Institute, Massey University, Palmerston North, New Zealand Sunantha Ketnawa Graduate School of Horticulture, Chiba University, Matsudo, Japan Nasir Mehmood Khan Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China; Department of Biotechnology, Shaheed Benazir Bhutto University, Sheringal, Pakistan Rie Kurata Division of Upland Farming Research, Kyusyu Okinawa Agricultural Research Center, NARO, Miyakonojo, Japan Peng-Gao Li Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China; School of Public Health, Capital Medical University, Beijing, People’s Republic of China; Beijing Key Laboratory of Environmental Toxicology, Capital Medical University, Beijing, People’s Republic of China Dai-Fu Ma Xuzhou Sweet Potato Research Center, Chinese Academy of Agricultural Sciences, National Sweet Potato Improvement Center, Xuzhou, People’s Republic of China Meng-Mei Ma Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China Tai-Hua Mu Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China

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Yoshiyuki Nakamura Division of Field Crop Research, Institute of Crop Science, National Agriculture and Food Research Organization (NARO), Tsukuba, Japan Yukiharu Ogawa Graduate School of Horticulture, Chiba University, Matsudo, Japan Fredrick O. Ogutu Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China; Food Technology Division, Kenya Industrial Research and Development Institute, Nairobi, Kenya Tomoyuki Oki Graduate School of Health and Nutrition Sciences, Nakamura Gakuen University, Fukuoka, Japan Shigenori Okuno Department of Planning, Kyusyu Okinawa Agricultural Research Center, NARO, Kumamoto, Japan Isela Carballo Pérez Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, P.R. China; Institute of Food Research, Havana, Cuba Jaspreet Singh School of Food and Advanced Technology and Riddet Institute, Massey University, Palmerston North, New Zealand Terumi Sugawara Crop Development and Agribusiness Research Division, Kyusyu Okinawa Agricultural Research Center, NARO, Kumamoto, Japan Hong-Nan Sun Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China Kazunori Takamine Division of Shochu Fermentation Technology, Education and Research Center for Fermentation Studies, Faculty of Agriculture, Kagoshima University, Kagoshima, Japan Miao Zhang Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China

Preface Sweet potato, commonly known in China as Hongshu, Baishu, Digua, Fanshu, Hongyu, and Hongshao, is an annual or perennial herb of the Convolvulaceae family. Sweet potato cultivation is characterized by lowinput, high-yield, drought tolerance, and resistance to infertility, and sweet potato is an important food crop after rice, wheat, corn, and potato. The production of sweet potatoes is highest in Asia, followed by Africa, then the Americas and Oceania. Sweet potato roots and leaves are rich in a variety of nutrients required by the human body, such as starch, protein, dietary fiber, lipids, polyphenols, carotenoids, vitamins, and mineral elements such as potassium and calcium, depending on different varieties. Based on its excellent nutritional value, research on the extraction and structural, physicochemical, and functional properties of functional components in sweet potato and its by-products from industrial processing have gained widespread attention. Sweet potato has also been processed into flour, flakes, granules, paste, purees, chips, canned products, beverages, and various snack foods. In addition, sweet potato is used as an important supplement for different staple products in the food industry, such as sweet potato steamed breads, baked breads, noodles, and pancakes, etc. To improve human dietary nutrition, it is important to support research, development, and application of the nutritional and functional components extracted from sweet potato and its by-products, as well as the existing and new snack and staple foods. This will be of great significance in increasing the consumption proportion of sweet potato in people’s daily diet. It is also an inevitable trend for the sustainable development of agricultural production and the improvement of human dietary habits. The book Sweet Potato: Chemistry, Processing, and Nutrition was compiled based on the research achievements of all the contributors. This book includes 14 chapters. Chapter 1, Sweet potato: chemistry, processing, and nutrition—an introduction, introduces overall information on sweet potato. Chapter 2, Sweet potato: origin and production, focuses on the origin and production of sweet potatoes in the world. Chapter 3, Sweet potato starch, focuses on the structure and physicochemical properties of sweet potato starch, as well as the effects of physical and chemical modifications. Chapter 4, Sweet potato protein and its hydrolysates, introduces the xi

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recovery and composition, gelation and emulsifying properties of sweet potato protein, as well as the preparation and antioxidant activity of its hydrolysates. Chapter 5, Sweet potato dietary fiber, introduces the efficient manufacture, and physicochemical and functional properties of sweet potato dietary fiber, as well as the extraction methods, composition, and physicochemical and functional properties of sweet potato pectin affected by ultrasonic modification. Chapter 6, Sweet potato lipids, introduces the lipid and fatty acid compositions of different varieties of sweet potato, and the antiproliferative effect and cell migration inhibition of sweet potato lipids. Chapter 7, Sweet potato polyphenols, introduces the main categories, the individual phenolic composition, physiological functions, processing stability, and utilization of polyphenols from sweet potato roots and tops. Chapter 8, Sweet potato carotenoids, describes the composition, content, analytical method, physiological functions, and retention during processing of carotenoids from sweet potato roots and tops. Chapter 9, Sweet potato microstructure, starch digestion, and glycemic index, introduces the microstructure and glycemic features of sweet potato. Chapter 10, Sweet potato staple foods, presents the definition, types, main raw ingredients, and development of sweet potato staple foods; Chapter 11, Sweet potato snack foods, introduces a series of common sweet potato snacks, including sweet potato chips, roast sweet potatoes, sweet potato biscuits, dried sweet potato slices, sweet potato cakes, doughnuts, extruded sweet potato snacks, and the packaging thereof. Chapter 12, Sweet potato fermentation food (sweet potato shochu), introduces the sweet potato shochu manufacturing methods and the factors affecting shochu aroma, and briefly discusses its potential health benefits. Chapter 13, Quality evaluation of sweet potato products, describes Japanese sweet potato varieties for food and their chemical properties on the sweetness and texture of steamed storage roots. Chapter 14, Global market trends, challenges, and the future of the sweet potato processing industry, introduces the global market trends, challenges, and future of the sweet potato processing industry. The purpose of this book is to provide a reference and guidance for the chemistry, processing, and nutrition of the sweet potato, and to provide basic information for scientific innovations in the sweet potato industry.

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We thank all the contributors, Ms. Susan Ikeda, and Elsevier who made this book possible. 1

Tai-Hua Mu1 and Jaspreet Singh2

Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, P.R. China 2 Massey Institute of Food Science and Technology, Massey University, Palmerston North, New Zealand

CHAPTER 1

Sweet potato: chemistry, processing, and nutrition—an introduction Tai-Hua Mu1 and Jaspreet Singh2 1

Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China 2 School of Food and Advanced Technology and Riddet Institute, Massey University, Palmerston North, New Zealand

Sweet potato (Ipomoea batatas [L.] Lam) is a dicotyledonous plant belonging to the family Convolvulaceae. The plant bears white and purple sympetalous flowers and has large nutritious storage roots. Sweet potato roots develop mainly as storage roots, bearing alternate heart-shaped or palmately lobed leaves, which are long and tapered. The sweet potato roots have a smooth skin that is yellow, orange, red, brown, purple, and/or beige, and has flesh that is beige to white, red, pink, violet, yellow, orange, or purple depending upon the cultivar. The roots, leaves, and stems of sweet potatoes are all edible and nutritious, and these play a vital role in ensuring the food security of many developing countries. The world’s annual production of sweet potatoes was 105 million metric tons in 2016, with approximately 95% of sweet potatoes being grown in developing countries, and with China ranked as the biggest producer (FAOSTAT, 2016). The world’s production of sweet potatoes is currently stable, as is the ranking of the regions and continents. Sweet potato production in Asia is considerably ahead of the other continents, followed by Africa, South America, Caribbean, North and Central America, Europe, and lastly Oceania (FAOSTAT, 2016). As a versatile crop, sweet potatoes can be used for a wide range of purposes. Sweet potato roots and leaves are rich in starch, protein, dietary fiber, lipids, polyphenols, carotenoids, vitamins, and mineral elements such as potassium and calcium depending on the different varieties. Sweet potatoes can also be utilized as a raw material to extract different functional components with higher nutritional value. Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00001-6

© 2019 Elsevier Inc. All rights reserved.

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Starch is the major component of sweet potato dry matter and consists of amylose and amylopectin with values ranging from 13.33% to 26.83% and 73.17% to 86.67%, respectively (Abegunde et al., 2013). Sweet potato starch granules vary from polygonal to round to cupuliform/bell shapes with granule sizes ranging from 2 to 42 μm (Chen et al., 2003). The digestibility, syneresis percentage, swelling power, and solubility of sweet potato starches are in the ranges of 10.35% 15.15%, 32.45% 44.68%, 13.46 26.13 g/g, and 8.56% 19.97%, respectively (Abegunde et al., 2013). Sweet potato starches can be modified by physical and chemical methods to improve their physicochemical characteristics and resistance towards digestive enzymes during starch digestion (Yu et al., 2015, 2016), which can aid in them being able to serve as functional additives in food. Sweet potato contains approximately 1.73% 9.14% of protein on a dry weight basis. Sweet potato protein (SPP) is mainly composed of sporamins, which are rich in essential amino acids, and SPP is comparable with other superior quality vegetable proteins (Mu et al., 2009). Unfortunately SPP is generally discarded as industrial waste during sweet potato starch processing. SPP possesses good gelation and emulsifying properties and can easily be recovered by isoelectric precipitation or ultrafiltration/diafiltration processed methods (Arogundade and Mu, 2012; Arogundade et al., 2012; Khan et al., 2014). SPP hydrolysates (SPPH) show noteworthy antioxidant activity, and SPPH could be obtained by enzymatic hydrolysis under high hydrostatic pressure (Zhang and Mu, 2017). Sweet potato dietary fiber could be extracted from sweet potato pulp, which is the dehydrated residue produced during sweet potato starch manufacturing. Sweet potato pulp consists of 49.7% dietary fiber, which is rich in pectin (39.5%), cellulose, hemicellulose, and lignin (Takamine et al., 2000). Sweet potato dietary fiber exhibits good physicochemical and functional properties. In particular, sweet potato pectin obtained by sonication-induced modification showed lower molecular weight, higher galacturonic acid content, and stronger antioxidant capacity when compared to unmodified samples and could induce apoptosis-like cell death in colon cancer (Ogutu and Mu, 2017; Ogutu et al., 2018). Lipids are present in almost all foods and play an important role in human nutrition and the sensory aspects of food. The total lipids (TLs) content of different cultivars of sweet potato normally ranges from 0.72% to 1.44%, consisting of 36.74% 61.04% of neutral lipids (NLs), 30.29% 49.25% of glycolipids (GLs), and 7.05% 17.07% of phospholipids (PLs).

Sweet potato: chemistry, processing, and nutrition—an introduction

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The fatty acids (FAs) in sweet potato TLs, NLs, GLs, and PLs include different percentages of palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:1), linoleic acid (C18:2), linolenic acid (C18:3), and arachidic acid (C20:0). Sweet potato lipids have been reported to have certain anticancer effects, especially GLs with the contribution of monogalactosyl diacylglycerol and digalactosyl diglyceride (Zhao, 2014). Polyphenols are composed of a variety of components, and their ingestion for health reasons is currently widely recommended in most developed countries. Sweet potato polyphenolic compounds are separated into two main categories: flavonoids and phenolic acids. Flavonoids are mainly found in sweet potato tuberous roots, and include anthocyanins, rutin, and quercetin, but are also found in sweet potato tops, for example, quercetin glycosides. Sweet potato phenolic acids consist of a mixture of caffeic acid and caffeoylquinic acid derivatives, which are commonly present in all parts of sweet potato leaves, petioles, stems, and tuberous roots. Sweet potato carotenoids are mainly distributed in orange- and yellow-fleshed sweet potato roots, as well as sweet potato leaves. Approximately 90% of the carotenoids in orange-fleshed sweet potatoes are β-carotene (Kimura et al., 2007). Lutein is a member of the xanthophyll family of carotenoids, and it has been found in the constituent parts of sweet potato tops. Sweet potato cultivars with a high content of carotenoids and deep orange or yellow flesh have been developed. Sweet potato carotenoids show good antioxidant activities and could be used as a solution to vitamin A deficiency. Nowadays sweet potatoes have been used as an important supplement for different staple products in the food industry, such as sweet potato steamed breads, baked breads, noodles, and pancakes. Sweet potatoes have also been used to produce snack foods, such as sweet potato chips, roast sweet potatoes, biscuits, dried slices, cakes, doughnuts, and extruded snacks. In addition, sweet potatoes are used to produce fermented foods, for example, sweet potato shochu, which is a distilled alcoholic beverage. Furthermore, to ensure the consumers get high quality products, the quality evaluation of sweet potato products is important. In summary, the sweet potato processing industry has good opportunities but at the same time some difficult challenges. The scientists and researchers working in the area of food processing may need to seek further cooperation with sweet potato breeders and growers to maximize the utilization of sweet potatoes for different food processing applications.

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References Abegunde, O.K., Mu, T.-H., Chen, J.-W., Deng, F.-M., 2013. Physicochemical characterization of sweet potato starches popularly used in Chinese starch industry. Food Hydrocol. 33 (2), 169 177. Arogundade, L.A., Mu, T.-H., 2012. Influence of oxidative browning inhibitors and isolation techniques on sweet potato protein recovery and composition. Food Chem. 134 (3), 1374 1384. Arogundade, L.A., Mu, T.-H., Añón, M.C., 2012. Heat-induced gelation properties of isoelectric and ultrafiltered sweet potato protein isolate and their gel microstructure. Food Res. Int. 49 (1), 216 225. Chen, Z., Schols, H., Voragen, A., 2003. Physicochemical properties of starches obtained from three varieties of Chinese sweet potatoes. J. Food Sci. 68 (2), 431 437. FAOSTAT, 2016. Production of crops. Available from: ,http://faostat3.fao.org/browse/ Q/QC/Ex . . Khan, N.M., Mu, T.-H., Zhang, M., Arogundade, L.A., 2014. The effects of pH and high hydrostatic pressure on the physicochemical properties of a sweet potato protein emulsion. Food Hydrocol. 35, 209 216. Kimura, M., Kobori, C.N., Rodriguez-Amaya, D.B., Nestel, P., 2007. Screening and HPLC methods for carotenoids in sweet potato, cassava and maize for plant breeding trials. Food Chem. 100 (4), 1734 1746. Mu, T.H., Tan, S.S., Xue, Y.L., 2009. The amino acid composition, solubility and emulsifying properties of sweet potato protein. Food Chem. 112 (4), 1002 1005. Ogutu, F.O., Mu, T.-H., 2017. Ultrasonic degradation of sweet potato pectin and its antioxidant activity. Ultrason. Sonochem. 38, 726 734. Ogutu, F.O., Mu, T.-H., Sun, H., Zhang, M., 2018. Ultrasonic modified sweet potato pectin induces apoptosis like cell death in colon cancer (HT-29) cell line. Nutr. Cancer 70 (1), 136 145. Takamine, K., Abe, J.-i, Iwaya, A., Maseda, S., Hizukuri, S., 2000. A new manufacturing process for dietary fiber from sweet potato residue and its physical characteristics. J. Appl. Glycosci. 47 (1), 67 72. Yu, S.X., Mu, T.H., Zhang, M., Ma, M.M., Zhao, Z.K., 2015. Effects of retrogradation and further acetylation on the digestibility and physicochemical properties of purple sweet potato flour and starch. Starch-Stärke 67 (9 10), 892 902. Yu, S.X., Mu, T.H., Zhang, M., Zhao, Z.K., 2016. Effects of inorganic salts on the structural and physicochemical properties of high-hydrostatic-pressure-gelatinized sweet potato starch. Starch-Stärke 68 (9 10), 980 988. Zhang, M., Mu, T.-H., 2017. Identification and characterization of antioxidant peptides from sweet potato protein hydrolysates by Alcalase under high hydrostatic pressure. Innovat. Food Sci. Emerg. Technol. 43, 92 101. Zhao, S.-T., 2014. Study on the Lipid Composition and Anti-cancer Activity of Sweet Potato. Xinjiang Agricultural University.

CHAPTER 2

Sweet potato: origin and production Tai-Hua Mu1 and Peng-Gao Li1,2,3 1

Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China 2 School of Public Health, Capital Medical University, Beijing, People’s Republic of China 3 Beijing Key Laboratory of Environmental Toxicology, Capital Medical University, Beijing, People’s Republic of China

Origin Botanical characteristics of sweet potatoes The sweet potato (Ipomoea batatas [L.] Lam) is a dicotyledonous plant that belongs to the family Convolvulaceae. It is an herbaceous perennial vine that has white and purple sympetalous flowers, large nutritious storage roots, and alternate heart-shaped or palmately lobed leaves. The large, starchy, sweet-tasting tuberous roots are by far the most important part of the plant. It is a storage root, not a tuber or thickened stem like the potato (Solanum tuberosum). It is also different from yams. They are not related to each other despite a physical and compositional similarity. The roots of sweet potatoes are long and tapered, with a smooth skin whose color ranges between yellow, orange, red, brown, purple, and beige. Its flesh ranges from beige to white, red, pink, violet, yellow, orange, and purple (Fig. 2.1). The storage root contains a large amount of starch that can be turned into energy in the human body so the root can be consumed as a staple food instead of other staple food crops such as rice, wheat, and corn. The roots, leaves, and stems of sweet potatoes are all edible and nutritious, and thus the cultivation of the sweet potato can play a vital role in ensuring the food security of many developing countries. Meanwhile the nutritional needs of the people in these countries can also be met by proper consumption of the whole plant. Each 100 g of the fresh root provides an energetic value of 85 kcal, which is greater than that for potatoes but less than for cereals. The root also provides a small amount of protein and relatively higher amounts of vitamins and minerals than cereals. Some varieties of sweet potatoes contain a very high amount Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00002-8

© 2019 Elsevier Inc. All rights reserved.

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Sweet Potato

Figure 2.1 Sweet potato with different flesh color.

of β-carotene that can be converted into vitamin A in the human body, thus the cultivation and consumption of these varieties have been recommended as an applicable approach to prevent the epidemic of vitamin A deficiency that is widespread in many developing countries. The young leaves, shoots, and vine of the plant are a good source of dietary fiber, as well as of many vitamins, minerals, and antioxidant molecules, which makes the sweet potato an excellent vegetable. Nowadays, to fully utilize the value of the plant, the young leaves have been made into teas and powder drinks for human consumption.

The origin and the dissemination of sweet potatoes around the world As to the origin of sweet potatoes, it is now widely accepted that the plant originated from the tropical regions in the South and Central America. Although it is now mainly produced in China, the plant is not native to China. In fact the plant was introduced to China only about 631 years ago. According to most of the sources, the origin and domestication of sweet potatoes are thought to be in either Central America or South America (Austin, 1988). In Central America, sweet potatoes were domesticated at least 5000 years ago (Bovell-Benjamin, 2014; Nishiyama, 2006). The plant was then brought to other tropical and subtropical regions around the world and became popular in the islands of the Pacific Ocean (Bovell-Benjamin, 2014; Nishiyama, 2006). Archeologists have found prehistoric remnants of sweet potatoes in Polynesia from about CE 1000 1100 according to radiocarbon dating. They have hypothesized that ancient Polynesians may have interacted with people on the west coast of South America and brought sweet potatoes to the islands of the Pacific Ocean long before the Europeans set foot on the continent.

Sweet potato: origin and production

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By analyzing the DNA of 1245 sweet potato varieties from Asia and the Americas, researchers have proven that the root vegetable had made it all the way from the Andes to Polynesia nearly 400 years before Christopher Columbus landed in the New World in 1492 (Roullier et al., 2013). Polynesian and European voyagers then spread sweet potatoes rapidly to other parts of the world. It was first diffused throughout Polynesia, in already populated islands such as Hawaii, Easter Island, and some other islands of eastern Polynesia, and then into New Zealand around CE 1150 1250. During the 16th century, Spanish and Portuguese traders and travelers introduced it into the Philippines and Indonesia. From these points, local traders, European travelers, or both, may have distributed sweet potatoes into the rest of the Asia. It was introduced from the Philippines as a food crop to Fujian Province of China in about 1387, to Japan in about 1735 (Takegoshi, 2003) and to Korea in about 1764 (Kim, 2012). Because its planting material can be multiplied greatly from a very few roots and due to its high adaptability, sweet potatoes spread quickly throughout Asia, Latin America, and Africa during the 17th and 18th centuries. Now the sweet potato is widely cultivated throughout tropical and warm temperate regions, wherever there is sufficient water to support its growth (O’Hair et al., 1990). Notably the introduction and cultivation of sweet potatoes in China in the late Ming dynasty greatly alleviated the food shortage at that time and prompted the central government of the following Qing dynasty to enlarge the sweet potato cultivation area in China and spread it to other farther north regions in Asia. Nowadays, sweet potatoes are planted in almost every province of China, even in the coldest Heilongjiang Province in the far north of China.

Production Sweet potatoes are now widely cultivated all over the world wherever the plant can survive and the sweet potato has become one of the most important food crops in terms of human consumption. As a high-yield tuberous crop, the plant can be cultivated in poor soil and water conditions, such as in sub-Saharan Africa, parts of Asia, and the Pacific Islands. With the advancement of agronomic technology, more and more varieties of sweet potatoes have been bred to meet the needs of either the consumer or the food processing industry.

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Worldwide production According to the statistics from the Food and Agriculture Organization of the United Nations (FAO), the world’s total production of sweet potatoes in 2016 was 105 million tons. In comparison to other crops that can be consumed as a staple food, sweet potatoes are the sixth largest crop, ranking just behind corn (1060 million tons), wheat (749 million tons), rice (741 million tons), potato (376 million tons), and cassava (277 million tons). Although the cultivation of sweet potatoes is widespread around the world, production is not evenly distributed around the world. China is the biggest producer of sweet potatoes in the world. In 2016 China alone produced 71 million tons of sweet potatoes, accounting for 68% of the world’s total production. Meanwhile a large number of the varieties of the plant have been cultivated in China. Main cultivation regions and countries Nowadays, more than 100 million tons of sweet potatoes are produced globally each year, approximately 95% of which are grown in developing countries. Asia is today the largest sweet potato-producing region in the world. More than 78 million tons were produced in Asia in 2016. Africa is the second largest producer, producing 21 million tons in 2016. The Americas, the original home of sweet potatoes, grows less than 5% of the world’s supply. Europe has only a very small sweet potato production, mainly in Portugal. To date the ranking of the regions and continents in terms of their production of sweet potatoes is very stable. Asia is far ahead of other continents, followed by Africa and South America. As shown in Table 2.1 Table 2.1 Annual production of sweet potatoes in different continents, 2014 16 (million tons). Continent

2014

2015

2016

Asia Africa South America and Caribbean North and Central America Oceania Europe Total

79.14 (75.67%) 20.67 (19.77%) 2.44 (2.33%)

78.96 (76.01%) 20.01 (19.26%) 2.49 (2.40%)

78.60 (74.72%) 21.32 (20.27%) 2.81 (2.67%)

1.39 (1.33%)

1.48 (1.42%)

1.51 (1.43%)

0.88 (0.84%) 0.05 (0.05%) 104.57 (100%)

0.90 (0.86%) 0.05 (0.05%) 103.88 (100%)

0.91 (0.86%) 0.05 (0.05%) 105.19 (100%)

Source: Data from FAOSTAT Database (http://www.fao.org).

Sweet potato: origin and production

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the world’s total production of sweet potatoes was 105.19, 103.88, and 104.57 million tons in the years 2014 16, respectively. Asia accounts for 74.72% of global production in 2016, followed by Africa (20.27%), South America and the Caribbean (2.67%), and North and Central America (1.43%). Europe and Oceania have only a very small sweet potato production, contributing only 0.86% and 0.05% of the world’s total in 2016, respectively. Thus it is obvious that the cultivation of sweet potatoes is not widespread in developed countries in spite of the fact that they have far more advanced agricultural technologies than most of the countries in Asia and Africa. In 2016 the total output of sweet potatoes in Asia was 78.60 million tons. Table 2.2 shows that 67.30% of the world’s total production was contributed by China, which produced 70.79 million tons in 2016. It means that practically 90.07% of the annual production in Asia was contributed by China. The second largest producer is Nigeria, which accounts for 3.72% of the global output in 2016. The output from the United Republic of Tanzania is near to that of Nigeria, contributing 3.63% of the world’s total output in 2016. The shares from other countries were all lower than 3%. In Asia there are 24 countries that plant sweet potatoes. Indonesia is behind China but head of India in the ranking, with the production of 2.27 million tons (2.16% of the worldwide output) in 2016. The output from India (1.47 million tons) is close to that of the United States, which is the only American country that ranks among the top 10 sweet potato producers in the world. The United States produced 1.43 million tons in 2016, accounting for 1.36% of the world total output. In Europe, there are only four countries that produce sweet potatoes. The leading European country in sweet potato production is Portugal, which produced 22,901 tons in 2016, followed by Spain (13,523 tons), Italy (12,456 tons), and Greece (3305 tons). Reasons for the imbalance in sweet potato production among different regions and countries The differences in the amounts of sweet potato produced in different regions and countries are caused by many factors. First, in China’s history the central government has played an important role in propagating and encouraging the cultivation of sweet potatoes in the vast land of this big country. As a result it soon became the world’s largest grower. Second, the demand for the production of sweet potatoes is closely related to its

Table 2.2 Top 20 countries in terms of the annual production (million tons) of sweet potatoes, 2014 16. 2014

2015

2016

Country

Value

%

Country

Value

%

Country

Value

%

China Nigeria The United Republic of Tanzania Ethiopia Indonesia Angola Uganda Vietnam The United States Madagascar India Rwanda Japan Kenya Papua New Guinea Burundi Brazil Philippines Cuba Haiti World total

71.54 3.75 3.50

68.41 3.59 3.35

71.36 3.83 3.45

68.69 3.69 3.33

67.3 3.72 3.63

2.58 2.28 1.84 1.74 1.34 1.28 1.09 1.04 0.9 0.85 0.73 0.64 0.64 0.5 0.5 0.49 0.49 100

2.30 2.05 1.93 1.51 1.41 1.34 1.23 1.23 1.06 0.93 0.81 0.69 0.60 0.58 0.54 0.51 0.49 103.88

2.21 1.97 1.86 1.46 1.35 1.29 1.19 1.18 1.02 0.9 0.78 0.66 0.57 0.56 0.52 0.49 0.48 100

China Nigeria The United Republic of Tanzania Indonesia Uganda Ethiopia Angola India The United States Vietnam Madagascar Rwanda Japan Mozambique Burundi Papua New Guinea Kenya Brazil Haiti Cuba

70.79 3.92 3.82

2.70 2.38 1.93 1.82 1.40 1.34 1.14 1.09 0.94 0.89 0.76 0.67 0.66 0.53 0.52 0.51 0.51 104.57

China Nigeria The United Republic of Tanzania Indonesia Uganda Angola Ethiopia The United States Vietnam Kenya India Madagascar Rwanda Japan Papua New Guinea Brazil Burundi Philippines Cuba Haiti

2.27 2.13 1.94 1.83 1.47 1.43 1.27 1.11 0.92 0.86 0.73 0.73 0.70 0.70 0.67 0.65 0.59 105.19

2.16 2.02 1.84 1.74 1.4 1.36 1.21 1.06 0.87 0.82 0.69 0.69 0.67 0.66 0.64 0.62 0.57 100

Source: Data from FAOSTAT Database (http://www.fao.org).

Sweet potato: origin and production

11

uses. In many countries where the annual production is small, sweet potatoes are mainly consumed as a vegetable, but in other regions, such as in some tropical areas, especially in Africa, almost all sweet potatoes are consumed as a staple food. As a result, Africa soon became the second largest grower and consumer of sweet potatoes. Third, the cultivation of sweet potatoes is closely related to the final uses. Chinese has developed a great number of uses for this starchy food crop. In particular, the invention of the starch vermicelli (“fentiao” in Chinese) has greatly increased the demand for this crop. Vermicelli made from sweet potatoes is widely welcomed in many kinds of Chinese cuisines, such as hot pots, stewed vegetables, and convenience fast food. The unique flavor, taste, and physicochemical properties make sweet potatoes irreplaceable by other starch materials such as corn, potato, and cassava starch. As a result, although sweet potatoes are no longer used as a staple food in China, there is still a huge market. Nowadays, about half of the total production of sweet potatoes is used for starch extraction, the remainder is used for fresh consumption, animal feed, or the manufacturing of other products. These factors keep the demand for the crop at a stable high level. There are several cultural factors that also influence the cultivation and production of sweet potatoes. It is not only in poor countries or regions that sweet potatoes are considered as a staple food. This also holds true for many other regions around the world in hard times when there is a crop failure of other staple foods, because sweet potatoes are productive and can be grown in poor soils with little fertilizer. Thus since they are a reliable crop even in cases of crop failures of other staple foods, sweet potatoes are often associated with hard times in the minds of some people. When these people became affluent enough to change their diet, they tend to abandon sweet potatoes and turn to other foods. As a result the cultivation area of the plant may diminish in these areas. The good news is that sweet potatoes are so delicious that they soon gain the favor of the young generation and sweet potatoes become popular again after a period of time. In the meantime, with the advancement of the food processing technology, more and more industrial uses of sweet potatoes have been found, ushering in a new era of development and planting of this highyield crop. Importation and exportation of sweet potatoes In 2016, there were 117 countries that imported a total amount of 306,056 tons of sweet potatoes from other countries. As shown in

12

Sweet Potato

Table 2.3, the leading countries in the importation of sweet potatoes are the United Kingdom (66,128 tons), followed by Canada (50,619 tons), the Netherlands (32,096 tons), and Japan (21,710 tons), clearly demonstrating that sweet potatoes are loved by people in developed countries too. Many of the countries in the importing countries’ list, including the United Kingdom, Canada, the Netherlands, France, and Germany, do not plant sweet potatoes themselves at all, so all of their needs are met by importation. Japan is an exception as it produced 942,300 tons of sweet potatoes itself. However, production could not meet its own demands and as a result it imported 21,710 tons from other countries. In 2013 the worldwide sweet potatoes imports amounted to US$ 252.11 million. As shown in Fig. 2.2, in the past 10 years before 2013, the amount was increasing continuously from US$ 85.13 million in 2004. After 2012 the growth rate seems to have increased (from US$ 210.15 million to US$ 252.11 million) and the figure may progressively increase in the future. According to the data from the FAOSTAT database, in 2013 the worldwide exports of sweet potatoes totaled 256,988 tons. Interestingly, as shown in Table 2.4, China is not the largest exporter. The leading country in the exportation of sweet potatoes is the United States (128,231 tons), followed by China (19,870 tons) and Egypt (15,651 tons). The reason may be due to that China does not export much of sweet potato, and some other countries do not have much to export. This also demonstrates that in countries that have a relatively large annual production, such as China and Nigeria, sweet potatoes are mainly for domestic consumption and the domestic needs in these countries are very stable. This also implies that the planting of sweet potatoes is very important for these countries. Whereas for many other countries, such as the countries in Europe, the cultivation of the sweet potato is barely significant compared with that of the potato, which is considered an integral part of the daily diet. In Europe, sweet potatoes are mainly consumed as a vegetable or confectionery. Sporadic importation in small amounts alone can meet their needs. In the last few years importation has undergone a gradual increase thanks to the demand in the industrial sector (confectionery). Trend in worldwide production of sweet potatoes The annual production of sweet potatoes is determined by the yield of the crop and the areas used to plant it. As shown in Fig. 2.3, in the last

Table 2.3 The amount (tons) of the imported sweet potatoes from other countries for the top 60 countries in 2013. Country

Value

Ranking

Country

Value

Ranking

Country

Value

Ranking

The United Kingdom Canada The Netherlands Japan Thailand

66,128

1

Lebanon

2032

21

400

41

50,619 32,096 21,710 15,301

2 3 4 5

1687 1485 1350 1320

22 23 24 25

381 360 291 289

42 43 44 45

The United States Italy France

15,220 14,274 14,208

6 7 8

Uruguay Sweden Kuwait Hong Kong SAR, P.R. China Republic of Korea Switzerland Poland

Macau SAR, P.R. China Oman Rwanda Hungary El Salvador

1253 1235 1232

26 27 28

276 252 249

46 47 48

Malaysia Germany Singapore

9809 7469 7428

9 10 11

1134 1105 1011

29 30 31

Bahamas Romania The Czech Republic Nepal Slovakia South Africa

237 233 217

49 50 51

Saudi Arabia Haiti Niger

4559 3682 2835

12 13 14

1000 906 600

32 33 34

Brunei Darussalam Mainland China New Zealand

212 187 183

52 53 54

Belgium Finland China Ireland Mexico The United Arab Emirates

2755 2572 2507 2404 2261 2217

15 16 17 18 19 20

505 477 443 429 425 403

35 36 37 38 39 40

Russian Federation Lithuania Bahrain Somalia Maldives Argentina

167 142 106 106 104 100

55 56 57 58 59 60

Chile Norway Plurinational State of Bolivia Spain Namibia Taiwan, Province of China Botswana Austria Greece Iceland Denmark Portugal

Source: Data from FAOSTAT Database (2013) (http://www.fao.org).

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Sweet Potato

Figure 2.2 The import value (US$1000) of sweet potatoes around the world from 2004 to 2013. Source: Data from FAOSTAT Database (http://www.fao.org).

decade, the worldwide area of sweet potato cultivation has increased from 8,141,292 ha in 2007 to 8,623,973 ha in 2016. Figs. 2.4 and 2.5 show that the increase in the worldwide cultivation area is mainly contributed by Africa, which has increased steadily from 2.86 million ha in 2008 to 4.19 million ha in 2016, an increase of 46.5%. The increase in the cultivation area in Africa has successfully counteracted the decrease in Asia, which has been progressively decreasing since 2008, from 4.51 million ha in 2008 to 3.91 million ha in 2016, a decrease of 13.3%. The year 2015 is a historic moment that marks a turning point. That was the year that Africa surpassed Asia in terms of the area used to plant sweet potatoes and became the world’s leading continent in terms of the area under sweet potato cultivation. Fig. 2.5 also shows that the cultivation area in the Americas has increased in recent years, increasing from 273,726 ha in 2013 to 362,126 ha in 2016, an increase of 32.3%. Considering that the Americas are the origin of sweet potatoes, the climate and soil conditions are most suitable for their growth, and there are huge areas of fertile, arable lands in the Americas, the prospects are very attractive for a greatly increased production of sweet potatoes in the Americas. In comparison the areas under cultivation in Europe and Oceania have been very steady over this same period, with no big changes, implying that the people in these

Table 2.4 The amount (tons) of the exported sweet potatoes from the top 60 countries in 2013. Country

Value

Ranking

Country

Value

Ranking

Country

Value

Ranking

The United States

128,231

1

Ecuador

1488

21

92

41

China

19,870

2

Japan

1029

22

87

42

Egypt The Netherlands Indonesia Spain Dominican Republic Honduras The United Kingdom

15,651 15,348 9797 9246 8556 7181 6345

3 4 5 6 7 8 9

611 589 552 552 483 436 311

23 24 25 26 27 28 29

62 59 57 53 53 39 38

43 44 45 46 47 48 49

Denmark Italy Peru Brazil

4994 4566 3575 2784

10 11 12 13

India Germany Australia Canada Portugal Albania Taiwan, Province of China Singapore Argentina Senegal Oman

The United Arab Emirates Islamic Republic of Iran Tonga Hungary Cameroon Mali Republic of Korea Slovenia Mexico

283 280 274 273

30 31 32 33

34 30 27 22

50 51 52 53

South Africa

1979

14

Madagascar

202

34

20

54

Turkey France Malaysia Belgium

1946 1894 1834 1583

15 16 17 18

159 136 119 116

35 36 37 38

19 18 17 16

55 56 57 58

Saint Vincent and the Grenadines Jamaica

1550

19

Costa Rica Guatemala Lithuania Hong Kong SAR, P.R. China Kenya

Austria Lebanon Colombia The Czech Republic Bosnia and Herzegovina Uganda Benin Yemen Philippines

112

39

Greece

13

59

1503

20

Rwanda

93

40

Swaziland

13

60

Source: Data from FAOSTAT Database (2013) (http://www.fao.org).

16

Sweet Potato

Figure 2.3 Worldwide cultivation area (ha) of sweet potatoes, 2007 2016. Source: Data from FAOSTAT Database (http://www.fao.org).

Figure 2.4 Cultivation area (hectare) of sweet potatoes in Asia and Africa, 2007 2016. Source: Data from FAOSTAT Database (http://www.fao.org).

continents still have not formed the habit of eating food that contains sweet potatoes. As we have mentioned earlier, Africa is now the largest sweet potato cultivation region in the world. However, the yield of sweet potatoes in Africa is far lower than that of other continents. As shown in Fig. 2.6 the

Sweet potato: origin and production

17

Figure 2.5 Cultivation area (hectare) of sweet potatoes in the Americas, Europe, and Oceania, 2007 2016. Source: Data from FAOSTAT Database (http://www.fao.org).

Figure 2.6 Comparison of the yield of sweet potatoes on different continents, 2007 2016. Source: Data from FAOSTAT Database (http://www.fao.org).

yield of the crop in Asia is relatively higher than that of other continents. In 2016 the yield in Asia was as high as 200,823 hg/ha. The yield in Europe was 198,252 hg/ha, which is also very high. But the yields in other continents were much lower than Asia and Europe. For instance, the yield in Africa was only 50,903 hg/ha, which is only about one fourth

18

Sweet Potato

of that in Asia. However, this also means that there is a great potential for Africa to increase its production level of sweet potatoes in the future.

The production of sweet potatoes in China Cultivation area and yield As we have mentioned earlier, the cultivation of sweet potatoes in Asia has been progressively decreasing in the last decade. It is mainly caused by the decline of the area under cultivation in China. As shown in Fig. 2.7, during this period the cultivation area in China has declined from 3.76 million ha in 2008 to 3.28 million ha in 2016, a decline of 12.8%. The decline of the area under cultivation for sweet potatoes in China is caused by many factors. First, China has a long history of massive cultivation of sweet potatoes and the land used to plant the crop is widespread all over the country, especially in the hard times before the country’s reform and opening up to the world. However, with the rapid development of the economy in China after the 1980s, the food choices of Chinese people have changed greatly. People in most parts of China now have access to a huge variety of vegetables, fruits, and almost every kind of food all year round. As a result the demands for staple foods such as rice, wheat, and corn have all declined proportionally. The people no longer need to grow so many sweet potatoes to keep them alive as they were forced to do during the previous hard times. According to the China

Figure 2.7 Cultivation area of sweet potatoes in China, 2007 2016. Source: Data from FAOSTAT Database (http://www.fao.org).

Sweet potato: origin and production

19

Table 2.5 The uses of sweet potatoes in China. Usages

Percentage in 2013

Predicted percentage, 2016 20

Quantity (million tons)

Extracting starch Consuming as fresh vegetable Animal feed Inevitable losses Reserved as seeds for the coming year Aggregate

50 25

55 30

44 55 24 30

10 10 5

5 5 5

4 5 4 5 4 5

100

100

80 100

Source: Data from Dai, Q.W., Niu, F.X., Sun, J., Cao, J., 2016. Changes analysis of sweet potato production and consumption structure in china. J. Agric. Sci. Technol. 18, 201 209.

Rural Statistical Yearbook (2014), the proportion of the land used to grow sweet potatoes has decreased from above 4% in 1995 to less than 2% in 2016. Second, as shown in Table 2.5, approximately 50% of sweet potatoes in China are used as a raw material for the extraction of the starch. However, during the extracting process, a huge volume of wastewater and fibrous residues are produced simultaneously. Both of the by-products still contain a significant amount of the starch and other nutrients, such as proteins, sugars, and a myriad of micronutrients. They are very prone to spoilage and unavoidably pollute the environment around the factory. In the past 20 years or so, many rivers and much arable land have been contaminated. As a consequence, the government has closed these factories to save the environment. This has also hit the farmers’ willingness to plant the crop and has led to the reduction in the area under cultivation for sweet potatoes in China. Third, with the advancement of agricultural technology, the annual yield of sweet potatoes per hectare has been increased significantly. Farmers can maintain sufficient production with less and less land. According to the data from the FAOSTAT database, in China’s history the area planted with sweet potatoes reached as high as 10.61 million ha in 1961, with an annual production of 74.02 million tons and a yield of 69,738 hg/ha. In contrast, the cultivation area in 2016 (3.28 million ha) had declined to only one-third of that in 1961 but the total production was almost the same (70.57 vs 74.02 million tons) because the yield has been increased tremendously (215,056 vs 69,738 hg/ha).

20

Sweet Potato

Main cultivation regions in China With data obtained from the China Rural Statistical Yearbook (2014), the major provinces growing sweet potatoes in China and the areas planted from 1982 to 2013 are displayed in Table 2.6. It can be found that, in 2013, Sichuan Province and Chongqing municipality together accounted for 25.2% of the total planting area of sweet potatoes in China. Guangdong and Hainan Provinces in combination contributed 3.62 million ha, accounting for 10.8% of the total area. Henan Province is the third largest region in the list, harvesting 3.02 million ha in 2013, corresponding to 9.0% of the total planted area in China. It is also obvious that the planting areas in all of the regions have been decreasing since 1982. Main uses of sweet potatoes in China Sweet potatoes are a versatile crop that can be used for a wide range of purposes. It is estimated that sweet potatoes can be converted to more than 2000 chemical products through various processes. Not only can sweet potatoes be used directly as foodstuffs or feedstuffs, but they also can be used as a raw material in order to extract other products with a higher added value, such as starch, pectin, β-amylase, β-carotene, anthocyanin, and dietary fibers, to name only a few. As shown in Table 2.7, it is predicted that during the 13th “Five-Year Plan” period (2016 20) of China, the total value of the sweet potato industry in China will reach 900 to 1080 billion Chinese Yuan, of which the value of sweet potatoes consumed as fresh vegetables will reach 750 900 billion Yuan. The second major use of sweet potatoes is the extraction of the edible starch that is widely used in Chinese cuisine. The third major use is the use of the extracted starch to further produce various types of the starch vermicelli that is widely welcomed in East Asian countries such as China, South Korea, and Japan. Various snacks made from sweet potatoes, such as dried or fried sweet potato chips, are also welcomed by Chinese people. They are predicted to contribute 20 25 billion Chinese Yuan during this period. The whole powder made from sweet potatoes has been gaining recognition in China in recent years. The powder can be stored for a much longer time than many other forms of the product and thus can be used all year round. In addition, the whole powder has the merit of maintaining most of the nutrients of the fresh sweet potatoes. This is very important from a modern nutritional point of view. Thus it is possible that this processing route will have a rapid development in the future. Other promising uses of sweet potatoes include producing convenient

Table 2.6 The areas (ha) planted in the major sweet potato growing provinces in Mainland China, 1982 2013. Regions

1982

1992

2002

2012

2013

Share in 2013 (%)

Sichuan 1 Chongqing Guangdong 1 Hainan Henan Shandong Guizhou Guangxi Hunan Fujian Anhui Jiangxi Yunnan Hebei Hubei Jiangsu Zhejiang Shaanxi Liaoning Shanxi Jilin Beijing Neimenggu Shanghai Tianjing Ningxia Aggregate

13,970,000 6,149,000 8,993,000 10,809,000 917,000 2,297,000 2,965,000 2,376,000 6,829,000 1,086,000 561,000 3,362,000 1,950,000 3,260,000 1,360,000 979,000 300,000 372,000 38,000 61,000 2000 1000 90,000 419,000 69,146,000

13,420,000 5,727,000 7,246,000 6,697,000 1,251,000 2,526,000 3,059,000 2,588,000 5,700,000 1,419,000 874,000 2,976,000 1,956,000 1,925,000 1,130,000 1,025,000 448,000 481,000 48,000 66,000 8000 5000 43,000

12,521,000 4,805,000 5,523,000 2,802,000 2,322,000 2,890,000 2,915,000 2,597,000 3,806,000 1,454,000 1,650,000 2,220,000 1,988,000 1,438,000 1,022,000 897,000 605,000 529,000 22,000 62,000 26,000 8000 19,000

8,485,000 3,595,000 3,119,000 2,450,000 2,431,000 2,111,000 1,553,000 1,618,000 1,478,000 1,254,000 1,425,000 1,006,000 786,000 602,000 533,000 500,000 271,000 214,000 55,000 21,000 400 10,000 11,000

8,418,000 3,623,000 3,019,000 2,485,000 2,484,000 2,004,000 1,863,000 1,664,000 1,443,000 1,320,000 1,297,000 962,000 658,000 585,000 545,000 542,000 242,000 219,000 55,000 14,000 11,000 10,000 7000

25.2 10.8 9.0 7.4 7.4 6.0 5.6 5.0 4.3 3.9 3.9 2.9 2.0 1.7 1.6 1.6 0.7 0.7 0.2 0.0 0.0 0.0 0.0

60,618,000

52,121,000

33,528,400

33,470,000

100

Source: Data from China Rural Statistical Yearbook 2014.

22

Sweet Potato

Table 2.7 Prediction of sweet potato processing industry in China, 2016 20.

Fresh consumption Edible starch Starch vermicelli Snacks Whole powder Industrial starch Total

Raw material consumption (million tons)

Products (million tons)

Value (billion Yuan)

24 30

25 30

750 900

40 50 25 30 4 5 5 10 15 68 85

8 5 1 1 2

150 200 100 120 20 25 20 10 15 900 1080

10 6 1.2 3

Source: Data from Dai, Q.W., Niu, F.X., Sun, J., Cao, J., 2016. Changes analysis of sweet potato production and consumption structure in china. J. Agric. Sci. Technol. 18, 201 209.

snack foods as well as more sophisticated industrial products such as alcohol, citric acid, food pigments, and modified starches. For example, sweet potatoes can be converted into ethanol through fermentation and dehydration and then used as the fuel for automobiles. It is estimated that each hectare of sweet potatoes can produce 4500 6000 kg of 95% ethanol. From 2018 traditional gasoline will gradually be replaced by ethanol gasoline to be used as the fuel for vehicles in China. By 2020 the whole country will achieve the goal of the full coverage of the use of the ethanol gasoline in all cars. This also provides an opportunity for the sweet potato processing industry. Output of the sweet potatoes processing industry in China Postharvest processing of sweet potatoes in China has several characteristics. First, in East, Central, and Southwest China, sweet potatoes are mainly used for starch extraction. The downstream utilization of the starch in these regions is mainly focused on the production of various types of starch vermicelli. However, in North and Southeast China, starch extraction is not widespread. Fresh sweet potatoes are usually directly dried by various means to produce the dried slice and strip products. These products go directly to the snack food market. In recent years many companies have been also exploring new processing and utilization methods to enrich their product lines as well as to prolong their production season. New product forms, such as sweet potato powder, frozen mashed sweet potato chips, steamed bread, and noodles, are also appearing

Sweet potato: origin and production

23

in the market. The second characteristic of the Chinese sweet potato processing industry is that it is dominated by small workshops. Most of the small workshops employ only dozens of or even just a few employees. Large-scale processing plants are still too few and their production capacity is limited. Moreover, these large-scale enterprises are mainly located in the relatively rich provinces in East and Central China. According to the statistics from the China Sweet Potato StarchSpecialized Committee, China Starch Industry Association (CSPSSCCSIA, www.zgganshu.com), there are only 38 relatively large sweet potato processing companies in China. In 2016 these companies in combination produced 294,800 tons of sweet potato starch, 395,100 tons of dried sweet potatoes, 6000 tons of whole sweet potato powder, and 29,900 tons of other sweet potato products. In terms of the starch production, there were only six companies with an output of 10,000 tons or more in 2016, including four in Shandong Province, one in Henan Province, and one in Jiangsu Province. The output of these six companies (218,300 tons) accounted for 74.05% of the total output. There were only eight enterprises with an annual output exceeding 5000 tons, of which five were from Shandong Province, and one each from Henan, Hubei, and Jiangsu Province, respectively. Their combined output (230,300 tons) accounted for 78.12% of the total output. In 2016 the member companies of the CSPSSC-CSIA in combination produced 162,800 tons of the vermicelli from sweet potato starch. There were only three companies with an output exceeding 10,000 tons, including one each in Shandong, Henan, and Sichuan Province. The output of these three companies (131,300 tons) accounted for 80.65% of the total output. There were only five companies with an annual output of more than 5000 tons, including two companies in Shandong, and one each in Henan, Sichuan, and Hubei Province. The output of these five companies (145,300 tons) accounted for 89.25% of the total output of sweet potato vermicelli. In comparison to the huge amount of fresh sweet potatoes harvested in China, it is obvious that the limited total output of the processed products from these relatively large-scale companies from the CSPSSC-CSIA cannot represent the total output of the country. It is known that there are numerous small- and microscale processing workshops that are widespread all over the country, and these have also produced an innumerable amount of starch, vermicelli, and snack food products from sweet

24

Sweet Potato

potatoes. However, it is too hard to record and/or calculate the total outputs of these small- and microscale companies. Prospects for the future As we have mentioned earlier, there are still many obstacles that hinder the development of the sweet potato processing industry. First, fresh sweet potatoes are perishable and it is very hard for ordinary peasants to preserve them for a long period of time. As a consequence most of the harvested sweet potatoes must be processed into other products as soon as possible; otherwise there will be serious economic loss. Second, the environmental pollution caused by the by-products of the starch-extracting factories must be solved properly; otherwise these factories will be shut down by the government. It means that a revolutionary processing roadmap must be conceived to completely utilize the whole value of the tuberous root without producing any by-products and pollutants. In 2018 a company in Henan Province took the initiative to change its mindset and adopted Peng-Gao Li’s innovative invention of a novel concentrated sweet potato extract production method to carry out the deep processing of sweet potatoes. Using this patented method, the factory can fully utilize the entire sweet potato root without producing any waste, and without any environmental pollution. During the process, three kinds of final products, namely starch, concentrated sweet potato extract, and high dietary fiber powder, can be simultaneously obtained. As a result the added value of the final products is tremendously augmented. With the promotion of this novel processing technology, China’s sweet potato processing industry is expected to usher in a new period of development. Of course, there are still many other problems that need to be solved. In this aspect the most imperative task is the comprehensive study of the physicochemical, functional, and nutritional properties of the whole plant. This is the prerequisite for the R&D of most of the food products. In early 2015 China launched a national project of producing potatobased staple foods, such as steamed bun, bread, and noodles. This government project is considered beneficial for ensuring the food security of the country and augmenting the nutritional status of the people. This policy is also beneficial for the development of the sweet potato industry. The fibrous residue produced during the starch extraction process still contains approximately 40% 50% of the starch and 20% 30% of the dietary fiber on a dry basis, so it can to be processed into a high fiber staple food powder. The powder can be used to produce steamed buns (mantou in

Sweet potato: origin and production

25

Chinese), pancakes (shaobing in Chinese), and biscuits as well as a myriad of local specialty staple foods that are enthusiastically welcomed by the local people in different parts of China. All in all, it is obvious that deep processing is vital for the growth of the sweet potato industry in the leading countries. New forms of the product must be developed from this crop to meet the upgraded demands of the modern market. Otherwise a decline in the production of sweet potatoes is predicted.

References Austin, D., 1988. Exploration, maintenance, and utilization of sweet potato genetic resources. International Potato Center, the Taxonomy, Evolution and Genetic Diversity of Sweet Potatoes and Related Wild Species. Lima, Peru. Bovell-Benjamin, A.C., 2014. Sweet Potato: Origins and Development. Springer, New York. Dai, Q.W., Niu, F.X., Sun, J., Cao, J., 2016. Changes analysis of sweet potato production and consumption structure in china. J. Agric. Sci. Technol. 18, 201 209. Kim, J., 2012. A history of korea: from “land of the morning calm” to states in conflict. Indiana University Press, Bloomington. Nishiyama, I., 2006. Evolution and domestication of sweet potato. Bot. Mag. Tokyo 84, 377 387. O’Hair, S.K., Janick, J., Simon, J.E., 1990. Tropical root and tuber crops. Advances in new crops. In: Proceedings of the First National Symposium ‘New Crops: Research, Development, Economics’, Indianapolis, IN, USA. Roullier, C., Benoit, L., Mckey, D.B., Lebot, V., 2013. Historical collections reveal patterns of diffusion of sweet potato in Oceania obscured by modern plant movements and recombination. Proc. Natl Acad. Sci. U.S.A. 110, 2205 2210. Takegoshi, Y., 2003. The Economic Aspects of the History of the Civilization of Japan. Taylor & Francis, London.

Further reading Van Tilburg, J.A., 1995. Easter Island: Archaeology, Ecology and Culture. British Museum Press and Smithsonian Institution Press, London and Washington, DC. Bassett, K.N., Gordon, H.W., Nobes, D.C., Jacomb, C., 2004. Gardening at the edge: documenting the limits of tropical Polynesian kumara horticulture in southern New Zealand. Geoarchaeol. Int. J. 19, 185 218.

CHAPTER 3

Sweet potato starch Tai-Hua Mu and Miao Zhang

Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China

Overview of starch Source of starch Starch is the most abundant storage reserve carbohydrate in plants. It is widely found in cereal grain seeds (e.g., corn, wheat, rice, and sorghum), tubers (e.g., potato), roots (e.g., sweet potato, cassava, and arrowroot), legume seeds (e.g., peas, beans, and lentils), fruits (e.g., green bananas, unripe apples, and green tomatoes), trunks (e.g., sago palm), and leaves (e.g., tobacco) (Chen et al., 2003). Generally root and tuber crops are rich sources of starch containing 70% 80% of water, 16% 24% of starch, and less than 4% of trace quantities of protein and lipids, beside other minerals and vitamins (Hoover, 2001). The major food consumed by human is starch, providing 75% 80% of the total caloric intake worldwide (Bemiller and Whistler, 1996). China is the second largest starch producing country in the world, after the United States, producing about 11.1 million metric tons of starch annually as of the year 2005 which principally comprises corn, cassava, potato, sweet potato, and wheat starch (Wang, 2005). Starch plays a vital role in developing food products either as a raw material or as a food additive, such as thickener, stabilizer, or texture enhancer (Aina et al., 2012). Starch is useful in maintaining the quality of stored food products; it improves moisture retention and consequently controls water mobility in food products. It could also be used as a delivery vehicle for substances of interest in the food and pharmaceutical industries such as antioxidants, colorants, flavors, and pharmaceutically active proteins (Guan et al., 2000). Starch is extracted from various starch-rich crops by wet separation techniques. The crops are harvested at full maturity, washed, and ground. The starch granules will settle in water due to their higher density. However, the sedimentation of starch granules in water is hindered by the presence of various nonstarch components like mucilage and latex, leading Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00003-X

© 2019 Elsevier Inc. All rights reserved.

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Sweet Potato

not only to the loss of starch, but also lowering of the quality of the extracted starch. In addition, the presence of nonstarch components and microorganisms affects the color of the starch, limiting its use in food and textile applications. Studies on the better recovery and economical extraction of starches with good functional properties indicated that lactic and citric acids improve the yield and color of starch from sweet potato tubers; on the other hand, an enzymatic method was developed for the enhancement of the recovery of starch from some roots and tubers (Balagopalan et al., 1996). In addition, sweet potato starch was also isolated by sour liquid processing and centrifugation (Deng et al., 2013).

Structural and physicochemical characteristics of starch Color and granule sizes of starch Native starch is whitish, odorless, bland, and insoluble in water. Color is an important criterion for starch quality, especially for use in the textile industries. Starch pastes should be clear and free from any off-color for better acceptability (Radley, 1976). Its granules exist in different ranges of size distribution, shapes, and dimensions (Table 3.1), which are greatly influenced by their botanical sources and growing and harvest conditions. Rice and oat starch have tiny granules ranging from 1.5 to 9 μm, whereas potato starch granules are as large as 100 μm (Chen et al., 2003). Suganuma and Kitahara (1997) reported the sizes of orange and purple Table 3.1 Characteristics of starch granules from different sources. Starch

Granule shape

Granule size range (µm)

Reference

Maize (waxy and normal)

Spherical, round, polygonal

2 30

Potato

Oval, spherical, lenticular Round, polygonal, oval, bell, round, Round, truncated, cylindrical, oval, spherical Spherical

5.5 72.2, 5 100

Chen et al. (2003); Tester and Karkalas (2002) Tester and Karkalas (2002) Chen et al. (2003); Moorthy (2002) Asaoka et al. (1992)

Sweet potato Cassava

Sorghum Mung bean Wheat

Round, oval Lenticular (A type), spherical (B type)

2 42, 3.4 27.5 4 35, 4 43, 5 45

5 20 1 45 15 35, 2 10

Tester and Karkalas (2002) Chen et al. (2003) Tester and Karkalas (2002)

Sweet potato starch

29

sweet potato starch to be in the range of 7 12 and 12 15 μm, respectively, with round, oval, and polygonal shapes. Granule size substantially affects the swelling power, solubility, and digestibility of starch. Proximate composition of starch Starch granules are composed of two major polymers: amylose and amylopectin, which are composed of glucose molecules linked together in linear and branched forms, respectively. Generally, depending on the plant, starch consists of 20% 25% of amylose and 75% 80% of amylopectin by weight. Waxy starches consist of less than 15% of amylose, and normal starches are 20% 35% amylose, while the high-amylose starches contain more than 40% amylose content (Tester et al., 2004). The structures and the relative amount of both polymers play an important role in determining starch properties. Amylose Amylose is a minor component of native starches; it forms a colloidal dispersion in hot water, whereas amylopectin is completely insoluble. The structure of amylose consists of long polymer chains of glucose units connected by α-acetal linkage. In an amylose chain all of the monomer units are α-D-glucose, and all the α-acetal links connect C-1 of one glucose molecule to C-4 of the next glucose molecule (Ophardt, 2003). The side chains range in length from 4 to over 100 (Hizukuri et al., 1981). Amylose content varies considerably among different starches, and genetic modifications have been carried out to obtain starch with amylose contents varying from 0% to .75%. The amylose fraction usually can be extracted by aqueous leaching procedures, dispersion, and precipitation (Adkins and Greenwood, 1969; Banks et al., 1971; Hizukuri, 1996). Considerable variations in the amylose content of sweet potato starches have been reported by various researchers (Chen et al., 2003; Noda et al., 1992; Zhu et al., 2011). Amylose content and degree of polymerization (DP) of amylose play a key role in influencing the physicochemical and technological properties of starch. Amylopectin Compared to amylose, the structure of amylopectin is more complex since 4% 5% of the total linkages form branches. Amylopectin molecules exist as heterogeneous mixtures and are thus usually characterized by the average values of DP and “chain length” (CL). CL is the total amount of

30

Sweet Potato

carbohydrate divided by the number of nonreducing end groups (Chen et al., 2003; Hoover, 2001). The distribution of CL can be determined by size-exclusion chromatography and high-performance anion exchange chromatography with pulsed amperometric detection after debranching of amylopectin with isoamylase or pullulanase (Hizukuri, 1996). The α-acetal linkages connect C-1 of one glucose to C-4 of the next glucose molecule. The branches are formed by linking C-1 to C-6 through acetal linkages (Ophardt, 2003). Others The moisture content of air-equilibrated starch from cereals, roots, and tubers varies from about 6% to 18% (Moorthy, 2002; Tester et al., 2004). The moisture content prescribed for safe storage of dry starch in most of the starch producing countries is 13%. Higher levels of moisture can lead to microbial damage and subsequent deterioration in quality (Moorthy, 2002). The quantities of other components, such as lipids, protein, and minerals, are strongly influenced by the extraction methods and several other factors including the age of the crops and environmental conditions (Nkala et al., 1994). Lipids form another important component that has a strong effect on the starch properties (Sriroth et al., 1998). Generally, cereal starches contain higher lipid content (0.2% 0.8%) and protein (0.2% 0.5%) than root and tuber starches (Chen et al., 2003). Phosphorus content in sweet potato starch is similar to that in cassava starch (Asaoka et al., 1992), but much less than that in potato starch. Takeda et al. (1986) also found that in sweet potato, amylose contains less phosphorus (3 6 μg/g) than amylopectin (117 144 μg/g). Unlike other minerals phosphorus exerts significant effects on the functional properties of starches. High phosphorus content can impart high viscosity to starch and also improve its gel strength. High phosphorus starches can find use in food applications requiring high gel strength, such as jelly, etc. (Moorthy, 2002). Structural characteristics of starch Numerous investigations have been done to establish the level of intergranular organization within starch granules. The techniques used vary from X-ray diffraction (XRD) to atomic force microscopy and Fourier transform infrared spectroscopy, etc. In the native form of starch, amylose and amylopectin molecules are organized in granules as alternating semicrystalline and amorphous layers that form growth rings (Jobling, 2004).

Sweet potato starch

31

The semicrystalline layer consists of ordered regions composed of double helices formed by short amylopectin branches, most of which are further ordered into crystalline structures known as the crystalline lamellae. The amorphous regions of the semicrystalline layers and the amorphous layers are composed of amylose and nonordered amylopectin branches. Starch crystallinity has been assigned to the well-ordered structure of the amylopectin molecules inside the granules (Moorthy, 2002). Starches present either A, B, or C crystalline types, and the C type is a mixture of the A and B type crystalline patterns. Most cereal starches display the A pattern, while tuber starches (e.g., potato, lily, canna, and tulip) exhibit the B pattern (Tester and Karkalas, 2002). Sweet potato starch was reported to possess the A and C patterns, or be intermediate between A and C (Moorthy, 2002). According to the report of Takeda et al. (1986), the A pattern was observed for two varieties, whereas another cultivar exhibited the C pattern. The distribution of crystallites in starch granules is an important factor controlling the rate of hydrolysis. Gérard et al. (2001) reported that B and C type crystalline patterns showed greater resistance to enzymatic hydrolysis compared to the A type pattern. Gelatinization and pasting properties of starch Gelatinization properties of starch are analyzed using differential scanning calorimetry. When starch is heated in the presence of abundant water, it results in the complete loss of crystallinity. This transformation process is referred to as “Gelatinization.” During gelatinization the structures of the starch granules are disrupted by extensive swelling of the granules. This is measured by the loss of birefringence and the point at which birefringence first disappears is regarded as the “gelatinization point” or “gelatinization temperature” (Whistler and Daniel, 1985). Gelatinization endotherms reflect the hydrogen-bond dissociation of double helices (Tester and Sommerville, 2003). Garcia and Walter (1998) investigated two Peruvian sweet potato varieties cultivated at different locations and found a range for the onset of gelatinization (To) to be between 58°C and 64°C, peak gelatinization (Tp) to be between 63°C and 74°C, and gelatinization conclusion (Tc) to be between 78°C and 83°C. The cultivation location was found to influence the thermal parameters. Various starch modification processes have also been reported to significantly affect the thermal properties of starches (Kawai et al., 2007; Umemoto et al., 2002).

32

Sweet Potato

Starch paste is an interesting system for rheological studies because of its viscoelastic behavior. The utilization of starch in the textile, paper, adhesives, and food industries depends on the pasting properties, which include pasting temperature, peak viscosity (PV), breakdown viscosity (BDV), final viscosity (FV), and setback viscosity (SBV). Different varieties of sweet potato starches exhibit considerable variation in their pasting characteristics. However, sweet potato and cassava starches have some similarities in their pasting properties (Moorthy, 2002). The pasting temperature is the temperature at which a perceptible increase in viscosity occurs. This is always higher than the gelatinization temperature and is usually measured using a viscometer, such as a Brabender Viscograph or Rapid Visco Analyzer (RVA). The pasting temperature of sweet potato starch obtained using a Brabender Viscograph varied between 66.0°C and 86.3°C, while microscopic determination gave values between 57 70°C and 70 90°C (Moorthy, 2002). Based on previous investigations, sweet potato starch pastes possess a rigid viscous behavior and low gel strength (Jangchud et al., 2003). Collado et al. (1999) studied the pasting properties of 44 different sweet potato genotypes at 7% and 11% concentrations using RVA and worked out the correlations among the RVA parameters. They observed wide variation not only in the PV but also the broadness of peak, which has been attributed to another parameter, that is, time elapsed from the start of gelatinization to the time PV is reached. And a significant negative correlation was observed between PV and amylose content. Srichuwong et al. (2005) also reported that the PV of starch paste increased with low amylopectin content ratio, low amylose content, and large average granule size for 15 starches from different origins. Swelling power and solubility of starch Starch granule swelling ability is usually quantified by swelling power (the weight of sedimented swollen granules per gram of dry starch) or swelling volume (the volume of sedimented swollen granules per gram of dry starch) at the corresponding temperature (Konik et al., 2001). Swelling power provides evidence of noncovalent bonding between starch molecules. The degree of swelling and solubility in starches depends on the following factors: amylose amylopectin ratio, CL and molecular weight distribution, degree/length of branching, and conformation. Srichuwong et al. (2005) reported that the swelling power of starch granules increased with amylopectin unit chain ratio, and the swelling power and pasting

Sweet potato starch

33

properties were associated with the ratio of the relative molar distribution of amylopectin branch-chains in the starches from different botanical sources including sweet potato starch. The solubility of starch extracted from seven sweet potato collections from Peru increased to about 10% with an increase in temperature, while a higher degree of solubility (28%) was observed for the commercial starches. It was found that selection identity did not have noticeable effect, but location had significant influence at temperatures above 60°C (Garcia and Walter, 1998). Retrogradation of starch Sol stability or paste stability reflects the retrogradation tendency of starch pastes (Moorthy, 2002). Starch granules undergo irreversible swelling when heated in excess water above their gelatinization temperature, resulting in amylose leaching into the solution. In the presence of a sufficient starch concentration, this suspension will form an elastic gel on cooling. The molecular interactions that occur after cooling are mainly hydrogen bonding within the starch chains, known as retrogradation (Ratnayake et al., 2002). Amylose is considered to be primarily responsible for the short-term retrogradation process due to the fact that the dissolved amylose molecules reorient in a parallel alignment. The long-term retrogradation is represented by the slow recrystallization of the outer branches of amylopectin (Daniel and Weaver, 2000). The rate and the extent of retrogradation increase with the increase in the amount of amylose. In addition to the origin of the starch, retrogradation also depends on starch concentration, storage temperature, pH, temperature procedure, and the composition of the starch paste. Retrogradation is generally stimulated by a high starch concentration, low storage temperature, and pH values between 5 and 7 (Chen et al., 2003; Moorthy, 2002). Ishiguro et al. (2000) studied the retrogradation tendencies of starch isolated from 10 sweet potato cultivars with different amylose contents and CL distributions. Starches with fewer amylose and amylopectin molecules and higher contents of extrashort chains (around DP 10) retrograded more slowly compared to the others. Digestibility of starch The digestibility of starch by enzymes is important for evaluating their nutritive value as well as their use for various industrial purposes. A number of researchers have reported the effect of the action of amylases on the sweet potato starch granules (Noda et al., 1992; Rocha et al., 2010;

34

Sweet Potato

Zhang and Oates, 1999; Zhu et al., 2011). These studies showed that starches varied in their susceptibility to enzymatic hydrolysis, and the variability in their susceptibilities to the interaction were attributed to various factors including the starch source, granule size, amylose lipid complex, type of enzymes, and hydrolysis conditions (Tester et al., 2006). Among noncereal starches, cassava starch had considerably higher susceptibility to enzymatic hydrolysis than other starches, such as potato and sweet potato starches (Zhang and Oates, 1999). The digestibility of raw starches from eight sweet potato varieties by glucoamylases was compared by Noda et al. (1992), who found that the digestibility was greater than 80% after 24 h of hydrolysis, and no significant correlation was observed between digestibility and amylose content of the starches.

Starch modification Starch modification involves the alteration of the physicochemical properties of native starch to improve its functional properties (Hermansson and Svegmark, 1996). The modification of native granular starches profoundly alters their gelatinization, pasting, and retrogradation properties (Choi and Kerr, 2003). Starch has been modified by various methods to achieve functionalities suitable for diverse industrial applications (Adebowale et al., 2006; Kawai et al., 2007; Lawal et al., 2005; Olayide, 2004). There are four broad kinds of modifications: chemical, physical, enzymatic, and genetic, and the main common modification methods are chemical, physical, and enzymatic. Different modification methods of starches Chemical modification of starches Chemical modification involves the introduction of functional groups into the starch molecules, resulting in markedly altered physicochemical properties. Such modification of native granular starches strongly changes the proximate compositions, gelatinization, retrogradation, and pasting characteristics. Modification is generally achieved through derivatization, for example, acetylation, cationization, oxidation, acid hydrolysis, and crosslinking. There has been a resistance toward the use of chemically modified starches in food applications since many chemicals are used for this kind of modification (Moorthy, 2002). Nevertheless, within the last decade researchers have developed an intense interest in developing novel methods of starch modification with greater emphasis on physical and enzymatic modifications.

Sweet potato starch

35

Physical modification of starches Physical modification techniques can be safely used for food products as they do not involve chemicals. The use of high hydrostatic pressure (HHP) treatment for the physical modification produces starches with unique retrogradation and thermal properties. HHP-gelatinized starches show a lower quantity of released amylose (Stolt et al., 2001). HHP processing has been previously reported to result in the partial gelatinization of starch water mixtures at room temperature without altering their granular integrity (Blaszczak et al., 2005; Kawai et al., 2007). The extent of starch gelatinization during HHP treatment has been reported to be influenced by the starch concentration, holding time, temperature, and the starch origin (Bauer and Knorr, 2004; Kawai et al., 2007). And the degree of gelatinization (DG) starch suspensions increased with the increase in the pressure and decrease in starch content (Katopo et al., 2002). Enzymatic modification of starches Enzymatic modification mainly involves the use of hydrolyzing enzymes to modify the properties of starches (Kaur et al., 2012). Enzymatic hydrolysis of starches could be very effective in understanding the internal structure of starch granules. Amylolytic enzymes are the major type of enzymes involved in the breakdown of starch molecules. The amylases can be categorized into α-amylases, β-amylases, glucoamylases, and debranching enzymes depending on the configuration of the substrate involved or products formed (Goesaert et al., 2010). Furthermore, the amylases have two main classes, endo- and exo-acting enzymes, according to their type of action on substrates. A common type of α-amylase is the endo-acting enzymes that hydrolyze α-D-(1,4)-glycosidic linkages specifically depending on its origin, and internally yielding soluble products such as oligosaccharides and branched and low molecular weight α-limit dextrins; β-amylases are exo-acting enzymes, which hydrolyze the α-(1,4)linkages, beginning at the nonreducing ends of starch molecules, to form β-maltose and β-limit dextrins; while isoamylase and pullulanase are debranching enzymes which hydrolyze only the α-D-(1,6)-glycosidic bonds in an amylopectin chain branch which elevate linear short chains of glucan polymers and form the high-amylose starch (Butler et al., 2004). Value addition of starches by modification Starch modification is aimed at overcoming one or some of the shortcomings of native starches to enhance their versatility and to satisfy consumer

36

Sweet Potato

demand, such as the loss of viscosity and thickening power, retrogradation characteristics, and syneresis (Tharanathan, 2005). In a way starch modification provides desirable functional attributes and economic alternative choices to other hydrocolloid ingredients, thus enhancing the versatility of starch and satisfying consumer demand. Nowadays, more concern has been shown to reducing the dietary calorie intake to avoid obesity complications by health-conscious people. Some of the starch derivatives are increasingly used as fat replacers or fat substitutes, and these derivatives are partially or totally undigested and contribute zero calories to the food consumption (Tharanathan, 2005). When hydrated starch-based fat replacers provide a slippery mouthfeel with various sensory perceptions depending on the different modification types, which work well in food systems with high moisture, such as meat emulsions, salad dressings, and some bakery products. Also resistant starch, an undigested starch fraction, has been found to have beneficial nutritional effects on humans. Resistant starch is a highly retrograded amylose fraction of starch, which can escape digestion in the small intestine but is fermented later in the colon to short-chain fatty acids (Weaver et al., 1992), and is considered to be a man-made dietary fiber with high physiological value and a potential ingredient of low calorie food. In addition, as a main source of starches, sweet potato has played an important role in the Chinese economy. Its characteristic high yield and wide adaptability once made great contributions to feeding the dramatically increasing Chinese population. The group members in the lab of the Potato and Sweet Potato Food Science Innovation Team, CAAS, China have investigated the structural and physicochemical characteristics of sweet potato starches and and/or their modified starches. The findings are summarized in the following sections.

Structural and physicochemical characteristics of sweet potato starch In China, the major commercial use of sweet potato is for its starch and the preparation of starchy food. As the physicochemical properties of starches dictate their functionality in various applications, this section therefore introduces the physicochemical properties of starches isolated from 11 sweet potato cultivars (Mixuan no.1, Chuangshu 217, Xichengshu 007, Xushu 28, Luoshu 10, Shangshu 19, Xushu 22, Xushu

Sweet potato starch

37

27, Chuanshu 34, Xushu 18, and Shi 5) popularly used for starch production in different regions of China.

Proximate composition The chemical composition of the different sweet potato starches is shown in Table 3.2. The purity of the starches was reasonably high ( . 91%). Moisture content (3.86% 6.52%) falls within the moisture level (,20%) recommended for commercial starches (Soni et al., 1993). It is also within the range (,13%) recommended for safe storage in most starch producing countries (ISI, 1970). The protein content of the starches varied between 0.28% and 0.75% with Mixuan no.1 having the highest protein content. A lower level of protein (0.23%) was previously reported for Xushu 18 by Chen et al. (2003), which could be ascribed to the extent of the removal of protein present in the starting material. The ash content varied significantly among the starches with values ranging from 0.10% to 0.47% (P , .05). This falls within the limit (#0.5%) recommended for grade A industrial starches (Radley, 1976). Many of the starches contained no lipid, except for starches of Shangshu 19 and Xushu 22 sweet potato cultivars. Amylose and amylopectin content varied significantly among the starches with values ranging from 13.33% to 26.83% and 73.17% to 86.67%, respectively. Among the starches, Chuanshu 217 showed the highest amylose content while the lowest was found in Chuanshu 34. Amylose and amylopectin content plays an important role in influencing the functional properties of starches. High-amylose starches are characterized by their high gelling strength which suggests their usefulness in the production of pasta, sweets, bread, and in coating fried products (Hung et al., 2005; Vignaux et al., 2005). Differences in the amylose content of sweet potato starches have been reported and ascribed to genotypic differences, environmental factors, and starch processing methods (Garcia and Walter, 1998; Oduro et al., 2000).

Thermal properties The thermal properties of the sweet potato starches are presented in Table 3.3. Gelatinization temperature is the temperature at which heated starch granules undergo the transition from a crystalline state to a gel. Starch gelatinization is an important parameter in starch characterization. The onset transition temperature (To) and peak temperature (Tp) of the starch ranged between 54.5 69.1°C and 62.5 75.9°C with mean values

Table 3.2 Chemical composition of sweet potato starches (w/w, %).a Cultivar

Origin

Moisture

Protein (dbb)

Ash (db)

Lipid (db)

Starch (db)

Amylose

Amylopectin

P3 (db)

Mixuan no.1 Chuanshu 217 Xichengshu 007 Xushu 28 Luoshu 10 Shangshu 19 Xushu 22 Xushu 27 Chuanshu 34 Xushu 18 Shi 5 Mean

Beijing

5.93 6 0.05cd

0.75 6 0.08a

0.15 6 0.01cd

0.00 6 0.00c

94.271.08abc

20.50 6 0.24f

79.5 6 0.24b

0.02 6 0.00

Sichuan

3.86 6 0.27h

0.33 6 0.01b

0.10 6 0.01d

0.00 6 0.00c

92.20 6 1.08dc

26.83 6 0.24a

73.17 6 0.24g

0.02 6 0.00

Sichuan

4.20 6 0.01g

0.34 6 0.09b

0.47 6 0.02a

0.00 6 0.00c

92.87 6 0.19bdc

24.17 6 0.24c

75.83 6 0.24e

0.02 6 0.00

Jiangsu Henan Henan

5.19 6 0.01f 5.77 6 0.03d 6.52 6 0.05a

0.31 6 0.06b 0.39 6 0.00b 0.39 6 0.01b

0.12 6 0.00d 0.31 6 0.11b 0.22 6 0.01c

0.00 6 0.00c 0.00 6 0.00c 0.05 6 0.01a

94.07 6 0.18abc 94.77 6 1.47ab 91.90 6 0.53d

22.00 6 0.47de 23.83 6 0.24c 21.50 6 0.24de

78.00 6 0.47dc 76.17 6 0.24e 78.50 6 0.24dc

0.02 6 0.01 0.02 6 0.00 0.02 6 0.00

Jiangsu Jiangsu Sichuan

6.12 6 0.00bc 5.79 6 0.03d 4.22 6 0.04g

0.32 6 0.10b 0.28 6 0.01b 0.29 6 0.02b

0.17 6 0.01cd 0.17 6 0.02cd 0.17 6 0.01cd

0.02 6 0.01b 0.00 6 0.00c 0.00 6 0.00c

93.17 6 1.49bdc 91.90 6 1.30d 93.13 6 0.58bdc

22.17 6 0.24d 21.33 6 0.47e 13.33 6 0.47g

77.83 6 0.24d 78.67 6 0.47c 86.67 6 0.47a

0.02 6 0.00 0.01 6 0.01 0.002 6 0.001

Jiangsu Sichuan

6.25 6 0.04b 5.53 6 0.04e 5.40

0.30 6 0.05b 0.30 6 0.05b 0.36

0.19 6 0.02cd 0.31 6 0.06b 0.214

0.00 6 0.00c 0.00 6 0.00c 0.01

95.60 6 1.10a 95.32 6 0.32a 93.56

23.50 6 0.24c 25.83 6 0.24b 22.27

76.50 6 0.24e 74.17 6 0.24f 77.73

0.01 6 0.00 0.01 6 0.00 0.02

P, phosphorus content. a Means in a column with the same letters are not significantly different at P , .05. b Dry basis.

Table 3.3 Physicochemical and thermal properties of various sweet potato starches.a Cultivar

Syneresis (%)

Swelling power (g/ g)

Solubility (%)

To (°C)

Tp (°C)

Tc (°C)

Mixuan no.1 Chuanshu 217 Xichengshu 007 Xushu 28 Luoshu 10 Shangshu 19 Xushu 22 Xushu 27 Chuanshu 34 Xushu 18 Shi 5 Mean

37.23 6 1.50fe 44.68 6 1.50a 41.22 6 1.13bc

13.46 6 0.22f 17.08 6 0.48e 17.43 6 0.55e

8.56 6 0.44e 11.56 6 0.23de 12.82 6 0.53d

68.18 6 0.36a 69.11 6 0.28a 68.55 6 0.07a

75.41 6 0.14ab 74.53 6 0.00bc 75.17 6 0.00ab

81.60 6 0.96cde 82.18 6 0.27cd 81.88 6 0.95dc

6.98 6 0.57c 6.40 6 0.48c 9.32 6 0.07b

13.43 6 1.32f 13.08 6 0.01f 13.33 6 1.02f

43.88 6 0.38ab 39.63 6 1.88cde 38.30 6 1.50de 37.23 6 0.00fe 39.89 6 0.75cde 32.45 6 0.75g 34.57 6 2.56cde 40.16 6 1.13cd 39.02

22.20 6 0.22dc 21.37 6 1.93d 23.95 6 1.70abc 26.13 6 0.99a 24.48 6 1.51abc 23.33 6 1.65bcd 25.62 6 0.22ab 23.63 6 1.19abcd 21.70

12.92 6 2.12d 12.35 6 0.37d 18.77 6 1.27ab 13.15 6 1.82cd 12.12 6 1.98d 11.62 6 2.65de 19.97 6 1.42a 16.20 6 0.58bc 13.64

54.54 6 0.08f 58.79 6 1.01d 63.93 6 0.10c 59.97 6 0.11d 64.36 6 0.71c 55.11 6 0.05ef 66.92 6 1.2b 56.00 6 0.0f 62.31

63.81 6 0.00f 72.74 6 1.42d 73.90 6 0.00c 66.75 6 0.42e 75.90 6 0.19a 66.91 6 0.00e 75.84 6 0.57a 62.54 6 0.21g 71.23

78.88 6 0.74fg 82.75 6 0.54cd 84.69 6 0.34a 81.26 6 0.34de 84.46 6 0.12ab 80.16 6 0.54ef 83.03 6 1.08bc 78.59 6 0.88g 81.77

11.64 6 0.54a 10.84 6 1.53a 9.99 6 0.40b 10.08 6 0.39b 9.78 6 0.89b 11.89 6 0.26a 8.88 6 0.53b 8.95 6 0.46b 9.52

24.34 6 0.82ab 23.97 6 1.55ab 20.76 6 0.44dc 21.29 6 0.23dc 20.10 6 0.59d 25.06 6 0.59a 16.12 6 2.28e 22.59 6 0.91bc 19.46

To, Onset transition temperature; Tp, peak transition temperature; Tc, conclusion transition temperature; R, gelatinization range (Tc a Means in a column with the same letters not significantly different at P , .05.

ΔHgel (J/g)

To); ΔHgel, enthalpy of gelatinization.

R (°C)

40

Sweet Potato

of 62.31°C and 71.23°C, respectively. Starches of Xushu 28, Xushu 22, Luoshu 10, Chuanshu 34, and Shi 5 showed significantly lower To, Tp, and conclusion transition temperature (Tc) compared to starches of other cultivars. Vasanthan et al. (1999) indicated starches with higher gelatinization transition temperatures (To and Tp) and enthalpy would require a higher heat of solubilization. However, sweet potato starches with higher transition temperatures, such as Mixuan no.1, Chuanshu 217, and Xichengshu 007, showed lower enthalpy of gelatinization. On the other hand, starches of Xushu 28, Luoshu 10, and Chuanshu 34 with lower transition temperatures showed higher enthalpy of gelatinization (WHgel) with values ranging from 11.64, 10.84, and 11.89 J/g, respectively. The gelatinization parameters (To, Tp, Tc, and WHgel) are strongly influenced by the molecular architecture of the crystalline region of starches (Noda et al., 1992). WHgel mainly reflects the loss of molecular order within the internal structure of starches (Cooke and Gidley, 1992). The gelatinization range (R) of the sweet potato starches varied significantly. Chuanshu 34 showed the highest gelatinization range, while the lowest was observed in Chuanshu 217. The gelatinization ranges showed that the numbers of double helices (in the amorphous and crystalline domains) that disentangled and melted during gelatinization were relatively similar in Mixuan no.1, Chuanshu 217, and Xichengshu 007 starches compared to the starches of other cultivars. It also showed that the degree of heterogeneity of the starch crystallites within Mixuan no.1, Chuanshu 217, and Xichengshu 007 starch granules was lower than those of other starches (Ratnayake et al., 2001). Furthermore, the variation in the gelatinization properties of the starches could be attributed to various factors including mineral composition, proportion of large and small granules, and the molecular architecture of the crystalline region of starches (Kaur et al., 2007).

Scanning electron micrograph The micrographs of the granules of the various sweet potato starches are shown on Fig. 3.1. The shapes of the various starch granules varied from polygonal, round, to cupuliform/bell shapes.

General physicochemical properties Color and granule sizes The color of the various sweet potato starches is presented in Table 3.4. There were significant differences in the color of the sweet potato starches

Sweet potato starch

41

Figure 3.1 Scanning electron micrographs of starch granules of various sweet potato cultivars, showing diversity in shapes and sizes. Values in parentheses denote the degree of magnification. (A) Mixuan no.1 ( 3 3500); (B) Xichengshu 007 ( 3 1000); (C) Xushu 28 ( 3 600); (D) Xushu 18 ( 3 3500); (E) Chuanshu 34 ( 3 3500); (F) Xushu 27 ( 3 1000); (G) Xushu 27( 3 600); (H) Shi 5 ( 3 3500).

Table 3.4 Color, granule size distribution, and shapes of various sweet potato starches.a Cultivar

Color 

L

Mixuan no.1 Chuanshu 217 Xichengshu 007 Xushu 28 Luoshu 10 Shangshu 19 Xushu 22 Xushu 27 Chuanshu 34 Xushu 18 Shi 5 Mean a

97.44 6 0.11fab 95.76 6 0.15g 96.75 6 0.15de 94.47 6 0.02h 97.19 6 0.32bc 97.61 6 0.16a 96.88 6 0.06de 96.44 6 0.06f 96.05 6 0.08g 96.65 6 0.08ef 96.98 6 0.09c 96.56

a



0.01 6 0.00cd 0.24 6 0.01b 0.02 6 0.01d 1.21 6 0.13e 0.03 6 0.01cd 0.03 6 0.01cd 0.22 6 0.02b 0.08 6 0.01cd 0.38 6 0.01a 0.00 6 0.00cd 0.09 6 0.01c 0.01



b

0.75 6 0.01de 0.61 6 0.03ef 0.68 6 0.01e 4.21 6 0.21a 0.89 6 0.00d 1.06 6 0.05c 0.51 6 0.06f 1.67 6 0.02b 0.63 6 0.02ef 1.62 6 0.05b 1.54 6 0.04b 1.29

Means in a column with the same letters are not significantly different at P , .05.

Diameter range (µm)

Average diameter (µm)

Shape

0.85 0.76 0.76 0.76 0.85 0.85 0.76 0.85 0.85 0.85 0.85

13.07 6 0.04a 8.83 6 0.23fg 9.43 6 0.24f 9.31 6 0.09f 8.67 6 0.00g 9.08 6 0.00c 8.10 6 0.03h 10.91 6 0.24d 12.37 6 0.12b 11.53 6 0.28cd 9.93 6 0.01e 10.31

Round, Round, Round, Round, Round, Round, Round, Round, Round, Round, Round,

44.69 26.17 29.12 29.12 26.17 36.08 23.51 36.08 40.15 40.15 32.41

cupuliform, cupuliform, cupuliform, cupuliform, cupuliform, cupuliform, cupuliform, cupuliform, cupuliform, cupuliform, cupuliform,

polygonal polygonal polygonal polygonal polygonal polygonal polygonal polygonal polygonal polygonal polygonal

Sweet potato starch

43

with starch of Shangshu 19 being the whitest. The starch of Chuanshu 34 was found to be redder than other starches, while Xushu 28 showed a greater yellowness. Color is an important criterion in evaluating starch quality. Any form of pigmentation in starch will negatively affect its acceptability and that of its products (Galvez and Resurrection, 1992). A high value of lightness is desired for starches. In the case of starch particle size distribution (Table 3.4), Mixuan no.1 cultivar showed the highest mean granule size (13.07 μm) and the widest granule size range of 0.85 44.69 μm. On the other hand, starch of Xushu 22 showed the lowest mean granule size (8.10 μm) and the narrowest granule size range (0.76 29.12 μm). Hoover (2001) reported sweet potato starch granules as round, oval, and polygonal with sizes ranging from 2 to 42 μm. In comparison to granules of starches from other sources, higher (25.8 μm), lower (1.05 1.32 μm), and relatively similar values (7.3 9.7 μm) values of mean granules sizes were reported for potato, amaranth, and cassava starches, respectively (Chen et al., 2003; Kong et al., 2009). The differences in the granule sizes of the starches are presumably attributed to cultivar differences, growing conditions, and plant physiology. Moreover, starch granule size plays a significant role in influencing the pasting parameters of starches (Noda et al., 2004; Zaidul et al., 2007). Fine starch granules could be used as fat substitutes in high fat foods (Ma et al., 2006). However, starches with larger proportions of small starch granules, like Xushu 22, Luoshu 10, and Chuanshu 217, will find use in applications requiring relatively small starch granules. Digestibility, syneresis, swelling power, and solubility The enzyme digestibility of raw sweet potato starches as measured by pancreatin hydrolysis exhibited significant differences as shown in Fig. 3.2. Enzyme digestibility of raw starches is an important factor to be considered when evaluating their usefulness in diverse food applications. The digestibility of the starches showed variations from 10.35% in Xichengshu 007 to 15.15% in Xushu 18 cultivar with a mean value of 14.00%. The variability in the digestibility of the various starches might be due to environmental conditions associated with the crop growth location, such as temperature, precipitation, and soil. In addition, granule size and the structural characteristics of starches have been previously observed to exert substantial influence on in vitro digestibility of starches (Jayakody et al., 2005; Szylit et al., 1978). Also the interactions of various factors, including starch source, amylose, lipid complex, binding site, hydrolysis condition,

44

Sweet Potato

Figure 3.2 Enzyme digestibility of starches of various sweet potato cultivars. Error bars represent standard deviations. Columns with the same letters are not significantly different at P , .05.

and type of hydrolyzing enzyme, influence starch digestibility (Rocha et al., 2010). However, no significant correlation was found between the digestibility and the amylose content of starches obtained from eight sweet potato cultivars hydrolyzed by glucoamylases (Noda et al., 1992). Syneresis, an index for the degree of starch retrogradation at low temperatures is presented in Table 3.4. After 7 days of storage, the percentage of syneresis varied significantly between 32.45% and 44.68% with a mean value of 39.02%. Starch paste of Chuanshu 217 exhibited a higher retrogradation tendency due to the large volume of water expelled during the retrograding process compared with other starches regardless of the storage period, while Chuanshu 34 showed the lowest syneresis. The higher retrogradation tendency observed in the starch pastes from Chuanshu 217 might be due to its higher amylose content (Singh et al., 2003). Syneresis exhibited a significant positive correlation with amylose content (r 5 0.70, P # .05), while it showed a negative correlation with mean granule size (r 5 20.59, P # .05). The structural arrangement of the chains within the amorphous and crystalline regions of starches has been reported to strongly influence the interaction that occurs between these starch chains during gel storage (Singh et al., 2006).

Sweet potato starch

45

To understand the interactions between the water molecules and the starch chains in the crystalline and amorphous regions during heating, the swelling power and solubility of the starches are shown in Table 3.4. The various sweet potato starches exhibited different swelling power and solubility when heated in water at 90°C. The swelling power of the starches from the various sweet potato cultivars ranged from 13.46 to 26.13 g/g. Starch of Xushu 22 showed the highest swelling power, while the lowest was observed in Mixuan no.1 cultivar. The solubility of the starches ranged from 8.56% to 19.97%, with the lowest found in Mixuan no.1, whereas Xushu 18 showed the highest solubility. Differences in the swelling power and solubility of the starches could be attributed to the variations in the associative bonding forces within the starch granules. Previous studies attributed differences in the swelling and solubility patterns of starches to differences in amylose content, phosphorus, and starch granular properties (Kaur et al., 2007). However, no significant correlation was observed between the swelling power and amylose content in our studies but the swelling power and solubility of the starches positively correlated with each other (r 5 0.64, P # .05).

Pasting properties Sweet potato starches exhibited significant variations in their pasting behaviors (Table 3.5). The plant source, starch purity, and the interactions among starch components strongly influence the pasting properties of starches. The PV of the starches varied from 134 to 255 BU, being lowest for Shangshu 19 followed by Xushu 28 and being highest for Chuanshu 34 followed by Xichengshu 007, with an average value of 209.09 BU. Starch of Xushu 28 showed the lowest hot paste viscosity (HPV), while the highest was observed in Xushu 18. Aina et al. (2012) stated that starches with high HPV would be preferred in applications which require high starch consistency during prolonged cooking. However, amylose leaching, amylose lipid complex formation, friction between swollen granules, and granule swelling have been reported as the key influencing factors of HPV (Singh et al., 2006). BDV, a measure of the starch paste resistance to heat and shear, varied significantly between 91 and 162 BU in the various sweet potato starches, the lowest and highest values being observed for Shangshu 19 and Chuanshu 34 cultivars, respectively. The lower BDV observed in Shangshu 19 starch cultivar suggested its greater resistance to shear as

46

Sweet Potato

Table 3.5 Pasting properties of various sweet potato starches.a Cultivar

PV (BU)

HPV (BU)

BDV (BU)

CPV (BU)

SBV (BU)

PT (Min)

Ptemp (°C)

Mixuan no.1 Chuanshu 217 Xichengshu 007 Xushu 28 Luoshu 10 Shangshu 19 Xushu 22 Xushu 27 Chuanshu 34 Xushu 18 Shi 5 Mean

236d 226d 248b 138i 202f 134j 200g 237c 255a 236d 188h 209.09

81f 82e 91d 8k 75g 43j 71h 100b 93c 101a 58i 73

155c 144d 157b 130h 127j 91k 129i 137e 162a 135f 130g 136.09

167d 166d 201b 120g 134e 69i 130f 200b 192c 211a 94h 153.1

86e 84f 110b 112a 59g 26i 59g 100c 99d 110b 36h 80.09

6.90b 6.67f 6.73e 6.37h 7.03a 6.47g 6.30i 6.77d 6.00j 6.80c 5.97k 6.55

73.20a 72.50c 72.30d 69.70i 71.00g 71.10f 70.20h 72.60b 67.20k 72.10e 67.90j 70.89

PV, peak viscosity; HPV, hot paste viscosity; BDV, breakdown viscosity; CPV, cold paste viscosity; SBV, setback viscosity; PT, peak time; Ptemp, pasting temperature. a Means with the same letters in the same column are not significantly different at P , .05.

compared to the starches of other cultivars. SBV showed the tendency of starch pastes to retrograde. Starches of Xichengshu 007, Xushu 18, Xushu 27, and Xushu 28 showed higher retrogradation tendency due to their higher SBV. On the other hand, starch of Shangshu 19, Shi 5, Luoshu 10, and Xushu 22 showed lower SBV suggesting lower retrogradation tendency. The peak time (PT) of the sweet potato starches ranged from 5.97 to 7.03 min. The highest value was observed in Shi 5, while the lowest was observed in Luoshu 10. Low swelling starches are characterized by high PTs. The pasting temperature (Ptemp) of the starches varied significantly from 67.20°C to 73.20°C in starches of Chuanshu 34 and Mixuan no.1, respectively, with a mean value of 70.89°C. Pasting and thermal properties are the most important properties when considering starches for use as gelling and thickening agents. Starches with relatively high PV, high BDV, and low SBV like Luoshu 10, Xushu 22, and Shi 5 could be considered for use as thickening or gelling agents. However, low PV starches like Xushu 28 and Shangshu 19 would be suitable for the manufacture of weaning foods where low paste viscosity food ingredients are required. At present cereals used in weaning food applications need to be malted to reduce the viscosity of their pastes (Akingbala et al., 2002). Compositional and morphological properties of starch, such as amylose content, phosphorus content, and mean granule

Sweet potato starch

47

size, play crucial roles in influencing the pasting and rheological properties of starches (Liu et al., 2003; Singh et al., 2006; Zaidul et al., 2007). Overall the variability observed among the physicochemical properties of these various sweet potato starches further illustrated their useful potentials in various food and nonfood applications.

Structural and physicochemical properties of retrograded chemically modified sweet potato starch Sweet potato starches can result in a high glycemic index (GI) after cooking or gelatinization. However, a food product with low GI is preferable, not only in obese patients and individuals with diabetes, but also in healthy individuals (Björck and Asp, 1994). Thus it is necessary to improve the starch digestion resistibility and physicochemical properties of sweet potato starches to enhance their health value. This section therefore introduces the enhancement of the starch digestion resistibility of retrograded chemically modified sweet potato starches by retrogradation and further acetylation, as well as the changes of their physicochemical and morphological properties.

Content of resistant starch There are four kinds of resistant starch: Type 1 is physically inaccessible starch; Type 2 is nongelatinized starch; Type 3 is retrograded starch; and Type 4 is physically or chemically modified starch (Haralampu, 2000; Englyst et al., 1983, 1992). Retrogradation is the most common method to make resistant starch, because the processing method is very simple (Bao et al., 2007; Wu et al., 2009; Bravo et al., 1998). Acetylation is generally used to prepare Type 4 resistant starch. Recently, it was found that resistance to amyloglucosidase activity of acetylated retrograded (Type 3/ Type 4-retrograded chemically modified) potato starch was higher than that of retrograded and acetylated potato starch (Zie˛ba et al., 2011a,b, 2014). The retrograded sweet potato starch (RS) was obtained by suspending native sweet potato starch (NS) in distilled water, heating in boiling water to gelatinize the starch, cooling, freezing first and then melting, rinsing with distilled water, centrifuging, oven-drying, grinding, and sieving, while acetylated retrograded sweet potato starch (ARS) was prepared with acetic anhydride (Yu et al., 2015). The NS had high resistance to α-amylase without any processing, of which the resistant starch content

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Sweet Potato

was 68.45%, and thus belonging to the goup of resistant starch granules (Type 2). The resistant starch content of RS was increased by retrogradation to 37.24% (Type 3). An acetyl residue can substitute the hydrogen of hydroxyl at the second and third carbon atoms that are neighboring the α-1,4 glycosidic bond, and thus it can inhibit the hydrolysis of amylase— enhance the hydrolysis resistance of starch (Sha et al., 2012). Therefore further acetylation dramatically increased the resistant starch content (Type 4, a kind of chemical modification starch due to joint chemical reagents) of ARS to 48.02%.

Thermal properties The gelatinization and retrogradation transition temperature and enthalpy of two times scanning were presented in Table 3.6. The retrogradation and further acetylation significantly influenced the To, Tp, and R1/2. The retrogradation decreased the WHgel of NS from 12.63 to 5.85 J/g, while further acetylation not only decreased the WHgel of RS from 5.85 to 2.64 J/g, but also significantly changed the To, Tp, and R1/2. After 2 days of retrogradation, the To, Tp, and WHgel of NS, RS, and ARS were significantly deceased; and the R1/2 of them became broader. The endothermic enthalpy and transition temperature of retrograded starch were usually lower than that for the native starch, and the temperature range of retrograded starch became broader (Singh et al., 2007). The retrogradation degree (RD) of NS was increased by retrogradation from 26.62% to 39.89%. Further acetylation treatment also increased the RD of RS from 39.89% to 52.30%, which was in agreement with the resistant starch content and digestion resistibility. Table 3.6 Gelatinization parameters of native, retrograded, and further acetylated sweet potato starch. Sample

To (°C)

NS

72.93 6 0.27 50.15 6 0.89b 52.98 6 0.24d 49.19 6 1.44c 53.60 6 0.25d 56.51 6 4.22a

RS ARS

1st 2nd 1st 2nd 1st 2nd

Tp (°C) a

R1/2 (°C)

79.93 6 0.18 61.75 6 0.56c 62.42 6 0.24e 61.69 6 0.48c 64.05 6 0.30d 64.40 6 0.50a b

WHgel (J/g)

7.29 6 0.18 12.23 6 0.87c 10.34 6 0.17c 13.97 6 1.42b 13.91 6 0.72b 12.65 6 1.18c d

RD (%)

12.63 6 0.31 3.36 6 0.59a 5.85 6 0.15b 2.33 6 0.28b 2.64 6 0.19c 1.39 6 0.24d a

26.62 6 4.62e 39.89 6 4.84c 52.30 6 5.62b

To, onset transition temperature; Tp, peak transition temperature; R1/2, half temperature range; WHgel, enthalpy of gelatinization; RD, retrogradation degree. Values followed by the different letter in the same column are significantly different (P , .05).

Sweet potato starch

49

Scanning electron micrograph Fig. 3.3 shows scanning electron micrograph (SEM) of both original and heated (boiling water for 20 min) NS, RS, and ARS. ARS is more compact than RS, because the acetylation will lead to the aggregation and fusion of starch, and acetylated granules also formed grooves on the surface of particles (Das et al., 2010). After heat treatment each starch sample changed to the porous structure, which gave them bigger surface area. However, with the retrogradation and further acetylation, the pores of heat-treated starches became bigger and the wall became more compact and thicker.

General physicochemical properties Digestion resistibility The resistance of enzyme digestibility of nonheat-treated and heat-treated NS, RS, and ARS are shown in Fig. 3.4. The digestion resistibility between nonheat-treated and heat-treated NS had more significant differences than RS and ARS, because the native starch in NS was totally gelatinized after heating in boiling water for 20 min since it was easily hydrolyzed by enzyme. The resistibility of RS also showed a significant decrease between the nonheat-treated and the heat-treated forms, but the resistibility of ARS showed the least difference. Therefore ARS were more resistant to heat treatment than NS and RS, and thus further acetylation can significantly increase the digestion resistibility and thermal stability of starch. Zie˛ba et al. (2011b, 2014) found that acetylated retrograded starch was more resistant to the hydrolysis of amyloglucosidase than native and retrograded starch. Water-soluble index, water absorption index, and swelling capacity The water-soluble index (WSI), water absorption index (WAI), and swelling capacity (SWC) of NS, RS, and ARS are shown in Table 3.7. The RS showed the lowest WSI (4.29/100 g dry solids) compared to NS and ARS (P , .05). It was reported that the solubility of retrograded starch was lower than that of native starch (Zie˛ba et al., 2011a). The WAI and SWC of starches were decreased with the retrogradation and acetylation treatment (Table 3.7). As commonly observed, the retrogradation of starch would make the gelatinized starch recrystallize to form a more compact and dense structure. Compared to the NS, the water binding capacity of acetylated starch was slightly higher (Das et al., 2010), which

50

Sweet Potato

Figure 3.3 Resistance of the activity of pancreatin of NS, RS, and ARS. Nonheattreated, before pancreatin treatment without any pretreatment; heat-treated, heated in boiling water for 20 min before pancreatin treatment. Values followed by different letters in the same composition and sample are significantly different (P , .05).

Sweet potato starch

51

Figure 3.4 Scanning electron micrographs of the NS, RS, ARS, and heat-treated sample, respectively. HNS, HRS, and HARS are heat-treated NS, RS, and ARS, respectively. Table 3.7 WSI (water-soluble index), WAI (water absorption index), and SWC (swelling capacity) of native, retrograded, and further acetylated sweet potato starch. Sample

WSI (g/100 g dry solids)

WAI (g/g dry solids)

SWC (g/g dry solids)

NS RS ARS

6.08 6 0.33e 4.29 6 0.18f 13.77 6 0.28c

16.49 6 0.55a 14.91 6 0.94b 13.18 6 0.29c

17.55 6 0.45a 15.58 6 0.38b 15.29 6 0.53b

Values followed by the different letters in the same column are significantly different (P , .05).

was due to the fact that the water could easily bind in the looser area in amylose and amylopectin (Singh et al., 2004). Particle size distribution The particle size distribution parameters are shown in Table 3.8. The starch particle size became bigger and the span (range) became broader after retrogradation and further acetylation. From the ratio of particle size distribution, the main particle sizes of NS, RS, and ARS were around 13, 20, and 30 μm, respectively. Acetylation could cause a slight aggregation or a cluster of starch particles (Das et al., 2010).

Pasting property Among starch samples, ARS showed the lowest viscosity, with PV of 24 mPa s, trough viscosity (TV) of 16 mPa s, and FV of 21 mPa s

52

Sweet Potato

Table 3.8 Particle size distribution parameters of native, retrograded, and further acetylated sweet potato starch (μm). Sample

NS RS ARS

Span (range)a

D50b

D[4,3]c

35.37 6 1.09 (0.69 36.09)e 39.39 6 1.25 (0.76 40.15)e 116.08 6 2.10 (1.05 117.13)c

9.83 6 0.39e 16.11 6 0.10d 20.27 6 0.26c

10.65 6 0.39e 18.09 6 0.14d 27.16 6 0.28c

Values followed by the different letter in the same column are significantly different (P , .05). a Range representing the particle size distribution range of sample and Span was the wide of the range. b D50 representing 50% of total particle size distribution. c D[4,3] representing particle size derived from the volume distribution.

Table 3.9 The viscosity parameters of native, retrograded, and further acetylated sweet potato starch. Sample

PV (mPa s)

TV (mPa s)

NS RS ARS

1909 6 33 1323 6 28c 24 6 5e a

1213 6 35 1298 6 43a 16 6 2e

b

BD (mPa s)

FV (mPa s)

SB (mPa s)

PT (min)

696 6 24 25 6 3cd 8 6 3cd

1784 6 34 1970 6 37e 21 6 6e

571 6 10 672 6 23a 5 6 3e

4.6 6 0.1 7 6 0.1a 2.1 6 1.4c

b

ab

b

Ptemp (°C) b

80 6 0.0c 70.2 6 0.3d ND

PV, peak viscosity; TV, trough viscosity; BD, breakdown viscosity; FV, final viscosity; SB, setback; Pt, peaking time; Ptemp, pasting temperature. Values followed by the different letter in the same column are significantly different (P , .05).

(Table 3.9). Thus further acetylation can significantly decrease the viscosity of RS. Colussi et al. (2014) found that the viscosity of acetylated potato starch was lower than that of native potato starch. Zie˛ba et al. (2014) also found that the viscosity of native potato starch was higher than that of retrograded starch and retrograded acetylated starch. Within the pasting property parameter, the PV represents the swelling ability of the samples, TV and BDV represent the stability and shear resistance of gelatinized sample, while FV and SBV represent the retrogradation properties of the sample and the ability to increase the consistency of the food system (Deng et al., 2013). The PV of NS was higher than RS and ARS, but RS showed the higher TV, SBV, and FV, which suggested that the RS had the higher retrogradation tendency. The RS showed lower amylographic viscosity parameters compared to NS. This attribute could be a result of the destruction of granules subjected to hydrolysis during heating, which improved the levels of linear short-chain molecules. Moreover, the molecular motion decreased by the retrogradation and the microcrystalline beam were formed by hydrogen bonding between amylose and amylopectin at the same time. These two reasons might lead to the reduction of the viscoelasticity properties.

Sweet potato starch

53

Structural and physicochemical properties of physically modified sweet potato starch HHP is a nonthermal food processing technique, which has interesting functional effects in foods (Kim et al., 2012). To produce high-quality HHP products, it is important to know the effects of different HHP treatment conditions on different food components, including starch. When scattered in aqueous solutions, starch is gelatinized by HHP treatment, resulting in structural and physicochemical changes (Kim et al., 2012; Vallons et al., 2014). Starch is present in complex food matrices, so it is necessary to evaluate the effects of other food components, especially of salts or ions (e.g., sodium, calcium, and chloride ions) on HHP-induced starch gelatinization, which is a hydration and hydrogen-bond disrupting process (Buckow et al., 2007; Rumpold and Knorr, 2005). This section therefore introduces the effects of different inorganic salts on the structural and physicochemical properties of HHP-gelatinized sweet potato starch.

Thermal properties The thermal properties of native and HHP-treated sweet potato starches are shown in Table 3.10. Compared with the native starch, HHP-treated sweet potato starches with and without inorganic salts had higher To and Tp, but lower R, WHgel, and peak height index (PHI) values. Due to the interference or strengthening of the water molecule hydrogen-bond network, the dissociative ions had an effect on the water absorption of sweet potato starch. It led to a greater requirement for calories in the melting process, and prevented the damage of the crystal structure. It was noteworthy that the starch suspensions with calcium chloride or sodium chloride had significantly higher To and Tp values than those suspensions without salts. Among the HHP-treated sweet potato starches, the highest To and Tp values were obtained with 0.01 M of calcium and 0.01 M sodium chloride (73.10°C and 72.68°C, respectively). It was shown that To and Tp values at 0.1 M (in both salts) were lower than those at 1 M. It might be caused by the relatively high concentrations of salt osmotic pressure, and the interaction between starch and salt would also reduce the liquidity of starch molecular chain segments. On the other hand, because of the combined impact of starch molecule and ions, particles would be stretched to a certain extent (Eleni et al., 2001). With increasing salt concentrations, the R of HHP-treated sweet potato starches decreased first and subsequently increased. The starch suspension with 0.001 M salt had

Table 3.10 The thermal property parameters of native and HHP-treated sweet potato starch. Pressure (MPa)

Salt (M)

To (°C)

Tp (°C)

R (°C)

WHgel (J/g)

PHI

0.1 600

0 (Native) 0 CaCl2 0.001 0.01 0.1 1 NaCl 0.001 0.01 0.1 1

65.02 6 0.21d 69.41 6 0.07b

74.90 6 0.3bc 75.47 6 0.28bc

19.75 6 0.86ab 12.12 6 0.43de

12.03 6 0.49a 0.64 6 0.13f

1.22 6 0.06a 0.11 6 0.02ef

70.23 6 1.36b 73.10 6 1.25a 67.53 6 0.32c 71.15 6 1.39b

77.91 6 0.11a 77.73 6 0.78ab 75.25 6 0.08bc 77.69 6 0.23ab

15.36 6 2.85cd 9.25 6 0.98e 15.45 6 0.49cd 13.07 6 2.32d

1.77 6 0.20e 2.43 6 0.15d 6.92 6 0.14c 9.37 6 0.16b

0.23 6 0.02ef 0.53 6 0.06d 0.90 6 0.05c 1.43 6 0.22b

68.53 6 0.71c 72.68 6 0.12a 70.82 6 0.92b 71.09 6 2.69b

77.73 6 0.04ab 78.34 6 0.85a 77.66 6 0.03ab 76.00 6 0.41b

18.41 6 1.34abc 11.33 6 1.47de 13.67 6 1.85cde 10.82 6 6.18de

1.21 6 0.14e 2.13 6 0.07d 6.03 6 0.22c 9.22 6 0.10b

0.13 6 0.02ef 0.38 6 0.05de 0.89 6 0.13c 0.89 6 0.22c

600 600 600 600 600 600 600 600

Values followed by the different letter in the same column are significantly different (P , .05).

Sweet potato starch

55

the highest R-value, while that with 0.01 M salt had the lowest R-value (Table 3.10). The addition of salts increased WHgel and PHI of HHPtreated sweet potato starch in a dose-dependent manner. The WHgel and PHI values of HHP-treated sweet potato starch with calcium chloride were higher than those with sodium chloride at similar concentrations, but with no significant difference (Table 3.10). HHP treatment induced gelatinization of starch when dispersed in excess water. Therefore the crystalline structure of starch granules changed from an ordered to a disordered conformation (Blaszczak et al., 2005, 2010). The DG of HHP-treated starches had the same tendency as WHgel and PHI. The HHP-treated sweet potato starch with no salt had the highest DG value (94.7%), while starch with 1 M calcium and sodium chloride had the lowest DG values (22.11% and 23.36%, respectively). A satisfactory correlation index (r2 5 0.96) obtained between DG and salt concentration with a mathematical model was y 5 223.214x 1 119.39. During the HHP-induced gelatinization process, the amorphous regions of starch granules were hydrated and formed lamellae among crystalline regions. Subsequently, granules swelled and gelatinized. In starch granules, sodium, calcium, and chloride ions had higher polarity than glucose molecules (Rumpold and Knorr, 2005). Therefore these ions had more strength to combine with water molecules and prevent the hydration of amorphous regions. Calcium ions had higher polarity than sodium and chloride ions, which had a higher capacity to prevent HHP-induced gelatinization or the loss of crystallinity of sweet potato starches. As we all know, partially gelatinized starch granules are more difficult to digest in the human body due to the integrity of the spherical structure compared to the gelatinized starches. Moreover, the partially gelatinized starch would slowly release glucose to provide energy continuously. So this could avoid the possibility of blood sugar rapidly reaching a high level after the intake of starchy foods. In other words, it could regulate the blood sugar metabolism. Therefore this kind of starch is expected to be applied as an auxiliary food for the improvement of hyperglycemia.

Scanning electron micrograph The SEM micrographs ( 3 3000) of native and HHP-treated sweet potato starch granules are shown in Fig. 3.5. HHP-treated starch granules without salts lost their characteristic shape and had irregular sections with smooth surfaces (Fig. 3.5). For 0.001 M calcium chloride and 0.001 and

56

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Figure 3.5 SEM micrographs of native and HHP-treated sweet potato starch samples.

Sweet potato starch

57

0.01 M sodium chloride, almost all starch granules disintegrated into small fragments or aggregated to larger sections with rough surfaces. With increasing salt concentrations, more granules remained intact. Compared with starch granules in sodium chloride, starch granules in calcium chloride had higher structural integrity, because calcium chloride could prevent HHP-induced starch gelatinization. Calcium ions as a kind of divalent cation showed a more significant cross-linking function and hydration ability in the starch system. Salt contended water molecules with starch granules, which led to more complicated expansion of starch granules (Cai et al., 2012). HHP-induced starch gelatinization has two stages. In the first stage, changes in starch granule morphology could not be observed; when pressure reached a certain value, the starch granules began to disintegrate (Wuzburge and Whistler, 1964). However, the starch granules almost remained intact at 0.1 and 1 M calcium chloride and 1 M sodium chloride (Fig. 3.5). Therefore the addition of inorganic salts increased the pressure of HHP-induced sweet potato starch gelatinization.

Confocal laser scanning microscopy and polarized light microscopy The confocal laser scanning microscopy (CLSM) (green) and polarized light microscopy (PLM) (black and white) micrographs of native and HHP-treated sweet potato starches are presented in Fig. 3.6. The growth ring in the CLSM optical slice represents the internal structure of the starch granules, while the Maltese cross of the starch granules under polarized light indicates the degree of molecular order and crystallinity (Suda et al., 2003). NS had a typical round growth ring and Maltese cross. At 600 MPa with no salt addition, only a small section of the starch fractions and granules was visible in CLSM due to starch gelatinization (Karim et al., 2000), while all the Maltese crosses disappeared. Almost all starch granules and Maltese crosses were detectable with 0.1 and 1 M calcium chloride and 1 M sodium chloride. Both the CLSM optical slice and Maltese cross represent the degree of molecular order and crystallinity (Suda et al., 2003). With increasing starch granule DG (or decreasing ion strength), fewer starch granules were visible in CLSM and more Maltese crosses disappeared from the inner section, suggesting that sweet potato starch granules were gradually gelatinized from the inner to the outer section, and the inner section was more sensitive to the HHP treatment. This explained why the starch granules

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Figure 3.6 CLSM and PLM micrographs of native and HHP-treated sweet potato starch samples.

retained their original shape in the first stage of the HHP-induced starch gelatinization process (Vallons and Arendt, 2009).

X-ray diffraction The XRD profiles and relative degrees of crystallinity of native and HHP-treated sweet potato starches are shown in Fig. 3.7. The XRD

Sweet potato starch

59

Figure 3.7 XRD curve and relative crystallinity of native and HHP-treated sweet potato starch samples.

pattern of sweet potato starch depended on the cultivar, which could be A, C, or a combination of A and C types (Schirmer et al., 2013). NS had peaks at 15.2°, 17.2°, 18.1°, and 22.86° with a characteristic A type XRD pattern (Fig. 3.6). Compared with the A type, the C type XRD pattern had an extra and weak peak around 5.6°, which might disappear after drying (Ann-Charlotte, 2009). In Fig. 3.6, no obvious diffraction peak change was observed at about 5.6°. Therefore seeking other changes was also important. A single diffraction peak at about 17.2° could act as forceful evidence to prove the pattern change (Ann-Charlotte, 2009). Following HHP treatment, the peak at 18.1° disappeared, which indicated that HHP treatment converted the XRD pattern of sweet potato starch from an A to a C type. The addition of salts did not affect the XRD pattern of HHP-treated sweet potato starch. The peak intensities in the XRD profile were indicative of the degree of crystallinity of the starch granule (Takeda et al., 1986). To assess the change in the degree of crystallinity after HHP treatment, the relative degree of crystallinity of each sample was estimated. Native starch showed the highest degree of crystallinity (30.53%), while the HHP-treated starch with no salt had the lowest

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degree of crystallinity (15.66%). With increasing salt concentration, the peaks in the XRD profiles became stronger, and the degree of crystallinity significantly increased. At similar concentrations, starches in calcium chloride (22.11% 28.64%) had stronger peaks and higher degrees of crystallinity than those in sodium chloride (16.07% 27.29%). However, the changes in the relative degrees of crystallinity did not correspond to changes in DG (Fig. 3.5), because rapid retrogradation occurred inside the starch granules following HHP treatment (Kawai et al., 2007).

Pasting properties Pasting property parameters of native and HHP-treated sweet potato starch samples are shown in Table 3.11. Compared with NS, the viscosity of HHP-treated sweet potato starch with no salt increased significantly (highest viscosity: 7357.33 cP). The addition of salts decreased viscosity and increased PT and pasting temperature of sweet potato starch pastes (Table 3.11). Salt could dissolve into anions and cations as one kind of electrolyte in water. Hydrogen bonds in starch molecules were damaged by these ions, as well as those between starch particles and water molecules. This led to more complications in terms of starch gelatinization and a significant reduction in the PV (Meera et al., 2008). With increasing salt concentrations the breakdown and pasting temperatures of starch pastes increased, while peak and hold viscosity of starch pastes from calcium chloride starch suspensions increased. It might be due to the increasing number of charged ions, which enhanced the adsorption capacity of starch. Final viscosity, setback, and PT of starch pastes reduced with sodium chloride starch suspensions (Table 3.11). The viscosity of HHPtreated starch in calcium chloride was lower than in sodium chloride at similar concentrations (Table 3.11), because calcium chloride has a higher ion strength than sodium chloride (Liu et al., 2010).

Swelling power and solubility The swelling power and solubility of native and HHP-treated sweet potato starch are shown in Fig. 3.8A and B. Compared with native starch, HHP-treated starch with no salt had lower swelling power and solubility (Fig. 3.8). However, the addition of salts increased the swelling power and solubility of HHP-treated sweet potato starch (Fig. 3.8). This change indicated that the cluster structure of amylopectin had been opened due to the influence of electronics carried by the ions, which enabled the

Table 3.11 The pasting property parameters of native and HHP-treated sweet potato starch. Pressure (MPa)

Salt (M)

Peak viscosity (cP)

Hold viscosity (cP)

Breakdown (cP)

Final viscosity (cP)

Setback (cP)

Peak time (min)

Pasting temp. (°C)

0.1 600

0 (Native) 0 CaCl2 0.001 0.01 0.1 1 NaCl 0.001 0.01 0.1 1

6024.67 6 102.77b 7357.33 6 0.58a

3526.67 6 12.70cd 5077.33 6 2.31a

2498.00 6 90.07a 2280.00 6 1.73ab

4480.33 6 15.01de 6822.67 6 4.62b

953.67 6 2.31d 1745.33 6 2.31c

4.42 6 0.03bcd 4.76 6 0.08bc

76.78 6 0.14ab 76.48 6 0.38ab

3661.33 6 69.14e 4273.67 6 96.50d 5687.67 6 74.27c 6175.00 6 1.73b

2751.67 6 60.34e 3546.33 6 14.15cd 3753.67 6 9.02c 3907.33 6 10.97c

909.67 6 13.80d 1127.33 6 84.81cd 1934.00 6 74.67b 2267.67 6 9.24ab

4460.67 6 89.03de 5298.33 6 5.51d 5234.33 6 32.62d 4883.00 6 19.05d

1709.00 6 28.69c 2152.00 6 18.03b 1480.67 6 40.50c 975.67 6 30.02d

4.84 6 0.08bc 5.00 6 0.12ab 4.49 6 0.04bcd 4.79 6 0.02bc

77.57 6 0.45ab 76.02 6 0.33ab 76.85 6 0.62ab 78.65 6 0.26a

6164.25 6 45.14b 4718.32 6 15.48cd 6086.09 6 29.22b 6179.20 6 3.45b

5153.51 6 12.55a 3802.15 6 25.35c 4690.00 6 31.26b 3742.55 6 19.87c

1011.54 6 49.55d 916.16 6 55.27d 1396.54 6 29.58c 2428.59 6 50.23a

7941.19 6 80.14a 6044.38 6 29.59c 7079.48 6 38.56b 4787.55 6 45.68d

2788.59 6 45.58a 2242.68 6 29.63b 2388.98 6 16.59ab 1047.79 6 41.56d

5.47 6 0.00ab 5.21 6 0.09ab 4.80 6 0.05bc 4.53 6 0.05bcd

70.71 6 0.55b 75.5 6 0.29ab 74.7 6 0.47ab 76.65 6 0.19ab

600 600 600 600 600 600 600 600

Values followed by the different letters in the same column are significantly different (P , .05).

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Sweet Potato

Figure 3.8 Swelling power (A) and solubility (B) of native and HHP-treated sweet potato starch samples. Values followed by the different letters are significantly different (P , .05).

release of more amylase (Tester and Morrison, 1990). With increasing salt concentrations, the swelling power of HHP-treated starch in sodium chloride increased (Fig. 3.8A), while the solubility of HHP-treated starch in calcium chloride decreased (Fig. 3.8B). The addition of calcium and

Sweet potato starch

63

sodium chloride to sweet potato starch suspensions not only inhibited the hydration of amorphous regions but might also limit the formation of amylose lipid complexes in starch granules during the HHP-induced starch gelatinization (Oh et al., 2008).

Research and development trend of sweet potato starch Research on starches from different food resources has attracted widespread attention around the world. In some areas or aspects, further studies are necessary as follows: (1) carry out pilot and industrial production demonstrations of processing technologies of modified sweet potato starches, to expand their application in the food system; (2) develop the application of sweet potato starches and/or their modified starches in snack foods, to make full use of their physicochemical properties; and (3) develop the application of sweet potato starches and/or their modified starches on staple foods, to take full advantage of their structural and physicochemical properties. In any case, the sweet potato is a potential source of high-quality starches and/or modified starches, and it is necessary to conduct continuous and in-depth studies on them.

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CHAPTER 4

Sweet potato protein and its hydrolysates Tai-Hua Mu1, Miao Zhang1, Lawrence Akinola Arogundade1,2 and Nasir Mehmood Khan1,3 1

Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China Chemistry Department, College of Physical Sciences, Federal University of Agriculture, Abeokuta, Nigeria 3 Department of Biotechnology, Shaheed Benazir Bhutto University, Sheringal, Pakistan 2

Overview of sweet potato protein and its hydrolysates Sweet potato protein The sweet potato ranks as the fifth highest food crop in China, which possesses 80% of the world’s total output. Sweet potato contains approximately 1.73%
9.14% of protein on a dry weight basis (FAOSTAT, 2016; Mu et al., 2009). Sweet potato protein (SPP) is mainly composed of sporamins, and the monomeric forms of sporamins A and B have similar compositions of amino acids, peptide maps, and characteristics (Maeshima et al., 1985). SPP is rich in essential amino acids and exhibits a higher nutritive value compared to most other plant proteins (FAO, 1990), but it is normally discarded as industrial waste in the process of sweet potato starch manufacturing. Therefore it would be meaningful to develop value-added products and/or functional ingredients in the food industry through the effective utilization of SPP. Nowadays the production of SPP from sweet potato starch wastewater is being given due attention in some starch industries in China for economic reasons and environmental concerns.

Sweet potato protein hydrolysates Protein hydrolysates, commonly generated from food proteins by enzymatic hydrolysis, gastrointestinal digestion, and food processing, present certain bioactivities that can be used in health care, such as antioxidant, antimicrobial, antihypertensive, immunostimulatory, antithrombotic, and antidiabetic activities (Valdez-Flores et al., 2016; Agyei et al., 2016). Antioxidant hydrolysates can scavenge free radicals and/or inhibit free Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00004-1

© 2019 Elsevier Inc. All rights reserved.

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Sweet Potato

radical formation, which play an important role in delaying the oxidation of food systems and are essential to human health. The author team prepared sweet potato protein hydrolysates (SPPH) by different enzymes, and found that SPPH generated by enzymatic hydrolysis with Alcalase presented noteworthy antioxidant activities and great protective effects against oxidative DNA damage (Zhang et al., 2012, 2014).

Recovery and composition of sweet potato protein SPP is of a comparable or superior nutritional quality to most vegetable proteins (Mu et al., 2009), but its characteristic discoloration stigma constitutes a major setback in its utilization in the food system. This discoloration problem arises from enzymatic oxidative browning caused by polyphenol oxidase (PPO). PPO is a copper-containing enzyme that catalyzes the hydroxylation of certain phenols, especially mono- and diphenols in the o-position adjacent to an existing
OH group to odiphenols, which further oxidize to o-quinones. These o-quinones condense and react nonenzymatically with amino acids and proteins, resulting in the formation of brown/dark melanin pigments (Severini et al., 2003). These reactions are undesirable in food systems because of their negative effect on food appearance, the development of off-flavors, and losses in nutritional quality (Severini et al., 2003). In this section the use of oxidative browning inhibitors on sweet potato to prevent darkening during SPP recovery is presented for its potential utilization in the food system.

Sweet potato oxidative browning inhibition Oxidative browning inhibition carried out in sodium metabisulfite, sodium bisulfite, ascorbic acid, and citric acid (Fig. 4.1A) showed that the reductions in oxidative browning in sodium metabisulfite, citric acid, sodium bisulfite, and ascorbic acid aqueous media (0.01
0.3 mol/L) were in the ranges of 61%
85%, 78%
85%, 40%
75%, and 76%
80%, respectively. The highest sweet potato oxidative browning inhibition was observed in citric acid solution (0.01
0.1 mol/L). Phenolics in the extracts in the presence of various oxidative browning inhibitors are shown in Fig. 4.1B. The various concentrations of citric and ascorbic acid used decreased the extracted phenolics in comparison to distilled water, while bisulfites increased the phenolics in the extract at 0.01 mol/L concentration, but decreased it thereafter with further increases in the bisulfites concentration. Extracted phenolics in the presence of the different

Sweet potato protein and its hydrolysates

0.7

6

(A) Extractable phenolics (mg GAE/g-dwb)

Browning index (OD 500 nm)

0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.01

0.05

0.1

0.2

Concentration of oxidative browning inhibitor (mol/L)

0.3

71

(B)

5 4 3 2 1 0 –1 0 0.01 0.05 0.1 0.2 0.3 Concentration of oxidative browning inhibitor (mol/L)

Figure 4.1 Sweet potato browning indices (A) and coextractable phenolics (B) in the presence of various browning inhibitors: (K) sodium bisulfite, (x) sodium metabisulfite, (W) ascorbic acid, and (▲) citric acid.

antibrowning inhibitors showed positive correlations with the observed oxidative browning (Fig. 4.1A). This further confirmed the claim of Severini et al. (2003) who reported that the enzymatic oxidative browning is caused by phenolic oxidation.

Sweet potato protein extractability and recovery The extractability profile of SPP in the presence of antibrowning agents is shown in Fig. 4.2A. In bisulfites aqueous media, consistent decreases in solubilized proteins were observed as the concentration of bisulfites increased, while in the presence of citric or ascorbic acid, the initial decrease in solubilized protein was followed by a slight increase as the antibrowning agent concentration increased. The pH trend of sweet potato extracts in the presence of various antibrowning agents at different concentrations (Fig. 4.2B) might account for this protein extractability profile. There was a positive correlation between extractable SPP and the extract’s pH. An increase in the antibrowning agent concentration continuously decreased the extract pH, which merely approached the isoelectric point (pH 4) in the presence of bisulfites, but in the presence of citric and ascorbic acids the extract pH was lowered beyond the isoelectric point, which would possibly be responsible for the observed increase with 0.05
0.3 mol/L citric acid and 0.1
0.3 mol/L ascorbic acid (below isoelectric point). The effectiveness of recovering sweet potato solubilized proteins from the various antibrowning aqueous media using ultrafiltration and isoelectric precipitation techniques are shown in Fig. 4.3A
D. In all cases, SPP

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120

7

(A)

100

5 80 4

pH

Extractable protein (%)

(B)

6

60

3 40 2 20

1

0

0 0

0.01

0.05

0.1

0.2

0.3

0

0.01

0.05

0.1

0.2

0.3

Concentration of oxidative browning inhibitor (mol/L) Concentration of oxidative browning inhibitor (mol/L)

Figure 4.2 Sweet potato extractable protein (A) and pH trend of sweet potato extracts (B) in various antibrowning aqueous media: (K) sodium bisulfite, (x) sodium metabisulfite, (W) ascorbic acid, and (▲) citric acid.

Figure 4.3 Sweet potato solubilized protein recovery by isoelectric precipitation (K) and ultrafiltration (x) techniques from various antibrowning aqueous media: (A) sodium metabisulfite, (B) sodium bisulfite, (C) citric acid, and (D) ascorbic acid.

recovered by ultrafiltration was significantly (P , .05) higher than that by the isoelectric precipitation technique. All the antibrowning agents considered in this study reduced the recoverable protein obtained by either ultrafiltration or isoelectric precipitation technique. This reduction

Sweet potato protein and its hydrolysates

73

in recoverable protein was however more pronounced with the isoelectric precipitation technique. The fractions of the extractable protein in the various extractants that were recovered by isoelectric precipitation from sodium metabisulfite, sodium bisulfite, ascorbic acid, and citric acid aqueous media were in the range of 0%
6.2%, 0%
40.3%, 10.6%
45.7%, and 4.2%
24.2%, respectively. However, with the ultrafiltration processing technique we recovered 89.2%
100%, 94.2%
98.7%, 76.9%
97.1%, and 49.5%
97.2% of the solubilized protein from sodium metabisulfite, sodium bisulfite, ascorbic acid, and citric acid aqueous media, respectively. Even though oxidative browning inhibition was least effective with sodium bisulfite compared with sodium metabisulfite, citric acid, and ascorbic acid, in view of the former’s higher protein extractability with relatively high protein recovery, it remains the best choice for optimum SPP yield by either isoelectric or ultrafiltration processing techniques. Sodium bisulfite (0.01 mol/L) with 40% browning inhibition and 97.9% extractable protein may be considered the most appropriate for SPP processing with a better yield and reduced oxidative browning.

Protein isolates yield, composition, and in vitro digestibility The protein recovery obtained by the membrane processing technique was more than double the recovery obtained by the isoelectric precipitation method (Table 4.1). SPP recovered by either isoelectric or ultrafiltration/diafiltration techniques (16%
51%) was a little lower than the recovery reported for some common legumes like soybean, lupin, and chickpea obtained by similar techniques (Chew et al., 2003; Papalamprou et al., 2009; Shallo et al., 2001). This might be due to SPP’s high solubility in aqueous media, since with our repeated washings and diafiltrations, more protein would have solubilized and washed off. Fewer washings could give a higher yield but the protein purity might be compromised. In addition to this, the possibility of some SPP fractions having their isoelectric point higher than pH 4 might have further affected SPP’s low recovery by isoelectric precipitation. The isoelectrically precipitated sweet potato (IPSP) protein which suffered low recovery syndrome also had the lowest purity (62.9%), while the ultrafiltration/diafiltration-processed sweet potato (UDSP) protein purity was in the range 76.0%
82.1%. The IPSP and UDSP proteins showed different chemical composition (Table 4.1). The Ca content of all the protein preparations was significantly (P , .05) higher than Zn. This might be due to Ca21 preferentially

Table 4.1 Yield and chemical composition of isoelectric and UDSP protein isolates.a Parameters

Yield (%) Purity (%dwbb) Moisture content (%) Ca (g/kg) Zn (g/kg) Total phenolics (mg GAE/g)c Tannin (mg GAE/g) Total flavonoids (mg CE/g)d Phytic acid (mg/100 g) Phytic acid-phosphorus (g/100 g) [Phy]/[Zn]f Trypsin inhibition (TIU/mg of protein)g In vitro digestibility (%)

Isoelectric precipitated protein

Membrane-processed proteins

IPSP

UDSP-4

UDSP-6

UDSP-7

16.0 62.9 6 0.2c 1.71 6 0.11c 2.31 6 0.07a 0.01 6 0.00b 3.76 6 0.03b 1.97 6 0.04b 3.48 6 0.05c 0.66 6 0.00a 0.19a 65.6a 0.034 6 0.003c 70.0 6 3.1c

41.3 76.0 6 0.2b 6.98 6 0.71a 0.29 6 0.01d 0.05 6 0.01a 4.08 6 0.07a 2.87 6 0.06a 2.98 6 0.13d NDe

0.033 6 0.004c 87.7 6 0.8a

41.3 82.0 6 0.5a 3.86 6 0.37b 0.82 6 0.01c 0.05 6 0.00a 2.06 6 0.03d 1.23 6 0.03d 4.04 6 0.10b 0.35 6 0.02b 0.10b 7.0b 0.111 6 0.011b 77.0 6 0.5b

51.3 82.1 6 0.9a 3.66 6 0.51b 1.96 6 0.09b 0.05 6 0.01a 2.87 6 0.04c 1.85 6 0.03c 4.56 6 0.31a 0.36 6 0.01b 0.10b 7.0b 0.143 6 0.012a 74.6 6 3.6bc

Mean values followed by different letter superscripts in the same row are significantly different (P , .05). dwb, dry weight basis. GAE, gallic acid equivalent. d CE, catechin equivalent. e ND, not detected. f Phytic acid (mg)/MW (molecular weight) of phytic acid: Zn (mg)/MW of Zn. g IU, trypsin inhibitor unit. a

b c

Sweet potato protein and its hydrolysates

75

binding to the protein and/or phytate content relative to Zn21. The Ca content was in the order UDSP-4 , UDSP-6 , UDSP-7 , IPSP. The increased Ca content of the UDSP proteins as pH increased might be due to the increased negative charges on the protein as the pH of processing media moves away from the isoelectric point, which in turn increased the Ca electrostatically binding to the proteins. Significantly higher phytic acid in IPSP (0.66 mg/100 g) compared to others (0
0.36 mg/100 g) might be responsible for the higher calcium content of IPSP than the other proteins. In addition, the [phytate]/[Zn] molar ratio of IPSP protein was much higher than that of UDSP (Table 4.1). Since a [phytate]/[Zn] molar ratio of 610 has been associated with good Zn bioavailability, Zn deficiency and its associated complications are not envisaged with UDSP protein formulated foods, while the reverse is the case with IPSP protein. The phenolic contents of SPP protein were in the order UDSP4 . IPSP . UDSP-7 . UDSP-6 (Table 4.1). The relatively high phenolic content of these proteins might be attributed to the oxidative browning inhibitor (0.01 mol/L NaHSO3) used, which was capable of extracting the tuberous root phenolics along with the protein. The variation of phenolic content among these proteins could be due to the various types of interaction they underwent with the protein and the caking problem associated with the ultrafiltration/diafiltration technique, particularly at pH 4 (UDSP-4). Phenolic compounds can undergo hydrogen bonding and electrostatic and hydrophobic interactions with proteins (Mondor et al., 2009). This might possibly be responsible for the high coprecipitation of phenolics by IPSP protein. The ultrafiltration/diafiltration technique produced proteins with significantly lower phenolics at pH 6 and 7 (UDSP-6 and UDSP-7). The severe caking problem associated with the ultrafiltration/diafiltration process at pH 4 might be responsible for UDSP-4 protein having the highest phenolics. Mondor et al. (2009) reported that cake formation during the diafiltration of chickpea protein also resulted in protein with high phenols. The protein deposited on the membrane could have rendered it less porous and thus retained polyphenols particles that would have permeated the membrane. The phenolic content of both IPSP and UDSP proteins was higher than that of chickpea (Mondor et al., 2009). Tannin content also followed the same trend as that observed with the total phenol. Protein
tannin complex formation has been associated with protein digestibility inhibition. This high tannin may be partly responsible for the fairly low SPP digestibility. There is, however, no significant

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Sweet Potato

(P , .05) correlation established between SPP digestibility and tannin, unlike the phytic acid content of these proteins which correlated positively with their in vitro digestibility. The total flavonoid content of these proteins was in the order UDSP-7 . UDSP-6 . IPSP . UDSP-4. The effect of pH treatment was significant (P , .05) on the total flavonoid content of ultrafiltration/diafiltration-processed proteins, that is, total flavonoids increased with pH. In view of the phytochemical antioxidative activities of these bioactives, retaining or eliminating them from protein entirely rests on the consumers’ needs, since their effectiveness in reducing cardio-cerebrovascular diseases and cancer mortality have been well established (Hertog et al., 1997). Phenolic acid and flavonoids have been reported to be the main phytochemicals responsible for the antioxidant capacity of plant foods (Sahreen et al., 2010). According to Sharififar et al. (2009), flavonoid-rich plants could be a good source of antioxidants that would help to increase the overall antioxidant capacity of an organism and protect it against lipid peroxidation; dietary intake of flavonoid-containing foods was therefore suggested to be of benefit. Trypsin inhibitory activities of IPSP and UDSP proteins (Table 4.1) were in the range of 0.03
0.14 TIU/mg. These values were much lower than the values reported for chickpea (18.88
20.03 TIU/mg) and mung bean (6.12 TIU/mg) protein obtained under similar processing conditions (El-Adaway, 2000; Mondor et al., 2009). SPP trypsin inhibitory activity was dependent on the pH rather than the processing techniques, it increased with pH, that is, UDSP4 , IPSP , UDSP-6 , UDSP-7. UDSP proteins had higher pepsin
pancreatin in vitro digestibility than those of IPSP (Table 4.1). This was in the order UDSP-4 . UDSP6 . UDSP-7 . IPSP. This trend might be a reflection of the proteins’ compositions and conformational attributes, since protein digestibility is a function of both the endogenous structural constitution and exogenous factors, like enzymatic inhibition, phytic acid, and polyphenol complexation. SPP digestibility correlated more negatively with its phytic acid content than tannin and trypsin inhibition activity, as noted earlier on.

Amino acid composition and nutritional quality The amino acid content of UDSP proteins (with the exception of glutamic acid, lysine, and arginine) was significantly higher than that of the IPSP protein isolate (P , .05, Table 4.2). This is an indication that UDSP

Table 4.2 Amino acid composition and nutritional quality of isoelectric and UDSP protein.a Amino acid

Alanine Arginine Aspartic acid Cysteinec Glutamic acid Glycine Histidine Isoleucine Leucine Lysine Methionine Phenylalanined Proline Serine Threonine Tyrosine Valine

Isoelectric protein

FAO/WHO patternb

Membrane-processed protein at different pH

IPSP

UDSP-4

UDSP-6

UDSP-7

32.73 6 0.38c 33.50 6 0.06b 81.60 6 0.82c 14.72 6 0.44d (99.12) 69.88 6 0.70a 30.19 6 0.27c 12.48 6 0.24c (50.63) 32.10 6 0.19d (88.57) 49.09 6 0.38c (57.20) 37.11 6 0.30a (46.24) 13.57 6 0.15c 37.54 6 0.29c (85.70) 24.30 6 0.21d 33.33 6 0.37c 32.65 6 0.30c (71.44) 31.52 6 0.16b 42.14 6 0.28c (90.43)

36.67 6 1.38b 33.97 6 1.32b 112.93 6 5.09b 18.20 6 0.95c (143.4) 66.43 6 2.59b 33.74 6 1.16b 13.53 6 0.55b (71.21) 36.97 6 1.02c (132.04) 52.70 6 1.82b (79.85) 33.78 6 1.26b (58.24) 17.65 6 1.14b 50.06 6 2.44b (144.0) 30.38 6 0.76c 42.38 6 1.72b 42.90 6 1.56b (126.2) 40.64 6 3.78a 50.93 6 1.52b (145.5)

39.96 6 0.69a 37.02 6 0.81a 126.55 6 2.08a 22.21 6 0.11a (164.8) 70.37 6 1.13a 37.17 6 0.69a 14.81 6 0.14a (77.05) 40.65 6 0.80a (143.89) 57.91 6 1.15a (86.91) 36.00 6 0.78a (61.38) 19.68 6 0.37a 55.17 6 1.45a (155.1) 34.79 6 0.77a 47.68 6 0.60a 47.55 6 0.70a (138.6) 43.68 6 1.51a 56.70 6 1.01a (160.4)

40.10 6 0.62a 36.86 6 0.75a 127.60 6 2.22a 20.41 6 0.35b (145.0) 72.69 6 1.15a 37.38 6 0.52a 14.47 6 0.32a (63.89) 38.45 6 0.70b (118.43) 56.59 6 1.03a (72.33) 35.94 6 0.67a (52.21) 18.94 6 0.29a 52.89 6 0.94ab (129.5) 32.94 6 0.43b 48.39 6 0.76a 47.69 6 0.68a (114.4) 42.46 6 0.29a 56.74 6 0.88a (133.3)

2
5 years old

Adult

2.5

1.7

1.9

1.6

2.8

1.3

6.6

1.9

5.8

1.6

6.3

1.9

3.4

0.9

3.5

1.3 (Continued)

Table 4.2 (Continued) Amino acid

Sulfur-containing amino acid Total essential amino acid Total amino acid E/Te (%) PDCAASf (%)

Isoelectric protein

Membrane-processed protein at different pH

IPSP

UDSP-4

UDSP-6

UDSP-7

2.83 6 0.05

3.58 6 0.18

4.19 6 0.03

3.94 6 0.03

30.29 6 0.24

35.64 6 1.29

39.44 6 0.73

38.46 6 0.54

60.85 6 0.47 49.79 44.79

71.25 6 2.44 50.06 51.08

78.79 6 1.35 50.05 47.79

78.05 6 1.11 49.27 45.73

FAO/WHO patternb 2
5 years old

Adult

Mean values followed by different letter superscripts in the same row are significantly different (P , .05). Values within parentheses are the essential amino acid scores calculated as a percentage ratio of each essential amino acid in the various proteins to their respective FAO/WHO (1991) requirement. b FAO/WHO (1991) requirement for respective essential amino acids. c Cystine 1 methionine. d Phenylalanine 1 tyrosine. e Essential (E) and total (T) amino acid ratio. f Protein digestibility-corrected amino acid score. a

Sweet potato protein and its hydrolysates

79

proteins are more nutritionally viable than their IPSP counterparts. With the exception of a few cases mentioned above, the amino acid distributions in the four preparations of SPP were in the order IPSP , UDSP4 # UDSP-7 # UDSP-6. Aspartic acid was significantly higher than all other amino acid constituents in both UDSP and IPSP proteins. This was similar to the report of Purcell et al. (1978) and Mu et al. (2009). Lysine was their limiting amino acid, which is similar to the report of Mu et al. (2009). The sulfur-containing amino acid contents (methionine 1 cysteine) were 2.83, 3.59, 4.19, and 3.94 g/100 g in IPSP, UDSP-4, UDSP-6, and UDSP-7, respectively, suggesting that IPSP and UDSP proteins had significantly (P , .05) higher sulfur-containing amino acid than the requirements of FAO/WHO for preschool children (FAO/ WHO, 1990). This is contrary to the report of Purcell et al. (1978) and Bradbury et al. (1984), who found sulfur-containing amino acids to be the SPP limiting amino acids and lysine to be in abundance. This difference can be attributed to the use of sodium bisulfite as an oxidative browning inhibitor in our SPP preparations. Protein rich in sulfurcontaining amino acids was also reported for 11S-rich hemp protein isolate prepared with low concentration (0.0094 mol/L) of sodium bisulfite (Wang et al., 2008). Mu et al. (2009) also reported that SPP prepared with 1% sodium bisulfite had sulfur-containing amino acids much higher than the FAO/WHO requirements for 2
5-year-old children. This is an added advantage for this tuberous root protein over most leguminous proteins. Deficiency in sulfur-containing amino acids is a common problem with the great majority of leguminous seed proteins. Much lower sulfurcontaining amino acids contents were reported for isoelectric protein isolates from soybean (1.99 g/100 g), chickpea (2.11
2.20 g/100 g), and lupin (1.61 g/100 g), and ultrafiltration/diafiltration-processed lupin had 1.4 g/100 g (Chew et al., 2003; Wang et al., 2010). Dietary protein quality assessment is of great importance since this gives information on the protein’s ability to meet human nutritional needs. In this study, SPP qualities were assessed by considering amino acid score, E/T ratio, and protein digestibility-corrected amino acid scores (PDCAAS), which correlate more directly with human requirements (Table 4.2). The amino acid score showed that SPP from either isoelectric or ultrafiltration/diafiltration preparation contained excess isoleucine, valine, methionine, and cysteine, and sufficient phenylalanine and tyrosine in comparison with the requirements of the FAO/WHO for preschool

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children, but could only supply 58.2%
64.0%, 65.7%
78.0%, and 74.4%
87.7% of their lysine, histidine, and leucine requirements, respectively FAO/WHO (1990). However, IPSP and UDSP proteins can satisfy adults’ nutritional needs conveniently. The amino acid scores of UDSP proteins showed that they can perform better than isoelectric preparation in meeting the nutritional needs of both preschool children and adults. The E/T (%) values for these SPP preparations were well above 36%, which is considered adequate for an ideal protein (FAO/WHO, 1990). The PDCAAS values obtained were in the range 44.89%
50.08% and 162.4%
185.2% for preschool children and adults, respectively, which indicated a low protein quality for children but the reverse for adults (Sarwar and McDonough, 1990). All these indices suggest that SPPs, especially ultrafiltration/diafiltration-processed species, can be utilized as protein sources for human nutrition with little or no supplementation from other protein sources.

Gelation properties of sweet potato protein The common method of producing protein gel is by heating the protein dispersion. On heating, protein molecules are unfolded, resulting in the exposure of the
S
H
,
S
S
, and hydrophobic amino acid side chains. This is followed by the rearrangement and aggregation of functional groups, that is, hydrophobic interaction and
SH/S
S interchange reactions, culminating in gel with a three-dimensional network structure (Liu et al., 2004). A systematic study on SPP gel’s formation mechanism and mechanical and structural attributes is necessary to showcase its gelling potential and utilization as a gelling agent in the food industry. This section therefore introduces flow, viscoelastic, and gelation properties of SPP dispersions as affected by ultrafiltration and isoelectric precipitation methods, as well as the rheological and mechanical properties of the protein gels, along with the mechanism of gelation.

Rheological properties of sweet potato protein dispersions The steady shear properties of both IPSP and UDSP protein dispersions were modeled with Power law and Casson equations. The correlation coefficient (R2) values for Power law equation were between 0.84 and 0.99, while those of the Casson model were in the range 0.72
0.92 (Table 4.3). The degrees of the proteins’ pseudoplastic behavior as depicted by flow index (n) values are shown in Table 4.3. IPSP protein

Table 4.3 Sweet potato proteins' flow properties.a Protein processing condition

Protein concentration (%)

Apparent viscosity, η100 (mPa s)

Consistence coefficient, k (Pa sn)

Flow behavior index, n

Yield stress, τ o (Pa)

Activation energy (J/mol)

Ultrafiltered/diafiltered sweet potato (UDSP) protein

2

1.87 6 0.07e

0.00 6 0.00b

0.81 6 0.03a

0.00 6 0.00b

NA

4 6 8 10 2

3.29 6 0.35d 4.57 6 0.18c 6.68 6 0.29b 10.5 6 0.46a 9.41 6 0.02d

0.04 6 0.00b 0.03 6 0.00b 0.09 6 0.00b 0.27 6 0.07a 1.00 6 0.16b

0.49 6 0.04bc 0.53 6 0.08b 0.40 6 0.01cd 0.28 6 0.05d 0.03 6 0.02a

0.02 6 0.00b 0.02 6 0.01b 0.05 6 0.00b 0.20 6 0.07a 0.92 6 0.07b

NA NA NA NA 9.44 6 1.17b

4 6 8 10

24.45 6 3.18cd 31.25 6 1.06c 66.95 6 4.74b 92.50 6 9.92a

2.89 6 0.48b 5.14 6 1.49b 33.20 6 3.24a 29.65 6 4.93a

2 0.03 6 0.01a 2 0.05 6 0.00a 2 0.29 6 0.03c 2 0.16 6 0.02b

2.74 6 0.56b 4.88 6 1.39b 23.17 6 3.29a 24.45 6 4.54a

9.81 6 1.01b 10.58 6 0.54ab 14.37 6 0.92a 11.34 6 0.40ab

Isoelectric precipitated (IPSP) protein

NA, Not applicable. a Significant differences between means were assessed using the Tukey test and values in a column followed by different letters in each protein preparation are significantly (P # .05) different.

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was significantly more pseudoplastic than UDSP protein. This is an indication that UDSP protein had a better flow tendency than IPSP protein (Marcotte et al., 2001). The mechanical spectra showing the viscoelastic properties of UDSP and IPSP protein dispersions are shown in Fig. 4.4A and B. IPSP protein dispersion exhibited a “solid-like” behavior with the magnitude of its

Figure 4.4 Mechanical spectra of (A) ultrafiltrated and (B) isoelectric protein showing the variation of storage modulus, G (’); loss modulus, Gv (K); and complex viscosity, η (%) with angular frequency.

Sweet potato protein and its hydrolysates

83

storage modulus (G0 ) values consistently higher than the loss modulus (Gv) within the frequency range considered. UDSP protein had G0 values higher than Gv only at a lower angular frequency range with a crossover at 82 s21, signifying the changeover from a “solid-like” nature to a “liquid-like” behavior at higher angular frequency. Such a crossover of some polymeric systems was attributed to the relaxation time and the onset of glassy behavior (Barnes, 2000). At low frequency the stress is applied over a long timescale which allows molecular interaction, but when the frequency exceeds certain values the applied stress is of a short timescale and there is no long-term interaction between molecules, which induces a change of state (Puppo and Añón, 1999). The difference in the mechanical spectra of IPSP and UDSP proteins could be attributed to the difference in their composition and physical structure (Steffe, 1996).

Gelation properties and network formation mechanism of isoelectrically precipitated sweet potato and ultrafiltration/ diafiltration-processed sweet potato proteins The IPSP and UDSP protein concentrations of 20 and 40 g/L, respectively, were the lowest concentrations required to form a gel at pH 7. Considering the least gelling concentration (LGC) as the gelation capacity index, IPSP protein could have a greater gelling capacity than UDSP protein, since proteins with lower LGC have a greater gelling capacity. These sweet potato tuberous root proteins (IPSP and UDSP) have similar gelation capacity to African yam bean protein (Arogundade et al., 2012), but are higher than most leguminous proteins, with LGC in the range 6%
18% (w/v) (Boye et al., 2010). The network development during gelation of IPSP and UDSP protein was determined by dynamic oscillatory measurement (temperature sweep) and the thermomechanical spectra as shown in Fig. 4.5Ai and ii. The gel network development was monitored with G0 , Gv, and tan δ as functions of temperature and time. According to Sun and Arntfield (2010), G0 values are a measure of the elastic component of the gel network structure and represent the strength of the structure contributing to the gel threedimensional network, while Gv is a measure of the viscous component and represents interactions which do not contribute to the threedimensional nature of the gel network. The change in phase angle (tan δ), which relates the viscous nature to the elastic behavior of the sample, predicts the type of network formed with lower tan δ values indicating a better three-dimensional structure.

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Sweet Potato

100 14000

(A)(i)

90 80

T G or G (Pa)

G or G (Pa)

10000 8000

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Tgel (79.4°C) G

G

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Temperature (°C)

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Time (min)

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(A)(ii)

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7000 Tgel (76.5°C) T

G or G (Pa)

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G G

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G

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Temperature (°C)

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Tan Δ

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Heating

0.2

Cooling

0.1

0.0 20

30

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90

100

90

100

0.16

Tan Δ

Heating

(B)(ii)

0.12 Cooling 0.08 20

30

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50 60 70 Temperature (°C)

80

Figure 4.5 Heat-induced gelation profile of 10% (w/v) sweet potato protein (SPP) dispersion: (A) thermomechanical spectra of (i) ultrafiltrated and (ii) isoelectric SPP showing development of gel structure with time and temperature; G0 , storage modulus; Gv, loss modulus; Tgel, gelation onset temperature; and T, temperature protocol. (B) Phase angle of ultrafiltered (Bi) and isoelectric (Bii) protein during gelation process.

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On heating the UDSP protein dispersion, the initially constant G0 and Gv started to increase at 79.4°C, which signaled the transition from a liquid-like to a solid-like (sol
gel) state (inset of Fig. 4.5Ai). This can be taken as the onset of gelation (Tgel), since one of the common methods of detecting the gelling point in the absence of crossover between G0 and Gv is the temperature at which G0 increases and becomes greater than the background noise (Lamsal et al., 2007). Tgel constitutes the initial stage of denaturation of the protein which exposes the hydrophobic patches as a preparatory stage for gel formation (Sun and Arntfield, 2010). This initial increase of UDSP dispersion G0 became more pronounced as temperature increased to 95°C. Increased thermal treatment might have caused more denaturation which could have promoted the unfolding of the buried hydrophobic residue inside the protein molecules. This favors hydrophobic peptide interactions and the formation of disulfide bonds that reinforce the gel network structure. The gradual development of the reinforced network was reflected by the progressive increase in G0 as the heating temperature increased to 95°C. Heating of the IPSP protein dispersion gave a decreased G0 until about 75°C and thereafter (76.5°C) it started to increase gradually (inset of Fig. 4.5Aii). This represents its Tgel. G0 of IPSP protein further increased with increased temperature, as reported for UDSP protein dispersion, but to a lesser extent. Tgel for UDSP and IPSP protein corresponded to 27 and 25 min of heat treatment, respectively. Cooling further enhances the network structure of both proteins as indicated by the steady increase in G0 for both proteins. This may be attributed to the consolidation of attractive forces like van der Waals and hydrogen bonding between proteins on cooling (Lamsal et al., 2007). According to Speroni et al. (2009), the stability of hydrogen bonds increases with a decrease in temperature. At the completion of the heating and cooling circle, the G0 value for UDSP protein gel was almost twice that of IPSP. This could be attributed to the difference in response of the protein's tertiary conformation to thermal treatment (Yin et al., 2009). The progress in gel network development was also monitored through the phase angle (tan δ) as the thermal treatment progressed (Fig. 4.5B). The phase angle, which is a better indicator of the viscoelasticity of biopolymer gel, confirmed the increased gel network elasticity as the temperature increased to or was maintained at 95°C with a decreased tan δ. This also suggested that more cross-linked networks were formed in both proteins as heating progressed. Tan δ of the UDSP dispersion did not however decrease initially until a temperature of 75°C was

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reached, indicating that the rapid reaction leading to gelation started at this temperature (75°C).

Ultrafiltration/diafiltration-processed sweet potato and isoelectrically precipitated sweet potato protein gel dynamic rheological properties The mechanical spectra of 10% (w/v) IPSP and UDSP protein gels showed typical gel structure with G0 values of both proteins maintaining consistently higher values than the loss modulus Gv within the frequency range tested (Fig. 4.6A and B). The larger difference between the G0 and Gv values of the UDSP protein gel than that of IPSP suggested that a higher percentage of the stored energy was recovered at each shearing cycle for UDSP protein than for the IPSP protein. The plots of G0 and Gv as a function of frequency do provide information on the gel structure, and help in determining whether the gel formed from entanglement networks, covalently cross-linked materials, or physical gels (Doucet et al., 2001). Entanglement networks are usually characterized with G0 Bω2 and GvBω1 at low frequency and a crossover between G0 and Gv at high frequency; covalently cross-linked gels are frequency independent; while physical gels are slightly frequency dependent (Doucet et al., 2001; Kavanagh and Ross-Murphy, 1998). The dependence of G0 on frequency can be obtained from the logarithmic plot of G0 5 K0 (ω)z0 (log G0 5 z0 log ω 1 K) as proposed by Egelandsdal et al. (1986), with physically and covalently linked gel having z0 . 0 and z0 5 0, respectively. The slopes (z0 ) from a logarithmic plot of G0 versus a logarithm of ω for UDSP and IPSP protein gels showed that both gels had G0 that was dependent on ω, but to a minimal extent with the low z0 values of 0.11 and 0.13 for UDSP and IPSP protein gels, respectively, signifying a covalent bond gel structure with some tendency for physical interaction (Doucet et al., 2001). Since the strong dependence of G0 on frequency indicates that there is no specific interaction between molecules (weak gel) while G0 independent of frequency is an indication of a strong gel network (Doublier, 1992), IPSP and UDSP gels formed from 10% (w/v) dispersion were therefore a gel structure which fell within strong and weak gels. K0 values showed the level of molecular interaction of the gel matrix, with higher K depicting higher interaction (Kim and Yoo, 2009). Therefore there was higher molecular interaction in UDSP than IPSP protein gel.

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Figure 4.6 Dynamic viscoelastic properties sweet potato protein gels made from 10% dispersion: (A) mechanical spectra of ultrafiltered protein gel showing variation of storage modulus, G0 (’); loss modulus, Gv (K); and phase angle, tan δ (▲) with angular frequency. (B) Mechanical spectra of isoelectric protein gel showing variation of storage modulus, G0 (’); loss modulus, Gv (K); and phase angle tan δ (▲) with angular frequency.

Mechanical properties of isoelectrically precipitated sweet potato and ultrafiltration/diafiltration-processed sweet potato protein gels The mechanical parameters derived from this texture profile analysis are shown in Fig. 4.7. Even though UDSP gel had a more highly covalently

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Mechanical parameter value

4.5 a∗

4 3.5 3 2.5 2

b∗

1.5 1

b

a a

0.5

a b

a

0 Hardness

Springiness

Cohesiveness

Resilence

Parameters

Figure 4.7 Texture profile analysis parameters of ultrafiltered ( ) and isoelectric ( ) sweet potato protein gels. Significant differences between means were assessed using the Tukey test and values of the histogram under each parameter with different letter labels are significantly (P # .05) different.  Values of hardness are 3 102.

linked
S
S
structure than the IPSP gel, the force needed to attain the given deformation (hardness) in the IPSP protein gel was higher than that of UDSP. The higher hardness observed with IPSP protein gel might be attributed to its higher carbohydrate or nonprotein content than the UDSP protein gel. According to Lamsal et al. (2007), the carbohydrate content of protein might cause a higher hardness due to interactions between proteins and carbohydrate particles. Springiness, which represents the rate at which deformed material goes back to its original height following removal of the applied force, was also higher with the IPSP gel than that of UDSP, but their cohesivenesses were not significantly different.

Isoelectrically precipitated sweet potato and ultrafiltration/ diafiltration-processed sweet potato protein gels microstructure Scanning electron microscopy revealed the clear differences in the network structure of UDSP and IPSP protein gels (Fig. 4.8). The dark areas represent the pores in the gel network. UDSP gel had a dense, spherical particulate appearance, while IPSP gel had a regular and repeating finestranded gel network structure with small voids distributed throughout the matrix. IPSP protein gel showed a more cross-linked particle network

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Figure 4.8 Scanning electron micrographs of ultrafiltered (A) and isoelectric (B) protein gels.

than that of UDSP which had an amorphous matrix appearance. The heavy cross-linking observed with IPSP gel might possibly reinforce the IPSP gel network structure, which in turn may account for the higher mechanical properties observed with IPSP protein gel compared to that of UDSP protein gel.

Emulsifying properties of sweet potato protein Proteins are amphiphilic molecules that can be used as emulsifiers to stabilize emulsions (Damodaran, 1996). SPP has good emulsifying activity but low stability (Mu et al., 2009). The maximum interfacial protein concentration reported for SPP was 1.81 mg/m2 with 2% (w/v) protein concentration; however, it has a poor stability compared to legume protein (Guo and Mu, 2010). High hydrostatic pressure (HHP) is an effective tool to destroy the microorganisms in foods; compared to treatments such as pasteurization and sterilization. HHP has a less severe effect on the stability and the gelation properties of protein emulsions (Anton et al., 2001). Hence, the appropriate structure modification of an SPP emulsion by HHPs might lead to enhanced functionality, which could increase the applications of SPP emulsions in the food industry. This section therefore introduces the effects of HHP treatment on the stability, viscosity, interfacial protein concentration, and droplet sizes of SPP emulsions (1% w/v) subjected to different HHPs and at different pH values (3, 7, and 8) for a better understanding of SPP as a functional agent in the food industries.

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Emulsifying activity index and emulsifying stability index The emulsifying activity index (EAI) and emulsifying stability index (ESI) of SPP emulsions subjected to the different HHP levels are shown in Fig. 4.9A and B. Compared to the control (0.1 MPa) emulsions, the

Figure 4.9 (A) Emulsifying activity index (EAI) and (B) emulsifying stability index (ESI) of control and high-pressure-treated SPP emulsions.

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Table 4.4 Effect of high hydrostatic pressure (HHP) treatment on the different parameter of emulsions prepared at pH 3, 7, and 8. Sample

HHP (MPa)

D4,3

D3,2

Г (mg/m2)

pH 3

0.1 200 400 600 0.1 200 400 600 0.1 200 400 600

8.27 6 0.08a 5.68 6 0.08b 3.42 6 0.01c 3.42 6 0.05c 7.10 6 0.04a 3.45 6 0.01b 3.41 6 0.0bc 3.40 6 0.02c 6.20 6 0.05a 3.39 6 0.05b 3.44 6 0.06b 3.43 6 0.03b

3.92 6 0.06a 3.17 6 0.04b 2.61 6 0.01c 2.61 6 0.03c 3.57 6 0.02a 2.68 6 0.01b 2.63 6 0.00c 2.62 6 0.01c 3.45 6 0.04a 2.59 6 0.00c 2.67 6 0.00b 2.66 6 0.02b

0.85 6 0.07c 0.86 6 0.04c 1.13 6 0.03b 1.67 6 0.03a 1.49 6 0.14b 1.51 6 0.02b 2.19 6 0.15a 2.43 6 0.09a 1.96 6 0.01c 2.02 6 0.01c 2.62 6 0.01b 3.82 6 0.03a

pH 7

pH 8

D4,3 (μm), volume-weighted means diameter; D3,2 (μm), volume-surface mean diameter; Г (mg/m2), interfacial protein concentration.

HHP treatments resulted in an EAI increase in the SPP emulsions at the pH values 3, 7, and 8. When the pressure was increased to 200 MPa, the emulsions had higher EAI values at pH 3 and 8 than that at pH 7, whereas higher-pressure treatments (400 and 600 MPa) decreased the EAI values (Fig. 4.9A). In contrast, at 600 MPa the ESI values were higher at all the pH values considered. The droplet size is important for the stability of emulsions; the smallest average size of the oil droplets for all the treated emulsions was obtained at 600 MPa (Table 4.4). The results suggest that the emulsifying properties of the SPP emulsions could be improved by the HHP treatment.

Droplet size distribution The emulsions had a monomodal droplet size distribution at the different pH values (Fig. 4.10). However, the HHPs affect the droplet size distribution with an increasing volume frequency of the smaller droplet size compared to the control (0.1 MPa) emulsions. In the presence of a deflocculating agent [e.g., slowly digestible starch (SDS)], the maximum volume frequency percentages obtained at 600 MPa for the droplets were approximately 3.42, 3.40, and 3.43 mm at pH values 3, 7, and 8, respectively (Fig. 4.10). Also in the presence of this deflocculating agent, HHPtreated (200
600 MPa) SPP stabilized emulsion had decreased droplet sizes compared to those of the control (0.1 MPa) at pH 3, 7, and 8. This

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(A)

8

pH 3 with 6 SDS 4 2 1

100

4 2

10

100

0.1

1

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(D) pH 7 without SDS

8

0.1 MPa 200 MPa 400 MPa 600 MPa

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4 2 0 1

100 10 Particle diameter (μm)

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8

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(C) pH 7 with SDS

0 0.1

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(B)

10 0.1 MPa 200 MPa 400 MPa 600 MPa

Volume frequency (%)

Volume frequency (%)

10

pH 8 without SDS

8 6

0.1 MPa 200 MPa 400 MPa 600 MPa

4 2 0 0.1

1

10 100 Particle diameter (μm)

1000

Figure 4.10 Effects of HHP on droplet size distribution (volume frequency) of SPP emulsion in pH 3 with SDS (A) and without SDS (B), 7 with SDS (C) and without SDS (D), and 8 with SDS (E) and without SDS (F) conditions in presence and absence of deflocculating agent (SDS).

corroborates the earlier observed increase in EAI of HHP-treated (200
600 MPa) SPP in comparison with those of 0.1 MPa at the various pHs. Since the higher the EAI values or the lower the emulsion droplet sizes then the higher the emulsifying properties of the protein, HHPtreated SPP gave better emulsifying properties than that of 0.1 MPa (native protein). The protein adsorbed to the interfacial film after the HHP treatment might have a sufficiently high zeta potential value, which most likely induced an effective electrostatic repulsion between the droplets. A similar behavior was observed by Le Denmat et al. (2000) in yolk emulsions at pH 3. The results suggest that a higher HHP treatment at pH 3 may enhance the stability of the emulsion. At pH 3, 7, and 8,

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HHP-treated SPP emulsions showed the same droplet size distribution with or without SDS except at 200 MPa (pH 3) and 0.1 MPa (pH 7 and 8), where an increase in emulsion droplet sizes was observed in the presence of SDS. Indeed, SDS, being a stronger surface active agent than vegetable protein, should displace the protein from the oil
water interface and thereby break down flocs (Demetriades and McClements, 2000), but the contrary was the case with SPP stabilized emulsion at 200 MPa (pH 3) and 0.1 MPa (pH 7 and 8). This could be due to formation of a mixed surfactant
polymer layer at the interface, with a diminished emulsifying capacity as compared to the pure polymer or SDS layers.

Emulsion microstructure The native SPP (0.1 MPa) emulsions at pH 3, 7, and 8 had fairly homogeneous structures (Fig. 4.11), which were in accordance with the results obtained in our previous study (Guo and Mu, 2010). The droplet sizes of SPP stabilized emulsion were larger in the continuous phase with the 0.1 MPa treatment than other HHP treatments, especially with application of 600 MPa (pH 3 and 8). Based on the microphotography, the oil

Figure 4.11 Microphotographs of freshly HHP-treated emulsions (15 min treatment). Images of control emulsions were made after 15 min of emulsion production. The length of a black bar is 20 mm.

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droplets were evenly distributed in the continuous phase, no coalescence was observed during the HHP treatments. With the exception of droplet size reduction, no change in the emulsion microstructure was detected with HHP treatment. In the oil
water emulsions, a decrease in the droplet size can affect the rheology and the shear thinning of the emulsion by increasing the apparent viscosity (Pal, 1996). HHP treatment enhanced the formation of smaller oil droplets at pH 3, which was most likely the cause of the rheological modifications and the increase in viscosity. Using microscopy we observed similar and important microstructural changes at pH 7 and 8 as the pressure was increased from 0.1 to 600 MPa (Fig. 4.11). Hydrophobic and electrostatic interactions of macromolecules (including proteins) are disrupted with high-pressure treatment. Such high-pressure-induced protein conformational changes have been associated with modifications of the food functional properties (Messens et al., 1997). In addition, the HHP-treated emulsion (600 MPa) at pH 3 showed a well-defined interfacial film around the oil droplets, whereas the pH 7 and 8 emulsions showed a microseparation of the phases, which was most likely a result of protein aggregation in the aqueous phase of the emulsion. The microphotography of the HHP-treated emulsions at each pH value also confirmed the reduction in the droplet sizes (D4,3 and D3,2; Table 4.4).

Interfacial protein concentration and composition The interfacial protein concentration (Г in mg/m2) of the HHP-treated emulsions is shown in Table 4.4. The Г for the HHP-treated emulsions at pH 3, 7, and 8 significantly increased (P , .05). The highest G values were obtained when the emulsions were treated with 600 MPa. The protein concentration at the oil
water interface was the critical factor in the stability of the emulsions because proteins can lower the interfacial tension at the oil
water interface of the droplets. This reduction in tension facilitates the formation of smaller droplets and increases their stability against coalescence by covering the droplets with a thin layer of protective coating (McClements, 2004). HHPs unfold protein structures by exposing the hydrophobic groups of the protein (Dickinson and James, 1998). Therefore one would expect that an increase in the surface hydrophobicity would be able to provide a better absorption potential at the oil
water interface. This hypothesis is consistent with the reduction in the droplet size (Table 4.4) and the increase in the Г as the pressure was

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increased from 200 to 600 MPa. Our previous study reported a saturated interfacial film at 1.81 mg/m2 of SPP, which was attributed to a closer packing of the adsorbed proteins in the monomolecular layer (Guo and Mu, 2010). However, the increase of the Г values by the HHP treatment might be a result of the protein in the aqueous phase, which is associated and/or aggregated to the previously formed monomolecular layer. This aggregation of the aqueous phase protein might affect HHP-induced unfolding, contributing to the formation of a secondary film where there are interactions between the proteins of the interfacial film and the aqueous phase. Our results agree with those of Puppo et al. (2011) who studied soybean protein emulsions. It has been reported that the net charge of the adsorbed protein layer is highly dependent on the pH (Dalgleish, 1997). At pH 3, proteins are positively charged, and this positive charge density decreases as the pH increases. In the HHP-treated emulsions, the interaction between SPP and the oil was stronger in the pH 3 emulsions, which was most likely due to a higher emulsion stability and lower value than for those in the pH 7 and 8 emulsions. The state of the adsorbed and the nonadsorbed proteins at the interface was analyzed by SDS-PAGE under nonreducing and reducing conditions (Fig. 4.12). Fig. 4.12A and B shows the electrophoretic profiles of the proteins belonging to the pH 3 SPP emulsion subjected to 200
600 MPa. Both sporamin A (31 kDa) and sporamin B (22 kDa), depending on the pressure level, were adsorbed to the interfacial layer after the HHP treatments (Fig. 4.12A). As the pressure was increased to 600 MPa, the sporamins were clearly visible on the SDS-PAGE (Fig. 4.12A, lane 5), which might be attributed to an increase in the interfacial protein concentration (Table 4.4). A similar pattern for the pH 7 emulsions was observed in the SDS-PAGE under nonreducing conditions (Fig. 4.12C). However, an interfacial layer, which was composed of higher molecular weight (MW) aggregates, was also observed in the pH 7 emulsions; this interfacial layer increased with increasing pressure. These results reveal that the HHP treatment contributed to the emulsion protein aggregation via disulfide bond formation. Upon the addition of the reducing agent (i.e., 2-mercaptoethanol), the higher MW aggregates, which were stabilized by the disulfide bonds, disappeared from the separating gel (Fig. 4.12D). The HHP treatment of the emulsions did not change and/ or displace the main electrophoretic bands of SPP. Furthermore, these electrophoretic bands were similar to those obtained in our previous study using SDS-PAGE (Guo and Mu, 2010).

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(A)

(B) 97.0 kDa 66.0 kDa

97.0 kDa 66.0 kDa 45.0 kDa Sporamin A 30.0 kDa

45.0 kDa 30.0 kDa

21.0 kDa 14.4 kDa STD 1

Sporamin B 2

3

4 5 6

7

8

9

(C)

21.0 kDa 14.4 kDa STD 1

Sporamin 2 3

4 5

6

7

8 9

(D)

97.0 kDa 66.0 kDa

97.0 kDa 66.0 kDa

45.0 kDa Sporamin A

45.0 kDa

30.0 kDa 30.0 kDa Sporamin B 21.0 kDa 14.4 kDa

21.0 kDa

Sporamin

14.4 kDa STD 1

2

3

4

5

6 7

8

9

STD 1 2

3 4

5

6 7

8

9

Figure 4.12 SDS-PAGE profile of proteins adsorbed and nonadsorbed at the oil
water interface derived from pH 3 and 7 emulsions. (A) and (C) are in nonreducing (without 2-mercaptoethanol), (B) and (D) in reducing conditions (with 2-mercaptoethanol) for pH 3 and 7, respectively. In each of the figures (A
D) STD, marker standard; lane 1, native SPP; lane 2, control adsorbed SPP; lane 3, 200 MPa adsorbed SPP; lane 4, 400 MPa adsorbed SPP; lane 5, 600 MPa adsorbed SPP; lane 6, control nonadsorbed SPP; lane 7, 200 MPa nonadsorbed SPP; lane 8, 400 MPa nonadsorbed SPP; lane 9, 600 MPa nonadsorbed SPP.

Using SDS-PAGE we observed that an aggregation could affect the appearance of the adsorbed sporamin A and B proteins at pH 3 and 7 (Fig. 4.12A and C, lanes 2
5). The appearance of sporamin A and B at pH 3 could be attributed to no and/or little aggregation; the disappearance of these proteins on the gels might be a result of protein hydrolysis in the acidic conditions (Fig. 4.12A). The SDS-PAGE results clearly demonstrate that the interfacial film is a result of protein aggregations and reveal the interaction between the adsorbed protein at the oil
water interface and the aggregated protein in the aqueous phase of the emulsion formed by the HHP treatment, which increased the interfacial protein concentration as shown in Table 4.4. Under reducing conditions, all the emulsions (at the different pH values) had a similar sporamin band (25 kDa), indicating that sporamin was well adsorbed to the interfacial

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layer (Fig. 4.12B and D). These results suggest that, with the exception of the disulfide bonds, the HHP treatment of the SPP emulsions did not lead to other structural changes that could alter their electrophoretic mobility.

Rheological properties Rheological behavior The control (0.1 MPa) and the HHP-treated emulsions (pH 3) were modeled using the Herschel
Bulkley model (Table 4.5), where τ c . 0 and n , 1. The pH 7 and 8 emulsions could be described using the power law model, where τ c 5 0 and n6¼1. In the case of the pH 7 emulsions, a shear-thinning behavior was observed in the control (0.1 MPa) and the HHP-treated emulsions, where n , 1 in all the pressure treatments. However, the 200 and 400 MPa-treated emulsions at pH 8 had a Newtonian behavior with a power law model (τ c 5 0 and n6¼1); the control (0.1 MPa) and the 600 MPa-treated emulsions showed a shearthinning behavior, with τ c 5 0 and n , 1. The shear rate versus the shear
strain curves of the pH 8 emulsions for the control (0.1 MPa) and the 600 MPa-treated emulsions explained the shear-thinning behavior of the emulsions (data not shown). For oil
water emulsions, a decrease in the droplet size can affect the rheological behavior (Pal, 1996). However, the Newtonian behavior of the pH 8 emulsions subjected to 200 and 400 MPa reveals that a reduction in the droplet sizes by the HHP treatment could not be the only factor that affected the rheological behaviors; the droplet size reduction might also be attributed to the properties of the stabilizing molecules. The factors that might affect the rheological behavior of the emulsions are mainly the viscosity of the aqueous phase, the interfacial film formed at the interface, the nature of the emulsifying agent, the volume frequency distribution of the droplets, the flocculated droplet network, and the electroviscous effect (Sherman, 1995). Flow index The flow indices (n) of each emulsion with respect to HHP treatment are shown in Table 4.5. The n values showed that the SPP emulsion had pseudoplastic flow behavior. The variability associated with these n values at pH 3 and 7 was high, this could be due to the higher flocculating tendency of the emulsion at pH 3 and 7 than at pH 8 on shearing. Pseudoplastic flow behavior is the most common type of nonideal behavior exhibited by food emulsions. It manifests itself as a decrease in the

Table 4.5 Modeling of the flow curves between 10 and 600 s21 of shear stress of control (0.1 MPa) and HHP-treated (200
600 MPa) emulsions prepared with SPP at pH 3, 7, and 8.

pH 3

pH 7

pH 8

Herschel
Bulkley factors

0.1 MPa

200 MPa

400 MPa

600 MPa

k (mPa sn) τ c (mPa) N R2 η (mPa s) 125 s21 k (mPa sn) τc (mPa) N R2 η (mPa s) 125 s21 k (mPa sn) τ c (mPa) N R2 η (mPa s) 125 s21

5.35 6 0.01 71 6 0.14 0.89 6 1.79 0.9998 6 0.0009 2.8 6 0.006 1.9 6 0.003 060 0.97 6 1.94 0.9992 6 0.0003 1.9 6 0.006 1.5 6 0.02 060 0.99 6 0.032 0.9993 6 0.0006 1.83 6 0.001

3.05 6 0.006 34 6 0.07 0.93 6 1.86 0.9991 6 0.0004 2.5 6 0.005 2.05 6 0.004 060 0.98 6 0.00 0.9991 6 0.0008 2.3 6 0.007 1.7 6 0.001 060 1.02 6 0.042 0.9960 6 0.0024 1.8 6 0.005

3.95 6 0.007 39.5 6 0.080 0.90 6 1.80 0.9992 6 0.0003 2.8 6 0.006 2.45 6 0.004 060 0.93 6 1.96 0.9998 6 0.0001 2.36 6 0.007 2.9 6 0.003 060 1.01 6 0.048 0.9982 6 0.0015 1.93 6 0.008

7.35 6 0.010 66 6 0.130 0.81 6 1.600 0.9993 6 0.0002 4.1 6 0.008 5.55 6 0.010 060 0.81 6 1.63 0.9998 6 0.002 3.3 6 0.009 4.4 6 0.00 060 0.87 6 0.064 0.9975 6 0.0016 2.56 6 0.005

k, viscosity coefficient (mPa sn); τ c, yield value (mPa); N, flow index; R2, regression coefficient of sample fitting plot; η (mPa s), viscosity at shear rate of 125 s21.

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apparent viscosity of a fluid as the shear rate increases and is referred to as shear-thinning. Pseudoplasticity might occur in food emulsions because of the spatial distribution of the particles, that is, nonspherical particles might align with the flow field, and/or flocs might be deformed and disrupted (McClements, 2005). The flow index is important when evaluating the emulsion viscosity, which mainly depends on the structure of the oil droplets and their attraction and aggregation under the application of stress. Viscosity The viscosity of the control (0.1 MPa) and the HHP-treated emulsions were considered at a 125 s21 shear rate (Table 4.5). The control (0.1 MPa) emulsion at pH 3 was initially more viscous than the emulsions at pH 7 and 8. The flocculation of the oil droplets in the emulsions played a key role in their viscosity. Anton et al. (2001) observed a higher flocculation and viscosity in emulsions treated with a 600 MPa-HHP treatment and prepared with hen egg yolk at pH 7 than at pH 3. Furthermore, pH 3 emulsions with low viscosity have been shown to have Newtonian behavior after HHP treatment (Anton et al., 2001), while our present study indicated that the emulsions stabilized by SPP had non-Newtonian behaviors before and after the HHP treatments in the acidic medium (Table 4.5). The increase in viscosity at pH 3 was observed at 600 MPa, which might be a result of a reduction in the flow index. In addition, the viscosity coefficient k (mPa sn), which is an indicator of the viscous behavior, was higher for the control (0.1 MPa) and the pressuretreated pH 3 emulsions (Table 4.5). The pH 7 and 8 emulsions had an increased viscosity at the 600 MPa HHP. At these pH values the protein had an almost zero or insignificant zeta potential value, which reduced the electrostatic repulsion potential and favored the droplet
droplet attraction. These results were similar to those obtained by Dickinson and James (1998, 1999), who reported that the high-pressure treatment of emulsions stabilized by β-lactoglobulin or bovine serum albumin induced a flocculation among the oil droplets leading to an increase in the viscosity. In addition to flocculation, the increase in viscosity at pH 7 and 8 might be attributed to the formation of a strong network of the aggregated proteins during the HHP treatments (Fig. 4.12C). Furthermore, the increased viscosity at 600 MPa had a positive correlation with the flow index (Table 4.5). A low flow index value at 600 MPa correlated with high viscosity in all cases, which indicates the non-Newtonian character of the emulsions. The results suggest that most of the rheological

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behavior modification took place using the 600 MPa-HHP treatment of the SPP emulsions.

Preparation and antioxidant activity of sweet potato protein hydrolysates Diverse hydrolysates/peptides, which can be obtained by enzymatic hydrolysis via commercial protease, gastrointestinal digestion, and food processing, have displayed potential health beneficial, one of the most researched being antioxidant activity (Singh et al., 2014; MazorraManzano et al., 2017). Antioxidant peptides can act as free radical scavengers, transition metal ion chelating agents, and lipid peroxidation inhibitors; they can protect cells from damage by reactive oxygen species and also show good nutritional and functional properties (Erdmann et al., 2008; Xie et al., 2008). HHP technology was designed to help improve enzymatic hydrolysis and digestibility of protein, and enhance the bioactivities of protein hydrolysates. HHP could also change the protein conformations, enable protein molecular chain extension, which is beneficial for enzymatic reactions due to the exposed new restriction sites (Bonomi et al., 2003), and might also produce some novel peptides with special physiological function. The author team investigated the effect of HHP on the degree of hydrolysis (DH), antioxidant capacity, and MW distribution of SPPH by Alcalase, separated and subsequently identified the antioxidant peptides from SPPH, and explored their structure
activity dependences (Zhang and Mu, 2017), which are introduced here.

Degree of hydrolysis and antioxidant activity of sweet potato protein hydrolysates DHs of SPPH by Alcalase under HHP were significantly different (P , .05, Table 4.6). DHs of SPPH by Alcalase under 0.1 MPa (atmospheric pressure) for 30 and 60 min were 20.83% and 23.17%, respectively. With the increase of pressure (100
300 MPa) and hydrolysis time (30
60 min), DHs of SPPH increased significantly, reaching the highest value of 31.68% under 300 MPa for 60 min. The improvement of enzymatic hydrolysis under HHP is supposed to be the result of the exposure of new cleavage sites through protein unfolding, and the enhancement of

Table 4.6 Degree of hydrolysis (DH), antioxidant activity, and ,3 kDa fractions percentage of sweet potato protein hydrolysates (SPPH) by Alcalase under high hydrostatic pressure (HHP). HHP (MPa)

Time (min)

DH (%)

 OH scavenging activity (%)

Fe21-chelating ability (%)

ORAC (μg TE/mL)

Percentage of ,3 kDa fractions (%)

0.1

30 60 30 60 30 60 30 60

20.83 6 0.10g 23.17 6 0.00e 22.65 6 0.00f 24.56 6 0.00d 23.34 6 0.20e 25.26 6 0.20c 25.78 6 0.20d 31.68 6 0.20a

41.91 6 1.32e 43.06 6 0.69e 41.45 6 1.47e 51.57 6 0.61c 45.82 6 1.41d 56.21 6 0.69a 50.08 6 0.82c 54.93 6 1.15a

91.87 6 1.01c 91.68 6 0.83c 89.38 6 0.18d 95.91 6 0.20a 92.19 6 0.40c 96.16 6 0.09a 93.47 6 0.52b 94.24 6 0.39b

123.93 6 2.67a 114.65 6 4.86bc 111.64 6 6.14bc 117.50 6 3.71ab 113.86 6 9.67bc 106.84 6 2.22c 113.96 6 2.20bc 116.67 6 4.32ab

28.50h 41.89f 36.56g 51.18c 41.98e 62.13b 50.89d 67.66a

100 200 300

 OH scavenging activity, Fe21-chelating ability, and oxygen radical absorbance capacity (ORAC) were tested at 1.0 mg/mL; values followed by different letters in the same column are significantly different (P , .05).

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enzyme activity and/or substrate
enzyme interaction (Belloque et al., 2007; Dufour et al., 1995). Antioxidant activities of SPPH were determined using  OH scavenging activity, Fe21-chelating ability, and oxygen radical absorbance capacity (ORAC) (Table 4.6). SPPH by Alcalase under HHP (100
300 MPa) had significantly higher  OH scavenging activity and Fe21-chelating ability than those under 0.1 MPa for 30 and 60 min (P , .05). SPPH by Alcalase under 200 and 300 MPa for 60 min showed the highest  OH scavenging activities, which were 56.21% and 54.93%, respectively, while SPPH by Alcalase under 100 and 200 MPa for 60 min showed the highest Fe21chelating abilities, which were 95.91% and 96.16%, respectively, with no significant difference between them (P..05). In addition, SPPH by Alcalase under 0.1 MPa for 30 min showed the highest ORAC value (123.93 μg of TE, P , .05), which was not significantly different to the SPPH under 100 MPa (117.50 μg TE/mL) and 300 MPa (116.67 μg TE/ mL) for 60 min (P..05).

Molecular weight distribution of sweet potato protein hydrolysates MW distribution profiles of SPPH by Alcalase under HHP were shown in Fig. 4.13. Effluent less than 3 kDa was eluted after 29.49 min based on the standard curve (shown as a dotted line). MW distribution was classified into two groups .3 and ,3 kDa, and their proportions regarding the total distribution were calculated based on the peak area. Obviously high performance liquid chromatography (HPLC) profiles of SPPH by Alcalase under 0.1 MPa and different high pressures (100
300 MPa) were different from each other, showing differences in MW distribution. Compared with SPPH by Alcalase under 0.1 MPa, SPPH by Alcalase under 100
300 MPa exhibited much smaller .3 kDa peaks, of which the ,3 kDa fractions (after 29.49 min) were prominent for both 30 and 60 min, respectively (Fig. 4.13A and B). The higher the pressure (100
300 MPa) and the longer the hydrolysis time (30
60 min), the greater amount of ,3 kDa peptides fraction were exhibited in SPPH (Fig. 4.13 and Table 4.6). Peptides exhibited in ,3 kDa fractions were significantly increased from 28.50% and 41.89% under 0.1 MPa to 50.89% and 67.66% under 300 MPa for 30 and 60 min, respectively (Table 4.6, P , .05). It was reported that chickpea hydrolysates by Alcalase under HHP at 200 MPa for 20 min showed high antioxidant activity, had the largest amount of ,3 kDa peptides fraction, and were mainly ranged

Sweet potato protein and its hydrolysates

103

(A) 1900 3 kDa Hydrolysis time 30 min

A215nm

1500 1100

0.1 MPa

100 MPa

200 MPa

300 MPa

700 300 –100 0

10

20

30 40 Retention time (min)

50

60

70

(B) 1900 3 kDa

A215nm

1500

Hydrolysis time 60 min 0.1 MPa

100 MPa

200 MPa

300 MPa

1100 700 300 –100 0

10

20

30 40 Retention time (min)

50

60

70

Figure 4.13 The molecular weight (MW) distribution profiles of sweet potato protein hydrolysates (SPPH) by Alcalase under high hydrostatic pressure (HHP) using HPLC. (A) Hydrolysis time of 30 min; (B) hydrolysis time of 60 min.

from 500 to 1000 Da (Zhang et al., 2012). In addition, enzymatic hydrolysis at 300 MPa induced an absolute degradation of lentil proteins and improved the concentration of the ,3 kDa peptides with all the enzymes used (Garcia-Mora et al., 2015).

Antioxidant activity of peptides by ultrafiltration Low MW peptide fractions were mainly the contributors to antioxidant activity of hydrolysates from protein (Garcia-Mora et al., 2015). Thus SPPH produced by Alcalase under 300 MPa for 60 min was selected for further characterization with respect to its higher DH, antioxidant activities, and ,3 kDa peptides proportion. SPPH was filtered via ultrafiltration to generate peptides with MW .10 kDa (FI), 3
10 kDa (FII), and ,3 kDa (FIII), respectively. Compared with .10 and 3
10 kDa peptide fractions, the antioxidant

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activity of peptide fractions with lower MW (,3 kDa) was significantly stronger, which was in line with our result previously (Zhang et al., 2014). The  OH scavenging activities of different fractions were 13.44%, 43.76%, and 60.50%, and the Fe21-chelating abilities were 10.77%, 77.49%, and 97.58%, while the ORAC values were 58.07, 66.06, and 93.54 μg TE/mL for FI, FII, and FIII, respectively. It was evident that the fraction with the lowest MW (FIII) showed higher antioxidant activities than the other higher MW fractions (P , .05). High antioxidant activities of low MW fractions were considered to be due to the easier reaction with and more effective elimination of free radicals (Ranathunga et al., 2006).

Separation and identification of peptides The FIII fraction was chromatographically fractionated into 54 fractions by semipreparative reverse-phase high-performance liquid chromatography (RP-HPLC) (Fig. 4.14 and Table 4.7). The low hydrophobicity peptides with low MW were eluted earlier, while high hydrophobicity peptides with high MW were eluted later (Wattanasiritham et al., 2016). ORAC values of fractions 1
54 ranged between 30.97 and 141.18 μg TE/mL. Among the 54 fractions, fraction 18 with an intermediate hydrophobicity exhibited the highest ORAC value (P , .05), which was 141.18 μg TE/mL, followed by those of fractions 31
34, 42, and 46 (119.23
127.80 μg TE/mL, Table 4.7). Based on their retention times, these fractions presented hydrophobic characteristics. It was also interesting that fractions 31 and 46 were dominant and showed high ORAC

Figure 4.14 RP-HPLC chromatography profile of active fraction FIII obtained by ultrafiltration from sweet potato protein hydrolysates (SPPH) at 300 MPa for 60 min.

Table 4.7 Oxygen radical absorbance capacity (ORAC) of RP-HPLC peaks from fraction FIII. Fraction no.

Time (min)

ORAC (μg TE/mL)

Fraction no.

Time (min)

ORAC (μg TE/mL)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

4.9
5.7 5.8
6.1 6.2
6.7 7.0
7.3 7.4
7.6 7.7
8.0 8.1
8.8 8.9
9.3 9.4
9.8 9.9
10.2 10.3
10.5 10.6
11.5 11.9
12.6 13.0
13.9 14.0
14.8 14.9
15.8 15.9
16.3 16.4
16.9 17.9
18.6 18.7
19.2 19.3
19.9 20.0
20.6 20.7
21.5 21.6
22.2 22.3
23.0 23.1
23.6 23.7
24.2

90.07 6 5.37qp 43.74 6 1.53v 45.27 6 0.11v 68.75 6 7.38tu 64.07 6 1.13u 30.97 6 1.24w 75.22 6 1.87rst 94.61 6 8.29nop 78.33 6 3.84rs 75.10 6 0.54rst 97.24 6 8.75mnop 114.83 6 4.26defg 111.93 6 6.36efgh 99.34 6 4.41lmno 101.99 6 5.26klmno 123.83.43 6 0.89bc 127.46 6 1.01b 141.18 6 10.84a 109.01 6 5.49fghijk 117.10 6 2.91cdef 109.16 6 3.68fghijk 79.34 6 6.38rs 71.44 6 4.64stu 100.13 6 1.42lmno 94.54 6 0.31nop 113.13 6 1.40efg 101.44 6 5.86klmno

28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

24.3
25.0 25.1
25.8 25.9
26.5 26.6
27.5 27.6
28.4 28.5
29.1 29.2
29.6 29.7
30.4 30.6
31.9 32.0
33.0 33.5
34.0 34.1
34.4 34.5
34.9 35.4
35.9 36.0
36.4 36.5
36.9 37.0
37.9 38.0
38.7 38.8
40.1 41.1
42.3 42.5
43.4 43.5
44.5 45.3
46.2 46.3
47.1 47.2
48.5 51.1
52.9 53.0
54.6

100.66 6 1.67klmno 88.70 6 1.63qp 103.10 6 1.19ijklmn 125.46 6 1.71bc 127.80 6 0.79b 124.51 6 2.01bc 123.90 6 3.07bc 106.85 6 1.61ghijkl 103.70 6 1.81hijklm 112.46 6 1.15efg 90.62 6 3.13qp 108.93 6 1.77fghijk 111.64 6 0.21efghi 118.31 6 1.20cde 122.59 6 2.20bcd 114.20 6 3.89defg 112.60 6 2.30efg 117.53 6 9.20cdef 119.23 6 2.13bcde 110.93 6 0.25efghij 114.25 6 0.43defg 102.43 6 0.45jklmn 77.39 6 15.05rst 63.82 6 0.53u 112.52 6 10.44efg 93.34 6 0.46op 83.43 6 10.39qr

Each peak (4.0 mL) was lyophilized and mixed with 200 μL of 75 mM phosphate buffer (pH 7.4) for the ORAC assay. Values followed by different letters in the same column are significantly different (P , .05).

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values of 125.46 and 119.23 μg TE/mL, respectively. Thus fractions 18, 31, and 46 were subsequently subjected to LC
MS/MS for peptide sequencing. As shown in Table 4.8, fractions 18, 31, and 46 were found to contain diverse peptides from sporamin A and B, with MW ranging from 600 to 2100 Da (5
19 amino acids). It was important to note that all the identified peptides belonged to sporamin A or B, and some of them belonged to both sporamins A and B. Sporamin, in its nonreduced form, exhibited two subunits of sporamins A and B, which had 219 and 216 amino acids according to the database from NCBI, respectively. In the case of fractions 18 and 31, the numbers of peptides from sporamin A were much less than those from sporamin B. While in the case of fraction 46, the numbers of peptides from sporamin A were similar to those from sporamin B. Orsini Delgado et al. (2016) found that diverse peptides were identified from gastrointestinal digestion of amaranth proteins, of which the MW ranged from 800 to 1700 Da with 7
15 amino acids, and the peptides mainly matched the sequence of 11S globulin, the subunit of amaranth proteins.

Synthesis of potential antioxidant peptides and conformation prediction Previously, researchers have evaluated the structure
antioxidant activity dependence of peptides. Hydrophobic amino acids (His, Tyr, or Pro) are present in the sequences of antioxidant peptides from soy β-conglycinin digests, and Leu and Val are located at the N-terminal domains (Chen et al., 1995). The His-His was necessary for the antioxidant activity of Leu-Leu-Pro-His-His and the Leu deletion of the N-terminal domains exhibited no significant effect on the activity, while the His deletion at the C-terminal domains decreased activity (Chen et al., 1996). One peptide with sequences of Ala-Thr-Ser-His-His from sandfish protein hydrolysates showed high antioxidant activity, which was due to the presence of His residues (Jang et al., 2016). Hernández-Ledesma et al. (2005) indicated that Trp, Tyr, and Met exhibited the strongest ORAC values followed by Cys, His, and Phe, while the others showed no detectable activity. The high antioxidant activities of Trp, Tyr, and Phe are based on their capability to donate hydrogen: Met can be oxidized to Met sulfoxide; Cys contributes sulfur to the hydrogen; His with the imidazole group has a proton-donating ability. Hernández-Ledesma et al. (2005) found that the conformation of peptides resulted in synergistic or

Table 4.8 Peptides sequences from selected fractions identified by LC
MS/MS. Fraction no.

Peptides

MW (Da)

Matched sequence in sporaminsa

18

GDEVRA

645.31

GDEVRAGE RLDSSSNE NIATNK

831.37 906.40 659.36

HDSASGQY HDSESGQY IKPTDM SDVIVS DVIVSPN ASDVIVSR SDVIVS PESTVVMPSTF PADPESTVVMPS RFNIATNK

863.34 921.34 719.35 618.32 742.38 845.46 618.32 1209.56 1244.56 962.53

KAGEFVSD

851.40

KAGEFVSDN

965.44

VVNDNL NDNLNAY SETPVLDINGDEVRAGENY YYMVSA EVRAGGNYYMVS RLAHLDTM RFNIA

672.34 822.35 2076.96 732.32 1344.61 955.49 619.34

RFNIATNK

962.53

f48
53 f46
51 f46
53 f73
80 f124
129 f121
126 f141
148 f138
145 f211
216 f84
89 f85
91 f82
89 f83
88 f105
115 f102
113 f122
129 f119
126 f151
158 f148
155 f151
159 f148
156 f164
169 f166
172 f37
55 f57
62 f50
61 f72
79 f122
126 f119
123 f122
129 f119
126

31

46

Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin

ORAC (μg TE/mL)b

A B B B A B A B B A A B B B B A B A B A B B B B A A A A B A B

123.06 6 13.13a 109.24 6 3.49b 68.47 6 2.94e

36.94 6 3.36fg 5.11 6 1.07h NA

95.74 6 9.69c 81.69 6 3.90d 117.30 6 6.02ab 30.54 6 4.55g NA

(Continued)

Table 4.8 (Continued) Fraction no.

a

Peptides

MW (Da)

Matched sequence in sporaminsa

NVNWGIKH NVNWGIQH NVNWGIQHD HDSESGQYF DSESGQYFVK HDSASGQYFLK HDSASGQYFLKAG SGQYFLK KAGEFVSDNSNQF

966.50 966.47 1081.49 1068.42 1158.52 1251.59 1379.65 841.43 1441.64

LKAGEFVSDNSNQFKIE RFHDPM RFHDPMLR RFHDPMLRT RFHDPMLRTT FVIKPT

1924.95 801.36 1086.54 1187.58 1288.63 703.43

YYIVS RSDFDNGDPITITPADPE DNGDPITITPA DNGDPITITPADPE NGDPITITPA NGDPITITPADPE NGDPITITPADPEST PITITPADPE VVNDNLNAYKIS RYYDPL RYYDPLTR

643.32 1958.88 1112.53 1453.66 997.51 1338.62 1526.71 1052.54 1348.70 825.40 1082.55

f131
138 f134
141 f134
142 f138
146 f139
148 f141
151 f141 2 153 f145
151 f151
163 f148
160 f150
166 f192
197 f192
199 f192
200 f192
201 f212
217 f209
214 f55
59 f89
106 f93
103 f93
106 f94
103 f94
106 f94
108 f97
106 f164
175 f189
194 f189
196

From National Center for Biotechnology Information (NCBI). ORAC of the synthesized peptides was tested at 0.2 mg/mL.

b

Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin Sporamin

ORAC (μg TE/mL)b

B A A B B A A A A B A A A A A A B B B B B B B B B B B B

98.87 6 0.92c 98.05 6 4.68c 76.31 6 2.85de NA 40.04 6 2.87f

4.46 6 0.94h 111.90 6 7.88b NA

113.97 6 7.10b

Sweet potato protein and its hydrolysates

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antagonist effects on their antioxidant activity compared to that of the amino acid itself. In addition, Li and Li (2013) detailed the structure
antioxidant activity dependence of peptides, showing the relationship between the antioxidant potentiality of peptides and the physicochemical characteristics ofthe C-terminal and N-terminal domains. In the case of ORAC, the importance of the positions were in the order C3 . C4 . C1 . N1. The bulky hydrophobic and polar/charged amino acids at the C-terminal domains contributed to ORAC, and those with minimal electronic property at the N-terminal domains (N1, N2) were also essential for it (Li and Li, 2013). Based on previous reports mentioned above, 20 peptides with potential antioxidant amino acids were selected and synthesized to verify their activity with an ORAC assay (Table 4.8, in bold). Peptides KAGEFVSD from fraction 31 and RFNIA, KAGEFVSDNSNQF, and RSDFDNGDPITITPADPE from fraction 46 did not present relevant activity when carried out at 0.2 mg/mL. Peptides RFNIATNK from fraction 31 and FVIKPT from fraction 46 had relatively low activity, while the other 14 peptides showed certain activity with ORAC ranging from 30.54 to 123.06 μg TE/mL (Table 4.8). Peptide HDSASGQY from fraction 18 presented the highest ORAC value, followed by peptides YYMVSA (fraction 46), RYYDPL (fraction 46), YYIVS (fraction 46), and HDSESGQY (fraction 18) (Table 4.8 and Fig. 4.14). The five peptides contained 5
8 amino acids, and had a MW range from 640 to 920 Da (Table 4.8), which was in line with the statement that short peptides with approximately 2
9 amino acids showed stronger antioxidant activity than larger polypeptides (Jeong et al., 2010), and further confirmed that most of the antioxidant peptides from food had an MW range from 500 to 1800 Da (Samaranayaka and Li-Chan, 2011). In addition, it was indicated that the stronger ORAC of short peptides might be due in part to the increased possibility to interact with and/or donate electrons to free radicals (Onuh et al., 2014). Peptide HDSASGQY showed His (positively charged, antioxidant) at the N1 position, Gly (hydrophobic) at the C3 region, and Tyr (bulky aromatic, antioxidant) at the C1 domain. However, its related peptide HDSASGQYFLK from fraction 46, containing the additional sequence of FLK at the C-terminal domain, presented much lower antioxidant potency than peptide HDSASGQY. Peptide HDSESGQY showed a little lower antioxidant activity than HDSASGQY, which might be because of the small difference at the N4 position. Peptide HDSESGQY presented

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Glu (negatively charged), while HDSASGQY had Ala (hydrophobic) in the N4 position. And it was interesting that its related peptide HDSESGQYF from fraction 46, containing the additional segment F at the C-terminal domain, also presented much lower antioxidant potency than peptide HDSESGQY. Peptide YYMVSA presented two Tyr (bulky aromatic, antioxidant) at the N1, N2 domain, and a Met (bulky hydrophobic, antioxidant) at the C4 region. Similarly, peptide YYIVS also contained two Tyr at the N1, N2 domain, and an Ile (bulky hydrophobic) at the C3 region. In addition, peptide RYYDPL presented an Arg (bulky positively charged) at the N1 domain, and two Tyr in the N2, N3 position. Obviously, His and Tyr were important to the antioxidant potency of the peptides, both in the N-terminal and C-terminal domains. And in the case of Tyr, the position inside the peptide sequence also contributed to its antioxidant activity. Due to the more active peptides being identified with 5
8 amino acids, some tridimensional conformations could be adopted to influence their activity. Conformations of peptides HDSASGQY (and HDSASGQYFLK for comparison), YYMVSA, RYYDPL, YYIVS, and HDSESGQY were carried out by PEPFOLD3 (Fig. 4.15). Peptide

Figure 4.15 Structures obtained by PEPFOLD3 for antioxidant peptides from SPPH: (A) HDSASGQY; (B) HDSASGQYFLK; (C) HDSESGQY; (D) YYMVSA; (E) YYIVS; (F) RYYDPL.

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HDSASGQY presented the most compact structure with four turns, exposing His and Tyr residues to the external medium (Fig. 4.15A). Compared with the overlapped peptide HDSASGQYFLK, His and Tyr residues were set out in the medium as well as a bulky Leu residue (Fig. 4.15B). Peptide HDSESGQY, presenting only one different amino acid from peptide HDSASGQY in the sequence, showed only three turns, with His and Tyr residues set out in the medium (Fig. 4.15C). Peptides YYMVSA, YYIVS, and RYYDPL showed only one turn (Fig. 4.15D
F). Peptide YYMVSA presented two Tyr with aromatic rings and one Met exposed to the external medium; peptide YYIVS also exposed two aromatic rings of Tyr to the external medium and a bulky hydrophobic residue Ile; while in peptide RYYDPL, a bulky positively charged Arg was located outside of the structure, with the two aromatic rings of Tyr revealed in the external medium (Fig. 4.15D, E, and F, respectively). Orsini Delgado et al. (2016) indicated that amaranth antioxidant peptides contained more than one bulky aromatic residue, and concluded that peptide AWEEREQGSR with the highest antioxidant activity presented the most compact structure with five folded turns, exposing Trp and Arg to the external medium.

Research and development trends of sweet potato protein and its hydrolysates Research on protein and its hydrolysates from different food resources has attracted widespread attention around the world. The following further studies are still necessary: (1) carrying out pilot and industrial production demonstrations of processing technology of SPP and its hydrolysates, in order to mitigate the environmental pollution and the waste of resources caused by waste liquid discharge from sweet potato starch processing, and to provide sufficient raw materials for the food industry; (2) developing the application of SPP and its hydrolysates in snack foods, in order to make full use of its gelation properties, emulsifying properties, antioxidant activity, etc.; and (3) developing the application of SPP and its hydrolysates in staple foods, in order to take full advantage of their nutritional value. In any case, sweet potatoes are a potential source of high-quality proteins and hydrolysates/ peptides, and it is necessary to conduct continuous and in-depth studies on them.

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912.

CHAPTER 5

Sweet potato dietary fiber Kazunori Takamine1, Meng-Mei Ma2 and Fredrick O. Ogutu2,3 1

Division of Shochu Fermentation Technology, Education and Research Center for Fermentation Studies, Faculty of Agriculture, Kagoshima University, Kagoshima, Japan 2 Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China 3 Food Technology Division, Kenya Industrial Research and Development Institute, Nairobi, Kenya

Overview of dietary fiber Definition of dietary fiber One definition that has been proposed for dietary fiber is “all of the poorly digestible ingredients in food, that cannot be digested by human digestive enzymes” (Kiriyama, 1980). Dietary fiber includes the structural polysaccharides (cellulose, hemicellulose) and lignin in plant cell walls, the viscous storage polysaccharides such as pectin and guar gum, the indigestible polysaccharides such as chitin and chitosan, as well as others. In addition, it includes starch-like polysaccharides, such as resistant starch and indigestible dextrose, and synthetic polysaccharides, such as polydextrose.

Classification of dietary fiber Dietary fiber is further divided into insoluble and soluble dietary fiber according to the solubility, with the former including cellulose, hemicellulose, lignin, and chitin, and the latter including pectin, plant gums, such as guar gum, and viscous substances, such as glucomannan. Cellulose is the principal component of plant cell walls, which comprises a linear-chain polysaccharide of D-glucose residues linked by β-1,4glycosidic bonds. Cellulose molecules have a degree of polymerization between approximately 3000 and 10,000, and in cell walls they associate with one another through hydrogen bonding to adopt a crystal structure. Cellulose molecules are insensitive to the hydrolytic activities of acid and digestive enzymes. Although cellulose is insoluble in water, it readily absorbs water, which causes it to swell. Hemicellulose is a matrix polysaccharide found in plant cell walls. Its main form is arabinoxylan, which consists of xylan, a polysaccharide with Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00005-3

© 2019 Elsevier Inc. All rights reserved.

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β-1,4-bonded-D-xylose residues, and side branches of L-arabinose, D -glucuronic acid, and 4-O-methylglucuronic acid. In cell walls hemicellulose binds to cellulose through hydrogen bonding and this contributes to the structural maintenance of the cell wall. Lignin is not a carbohydrate, but instead is an aromatic polymer that is strongly associated with cellulose through chemical bonds. Wood, for example, contains approximately 20% 30% lignin. Lignin is chemically and enzymatically difficult to break down, and it can be separated as a residue that is insoluble in the presence of 72% (w/w) sulfuric acid. Chitin is a polysaccharide of β-1,4-linked N-acetyl-D-glucosamine residues which is found in a diverse array of organisms, where it plays important structural roles. The shells of crabs and shrimp contain 10% 30% chitin, for example. Chitin is also present in the cell walls of molds, yeast, mushrooms, etc., often in a complex with protein. Chitin is insoluble in water, organic solvents, dilute acids, and dilute alkalis. Pectin is an acidic polysaccharide whose primary constituent is galacturonan, a polysaccharide containing α-1,4-linked residues of the acidic sugar galacturonic acid. Pectin is water soluble, but within cell walls it binds calcium, which renders it insoluble in water. Another form of pectin is rhamnogalacturonan, which has a galacturonan backbone containing L-rhamnose residues interspersed along the chain. The rhamnose residues can also be bound to neutral sugars such as L-arabinose, D-xylose, and D-galactose to create heteropolysaccharides. The carboxyl groups in polygalacturonic acid are also partially methyl-esterified. Guar gum is obtained from the endosperm of the seed of the legume guar. It is a galactomannan composed of galactose and mannose in a 1:2 ratio. Polydextrose is a water-soluble, indigestible synthetic polymer, obtained by mixing glucose, sorbitol, and citric acid, and conducting a condensation polymerization reaction under high temperature and high vacuum conditions.

Uses of dietary fiber In addition to its use as a bulking agent in food, dietary fiber has various physiological effects that allow it play an important role in maintaining health (Ebihara and Kiriyama, 1990). This role is different from that of the essential nutrients such as water, carbohydrates, protein, fats, vitamins, and minerals; thus dietary fiber is often considered to be the seventh

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nutrient. For example, insoluble dietary fiber, when consumed, needs to be chewed more than other types of food. It causes an increase in the secretion of digestive fluids, and suppresses the onset of colorectal cancer by increasing the amount of stool produced, and preventing and reducing the occurrence of constipation (Ebihara and Kiriyama, 1990). When dietary fiber reaches the large intestine, it is fermented by intestinal bacteria in the colon, producing carbon dioxide and organic acids, such as shortchain fatty acids. Among dietary fibers, water-soluble dietary fibers, such as some soluble hemicellulose and pectin, can be easily metabolized to organic acids by intestinal bacteria. It has been suggested that these organic acids are involved in suppressing the onset of colorectal cancer and decreasing plasma cholesterol (Bugaut and Bentéjac, 1993; Ebihara et al., 1993a,b; Salyers et al., 1977). In addition, dietary fiber has a protective effect on the mucosa of the gastrointestinal tract and also suppresses hunger, because it is capable of absorbing water and adopting a gel-like form that contributes to a feeling of fullness.

Dietary fiber from sweet potato Sweet potato pulp Sweet potato pulp is a by-product generated during the production of sweet potato starch (Fig. 5.1). Sweet potato starch is produced by pulverizing the highest quality sweet potatoes in a pulverizer, sequentially wetsieving the material through 80, 150, and 200 mesh sieves, and separating it into the starch fraction and the residue. Foreign substances are removed from the starch fraction with a nozzle separator, and the fraction is refined with a 300-mesh sieve to obtain sweet potato starch. Sweet potato starch is then either dried with warm air or sun-dried, before it is used. Filtrate (300-mesh)

Nozzle separate

Filtrate (200-mesh) Starch

Filtrate (150-mesh) Sweet potato

Mill

Filtrate (80-mesh)

Residue

Dehydrate

Figure 5.1 Process of sweet potato starch and sweet potato pulp.

Sweet potato pulp

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Figure 5.2 Microscopic observation of sweet potato pulp.

Meanwhile the residue is dehydrated to approximately 75% water content using a dehydrator to form the final sweet potato pulp. When sweet potato pulp is observed under a biological microscope, it appears as uncrushed, egg-shaped cell walls packed with starch or as crushed cell walls, in which the starch only partially remains (Fig. 5.2). The dry weight of sweet potato pulp consists of 43.5% starch, 2.0% protein, 0.4% lipids, 4.4% ash, and 49.7% dietary fiber. Dietary fiber can be manufactured by efficiently removing only the starch from sweet potato pulp. This chapter describes the efficient manufacturing of dietary fiber from sweet potato pulp (Takamine et al., 2000a), the physical properties of dietary fiber (Mei et al., 2010; Takamine et al., 2000a), a functional evaluation of dietary fiber in rats (Takamine et al., 2005), the extraction of pectin, an ingredient in dietary fiber, and the properties of pectin (Takamine et al., 2000b, 2007; Ogutu and Mu, 2017).

Efficient manufacture of dietary fiber from sweet potato pulp The particle size of sweet potato pulp ranges from 45 to 710 μm, with approximately 80% of the particles being between 150 and 350 μm, and most particles are at least 100 μm. Meanwhile, the core particles of sweet potato starch range from 15 to 20 μm, and at most 30 μm. Based on this, a method has been developed for manufacturing dietary fiber that relies on the difference in particle size between sweet potato pulp and starch (Takamine et al., 2000a). Specifically, the method of obtaining sweet potato-derived dietary fiber (SPDF) involves pulverizing sweet potato pulp, and then sorting it using sieves with pore sizes of 45 and 100 μm,

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Table 5.1 Components of sweet potato pulp and sweet potato dietary fiber (%, DW). Components

Sweet potato pulp

Sweet potato dietary fiber

Starch Crude protein Crude lipids Ash Dietary fiber Cellulose Hemicellulose Pectin Lignin

43.5 2.0 0.4 4.4 49.7 32.9 22.8 41.3 3.0

4.7 1.1 0.2 5.9 88.1 33.6 23.4 39.5 3.5

respectively; the material that passes through the 100 μm sieve, but remains on the top of the 45 μm sieve, is the SPDF. The composition of SPDF is shown in Table 5.1. The starch content by dry weight is 4.7%, being approximately one tenth of the quantity of starch found in sweet potato pulp. Proteins and lipids are reduced by approximately a half and the ash content increased slightly to 5.9%. This latter increase arises because of the reduction in protein and lipids which causes a corresponding relative increase in the percentage of ash. The dietary fiber in SPDF is 88.1%, an approximate 1.8-fold improvement over that of sweet potato pulp, a value that compares favorably with that of beet fiber (Nippon Beet Sugar Mfg. Co., Ltd.) and corn fiber (Nihon Shokuhin Kako Co., Ltd.). Also, the cellulose, hemicellulose, pectin, and lignin components of the dietary fiber are almost unchanged in abundance between SPDF and sweet potato pulp dietary fiber, as shown in Table 5.1. The SPDF therefore retains the dietary fiber derived from the sweet potato in an essentially unaltered form.

Physicochemical and functional properties of sweet potato dietary fiber Water and oil retention capacity Both water retention capacity and oil retention capacity are important parameters in the preparation of SPDF. A comparison of the water retention capacity and the oil retention capacity of SPDF obtained by air-dried at 60°C (AD-SPDF), freeze-dried SPDF (FD-SPDF), beet fiber, and corn fiber was shown in Fig. 5.3. The water retention capacity and oil retention capacity are both markedly superior in FD-SPDF, whereas ADSPDF exhibits retention capacities on a par with that of beet fiber. The

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(A) FD-SPDF

Water-holding capacity (g water / g dietary fiber )

50

AD-SPDF

40

Corn fiber Beet fiber

30 20 10 0 64

417

1670

4100

Centrifugal force (xg)

(B) FD-SPDF

Oil-holding capacity (g oil / g dietary fiber )

35

AD-SPDF

30

Corn fiber

25

Beet fiber

20 15 10 5 0 64

417 1670 Centrifugal force (xg)

4100

Figure 5.3 Water retention capacity (A) and oil retention capacity (B) of dietary fiber.

water retention capacity and oil retention capacity of FD-SPDF decrease as the centrifugal force increases, whereas for AD-SPDF, beet fiber, and corn fiber, the water and oil retention capacities are unaffected by the centrifugal force. The water retained in dietary fiber can be divided into three types: Type 1, water absorbed at the tissue surface; Type 2, water taken up into the tissue pores; and Type 3, free water implanted between the dietary fiber particles (Takeda and Kiriyama, 1995). The majority of water or oil content taken up by FD-SPDF is Type 2, and all the other samples are Type 1. These differences in water-holding capacity and oilholding capacity are thought to be due to the fact that FD-SPDF has a sponge-like, soft structure, as compared with AD-SPDF, beet fiber, and corn fiber, which all have a rigid surface, perhaps as a result of having been air-dried.

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Table 5.2 Color properties.

b

SPDF Beet fiber Corn fiber a

ΔEa

Index of whiteness

Index of yellowness

9.6 15.8 19.7

81.5 75.0 72.3

16.7 23.5 39.5

Value of a different from a standard white plate. Sweet potato dietary fiber.

b

Table 5.3 Composition of control and sweet potato dietary fiber diets (g/kg). Constituent

Control

Sweet potato dietary fiber

Casein α-Corn starch Corn oil Sucrose Cellulose powder Sweet potato dietary fiber Mineral mixture Vitamin mixture Choline chloride

200 299 60 300 100 0 30 10 1

200 299 60 300 0 100 30 10 1

Color and sensory evaluation The color difference from a standard white plate (ΔE) and the whiteness value were 9.6 and 81.5, respectively, for AD-SPDF, which was the closest to white, compared with beet fiber and corn fiber. Also AD-SPDF had the lowest yellowness, at 16.7, a value half or less than half that of corn fiber (Table 5.2). In addition, AD-SPDF was virtually odorless, since it contains almost no protein or lipids.

Effects of sweet potato dietary fiber on cecal fermentation products and intestinal flora in rats Four-week-old male Wister-Hanover rats (BrlHan:WIST@Jcl) were reared in individual cages at room temperatures between 22°C and 24°C, with 12 h alternating light and dark cycles (light from 8:00 to 20:00). Commercially available solid feed (CE-2, Clea Japan, Inc.) was given for 1 week to allow acclimatization to the environment. Following this, the rats were divided into two groups (n 5 7 each) and provided with feed containing 10% cellulose or SPDF, respectively, for 27 days. The detailed composition of the experimental feed is shown in Table 5.3

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(Osaki et al., 2001). Rats fed the cellulose-containing feed were used as controls. Free access to feed and drinking water was provided, and body weight was measured every 2 3 days. The quantity of stools produced and the quantity of feed consumed were measured in aggregate in 2- to 3-day increments. The water content of the stools was obtained by heating the stools for 17 h at 105°C. After 27 days of feeding, the rats were anesthetized with diethyl ether, and necropsies were performed. The SPDF group gained less weight than the control group, although this was not a statistically significant difference (Table 5.4). The SPDF group also consumed significantly less feed than the control group, so SPDF has the effect of suppressing appetite. A significant difference was not seen with respect to feed efficiency or cecal content pH. The SPDF group had a higher wet weight of cecal contents and wet weight of cecal tissue, but these differences were not significant. In other studies, rats given feed containing dietary fiber have been shown to generally consume less feed than rats fed standard feed, and large differences in feed efficiency have not been observed (Aritsuka et al., 1992; Hara et al., 1994). In our study the SPDF group also consumed a significantly lower quantity of food than the control group, but the feed efficiency was almost the same in both groups. The stool wet weight was lower in the SPDF group than in the control group, but despite this, the stool water content was significantly higher in the SPDF group than in the control group. The stool dry weight divided by the feed intake quantity was lower in the SPDF group compared with the control group, being 0.21 and 0.26, respectively. When rats are fed a large quantity of dietary fiber, the cecum and colon expand, but because the most marked expansion is observed in rats reared with feed containing an α-glucosidase inhibitor, it is believed that the expansion of the cecum and colon is not a phenomenon specific to dietary fiber, but rather, that expansion depends on an increase in the quantity and water retention properties of undigested material passing through the small intestine into the cecum (Aritsuka et al., 1992; Hara et al., 1994; Oku et al., 1981). Also because both the solid content and water content of the stools increase as dietary fiber intake increases, the stool volume and weight also increase, an effect that is reported to be greater with insoluble dietary fiber than with soluble dietary fiber (Hayakawa et al., 2003; Takahashi et al., 1999). In our study, the wet weight of both the cecal contents and cecal tissue increased with the addition of SPDF, an effect which likely arises because SPDF has a high water

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Table 5.4 Influence of the sweet potato dietary fiber on body weight gain, food intake, cecum, feces, microflora, and organic acid in cecum contents of rats.

Initial body weight (g) Body weight gain (g/27 d) Food intake (g/d) Food efficiencyc Cecum (g/100 g of body weight) Content of cecum (g/100 g of body weight) Tissue of cecum (g/100 g of body weight) pH of cecum Fecal weight (wet, g/d) Fecal weight (dry, g/d) Moisture (%) Clostridium Lactobacillus Bacteroides Peptostreptococcus Enterococcus Bifidobacterium Eubacterium Enterobacteriaceae Total aerobes Total anaerobes Total anaerobes/total aerobes Succinic acid Lactic acid Formic acid Acetic acid Propionic acid Isobutyric acid n-Butyric acid Isovaleric acid Total

Control

SPDFa

129 6 7 150 6 16 24.5 6 2.1 0.22 6 0.01 1.27 6 0.29

130 6 6 136 6 10 21.8 6 1.2b 0.23 6 0.01 1.55 6 0.50

0.87 6 0.25

0.99 6 0.48

0.40 6 0.07

0.56 6 0.12

6.95 6 0.20 7.20 6 1.66 6.38 6 1.52 11.6 6 1.85 5.68 6 0.8d 7.20 6 0.8 8.18 6 0.3 7.59 6 0.7 6.41 6 1.1 n.d.f n.d. 6.23 6 1.4 6.60 6 0.7 8.34 6 0.3 23 1.9 6 1.4 3.0 6 2.7 5.9 6 3.8 28.0 6 7.7 11.5 6 4.0 1.5 6 1.3 5.4 6 3.4 3.1 6 1.6 60.2 6 18.3

6.90 6 0.18 5.48 6 1.36 4.60 6 1.19b 16.0 6 3.42b 7.12 6 1.0 8.00 6 0.8 9.24 6 0.4 8.80 6 0.3 6.70 6 1.2 9.15 n.d. 6.78 6 1.4 7.28 6 0.5 9.45 6 0.3 105 2.6 6 4.0 4.1 6 5.8 5.4 6 6.6 33.3 6 7.8 25.2 6 16.7b 1.8 6 1.2 9.2 6 5.3 6.1 6 5.0 87.6 6 29.8

Data are means 6 SD (n $ 2). a Sweet potato dietary fiber. b Significantly different from the control group (P , .05). c Body weight gain (g)/food intake (g). d Values are expresses as mean 6 SD of bacterial counts. e Frequency of occurrence. f Not detected.

100e 100 86 86 86

100

100e 100 100 29 57 14 100

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retention ability. In fact it has been reported that 9 g, or more, of water can be retained per gram of SPDF (Takamine et al., 2005). In addition, the stool weight in the SPDF group significantly decreased compared with the control group, a result that is likely due to a reduction in feed intake. Similar to the effect on the wet weight of the cecal contents and cecal tissue, the increase in stool water content in the SPDF group compared with the control group is likely to have arisen due to the higher water retention ability of SPDF. Rats in the SPDF group also had a higher bacterial count in their intestinal flora than rats in the control group, and Bifidobacterium, in particular, which was not detected in the control group, was detected in the SPDF group, although only in the intestinal microflora of one rat out of seven (14%). The SPDF group also had a higher percentage of anaerobic bacteria than aerobic bacteria. The concentration of ammonia per gram of cecal contents did not differ much between the control group and the SPDF group; the values being 0.20 6 0.06 and 0.26 6 0.15 mg, respectively. Useful bacteria in the intestinal flora synthesize vitamins and proteins useful to the host, and are intimately involved in the digestion and absorption of food. Useful intestinal bacteria, such as bifidus, also inhibit the proliferation of harmful bacteria and work to purify the intestinal environment. Harmful intestinal bacteria have been shown, for example, to produce decomposition products, such as ammonia, as well as carcinogenic substances (Mitsuoka, 1995). Soluble fiber has been shown to be easily fermented by intestinal bacteria (Hayakawa et al., 2003) and to significantly increase the bacterial bifidus count (Aoe et al., 1988). The total bacterial count was higher in the SPDF group, but because the concentration of ammonia per gram of cecal contents was almost the same between the two groups, it appears that the proliferation of harmful bacteria was not promoted in the SPDF group. In contrast, bifidus was detected in the SPDF group, and it is likely that this is due to the presence of the soluble dietary fiber components hemicellulose and pectin contained in the SPDF. The concentrations of organic acids, including acetic acid, butyric acid, and isovaleric acid, in the intestinal contents were also numerically higher in the SPDF group than in the control group, but these differences in concentrations were not considered to be significant, because of large individual differences. In contrast, the concentration of propionic acid was significantly higher in the SPDF group. Propionic acid is known to inhibit HMG-CoA synthase, HMG-CoA reductase, and acetyl-CoA synthetase,

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and has been reported to be involved in suppressing cholesterol biosynthesis (Ebihara and Kiriyama, 1990; Mitsuoka, 1995). One study has reported that rats that consumed feed containing beet dietary fiber had significantly lower plasma cholesterol than those that consumed feed containing cellulose, but this correlation between short-chain fatty acid content and serum lipid concentration has not been confirmed (Arimandi et al., 1992). Although the propionic acid concentration was significantly higher in the SPDF group than in the control group, an analysis of serum lipids (values are not shown) showed no large differences in total cholesterol, HDL-cholesterol, or triglyceride levels between the two groups. Butyric acid is an energy source used by epithelial cells in the large intestine, and it is also known to inhibit the proliferation of cancer cells (Morishita, 1990). The concentration of butyric acid in the SPDF group was 1.7 times higher than in the control group, although there were large individual differences, and so this difference was not significant. Overall, the SPDF group had demonstrably higher concentrations of propionic acid, and numerically higher levels of butyric acid, and isovaleric acid, so it is possible that SPDF could be expected to have beneficial intestinal regulatory effects through increasing the levels of these organic acids.

Sweet potato pectin Overview of pectin Pectin is found between the cell walls of plant tissues in its calciumbound, water-insoluble state. Pectin is composed of an α-1,4-linked linear chain of the acidic sugar galacturonic acid (GalA), with side chains consisting primarily of the neutral sugars galactose and arabinose (Rolin and Vries, 1990). The rate at which the carboxyl group of GalA in pectin forms an ester bond with methanol differs depending on the plant source or extraction method. Pectin is primarily used as a gelling agent in raw food materials. Whether it forms a gel, and how its gel properties change, generally depends on the pH, the temperature, sugar concentration, calcium concentration, and pectin concentration (EI-Nawai and Heikal, 1995). In addition, the gelling mechanism of pectin is also determined by its degree of esterification. High methoxyl pectin (HM pectin), which is 50% esterified or more, forms a gel in the presence of acid and sugar. Low methoxyl pectin (LM pectin), which is 50% esterified or less, forms a gel in the presence of calcium in particular, as well as in the presence of

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alkaline earth metals (Grant et al., 1973; Oakenful, 1991; Walkinshaw and Arnott, 1981). In industry pectin is primarily extracted from the skin of citrus fruits, or the pulp of pressed apple juice (May, 1990). Beet pulp, potato pulp, sunflower, and others have also been reported as being good pectin sources. Pectin can also be used as a food bulking agent in the dieting industry to decrease calories, reduce cholesterol, and improve insulin sensitivity (Hayashi, 1996; Rombouts and Thibault, 1986; Sosulski et al., 1978; Turquois et al., 1999). Dietary fiber manufactured from sweet potato pulp contains approximately 40% pectin. This value is much higher than the 30% (Rombouts and Thibault, 1986) pectin content of the dietary fiber fraction of sugar beets, which are widely considered to be a useful source of pectin. Pectin has been widely used as a stabilizer in the food industry, but in recent years it has become highly prized for its ability to improve bowel regularity, lower blood cholesterol, and lower blood pressure (Yamaguchi et al., 1993).

Extraction of sweet potato pectin To extract pectin from plant cell walls, cool water (Li et al., 1998), warm dilute hydrochloric acid (Ootsuka et al., 1995), or an aqueous solution of ammonium oxalate (Noda et al., 1994) have all been used. However, pectin in sweet potato pulp cannot be efficiently extracted with warm water and warm dilute hydrochloric acid. This is because, when the starch is separated from the sweet potato, a solution of calcium oxide is added to the pulverized sweet potato to facilitate separation, and most of the carboxyl groups in the pectin form calcium salts. Also because sweet potato pulp contains approximately 44% starch (Takamine et al., 2000a), the starch gelatinizes at high temperature, so the starch and pectin must be fractionated in advance. Uronic acid content of sweet potato pulp Sweet potato dietary fiber contains approximately 39.5% pectin (Table 5.1). To extract the pectin, the dietary fiber is mixed with 0.5% of ammonium oxalate solution and stirred for 3 h at 95°C. An ethanol solution containing 1% of hydrochloric acid is added to the extraction solution at twice the volume of the extraction solution and the resulting solution is centrifuged for 20 min at 5400 3 g. The sedimented fraction is then placed in an evaporator to distill away the ethanol, and then the

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129

fraction is freeze-dried to obtain the final pectin preparation. The quantity of uronic acid in the pectin can be measured as anhydrous GalA using the m-hydroxydiphenyl method (Blumenkrantz and Asbsoe-hensen, 1973). The quantity of uronic acid in pectin extracted from starch has been measured as being 68.5%. This value has been used to calculate the dry weight percentage of uronic acid in sweet potato pulp as being 14.1%. Extraction reagents Pectin can be extracted from sweet potato pulp by adding a reagent to the sweet potato pulp suspension until the concentration reaches 50 mM, and then allowing extraction to proceed at 60°C for 24 h (Table 5.5). When pectin is extracted from citrus fruit using citric acid, hydrochloric acid, or phosphoric acid, the uronic acid extraction rate ranges from approximately 5% to 11%. Meanwhile with ethylenediaminetetraacetic acid (EDTA), the only reagent that can be used for canned or bottled food products, or with ammonium oxalate, which is normally used for pectin analysis, uronic acid can be extracted at rates that range from 82.8% to 94.2%, respectively. In addition, using dipotassium phosphate, tripotassium phosphate, and disodium phosphate, which cause the final pH of the extraction solution to change from neutral to alkaline, uronic acid can be extracted at rates of 80.9%, 76.9%, and 81.8%, respectively. However, hardly any uronic acid can be extracted using monopotassium Table 5.5 Extraction of pectin from sweet potato pulp. Reagent

Final pH

Extraction (%)

Potassium dihydrogenphosphate Dipotassium hydrogenphosphate Tripotassium phosphate Sodium dihydrogenphosphate Disodium hydrogenphosphate Trisodium phosphate Trisodium citrate Sodium carbonate Sodium chloride Citric acid Hydrochloric acid Phosphoric acid EDTA Ammonium oxalate

5.7 7.5 10.0 5.9 7.6 10.5 7.6 10.0 6.3 2.5 1.6 1.9 8.4 6.2

2.3 80.9 76.9 2.2 81.8 79.3 81.4 66.4 0.5 5.4 8.1 10.8 94.2 82.8

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phosphate, which produces a final extraction solution pH of 6 or below. Thus the extraction rate differs greatly, even among the different forms of phosphate. Extraction temperature and pH Pectin was extracted from 1% of sweet potato pulp suspension by adding 50 mM disodium phosphate. Extraction did not occur at temperatures ranging from 5°C to 30C, but the extraction rate increased significantly at 40°C and above, furthermore at 70°C approximately 82% of the pectin could be extracted. However, the starch in the sweet potato pulp was also extracted at temperatures of 66°C and above (Takamine et al., 2000b). This is because sweet potato starch begins to gelatinize at around 65°C, and is completely gelatinized at 78°C (Takeda and Hizukuri, 1974). Based on these facts, 63°C can be considered as the optimal temperature for extracting pectin without starch gelation occurring. In industry pectin is extracted in a metal acid, such as hydrochloric acid, diluted with warm water at a pH of 1 2 (May, 1990). The pectin solution is stable between pH 3 and 4; when the pH is lower, hydrolysis occurs, and when the pH is higher, the pectin is broken down by deesterification or a β-elimination reaction (Hayashi, 1996). In addition, pectin breaks down more easily as the extraction temperature increases. Interestingly high molecular weight pectin could be extracted, despite the extraction conditions being 63°C and a pH near 7.5. This is believed to be due to the fact that the raw materials for pectin extraction were different, and that calcium was used (Takamine et al., 2000a) during the sweet potato starch manufacturing process. Calcium forms a calcium salt with carboxyl groups and as a result the β-elimination reaction, which would normally occur at this pH and temperature, is inhibited (Hayashi, 1996). Recovering pectin from the extraction solution The amount of ethanol required to recover the pectin fraction from the pectin extract was determined. The data showed that when ethanol was added at 0.3 times the volume of the pectin extract, then complete recovery of the pectin was achieved. However, if higher levels of ethanol were added this caused the extraction reagent (disodium phosphate) to become insoluble and mix with the pectin, so extreme care must be taken to avoid adding too much ethanol in this procedure (Takamine et al., 2007).

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131

Properties of sweet potato pectin The chemical composition of sweet potato pectin Following pectin extraction from sweet potato pulp using disodium phosphate and its recovery by ethanol precipitation, the pectin was dried and pulverized. When this pectin preparation was treated with pectinase and analyzed by HPLC, 3.9% arabinose, 5.1% galactose, 1.7% rhamnose, 0.1% xylose, 0.1% mannose, and 0.3% glucose was found in sweet potato pectin. Furthermore, the water content was 3.0%, the GalA content was 64.8%, the degree of esterification was 1.4%, and the ash content was 21.6%. The vast majority (84%) of the ash content was sodium. The use of a metal ion-containing reagent such as EDTA to extract pectin can cause some carboxyl groups of the GalA in pectin to become neutralized by positive ions, which increases the ash content (Arslan, 1995). For example, pectin extracted from sugar beet residue using EDTA had a higher ash content than pectin extracted using hydrochloric acid or ammonium oxalate (Phatak et al., 1988; Sun and Hughes, 1998). Structure of sweet potato pectin determined by Fourier transform infrared spectroscopy The infrared spectra of pectin which were extracted by sodium polygalacturonate from sweet potato pulp are shown in Fig. 5.4. The peak at 1740 cm21 is due to the CQO stretching vibration of the carboxyl group that had formed a methyl ester, but when the carboxyl group was bonded to a salt, this absorbance was shifted to 1610 cm21 (Chatjigakis et al., 1998; Kamnev et al., 1998). The absorbance at 1200 1000 cm21

Transmittance (%)

100

(1)

3400 cm–1

80

2940 cm–1

(2) 60

40 4000

837 cm–1

1610 cm–1

3500

3000

2500

2000

1500

1000

450

Wavenumber (cm–1)

Figure 5.4 Infrared spectra of the pectin prepared from sweet potato pulp (1) and sodium polypectate (2).

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occurred due to the presence of oxygen in the glycosidic bonds and the stretching vibration of C O in the hydroxyl groups (Usui, 1986). In addition, the absorbance seen in the 833 855 cm21 range can be ascribed to the α-1,4-glycosidic bonds (Usui, 1986). The infrared spectra of pectin and sodium polygalacturonate extracted from sweet potato pulp were found to be almost identical. Molecular weight of sweet potato pectin From both the ash composition and infrared spectra data, most of the carboxyl groups in the pectin extracted from the sweet potato pulp using disodium phosphate were believed to have been substituted with sodium. In addition, the molecular weight of the prepared pectin showed the presence of two forms of pectin with peaks at 785,000 and 242,000 (Fig. 5.5) (Takamine et al., 2007). This pectin was therefore found to be more highly polymerized than commercially available pectin, which has a molecular weight of 50,000 150,000 (Hayashi and Hoshino, 2003). Breaking strength of pectin gel In the presence of divalent ions, such as calcium, it is the accepted belief that a junction zone, which is the fundamental structure of the gel, called an egg box (Oakenful, 1991), is formed between pectin molecules. A three-dimensional network is formed through this junction zone, and water and sugar molecules become trapped in this network completing the gel. In addition, pectin gelling and gel properties are also affected by the degree of methylation, molecular weight, pH, temperature, sugar concentration, pectin concentration, and others (Walkinshaw and Arnott, 1981).

105

104

103

RI response

106

10

15

20

25

30

35

Elution time (min)

Figure 5.5 Molecular weight distribution of sweet potato pectin.

Sweet potato dietary fiber

(B) 25,000

15,000

Breaking pressure of gel (N/m2)

Breaking pressure of gel (N/m2)

(A)

12,000 9000 6000 3000 0

20,000 15,000 10,000 5000 0

2

4

3

5

6

0

PH

0.5

1

1.5

2

2.5

Concentration of pectin (%)

(D)

(C)

Breaking pressure of gel (N/m2)

15,000 Breaking pressure of gel (N/m2)

133

12,000 9000 6000 3000

15,000 12,000 9000 6000 3000 0

0 0

40

20 2+

Concentration of Ca (mg/g-pectin)

60

0

10

20

30

40

50

Concentration of sucrose (%)

Figure 5.6 Effects of pH (A), pectin concentrations (B), calcium (C), and sucrose (D) on breaking pressure of pectin gel.

To study the effects of pH, pectin concentration, calcium concentration, and sugar concentration on the gel breaking strength of the pectin extracted from sweet potato pulp, the standard conditions used were a pH of 3.5, a sugar concentration of 30%, a pectin concentration of 1%, and a calcium concentration of 25 mg/g pectin. The effects of pH on the breaking pressure of pectin gel are shown in Fig. 5.6A. At pH 2.5 the gel was soft and not well formed. At pH 3.0 a gel was formed, and the strength and elasticity improved. At pH 4.0 the gel breaking strength was 1.3 3 104 N/m2, which remained almost constant up until pH 5.5. As shown in Fig. 5.6B, at pectin concentrations of 0.1% and 0.25%, a gel could not be formed. At 0.5% the gel form could not be maintained and became jam-like. At a pectin concentration of 2%, the fluid nature of the gel was absent and the gel contained air bubbles. When the pectin concentration increased from 0.5% to 1.5%, the gel breaking strength increased almost linearly. As calcium was changed from a concentration of 0 to 25 mg/g pectin, the breaking strength increased linearly, as shown in Fig. 5.6C. The maximum breaking strength of 1.1 3 104 N/m2 occurred at a calcium

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concentration of 25 mg, beyond which the gel became brittle and easily broken; the breaking strength was then found to rapidly decrease at concentrations higher than 25 mg. In order to form a stable gel with LM pectin, a junction zone of an appropriate length is thought to be required (Axelos and Thibault, 1991; Rees, 1982). The gel formation process is thought to be affected by calcium ions (Hayashi and Hoshino, 2003), so that when excess calcium ions are supplied, the junction zone becomes too long, and the quantity of liquid that can be confined in the network tends to decrease. Consequently, when the calcium concentration exceeds 25 mg/g pectin, the gel breaking strength is believed to rapidly decline. As shown in Fig. 5.6D, a gel was also formed without the addition of sugar. The breaking strength of this gel was 8.1 3 103 N/m2, and as the sugar concentration was increased from 0% to 30%, the breaking strength increased with increasing sugar concentration, although only slightly. Compared with the effects of pectin and calcium concentration, the effect of sugar concentration on the breaking strength was minimal.

Ultrasonic modification of sweet potato pectin Introduction of ultrasonic technology Ultrasonics is the science and technology of applying sound waves with frequencies above human hearing ability, or energy generated by sound waves of frequencies in the range of 18 kHz to 1 GHz. Sweet potato pectin exhibits a low degree of methyl esterification, which is rich in galactose and arabinose (Takamine et al., 2007). Moreover, application of sonication in pectin offers a novel, green, cost-effective, and easy to upscale method to modify pectin, which could yield better products for wider applications (Rastogi, 2011). Indeed, several low molecular weight polymers have shown higher activity than their large molecular weight counterparts.

Effect of pectin concentration and sonication time on sweet potato pectin sonolysis Concentration is one of the key factors that affect sonication efficiency. The pectin at concentration 2.5 mg/mL showed higher sonolysis, followed by 5 and 10 mg/mL. A 2.5 mg/mL pectin dispersion treated for 20 min at 200 W had a molecular weight reduced about twofold compared to 5 and 10 mg/mL pectin solutions, which are shown in Fig. 5.7A.

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Sweet potato dietary fiber

(B) 2.5 mg/mL pectin 5 mg/mL pectin 10 mg/mL pectin

800

1.5 Polydispersity %

600 400 200

1.0

0.5

0 min 5 min 10 min 20 min

0 min 5 min 10 min 20 min

0 0 min 5 min 10 min 20 min

Molecular weight (kDa)

(A)

Sonication time

0.0 0

5

10

20

Time (min)

Figure 5.7 Effects of pectin concentration and sonication time on molecular weight (A) and polydispersity (B) of sweet potato pectin at constant power 200 W and duty cycle 60%.

Pectin polydispersity did not show consistent change with increasing sonication time, which characterizes random scission of pectin chains (Fig. 5.7B), such a phenomenon was similar to that previously reported by Huang et al. (2015). In addition, increased sonication time resulted in clearer liquid compared to initial pectin dispersion (data not shown). Both pectin concentration and sonication time had significant (P , .0001) influences on pectin molecular weight change, with concentration contributing 77.50% influence, whereas sonication time had 18.69% influence, and interaction of the factors had insignificant (P 5 .1667) influence on molecular weight change according to two-way analysis of variance. Sonolysis had a direct correlation with time as shown in Fig. 5.7A. However, pectin concentration had a higher effect than that of sonication time. Concentration is known to affect cavitation, which in turn influences sonochemical impact.

Effect of sonication power on sweet potato pectin sonolysis Pectin molecular weight reduction had a positive correlation with sonication power, that is, increasing the power led to a decrease in molecular weight, which is shown in Fig. 5.8A. Polydispersity did not show a consistent trend with the increasing power, which corresponded to a random scission of pectin (Fig. 5.8B). Pectin molecular weight decreased rapidly between 100 and 400 W. In addition, the polydispersity of pectin was increased when the power increased to 100 W, and then decreased when the power further increased to 400 W.

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(A)

(B) 800

600

Polydispersity

Molecular weight (kDa)

2.0

400

200

0

1.5

1.0

0.5

0.0

0

100

200

300

Power (W)

400

500

0

100

200

300

400

Power (W)

Figure 5.8 Effects of ultrasonic power on molecular weight (A) and polydispersity (B) of sweet potato pectin.

Sonication reduces the molecular weight of a polymer to a definite point and further increases in the sonication power or time will not lead to an additional reduction in molecular weight. High intensity produced by high sonic power induces higher cavitation in the medium, hence a higher cavitation yield, leading to stretching and uncoiling in molecules, and hence the structure and flexibility of molecules determine their susceptibility to sonication. Cavitation growth depends on ultrasonic intensity, as high intensity ultrasound causes bubbles to have cavities that result in faster and more violent bubble growth and bursting. High-intensity ultrasound expands the cavity so rapidly during the negative-pressure cycle that the cavity does not shrink during the positive-pressure cycle (Kasaai, 2013). In this process therefore cavities grow faster in a single cycle, thus increasing sonochemical activity. On the other hand, low intensity ultrasound leads to the size of the cavities oscillating in phase with the expansion and compression cycles, hence less impact (Suslick, 1989). Overall the study results lend credence to the growing body of evidence showing that acoustic power is a vital parameter for the sonochemical effect, with increasing power resulting in an increased sonochemical effect.

Effect of sonication duty cycle on sweet potato pectin sonolysis Duty cycle is related to sonication time, the higher the duty cycle, the longer the sonication time. There was a direct relationship between duty cycle and molecular weight reduction. As expected, molecular weight decreased with increasing duty cycle, and the highest ultrasonic activity

137

Sweet potato dietary fiber

(B) 500

Polydispersity

Molecular weight (kDa)

(A) 400 300 200

1.5

b

1.0

0.5

100 0.0

0 20

40

60

Duty cycle (%)

80

20

40

60

80

Duty cycle (%)

Figure 5.9 The effects of ultrasonic duty cycle on molecular weight (A) and polydispersity (B) of sweet potato pectin.

was obtained at 80% duty cycle. The average molecular weight of pectin sonolyzed at 80% duty cycle was 140.41 6 11.49 kDa, while that at 20% duty cycle was 391.15 6 63.14 kDa, that is, a fourfold increase in duty cycle resulted in an approximately 2.8-fold decrease in molecular weight (Fig. 5.9A). Increasing the duty cycle led to an increased sonolysis effect, causing a significant decrease in pectin molecular weight with P 5 .0475 and the correlation coefficient (R2 5 0.9073) at α 5 0.05. When the duty cycle was lower than 60%, such as 20%, and 40%, the total sonication time could have been less to degrade substantial amount of pectin. However, above 60% duty cycle a significant molecular weight reduction occurred, which could be due to sufficient time being allowed for cavitation formation. Based upon our results on duty cycle, we selected 60% duty cycle for the experiments on the effect of ultrasonic power, time, and concentration on pectin sonolysis. It was reported that in pulse ultrasound treatment, the duty cycle length and interval had a significant effect on sonochemical activity, and from 50% duty cycle significant sonolysis was realized (Sun and Ye, 2013). The pulse duration is related to cavitation, hence the longer the duty cycle, the higher the cavitation and consequently, higher sonolysis. The polydispersity did not show a clear trend in the tested duty cycle range (Fig. 5.9B), indicating that sonolysis was most probably random.

Effect of sonication power on neutral sugar composition of sweet potato pectin The neutral sugar composition of sonicated pectin showed that there was a slight change in pectin structure at different sonication power levels at 60% duty cycle for 20 min (Table 5.6). The most noticeable change

Table 5.6 Effect of ultrasound power on neutral sugar content of sweet potato pectin at 60% duty cycle for 20 min. Treatment

Rha

Ara

Native 100 W 200 W 400 W

7.23 6 1.34 4.92 6 1.96c 6.64 6 0.08b 11.10 6 0.12a b

Gal

17.73 6 0.62 16.57 6 1.01c 18.37 6 0.53bc 30.49 6 1.85a bc

32.73 6 0.15 33.01 6 0.73c 39.72 6 1.41b 47.91 6 0.32a

Glc cd

Xyl

32.77 6 1.09 34.62 6 6.51a 28.31 6 2.10b 1.80 6 0.03c a

Ara 1 Gal/Rha

9.54 6 0.78 10.86 6 0.92a 6.96 6 1.13c 8.69 6 0.69b a

6.81 9.92 8.74 7.06

Data are means 6 SD (n $ 2). Values within columns with different letters are significantly different (P , .05). Rha, Rhamnose; Ara, arabinose; Gal, galactose; Glc, glucose; Xyl, xylose.

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occurred in 400 W-treated pectin, which showed a general increase in galactose content and a decrease in glucose content with increasing sonication intensity, as well as a decrease in arabinose and rhamnose content, while xylose content had less change. The glucose content was lowest (1.80%). The most dominant neutral sugars in pectin were arabinose and galactose, with glucose possibly coming from starch and cellulose hydrolysis during extraction. Based upon the neutral sugar composition, pectin is a copolymer of homogalacturonan and rhamnogalacturonan, with arabinose and galactose originating from a rhamnogalacturonan side chain, while rhamnose was from the parental chain joined to GalA and forming the branching point, and xylose could have come from xylogalactan (Yapo and Koffi, 2013). The degree of branching of rhamnogalacturonan I, represented as (Ara 1 Gal)/Rha ratio, is indicative of the number of Rha residues branched with Ara and Gal residues, irrespective of the length of Ara and/or Gal residues-containing side chains. Hence the greater the quantity of (Ara 1 Gal) than Rha the lower the amount of branching in the pectin polymer (Sila et al., 2009). According to Table 5.5, sonication at 200 and 400 W led to reduced branching compared to native pectin and pectin sonolyzed at 100 W. In a previous study it was observed that there was no change in the main chain while the decrease of pectin side chains was noted (Liu et al., 2013). The main chain is mostly composed of GalA which is resistant to sonication compared to the neutral sugars.

Effect of sonication power on the degree of methoxylation and galacturonic acid content of sweet potato pectin Sonication led to increased GalA content and decreased degree of methoxylation (DM), as shown in Table 5.7. GalA content increased significantly (P , .0001) with the increase in sonication power applied. Table 5.7 Effect of ultrasound power on GalA and DM of sweet potato pectin at 20 min and 60% duty. Sample treatment

GalA content

DM

Native pectin 100 W 200 W 400 W

72.0 6 2.1 85.6 6 1.6b 89.29 6 3.2ab 92.00 6 2.7a

12 6 3.0a 5.46 6 1.1b 6.28 6 0.92b 5.25 6 1.3b

c

Data are means 6 SD (n $ 2). Values within columns with different letters are significantly different (P , .05).

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The increase in GalA content could be attributed to the scission of the pectin side chain, which resulted in more GalA-rich homogalacturonan chains, hence GalA increased from 72.0% 6 2.1% in native pectin up to 92.00% 6 2.7% in 400 W-sonicated pectin. The DM of the sonicated pectin decreased significantly (P , .0001) compared to that of native pectin, but there was no significant difference between three different sonication treatments (100, 200, and 400 W). Liu et al. (2013) noted that GalA was more resistant to sonication compared to neutral sugars, which could explain the increasing GalA content with increased sonication intensity. Reduced DM could have been due to hydrolysis of ester groups by cavitation, or ester groups reacting with ionized groups generated during sonolysis. For instance, it was noted that chitosan’s degree of esterification reduced during sonolysis possibly due to the sonic hydrolysis of ester bonds (Baxter et al., 2005). In the same manner ester bonds in pectin could have been hydrolyzed leading to the formation of more free carboxyl groups, and hence increasing GalA content. The effect of sonication on ester groups was reported, and the sonication effect was found to be more pronounced in more hydrophobic ester groups (Piiskop et al., 2007). The team noted that ester groups are vulnerable to sonolysis, notwithstanding the solvent, and that long carbon chain length increases led to increased sonolysis in alcohol (ethyl, propyl, and butyl) esters.

Effect of sonication power on the structure of sweet potato pectin Fourier transform infrared spectroscopy (FTIR) was used to assess the changes in pectin structure due to sonication. Assignment of peaks was based upon the work of Filippov (1974), the peaks representing various groups within pectin chain were noted. The pectin peaks were similar to those reported by Sato et al. (2011) on sweet potato pectin. The pectin FTIR profile was shown in Fig. 5.10. The band at 3400 cm21 represented O H stretching, and the band at 2940 cm21 represented C H stretching of the CH2 group. A similar band profile was reported in sweet potato pulp pectin by Takamine et al. (2007). The FTIR spectra of all pectin showed a lower absorbance at 1750 cm21 (COOR) than at 1650 cm21 (COO ), indicating a LM pectin which corroborated the DM measured via the titration method. The bands at 1100 and 1070 cm21 represented rhamnogalacturonan, and there were bold bands at 1070 and 1043 cm21. The band at 1063 cm21 represented monopyranose component, and a higher peak at between 1250 to 1486 cm21 showed CH deformation

Sweet potato dietary fiber

Transmitance (T)

(A) 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 4000

141

200 W 100 W 400 W Native CH2

OH 1745

1645

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm–1) (B)

20% 40% 60% 80% Native

0.8

Transmitance (T)

0.7 0.6 0.5 0.4 0.3 0.2 0.1 4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm–1) Native 5 min 10 min 20 min

Transmitance (T)

(C)

0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 4000

1625 CH2

1745

OH

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm–1)

Figure 5.10 Effects of sonication power (100, 200 and 400 W) (A), duty cycle (20, 40, 60 and 80%) (B), and sonication time (5, 10 and 20 min) (C) on pectin structure of sweet potato pectin determined by Fourier transform infrared spectroscopy (FTIR).

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vibration, with three peaks associated with the carboxylic acid (COO ), the methyl ester (COOCH3), and the primary amide (CO NH2) groups. The fingerprint region below 1500 cm21 corresponded principally to coupled C C, C O C, and C OH vibration modes of the carbohydrate ring and to the glycosidic linkage vibrations. Ultrasound degradation had no recognizable effect on the absorption at around 3400 cm21 (OH), 2938 cm21 (CH), 1148.7 cm21 (COC), and 1101.3 cm21 (C C), which indicated that sonic power, time, and duty cycle had no noticeable effect on the main chain and the pectin side chain could have been sonolyzed by scission (Chen et al. 2012). The peak at 1750 cm 1 for native pectin was relatively smaller than that of other pectin, which could explain the reduced DM and increased GalA content during sonication.

Effect of sonication power on the antioxidant capacity of sweet potato pectin Oxidative radical absorbance capacity The antioxidant capacity of sweet potato pectin determined by oxidative radical absorbance capacity (ORAC) is shown in Fig. 5.10. The Trolox standard curve was plotted between 0 and 60 μg/concentration, and its NetAUC standard equation was Y 5 0.899x 1 2.581 (R2 5 0.993). Trolox equivalent (TE) is expressed in micromoles of TE/100 g of sample (Progress et al., 2011). ORAC values of 200 and 400 W-sonicated pectin were higher than those of native and 100 W-sonicated pectin. While 400 W-sonicated pectin at 4 mg/mL had 104,000 6 220 μM TE/100 g, which was fivefold higher than native pectin, which was higher than that of honey and red wine, but in the same range as acacia fruit/pulp/skin. On the other hand, the 200 W-sonicated pectin had ORAC value 34,420 6 180 μM TE/100 g as shown in Fig. 5.11, which was equally higher than that of many foodstuffs and spices according to the United States Department of Agriculture (USDA) ORAC value of foods (Haytowitz and Bhagwat, 2010). The 100 W-treated and native pectin showed almost equal ORAC values; this could have been due to less sonication at 100 W compared to 200 and 400 W, and hence pectin had insignificant structural change to improve its antioxidant activity relative to native pectin. Sonication increased the ORAC value of pectin and its effect was more pronounced at 200 and 400 W sonication power. Previous studies had shown that low molecular weight polysaccharide derivatives had higher bioactivity than high molecular weight ones

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%Activity compared to 1mM FeSo4 set at 100%

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Figure 5.11 Effects of sonication power on the antioxidant capacity of sweet potato pectin determined by oxidative radical absorbance capacity (ORAC) (A) and ferric reducing antioxidant power (FRAP) (B).

(Xia et al., 2011), which could explain the increased ORAC value of sonicated sweet potato pectin in the present study. Ferric reducing antioxidant power The ferric reducing antioxidant power (FRAP) values of pectin are shown in Fig. 5.11. The standard curve was plotted for FeSO4 with a concentration range of 0.2 1 mM. There was a strong correlation between FeSO4 concentration and antioxidant capacity (R2 5 0.992) and the equation was Y 5 0.1958x 2 0.2452. The standard curve displayed a linear trend

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between 0.2 and 1 mM FeSO4. There was a strong correlation between pectin concentration and antioxidant activity for all the pectin samples tested (R2 5 0.9778, 0.9885, 0.9742, and 0.9954), for the native, 100, 200, and 400 W-degraded pectin, respectively. Sonicated pectin had increased antioxidant activity with 400 W-treated pectin having 43% relative FRAP activity at 4 mg/mL; the same concentration of native pectin had a lower relative FRAP activity 16.4% compared to FeSO4. There was generally increased antioxidant activity with increasing sonication power applied. The results are consistent with the observations of Pokora et al. (2013), who reported that enzymatic hydrolysis of egg yolk protein and egg white protein improved their radical scavenging (DPPH) capacity, ferric reducing power, and chelating of iron activity. Native pectin is a complex molecule with a complex side group structure, and during sonolysis the large molecule is depolymerized yielding a low degree of polymerization of pectin, thus exposing previously hidden functional groups and creating functional groups at the scission sites, for example, carbonyl groups. The reducing agents mostly act as hydrogen/electron atom donors, thus reducing the radical species. Hydrolysis of protein into peptides is also known to increase its antioxidant capacity, because peptides have more functional groups per surface area than native protein, due to exposure of hidden functional groups. Increased FRAP activity correlated with GalA content, and reduced molecular weight due to more COO-groups. Moreover, sonication caused depolymerization which possibly created new functional groups within the low molecular weight pectin formed. Another possibility is that the hydrogen ion (H1) formed during water sonolysis is very reactive and a strong reducing agent. In line with this argument, it was reported that sonication of Ƙ-carrageenan yielded more oligo-carrageenan with a stronger reducing power. On the contrary, Zhou et al. (2014) found that the antioxidant capacity of proanthocyanidins increased with the increasing degree of polymerization, which was partly attributed to its numerous functional groups which increase proportionally with molecular size, hence large phenolic compounds quench more radicals than smaller molecules. Even though the two antioxidant assays work differently, the results show that sonicated pectin is an effective radical scavenger and oxidative species reducing agent. Taken together, the FRAP and ORAC results demonstrate that sonication serves as an innovative and green method of not only pectin molecular size reduction but also of enhancing the antioxidant activity of pectin.

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Application prospect of sweet potato dietary fiber Sweet potato starch residues contain large amounts of dietary fiber, which is rich in cellulose, hemicellulose, lignin, and pectin, and exhibits good physicochemical and functional properties. In particular, sweet potato pectin obtained by sonication modification showed high antioxidant capacity. These results showed that sweet potato dietary fiber could be potentially used in normal foods, functional foods, health care products, and pharmaceuticals in the near future.

References Aoe, S., Ohta, F., Ayano, Y., 1988. Effect of water-soluble dietary fiber on intestinal microflora in rats. J. Nutr. Sci. Vitaminol. 41, 203 211. Arimandi, B.H., Ahn, J., Nathani, S., Reeves, R.D., 1992. Dietary soluble fiber and cholesterol affect serum cholesterol concentration, hepatic portal venous short-chain fatty acid concentrationa and fecal sterol excretion in rats. J. Nutr. 122, 246 253. Aritsuka, T., Tanaka, K., Kiriyama, S., 1992. Long-term effects on serum cholesterol concentration of beet dietary fiber in normal and cecectomized rats fed a cholesterol-free casein diet. Nippon Nogeik Kaishi 66, 881 889. Arslan, N., 1995. Extraction of pectin from sugar-beet pulp and intrinsic viscositymolecular weight relationship of pectin solutions. J. Foood Sci. Technol. 32, 381 385. Axelos, M.A.V., Thibault, J.F., 1991. The chemistry of low-methoxyl pectin gelation. In: Walter, R.H., Taylor, S. (Eds.), The Chemistry and Technology of Pectin. Academic Press, New York, pp. 109 118. Baxter, S., Zivanovic, S., Weiss, J., 2005. Molecular weight and degree of acetylation of high-intensity ultrasonicated chitosan. Food Hydrocoll. 19, 821 830. Blumenkrantz, N., Asbsoe-hensen, G., 1973. New method for quantitative determination of uronic acids. Anal. Biochem. 54, 484 489. Bugaut, M., Bentéjac, M., 1993. Biological effects of short-chain fatty acids in nonruminant mammals. Annu. Rev. Nutr. 13, 217 241. Chatjigakis, A.K., Pappas, C., Proxenia, N., Kalantzi, O., Rodis, P., Polissioua, M., 1998. FT-IR spectroscopic determination of the degree of esterification of cell wall pectins from stored peaches and correlation to textural changes. Carbohyd. Polym. 37, 395 408. Chen, J., Liang, R., Liu, W., Liu, C., Li, T., Tu, Z., 2012. Degradation of high-methoxyl pectin by dynamic high pressure micro fluidization and its mechanism. Food Hydrocolloid. 28, 121 129. Ebihara, K., Kiriyama, S., 1990. Physico-chemical property and physiological function of dietary fiber. J. Japan Soc. Food Sci. Technol. 37, 916 933. Ebihara, K., Miyada, T., Nakajima, A., 1993a. Hyprocholesterolemic effect of cecally infused propionic acid in rats fed a cholesterol-free, casein diet. Nut. Res. 13, 209 217. Ebihara, K., Miyada, T., Nakajima, A., 1993b. Compression of propionic acid and sodium propionate infused to the cecum and stomach on hypocholesterolemic effect in rat fed a cholesterol-free, casein protein diet. Nut. Res. 13, 1305 1311. EI-Nawai, S.A., Heikal, Y.A., 1995. Factors affecting the production of low-ester pectin gels. Carbohyd. Polym. 26, 189 193.

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Filippov, M.P., 1974. IR spectra of pectin films. J. Appl. Spectrosc. (Zhurnal Prikl Spektrosk) 17, 1052 1054. Grant, G.T., Morris, E.R., Rees, D.A., Smith, P.J.C., Thom, D., 1973. Biological interactions between polysaccharides and divalent cations: the egg-box model. FEBS Lett. 32, 195 198. Hara, H., Saito, Y., Nagata, M., Tsuji, M., Yamamoto, K., Kiriyama, S., 1994. Artificial fiber complexes composed of cellulose and guar gum or psyllium may be better source of soluble fiber for rats than comparable fiber mixtures. J. Nutr. 124, 1238 1247. Hayakawa, T., Yamashita, K., Nakano, B., Miyake, Y., Yamamoto, K., Tsuge, H., 2003. Effects of ingestion of prune dietary fiber on cecal fermentation and fecal output in rats. J. Nutr. Sci. Vitaminol. 56, 17 22. Hayashi, Y., 1996. Property and application of pectin. Foods Food Ingred. J. Japan 167, 22 30. Hayashi, Y., Hoshino, T., 2003. Pectin application for fruit processed food products. Foods Food Ingred. J. Japan. 208, 903 912. Haytowitz, D.B., Bhagwat, S., 2010. USDA database for the Oxygen Radical Absorbance Capacity (ORAC) of selected foods, Release 2. US Dep Agric 2010, 10 48. Available from: ,http://www.pureacaiberry.com/testimonials/ORAC_R2.pdf.. Huang, C., Miao, M., Jiang, B., Cui, S.W., Jia, X., Zhang, T., 2015. Polysaccharides modification through green technology: role of ultrasonication towards improving physicochemical properties of (1-3)(1-6)-a-D-glucans. Food Hydrocoll. 50, 166 173. Kamnev, A.A., Colina, M., Rodriguez, J., Ptitchkina, N.M., Ignatov, V.V., 1998. Comparative spectroscopic characterization of different pectins and their sources. Food Hydrocolloid. 12, 263 271. Kasaai, M.R., 2013. Input power-mechanism relationship for ultrasonic irradiation: food and polymer applications. Nat. Sci. 5, 14 22. Kiriyama, S., 1980. Nutritional effect of dietary fiber. Chem. Biol. 18, 95 105. Li, T., Yamauchi, R., Kato, K., 1998. Fractionation and characterization of haw pectin. J. Appl. Glycosci. 45, 27 32. Liu, D., Zhang, L., Xu, Y., 2013. The influence of ultrasound on the structure, rheological properties and degradation path of citrus pectin. In: Proc Meet Acoust ICA 2013, 2 7 June 2013 Phys Acoust Montreal, Canada.Acoustical Society of America through the American Institute of Physics, Canada, pp. 1 9. May, C.D., 1990. Industrial pectins: sources, production and applications. Carbohyd. Polym. 12, 79 99. Mei, X., Mu, T.H., Han, J.J., 2010. Composition and physicochemical properties of dietary fiber extracted from residues of 10 varieties of sweet potato by a sieving method. J. Agric. Food Chem. 58, 7305 7310. Mitsuoka, T., 1995. Intestinal bacteria and dietary fiber. In: Inami, S., Kiriyama, S. (Eds.), Dietary Fiber. Dai-ichi Press, Tokyo, pp. 251 280. Morishita, Y., 1990. Structure and Function of Intestinal Flora. Asakura Publishing, Tokyo, pp. 177 181. Noda, T., Takahata, Y., Nagata, T., Shibuya, N., 1994. Chemical composition of cell wall material from sweet potato starch residue. Starch/Stärke 46, 232 236. Oakenful, D.G., 1991. The chemistry of high-methoxyl pectins. In: Walter, R. (Ed.), The Chemistry and Technology of Pectin. Academic Press, New York, pp. 88 108. Ogutu, F.O., Mu, T., 2017. Ultrasonic degradation of sweet potato pectin and its antioxidant activity. Ultrason. Sonochem. 38, 726 734. Oku, T., Konishi, F., Hosoya, N., 1981. Effect of various unavailable carbohydrates and dministrating periods on several physiological functions of rat. J. Japanese Soc. Nutr. Food Sci. 34, 437 443.

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Ootsuka, Y., Sawayama, S., Kawabata, A., 1995. Some properties of the pectic substances fordietary fiber in rhubarb. J. Soc. Cookery Sci. Japan 28, 146 150. Osaki, S., Kimura, T., Sugimoto, T., Hizukuri, S., Iritani, N., 2001. L-Arabinose feeding prevents increases due to dietary sucrose in lipogenic enzymes and triacylglycerol level in rats. J. Nutr. 131, 796 799. Phatak, L., Chang, K.C., Brown, G., 1988. Isolation and characterization of pectin in sugar-beet pulp. J. Food Sci. 53, 830 833. Piiskop, S., Hagu, H., Jarv, J., Salmar, S., Tuulmets, A., 2007. Sonification effects on ester hydrolysis in alcohol-water mixtures. Proc. Est. Acad. Sci. Chem. 56, 196 206. Pokora, M., Eckert, E., Zambrowicz, A., Bobak, è., Szołtysik, M., Da˛browska, A., et al., 2013. Biological and functional properties of proteolytic enzyme-modified egg protein by-products. Food Sci. Nutr. 1, 184 195. Progress, A., Prakash, A., Rigelhof, F., Miller, E., 2011. Antioxidant activity. Eur. Rev. Med. Pharmacol. Sci. 15, 376 378. Available from: http://www.ncbi.nlm.nih.gov/ pubmed/21621684. Rastogi, N.K., 2011. Opportunities and challenges in application of ultrasound in food processing. Crit. Rev. Food Sci. Nutr. 51, 705 722. Rees, D.A., 1982. Polysaccharide conformation in solutions and gels-recent results on pectins. Carbohyd. Polym. 2, 254 263. Rolin, C., Vries, J.D., 1990. Pectin. In: Harris, P. (Ed.), Food Gels. Elsevier Applied Science Series, London, pp. 401 434. Rombouts, F.M., Thibault, J.F., 1986. Sugar beet pectins: chemical structure and gelation through oxidative coupling, Chemistry and Function of Pectins, vol. 310. ACS Symposium Series, pp. 49 60. Salyers, A.A., West, S.H.E., Vercellotti, J.R., Wilkins, T.D., 1977. Fermentation of mucins and plant polysaccharides by anaerobic bacteria from the human colon. Appl. Environ. Microb. 34, 529 533. Sato, M.D.F., Rigoni, D.C., Canteri, M.H.G., Petkowicz, C.L.D.O., Nogueira, A., Wosiacki, G., et al., 2011. Chemical and instrumental characterization of pectin from dried pomace of eleven apple cultivars. Acta Sci. Agron. 33, 383 389. Sila, D.N., Van Buggenhout, S., Duvetter, T., Fraeye, I., De Roeck, A., Van Loey, A., et al., 2009. Pectins in processed fruits and vegetables: part II—structure function relationships. Compr. Rev. Food Sci. Food Saf. 8, 86 104. Sosulski, F., Lin, M.J.Y., Humbert, E.S., 1978. Gelation characteristics of acid-precipitated pectin from sunflower heads. Can. Inst. Food Sci. Technol. J. 11, 113 116. Sun, R., Hughes, S., 1998. Extraction and physico-chemical characterization of pectins from sugar beet pulp. Polym. J. 30, 671 677. Sun, Y., Ye, X., 2013. Enhancement or reduction of sonochemical activity of pulsed ultrasound compared to continuous ultrasound at 20 kHz? Molecules 18, 4858 4867. Suslick, K.S., 1989. The chemical effects of ultrasound. Sci. Am. Springer New York 260, 80 86. Takahashi, T., Maeda, H., Aoyama, T., Yamamoto, T., Takamatsu, K., 1999. Physiological effects of water-soluble soybean fiber in rats. Biosci. Biotech. Biochem. 63, 1340 1345. Takamine, K., Iwaya, A., Maseda, S., Abe, J., Hizukuri, S., 2000a. A new manufacturing process for dietary fiber from sweet potato residue and its physical characteristics. J. Appl. Glycosci. 47, 67 72. Takamine, K., Iwaya, A., Shimono, K., Maseda, S., Abe, J., Hizukuri, S., 2000b. Preparation of pectin from sweet potato residue and its characterization. J. Appl. Glycosci. 47, 201 206.

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Takamine, K., Hotta, H., Degawa, Y., Morimura, S., Kida, K., 2005. Effects of dietary fiber prepared from sweet potato pulp on cecal fermentation products and microflora in rats. J. Appl. Glycosci. 52, 1 5. Takamine, K., Abe, J., Shimono, K., Sameshima, Y., Morimura, S., Kida, K., 2007. Physicochemical and gelling characterizations of pectin extracted from sweet potato pulp. J. Appl. Glycosci. 54, 211 216. Takeda, C., Hizukuri, S., 1974. Characterization of the heat dependent pasting behavior of starches. Nippon Nogeik Kaishi 48, 663 669. Takeda, H., Kiriyama, S., 1995. Physicochemical properties of dietary fiber. In: Innami, S., Kiriyama, S. (Eds.), Dietary fiber. Dai-ichi Press, Tokyo, pp. 59 60. Turquois, T., Rinaudo, M., Taravel, F.R., Heyraud, A., 1999. Extraction of highly gelling pectic substances from sugar beet pulp and potato pulp: influence of extrinsic parameters on their gelling properties. Food Hydrocoll. 13, 255 262. Usui, T., 1986. Binding mode between glucose residues. In: Nakamura, M., Kainuma, K. (Eds.), Experiment of Starch and Polysaccharaides. Academic Press Center, Tokyo, pp. 59 60. Walkinshaw, M.D., Arnott, S., 1981. Conformations and interaction of pectin. II. Models for junction zones in pectinic acid and calcium pectate gels. J. Mol. Biol. 153, 1075 1085. Xia, W., Liu, P., Zhang, J., Chen, J., 2011. Biological activities of chitosan and chitooligosaccharides. Food Hydrocoll. 25, 170 179. Yamaguchi, F., Uchida, S., Shimizu, N., Maeda, S., Hatanaka, C., 1993. Drinks and foods containing low molecular weight hectic acid. Japanese Unexamined Patent Application Publication, No. Hei 05-192108. Yapo, B.M., Koffi, K., 2013. Extraction and characterization of highly gelling low methoxy pectin from cashew apple pomace. Foods 3, 1 12. Zhou, H.-C., Tam, N.F., Lin, Y.-M., Ding, Z.-H., Chai, W.-M., Wei, S.-D., 2014. Relationships between degree of polymerization and antioxidant activities: a study on proanthocyanidins from the leaves of a medicinal mangrove plant ceriops tagal. PLoS One 9, e107606.

Further reading Aoe, S., 1995. Dietary fiber materials used for food. In: Inami, S., Kiriyama, S. (Eds.), Dietary Fiber. Dai-ichi Press, Tokyo, p. 341. Renard, C.M.G.C., Thibault, J.F., 1993. Structure and properties of apple and sugar-beet pectins extracted by chelating agents. Carbohyd. Res. 244, 99 114.

CHAPTER 6

Sweet potato lipids Tai-Hua Mu and Miao Zhang

Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China

Overview of lipids Definition of lipids Lipids, as a potential functional ingredient, are present in almost all foods and play an important role in nutrition and the sensory aspects of food. Lipids are mainly divided into neutral lipids (NLs), glycolipids (GLs), and phospholipids (PLs). It was reported that the contents of NLs, GLs, and PLs in total lipids (TLs) of potato tubers were 21%, 22%, and 47%, respectively (Ramadan and Oraby, 2016). The NLs, GLs, and PLs contents in the major Indian Garcinia fruit, which has different varieties, range from 37.6% to 95.8%, 3.2% to 55.9%, and 0.8% to 6.8%, respectively (Patil et al., 2016). Fatty acids (FAs) play an important role in lipids, such as linoleic acid (C18:2), linolenic acid (C18:3), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA), showing effects on diverse physiological processes and thus have an impact on normal health and chronic diseases (Benatti et al., 2004).

Extraction methods of lipids Mechanical squeezing method Mechanical squeezing is one of the most long-standing methods for obtaining lipids. Lipids are obtained by being squeezed out, mainly by mechanical force, and this process mainly involves physical changes, such as friction heating, material deformation, oil separation, water evaporation, etc. At present, this method is still widely used by some enterprises. The advantages of the mechanical squeezing method are that it is convenient to operate and requires less investment. However, there are still some shortcomings, such as the low content of bioactive substances in the oil and the low yield of oil. At the same time, due to the effects of changes in water and temperature, some biochemical reactions will Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00006-5

© 2019 Elsevier Inc. All rights reserved.

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occur, for example, the oil is easily oxidized, and the protein is prone to denaturation. Bligh-Dyer method The Bligh-Dyer method is a widely used method for lipid determination in biologics and food. The outstanding advantages of this method are the high extraction rate of oil, low residual rate, and high production efficiency—it can achieve continuous processing with low energy consumption. However, some dangerous flammable solvents are used in this method, and the health problems caused by residual solvents in oil meal are also serious (Radin, 1988). Other novel lipid extraction methods Some novel lipid extraction methods have been investigated, for example, simultaneous distillation and extraction (Tanzi et al., 2013), ultrasoundassisted extraction (Adam et al., 2012), microwave-assisted extraction (Cheng et al., 2013), and supercritical fluid extraction (Halim et al., 2011). However, the methods above still require high energy inputs, high temperature, and take a long time, and are currently limited at the laboratory scale. Yang et al. (2014) extracted lipids from wet microalga Picochlorum sp. by using ethanol at room temperature, and indicated that the yield of lipids could be comparable to that by the Bligh-Dyer method without significant differences in FA composition and lipid classes distribution.

Physiological functions of lipids Lipids present different physiological functions, including improvements in the bioavailability of functional components, as well as having hypolipidemic, antiatherosclerosis, antimicrobial, antiinflammatory, memory improving, diabetes prevention, and anticancer properties, which have been summarized here. Improvement on the bioavailability of functional components It has been reported that some kinds of lipids could improve the bioavailability of some functional components. PLs are the major carriers for active molecules of plants; they interact with plant constituents, protect plant active components from degradation, and increase their bioavailability by imparting lipid solubility to them (Khan and Krishnaraj, 2014).

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PLs and glyceroglycolipids could markedly affect the uptake of carotenoids by human intestinal Caco-2 cells by solubilizing in mixed micelles via decreasing intercellular barrier integrity, suggesting their potential to modify the intestinal uptake of carotenoids (Kotake-Nara et al., 2015). The stability and biological availability of carotenoid fucoxanthin was improved when encapsulated in chitosan-glycolipid nanogels (Ravi and Baskaran, 2015). Hypolipidemic effect Yang and Jiao (2008) investigated the effects of soy lecithin on blood lipids in patients with hyperlipidemia. According to the blood lipid levels, 100 patients with hyperlipidemia were divided into the control and experimental groups, with 50 cases in each group. The patients in the experimental group took soy lecithin 20 g per day, and the control group received the same dose of placebo for 8 weeks. Serum total cholesterol (STC) and triglyceride (TG) levels were measured before and after taking soy lecithin or placebo. The results showed that no significant difference was observed in levels of STC and TG between the experimental group and control group before taking. However, after taking soy lecithin for 8 weeks, STC and TG levels in the experimental group were significantly lower than the starting levels and the levels in the control group, suggesting that soybean lecithin could significantly decrease the serum lipid level in the hyperlipidemic population. The lipid regulation function might be through inhibiting TG synthesis and eliminating cholesterol in vivo and in TC emulsions. Waststrate and Meijer (1998) found that STC and TC levels decreased significantly with an intake of 1.6 2 g/person/day of plant sterols. Nyugen et al. (1999) studied TC lowering effects of plant stanol esters, and the results showed that the total STC and low-density lipoprotein (LDL) cholesterol decreased by 6.4% and 10.1% after intake of 2 3 g/ person/day of plant stanol esters, respectively. Gylling and Miettinen (2005) indicated that plant sterols and stanols could inhibit the absorption of cholesterol that was endogenous and produced by diet in the small intestine. The authors indicated that the main reason is that free sterols and stanols could replace cholesterol from microcapsules mixed with bile acid, thus reducing the intestinal absorption of cholesterol, and also could compete with cholesterol during its absorption in the microvillous membrane.

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Antiatherosclerosis effect Karantonis et al. (2002) separated the TLs of oils from corn, olive, soybean, sunflower, and sesame into PLs and NLs by using a new extraction technique, which were further separated into different components by high-performance liquid chromatography. Among the different components of lipids, PL showed strong inhibitory effects on the plateletactivating factor, suggesting the preventive effects of PL on atherosclerosis. Volger et al. (2001) found that plant sterols had inhibitory effects on atherosclerosis, and its mechanism of action was mainly by reducing the STC. LDL oxidation was a major cause of atherosclerosis, and there was a positive correlation between the oxidation degree of LDL and STC content, so reducing STC content could inhibit the occurrence of atherosclerosis effects. Antimicrobial effect Jing and Qi (2001) isolated three kinds of glyceroglycolipids from Serratula strangulate, and determined their antibacterial activities. The results showed that three kinds of glyceroglycolipids had significant inhibitory effects on Bacillus subtilis, Escherichia coli, and Staphylococcus aureus. Zhang et al. (2013) used Candida albicans and E. coli as the tested bacteria, and determined the antibacterial activities of 10 saturated fatty acids (SFAs), 6 unsaturated fatty acids (UFAs), 2 FA methyl ester, 3 FAs, 4 fatty alcohols, and 9 FA monoglycerides. It was suggested that the antimicrobial activity of FAs and their derivatives was related to the carbon chain length of FAs, the number and position of double bonds in UFAs, and the types of substituent groups. Qian et al. (2012) found that the minimum inhibitory concentration of dirhamnoside derivative compound 7 (DDC7) and cleistrioside-5 (C5) on S. aureus (MIC) is 16 μg/mL, and the mechanism might be due to DDC7 and C5 having a similar chemical structure with the sugar chain of the β-1,4 glycosidic bond of the S. aureus peptidoglycan, which could destroy the biosynthesis or glycosidic linkage of bacterial polysaccharides. Hu et al. (2012) indicated that sophorolipids could effectively inhibit the growth of S. aureus in a concentration-dependent manner, even in acidic and high temperature conditions. Antiinflammatory effect GLs from spinach could suppress lipopolysaccharides-induced vascular inflammation through endothelial nitric oxide synthase and NK-κB signaling in human umbilical vein endothelial cells, suggesting that GLs from

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spinach might have a potential therapeutic utilization for inflammatory vascular diseases (Ishii et al., 2017). Milo et al. (2002) investigated the effects of short-chain FAs on the levels of proinflammatory cytokines in miniature pigs, and indicated that short-chain FAs could increase the intestinal proinflammatory cytokines IL-1 and lL-6 level to inhibit inflammation. Larsen et al. (2003) isolated monogalactosyl diacylglycerol (MGDG) from rose hip through active tracking detection technology, and found that MGDG presented an antiinflammatory effect without cell toxicity in an in vitro experiment. Bruno et al. (2005) isolated MGDG, digalactosyl diglyceride (DGDG), and sulfoquinovosyl diacylglycerol (SQDG) from thermophilic cyanobacteria, and showed that MGDG, DGDG, and SQDG could inhibit the swelling of mouse ear in a dosedependent manner. In addition, some clinical trials showed that a diet containing cod liver oil could significantly reduce the joint swelling in patients with rheumatoid arthritis, while the intake of an ordinary diet had no effect (James et al., 2000; Kremer, 2000). The mechanism of antiinflammation activities of cod liver oil was as follows: (1) through influencing the metabolism of peanut arachidonic acid (Simopoulos, 2002); (2) by changing the structure of PLs in the cell membrane (Calder and Grimble, 2002); and (3) by acting on the mediators of inflammation and immunity (Adam et al., 2003). Memory improving effect It was well known that EPA and DHA could improve memory and eyesight, especially to promote the growth and development of infant brain cells. Avrova et al. (2002) found that ganglioside GM1 could improve the memory function, and the mechanism was that GM1 improved the neuroprotective effects of the glial cell line and brain-derived neurotrophic factor, decreased the concentration of free radicals, thereby reducing the generation of nitric oxide, and then decreased the death of nerve cells. Chung et al. (1995) indicated that egg yolk lecithin could improve the memory of mice with dementia, and improve the concentration of acetylcholine in the brain of mice. Diabetes prevention effect Suresh and Das (2001, 2003) found that cooking oils rich in n-3 PUFA (EPA and DHA) and n-6 PUFA (GLA and AA) could prevent diabetes, and alleviate oxidative stress induced by diabetes. Delarue et al. (2004) indicated that the mechanism of n-3 PUFA on diabetes protection was as

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follows: (1) inhibiting the activity decrease of phosphatidylinositol 3kinase (PI3-K) and the failure of glucose transporter (GLUT4) in muscle; (2) preventing the decrease of GLUT4 expression in adipose tissue; and (3) inhibiting the activity of hepatic 6-glucokinase. Anticancer effect The anticarcinogenic effect of lipids was shown in several types of cancers including skin, melanoma, colon, and breast cancers. The addition of n-3 PUFAs in the diet could reduce the number of epithelial degeneration crypt lesions in mice induced by reactive oxygen species, thus preventing the development of colon cancer (Murray et al., 2002). DHA could reduce the positioning function of sarcoma protein (RSP) in epithelial cells of rats, inhibit the activation of RSP, reduce the binding level of RSP by guanosine triphosphate, and thus reduce the incidence of colon cancer in rats (Collett et al. 2001). It had been proved that the GLs components from plants could inhibit the proliferation and promote the differentiation of cancer cells. Morimoto et al. (1995) found that MGDG isolated from green algae had a strong inhibitory effect on lymph cancer cells. Maeda et al. (2008) indicated that GLs isolated from spinach presented a strong inhibitory effect on cervical cancer cells with the median lethal dose (LD50) of 57.2 μg/ mL. In addition, it was reported that MGDG, DGDG, and SQDG from freshwater cyanobacterium showed a certain inhibition effect on lymph cancer cells, among which MGDG and DGDG showed a higher inhibition effect than SQDG; and DGDG also presented a strong inhibitory effect on skin papilloma (Tokuda et al., 1996).

Lipids and fatty acid composition of different varieties of sweet potato Overview of sweet potato Sweet potato (Ipomoea batatas (L.) Lam.) is an important economic crop that can successfully adapt to a wide range of habitats, including marginal regions. It is a dicotyledonous plant belonging to the family Convolvulaceae with approximately 50 genera and over 1000 species (Woolfe, 1992). Artificial selection of sweet potatoes, as well as the occurrence of natural hybrids and mutations, has resulted in the existence of a very large number of cultivars, which differ in many of their properties, including physical appearance and texture of the tuber (Zhang and Oates,

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155

1999). Sweet potatoes are widely cultured in China, which has the highest production in the world with approximately 71.54 million tons produced annually, resulting in 68% of the world’s production, and sweet potatoes are the fifth most common food crop after rice, wheat, maize, and potato (FAOSTAT, 2017). Sweet potato has unique nutritional and functional properties (Wang et al., 2016), and is a valuable source of food, animal feed, and industrial raw materials.

Composition of sweet potato lipids To better understand the lipids and FA composition of different varieties of sweet potato, sweet potatoes from 11 representative cultivars were obtained from Zhejiang (Xinxiang No.1, Zheshu 7518), Hebei (Beijing 553, Jishu 98, Wutang No.1), Jiangsu (Xushu 18, Xushu 22, Xushu 27, Xushu 28, Shangshu 19), and Beijing (Mixuan No. 1), respectively, of which the proximate composition, lipids, and FA composition were determined. Table 6.1 shows the proximate composition of sweet potatoes. The moisture content ranged from 60.13% to 78.43%. The moisture content in Xushu 27 (78.43%) and Beijing 553 (75.28%) was significantly higher than those in Zheshu 7518 (60.13%) and Mixuan No.1 (61.21%). Starch was the main component of sweet potato, and the starch content ranged from 54.59% to 76.95%, which was similar to the values reported by Ravindran, Sivakanesan, and Rajaguru (1995). Xushu 27 had the highest starch content (76.95%), whereas Jishu 98 had the lowest starch content (54.59%). Protein content ranged from 3.53% to 9.13%. Woolfe et al. (1992) reported that the protein content in sweet potato was higher than cassava, plantain, and taro, but lower than potato and yam. Crude fiber content ranged from 1.80% to 3.65%, which was similar to the report by Zakir et al. (2006), where Xushu 27 had the highest crude fiber content (3.65%) and Wutang No.1 had the lowest (1.80%). Similarly, there were significant differences in ash content among the sweet potato cultivars (P , .05), and the content ranged from 2.32% (Zheshu 7518) to 3.95% (Shangshu 19). Ash content is an important indicator in evaluating mineral element content, and the minerals in sweet potato have been identified as Ca, P, Mg, Na, K, Fe, Zn, and Cu (Bouwkamp, 1985). The TLs content of sweet potatoes of different cultivars ranged from 0.72% to 1.44%, and these results were similar to those of Ravindran et al. (1995). Fat is involved in the insulation of body organs and in the maintenance of body

Table 6.1 Proximate composition of 11 sweet potato cultivars (%, DW). Cultivar

Moisturea

Starch

Protein

Crude fiber

Ash

Crude lipid

NLs

GLs

PLs

Xushu 28 Xushu 27 Mixuan No.1 Jishu 98 Xushu 22 Xushu 18 Shangshu 19 Beijing 553 Xinxiang No.1 Zheshu 7518 Wutang No.1

67.29 6 0.14def 78.43 6 0.13a 61.21 6 0.07g

72.25 6 1.39ab 76.95 6 1.53a 65.43 6 1.02b

6.23 6 0.26e 5.42 6 0.07f 7.00 6 0.01d

2.64 6 0.79c 3.65 6 1.19a 2.27 6 0.68d

2.46 6 0.81g 3.83 6 0.65b 2.41 6 0.14g

0.97 6 0.11cd 0.93 6 0.06d 1.06 6 0.04c

54.18 6 1.82ab 56.76 6 2.23ab 55.26 6 6.67ab

37.65 6 0.88cde 36.10 6 0.54e 36.08 6 4.32ef

8.17 6 0.13bc 7.14 6 0.26c 8.69 6 7.15bc

67.94 6 0.13de 72.3 6 0.67c 68.94 6 0.13d 72.40 6 0.50c 75.28 6 0.44b 65.15 6 0.17f

54.59 6 0.49c 66.33 6 0.53b 71.47 6 0.23ab 67.47 6 0.58ab 66.87 6 0.67b 73.88 6 0.72ab

9.13 6 0.35a 7.90 6 0.33b 3.72 6 0.26h 4.27 6 0.11g 5.32 6 0.13f 3.65 6 0.19hi

2.17 6 0.72d 3.51 6 0.76a 2.35 6 0.49cd 3.13 6 0.35b 3.76 6 1.29a 3.05 6 1.80b

2.73 6 0.22e 3.63 6 1.02c 2.33 6 1.16h 3.95 6 0.62a 3.15 6 0.87d 2.60 6 0.20f

0.72 6 0.08e 0.99 6 0.10cd 1.00 6 0.01cd 1.44 6 0.03a 0.98 6 0.12cd 1.25 6 0.07b

36.74 6 0.42d 61.04 6 1.87a 50.95 6 1.19bc 52.96 6 7.13b 55.79 6 0.21ab 44.90 6 0.12c

45.49 6 0.34ab 31.24 6 0.18g 42.00 6 0.22bc 31.30 6 0.10fg 36.38 6 3.06ed 41.03 6 4.44bcd

17.79 6 2.98a 7.72 6 0.16c 7.05 6 0.22c 15.73 6 2.49ab 7.85 6 2.19c 14.07 6 7.89abc

60.13 6 2.29g 66.42 6 3.53ef

68.59 6 0.35ab 72.71 6 0.68ab

3.53 6 0.06i 7.88 6 0.08c

2.08 6 0.67de 1.80 6 0.58e

2.32 6 0.34h 2.79 6 0.19e

0.95 6 0.16d 0.91 6 0.03d

55.56 6 0.50ab 33.70 6 5.15d

30.29 6 0.74g 49.25 6 1.62a

14.14 6 0.66abc 17.07 6 0.53a

Values within columns with different letters are significantly different (P , .05). NLs, Neutral lipids; GLs, glycolipids; PLs, phospholipids. a Moisture content was expressed in g/100 g FW.

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157

temperature and cell function, and sources of fats such as omega-3 and omega-6 FAs are required for the digestion, absorption, and transport of vitamins A, D, E, and K. To further understand the differences of lipids composition of sweet potatoes of different varieties, the TLs were separated into NLs, GLs, and PLs using silica gel column chromatography. The composition of lipid classes is shown in Table 6.1. NLs were the main component of sweet potato TLs, and TGs were most abundant in NLs (Walter, Hansen and Purcell, 1971); these play an important role in maintaining energy metabolism. The content of NLs in sweet potatoes ranged from 36.74% to 61.04%. Wutang No.1 had the highest content of NLs (61.04%), while Jishu 98 had the lowest (36.74%). The content of GLs ranged from 30.29% to 49.25%, and Zhushu 7518 had the highest content of GLs (49.25%) and Jishu 98 had the lowest (30.29%). It has been reported that GLs have significant antiproliferative, antimicrobial, antiviral, and antiinflammatory effects (Da Costa et al., 2016; Varamini et al., 2017). The content of PLs in sweet potato was limited, and ranged from 7.05% (Xushu 27) to 17.07% (Zheshu 7518). PLs from milk reduced plasma cholesterol concentrations, but did not change the low-density lipoprotein/high-density lipoprotein (LDL/HDL) ratio (Keller et al., 2013). Previous research indicated that the lipid composition of sweet potato was 42.1% NLs, 30.8% GLs, and 27.1% PLs (Walter et al., 1971). It has been reported that the composition of lipids in rice bran consists of 88.1% 89.2% NLs, 6.3% 7.0% GLs, and 4.5% 4.9% PLs (Hemavathy and Prabhakar, 1989). The composition of lipids extracted from millet seeds consisted of 85% NLs, 3% GLs, and 12% PLs (Osagie and Kates, 1984). Hemavathy and Prabhakar (1987) also reported that the TLs in fenugreek seeds consisted of 84.1% NLs, 5.4% GLs, and 10.5% PLs. Thus sweet potatoes are a good source of essential GLs and PLs.

Fatty acid composition of total lipids The composition of FAs in TLs is presented in Table 6.2. The FAs in sweet potatoes include palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:1), linoleic acid (C18:2), linolenic acid (C18:3), and arachidic acid (C20:0), of which C16:0, C18:2, and C18:3 are abundant. The content of C16:0 ranged from 35.92% to 42.57%, and Xushu 27 had the highest C16:0 content (42.57%), whereas Jishu 98 had the lowest content (35.92%). C18:2 was the most abundant FA in sweet potato TLs, which

Table 6.2 Fatty acids composition of TLs from 11 sweet potato cultivars (%). Cultivar

Palmitic acid C16:0

Stearic acid C18:0

Oleic acid C18:1

Linoleic acid C18:2

Linolenic acid C18:3

Arachidic acid C20:0

SFA

UFA

SFA/ UFA

Xushu 18 Mixuan No.1 Xinxiang No.1 Wutang No.1 Xushu 28 Xushu 27 Xushu 22 Jishu 98 Shangshu 19 Beijing 553 Zheshu 7518

38.53 6 0.07de 41.34 6 1.62ab 40.42 6 0.25bc 39.78 6 0.25cd 38.34 6 0.20e 42.57 6 0.10a 39.96 6 0.12c 35.92 6 0.39g 37.55 6 0.88ef 38.4 6 0.18e 36.52 6 0.25fg

2.54 6 0.03de 2.62 6 0.14d 3.43 6 0.03a 2.38 6 0.03gf 2.61 6 0.01d 2.74 6 0.03c 2.95 6 0.00b 2.3 6 0.04g 2.47 6 0.01ef 2.12 6 0.01h 3.04 6 0.01b

1.68 6 0.01b 1.18 6 0.04d 1.36 6 0.01c 0.69 6 0.22f 1.27 6 0.01cd 1.21 6 0.14cd 1.2 6 0.01d 0.62 6 0.01f 2.11 6 0.02a 0.92 6 0.00e 1.63 6 0.04b

37.78 6 0.18d 37.96 6 0.78d 39.67 6 0.13c 38.43 6 0.02d 41.45 6 0.08a 35.2 6 0.28f 36.73 6 0.07e 39.48 6 0.20c 41.67 6 0.50a 40.48 6 0.15b 40.61 6 0.19b

18.95 6 0.25b 16.28 6 1.01e 14.54 6 0.16f 18.16 6 0.04cd 15.88 6 0.12e 17.7 6 0.28d 18.57 6 0.03bc 21.21 6 0.19a 15.65 6 0.32e 17.54 6 0.04d 17.73 6 0.01d

0.30 6 0.01cd 0.36 6 0.02a 0.35 6 0.00a 0.31 6 0.03bcd 0.26 6 0.01e 0.31 6 0.04bcd 0.36 6 0.01a 0.28 6 0.03de 0.33 6 0.03abc 0.35 6 0.01ab 0.29 6 0.01de

41.61 6 0.09de 44.61 6 1.74ab 44.45 6 0.28b 42.74 6 0.25cd 41.43 6 0.21de 45.87 6 0.17a 43.53 6 0.11bc 38.71 6 0.40g 40.59 6 0.86ef 41.09 6 0.17ef 40.06 6 0.23fg

58.41 6 0.08cd 55.42 6 1.75fg 55.57 6 0.28f 57.28 6 0.23de 58.6 6 0.21cd 54.11 6 0.71g 56.5 6 0.11ef 61.31 6 0.38a 59.43 6 0.84bc 58.94 6 0.18bc 59.97 6 0.23ab

0.71 0.80 0.80 0.75 0.71 0.85 0.77 0.63 0.68 0.70 0.67

Values within columns with different letters are significantly different (P , .05). SFA, Saturated fatty acid; UFA, unsaturated fatty acid.

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159

ranged from 35.2% to 41.45%, and Xushu 28 had the highest content (41.45%) and Xushu 27 had the lowest content (35.2%). The content of C18:3 ranged from 14.54% to 21.21%, and Jishu 98 had the highest content (21.21%) and Xinxiang No.1 had the lowest content (14.54%). The content of C18:0, C18:1, and C20:0 were relatively low, and ranged from 2.12% (Beijing 553) to 3.43% (Xinxiang No.1), 0.62% (Jishu 98) to 2.11% (Shangshu 19), and 0.26% (Xushu 28) to 0.36% (Xushu 22), respectively. The content of UFAs in sweet potatoes was greater than SFAs. Jishu 98 had the highest content of UFA (61.31%), while Xushu 27 had the lowest content (54.11%). It was reported that the FAs in rice mainly consisted of C16:0, C18:2, and C18:3 (Yoshida et al., 2012), and the FA composition was very similar to that of sweet potatoes.

Fatty acids composition of neutral lipids The FAs composition of TLs is presented in Table 6.3. The UFA in sweet potato TLs primarily consisted of palmitic acid (C16:0), linoleic acid (C18:2), and linolenic acid (C18:3). C18:2 was the most abundant UFA in sweet potato NLs, which ranged from 43.79% to 54.47%. Xushu 28 had the highest UFA content (54.47%), while Xinxiang No.1 had the lowest content (43.79%). The contents of C18:1 and C18:3 ranged from 0.50% (Wutang No.1) to 1.43% (Mixuan No.1) and 7.35% (Wutang No.1) to 15.57% (Jishu 98), respectively. The SFA in sweet potato NLs primarily consisted of C16:0, C18:0, and C20:0. C16:0 was the most abundant SFA in sweet potato NLs, which ranged from 26.90% (Jishu 98) to 36.84% (Xushu 18). The contents of C18:0 and C20:0 ranged from 3.11% (Jishu 98) to 4.52% (Xushu 18) and 1.02% (Xushu 27) to 1.57% (Xushu 18), respectively. Compared with the TLs (Table 6.2), the UFA contents in NLs were significantly higher. The contents of UFA in NLs ranged from 55.89% (Xushu 18) to 66.18% (Xushu 28). UFA has positive effects on reducing blood cholesterol and fat contents, and in the prevention of cardiovascular diseases and the protection of the brain and nervous system (Torres et al., 2000).

Fatty acids composition of glycolipids The FAs composition of GLs is shown in Table 6.4. The content of GLs in sweet potato was approximately 8 10 times that in rice bran, fenugreek seeds, and Perilla seed (Hemavathy and Prabhakar, 1987, 1989; Shin

Table 6.3 Fatty acids composition of NLs from 11 sweet potato cultivars (%). Cultivar

Palmitic acid C16:0

Stearic acid C18:0

Oleic acid C18:1

Linoleic acid C18:2

Linolenic acid C18:3

Arachidic acid C20:0

SFA

UFA

SFA/ UFA

Xushu 28 Shangshu 19 Xinxiang No.1 Xushu 18 Wutang No.1 Xushu 27 Xushu 22 Mixuan No.1 Beijing 553 Jishu 98 Zheshu 7518

27.50 6 7.26d 31.35 6 0.17bcd 29.88 6 0.03cd 36.84 6 0.27a 31.39 6 0.22bcd 33.11 6 0.04abc 35.45 6 0.02ab 30.81 6 0.29bcd 34.26 6 2.03abc 26.90 6 0.24d 30.32 6 0.11cd

4.26 6 0.36abc 3.22 6 0.00f 4.41 6 0.00ab 4.52 6 0.01a 4.03 6 0.04cd 3.26 6 0.010f 4.56 6 0.03a 3.46 6 0.00ef 3.78 6 0.39de 3.11 6 0.04f 4.08 6 0.04bcd

1.14 6 0.12c 0.90 6 0.00e 1.17 6 0.01c 1.18 6 0.03c 0.50 6 0.06f 1.61 6 0.02a 1.07 6 0.00cd 1.43 6 0.07b 0.97 6 0.17de 0.65 6 0.04f 1.39 6 0.05b

54.47 6 5.46a 53.43 6 0.22ab 52.12 6 0.17abc 43.79 6 0.45d 54.00 6 0.10a 48.96 6 0.06c 44.56 6 0.10d 49.46 6 0.31c 48.95 6 1.30c 52.08 6 0.02abc 49.85 6 0.19bc

10.57 6 1.13ef 8.42 6 0.14h 9.96 6 0.12fg 10.91 6 0.16de 7.35 6 0.08i 11.40 6 0.08cde 11.49 6 0.04cd 13.11 6 0.06b 9.50 6 0.44g 15.57 6 0.33a 11.84 6 0.08c

1.23 6 0.08ab 1.36 6 0.00ab 1.36 6 0.06ab 1.57 6 0.01a 1.10 6 0.07b 1.02 6 0.00b 1.29 6 0.05ab 1.34 6 0.02ab 1.16 6 0.65ab 1.07 6 0.00b 1.40 6 0.04ab

32.98 6 6.82de 35.93 6 0.17cde 35.65 6 0.09cde 42.93 6 0.25a 36.52 6 0.33bcd 37.39 6 0.05bcd 41.31 6 0.06ab 35.61 6 0.27cde 39.20 6 3.07abc 31.08 6 0.28e 35.80 6 0.03cde

66.18 6 6.72ab 62.75 6 0.36bc 63.26 6 0.30bc 55.89 6 0.58d 61.86 6 0.09bc 61.97 6 0.03bc 57.13 6 0.07d 64.00 6 0.32abc 59.43 6 1.92cd 68.31 6 0.31a 63.08 6 0.06bc

0.50 0.57 0.56 0.77 0.59 0.60 0.72 0.56 0.66 0.45 0.57

Values within columns with different letters are significantly different (P , .05). SFA, Saturated fatty acid; UFA, unsaturated fatty acid.

Table 6.4 Fatty acids composition of GLs from 11 sweet potato cultivars (%). Cultivar

Palmitic acid C16:0

Stearic acid C18:0

Oleic acid C18:1

Linoleic acid C18:2

Linolenic acid C18:3

Arachidic acid C20:0

SFA

UFA

SFA/ UFA

Xushu 28 Shangshu 19 Xinxiang No.1 Xushu 18 Wutang No.1 Xushu 27 Xushu 22 Mixuan No.1 Beijing 553 Jishu 98 Zheshu 7518

28.04 6 0.09c 20.09 6 0.01e 24.06 6 4.88de 32.39 6 0.04ab 23.37 6 0.85de 29.46 6 0.18bc 31.04 6 0.19abc 24.08 6 0.28d 33.42 6 2.39a 21.37 6 0.51de 21.24 6 0.26de

5.43 6 0.06ab 3.13 6 0.01f 4.28 6 0.76def 5.04 6 0.06abc 4.81 6 0.36bcd 4.38 6 0.01cde 5.90 6 0.04a 4.37 6 0.10cde 3.78 6 1.25ef 3.94 6 0.03def 4.90 6 0.08abcd

0.49 6 0.01cdef 0.56 6 0.03cde 0.52 6 0.22def 0.85 6 0.07ab 0.25 6 0.01f 1.02 6 0.11a 0.72 6 0.01bc 1.00 6 0.26a 0.67 6 0.08bcd 0.37 6 0.01ef 0.57 6 0.08cde

57.72 6 0.02abc 65.20 6 0.41a 61.46 6 1.36ab 50.47 6 0.26bc 64.20 6 0.67a 54.28 6 0.06abc 50.54 6 0.30bc 52.80 6 0.01bc 52.46 6 3.48bc 59.18 6 0.20ab 61.35 6 0.12ab

8.04 6 0.04ef 10.58 6 0.06cd 9.31 6 3.47f 10.59 6 0.02cd 7.01 6 0.26f 10.23 6 0.05cde 11.36 6 0.16c 17.47 6 0.09a 8.28 6 1.03def 14.55 6 0.31b 11.66 6 0.14c

ND 0.22 6 0.31a 0.11 6 0.35a 0.31 6 0.44a 0.21 6 0.30a ND ND ND 0.47 6 0.67a ND ND

33.46 6 0.03a 23.44 6 0.32c 25.38 6 6.00bc 37.75 6 0.34a 28.39 6 0.91bc 33.84 6 0.19a 36.95 6 0.15a 28.45 6 0.18b 37.67 6 4.31a 25.32 6 0.54bc 26.14 6 0.17bc

66.25 6 0.03c 76.34 6 0.32a 71.02 6 1.19b 61.92 6 0.31e 71.46 6 0.91b 65.52 6 0.09cd 62.62 6 0.13de 71.27 6 0.18b 61.41 6 4.44e 74.10 6 0.50ab 73.58 6 0.19ab

0.51 0.31 0.36 0.61 0.40 0.52 0.59 0.40 0.61 0.34 0.36

Data are means 6 SD (n $ 2). Values within columns with different letters are significantly different (P , .05). SFA, Saturated fatty acid; UFA, unsaturated fatty acid; ND, not detected.

162

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and Kim, 1994). The most abundant FA in sweet potato GLs was C18:2, which ranged from 50.47% (Xushu 18) to 65.20% (Shangshu 19), followed by C16:0, which ranged from 20.09% (Shangshu 19) to 33.42% (Beijing 553). The contents of C18:0 and C18:3 ranged from 3.13% (Shangshu 19) to 5.90% (Xushu 22) and 9.31% (Xinxiang No.1) to 17.47% (Mixuan No.1), respectively. The contents of C18:1 and C20:0 were relatively low; C18:1 ranged from 0.25% (Wutang No.1) to 1.02% (Xushu 27), and C20:0 was not detected in some sweet potato cultivars, including Xushu 22, Mixuan No.1, Jishu 98, and Beijing 553. GLs derived from plants exhibit various biological properties in vitro and/or in vivo, including antitumor activity (Kuriyama et al., 2005; Hou et al., 2007) and antiinflammatory activity (Leth-Larsen et al., 2003). The GLs in sweet potatoes consisted mainly of MGDGs and DGDGs (Walter et al., 1971). Plant GLs are characterized by one or two UFAs with chain lengths typically varying from C16 to C20 linked to the glycerol moiety, and this is why the UFAs in GLs increased significantly.

Fatty acids composition of phospholipids The composition of FAs in PLs was similar to the other lipid classes (Table 6.5). The UFA content in sweet potato PLs was much higher than the SFA content. The content of UFA ranged from 56.53% (Beijing 553) to 75.37% (Shang 19). The UFAs mainly consisted of C18:1, C18:2, and C18:3, and their contents ranged from 0.59% (Wutang No.1) to 3.08% (Beijing 553), 48.02% (Mixuan No.1) to 67.74% (Shang 19), and 4.40% (Wutang No.1) to 8.84% (Xushu 22), respectively. The contents of SFA in sweet potato PLs primarily consisted of C16:0, C18:0, and C20:0. C16:0 was the most abundant SFA in sweet potato PLs, and ranged from 21.92% (Shangshu 19) to 39.33% (Beijing 553). The content of C18:0 ranged from 1.82% (Shangshu 19) to 4.53% (Xinxiang No.1). C20:0 was not found in Xushu 27 and Mixuan No.1. PLs were the predominant lipid class in biological membranes, although the content of PLs was limited.

Anticancer effects of sweet potato lipids As mentioned in the first section, lipids from different sources exhibited certain anticancer effects. However, reports on the anticancer effects of sweet potato lipids were rare. Thus the antiproliferative effects and cell migration inhibition on cancer cells of sweet potato lipids were studied,

Table 6.5 Fatty acids composition of PLs from 11 sweet potato cultivars (%). Cultivar

Palmitic acid C16:0

Stearic acid C18:0

Oleic acid C18:1

Linoleic acid C18:2

Linolenic acid C18:3

Arachidic acid C20:0

SFA

UFA

SFA/ UFA

Xushu 28 Shangshu 19 Xinxiang No.1 Xushu 18 Wutang No.1 Xushu 27 Xushu 22 Mixuan No.1 Beijing 553 Jishu 98 Zheshu 7518

34.48 6 0.92e 21.92 6 0.25h 31.76 6 1.25g 36.29 6 0.34cd 37.08 6 0.26bc 34.66 6 0.56e 33.36 6 0.32f 37.74 6 0.23b 39.33 6 0.66a 35.30 6 0.13de 32.60 6 0.06fg

3.45 6 0.20b 1.82 6 0.08c 4.53 6 0.99a 3.51 6 0.07b 3.57 6 0.07b 3.23 6 0.10b 3.63 6 0.09b 3.62 6 0.00b 3.36 6 0.30b 3.22 6 0.06b 3.70 6 0.16b

1.12 6 0.09d 1.10 6 0.15de 1.17 6 0.14d 1.27 6 0.09d 0.59 6 0.16g 2.11 6 0.05b 0.92 6 0.00ef 1.83 6 0.06c 3.08 6 0.73a 0.74 6 0.06fg 1.20 6 0.00d

53.16 6 1.06d 67.74 6 0.40a 56.38 6 1.23b 49.51 6 0.26g 53.15 6 0.11de 51.81 6 0.30f 52.30 6 0.02ef 48.02 6 0.64h 47.00 6 0.63i 51.92 6 0.06f 54.92 6 0.02c

6.76 6 0.16d 6.53 6 0.10d 5.62 6 0.33e 8.37 6 0.25b 4.40 6 0.18f 8.18 6 0.11bc 8.84 6 0.02a 8.21 6 0.01bc 5.89 6 0.58e 7.89 6 0.09c 6.91 6 0.05d

1.03 6 0.07a 0.72 6 0.01ab 0.55 6 0.78ab 0.98 6 0.01a 1.14 6 0.01a ND 0.87 6 0.07a ND 1.34 6 0.06a 0.93 6 0.02a 0.67 6 0.07ab

39.41 6 0.65d 24.46 6 0.35f 36.84 6 1.04e 40.78 6 0.29c 41.79 6 0.34bc 37.89 6 0.47e 37.86 6 0.16e 41.94 6 0.58b 43.73 6 0.42a 39.45 6 0.21d 36.97 6 0.03e

61.75 6 0.99cd 75.37 6 0.35a 63.16 6 1.04b 59.14 6 0.42e 58.14 6 0.45e 62.11 6 0.46bc 62.06 6 0.05bc 58.06 6 0.59e 56.53 6 0.78f 60.55 6 0.21d 63.03 6 0.03bc

0.64 0.32 0.58 0.69 0.72 0.61 0.61 0.72 0.77 0.65 0.59

Data are means 6 SD (n $ 2). Values within columns with different letters are significantly different (P , .05). SFA, Saturated fatty acid; UFA, unsaturated fatty acid; ND, not detected.

164

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to provide basic information for sweet potato lipids potentially to be used in functional foods.

Antiproliferative effect of sweet potato lipids on cancer cells Cancer is proposed to be an aftereffect of the accumulation of multiple genetic alterations, of which the developing risk could be regulated by bioactive components from food (Kim and Milner, 2011). Colorectal cancer is one of the commonest malignancies of the gastrointestinal tract, which ranks as the fourth and third most common cause of cancer deaths in the world and the United States, respectively, while breast cancer is the main cause of cancer deaths among women (Greenlee et al., 2000; Faghfoori et al., 2015; Brosens et al., 2015). Two cancer cells, HT-29 colon cancer cells and Bcap-37 breast cancer cells, then were chosen to study the antiproliferative effect of sweet potato lipids. At the same time, since GLs from other sources showed significant antiproliferative effect on different cancer cells, GLs from sweet potato lipids were separated by macroporous resin using different proportions of ethanol solutions into GLs (I, II, III) to better understand their antiproliferative effect. As shown in Figs. 6.1 and 6.2, both TLs and GLs (I, II, III) showed dose-dependent antiproliferative effects on HT-29 and Bcap-37 cells as shown using the MTT assay. Stronger inhibitory effects were observed on HT-29 than Bcap-37 cells. Compared with TLs, GLs I, II, and III had greater antiproliferative effects on HT-29 cells, of which GL III exhibited the highest effects (Fig. 6.1A, P , .05). The antiproliferative effects of GL III on HT-29 cells were 59.66%, 68.66%, 78.12%, 82.33%, 88.23%, and 92.21% at concentrations of 50, 100, 200, 400, 800, and 1000 μg/mL, respectively (Fig. 6.1A, P , .05). In the crystal violet study, dosedependent inhibitory effects of TLs and GLs (I, II, III) on HT-29 cells were also observed (Fig. 6.1B). In the case of Bcap-37 cells, GL III also exhibited the highest antiproliferative effects, which were 18.83%, 30.48%, 46.62%, 58.26%, 66.87%, and 76.42% at concentrations of 50, 100, 200, 400, 800, and 1000 μg/mL, respectively (Fig. 6.2A). And dosedependent inhibitory effects of TLs and GLs (I, II, III) on Bcap-37 cells were also observed in the crystal violet study (Fig. 6.2B). In addition, the maximum effects were seen in HT-29 and Bcap-37 cells treated with GLs III when concentrations reached 1000 μg/mL (Figs. 6.1B and 6.2B), which was in accordance with the results of the MTT assay (Figs. 6.1A and 6.2A). When the GLs fraction from spinach was incubated with

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Figure 6.1 Antiproliferative effects of sweet potato lipids on HT-29 cancer cells. (A) MTT assay; (B) crystal violet study.

mouse colon-26 cell lines for 24 h, the growth inhibition rate of colon-26 cells decreased to 75.7% at 100 μg/mL (Maeda et al., 2008), which was similar to the antiproliferative effect of sweet potato GLs on HT-29 cells. The treatment of cardiovascular diseases and cancers with natural food ingredients, such as GLs, has gained increased research attention. GLs are a class of metabolites and mainly comprise MGDG, DGDG, and SQDG, which have numerous biological properties, such as antitumor, antimicrobial, antimicrofouling, and antiinflammatory activities (Da Costa et al., 2016). MGDG and DGDG have shown potential anticancer activities due

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Figure 6.2 Antiproliferative effects of sweet potato lipids on Bcap-37 cancer cells. (A) MTT assay; (B) crystal violet study.

to DNA polymerase inhibition, antiproliferation of cancer cells, and antitumor growth (Maeda et al., 2008). SQDG has also shown antiproliferative effects on gastric cancer cells (Quasney et al., 2001).

Cell metastasis inhibition of sweet potato lipids on cancer cells Metastasis of cancer cells is the process of tumor cells transferring from primary tumor blocks to distant target tissues. It is a difficult point in tumor therapy and is also the main cause of cancer-related morbidity and

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mortality. The cell metastasis inhibition of sweet potato lipids on cancer cells was studied via their effects on the adhesion and migration of cancer cells to provide the basic data for the application of sweet potato lipids in functional foods. Effect of total lipids and glycolipids on adhesion of HT-29 and Bcap-37 cells The effects of TLs on the adhesion of HT-29 and Bcap-37 cells are shown in Fig. 6.3A. Compared with the untreated cells, the adhesion time for cells treated with phorbol 12-myristate 13-acetate (PMA) increased significantly, that is, 1.7 times that of untreated cells. With the increase of TLs concentration (100, 400, and 1000 μg/mL), the adhesion effect of cancer cells decreased significantly (P , .05). The cell adhesion time for cells treated with TLs at 100 and 400 μg/mL was higher than that of untreated cells, but lower than that of cells treated with PMA

Figure 6.3 Cell adhesion inhibition effects of sweet potato lipids on HT-29 and Bcap37 cancer cells. (A) TLs; (B) GLs I; (C) GLs II; (D) GLs III.

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(P , .05). The cell adhesion time for HT-29 and Bcap-37 cells treated with TLs at 1000 μg/mL is 7.5 and 7.0 min, separately, showing no significant difference with untreated cells (P..05). The effects of GLs I, II, and III on the adhesion of HT-29 and Bcap37 cells were shown in Fig. 6.3B, C, and D. The cell adhesion time of both HT-29 and Bcap-37 cells by adding PMA without GLs was significantly higher than those of the untreated cells and the cells with GLs (P , .05). Among the cells treated with GLs, the cell adhesion time decreased significantly with the increase of GLs concentration (100, 400, and 1000 μg/mL), of which GLs III showed the highest effect, followed by GLs II (P , .05). Compared with the untreated cells, adhesion time of cells treated with GLs was higher with the concentration less than 400 μg/mL. When the concentration of GLs increased up to 1000 μg/ mL, there was no significant difference between the adhesion time between the untreated cells and cells treated with GLs I, while the adhesion time of cells treated with GLs II and III was much lower than that of untreated ones (P , .05). For HT-29 cells, when the concentration of GLs was 1000 μg/mL, the adhesion time of cells treated with GLs I, II, and III were 7.1, 6.1, and 5.3 min, which reduced by 42.7%, 50.8%, and 57.3% compared with that of the PMA-treated cells, respectively. For Bcap-37 cells, when the concentration of GLs was 1000 μg/mL, the adhesion time of cells treated with GLs I, II, and III were 6.1, 5.5, and 4.7 min, which reduced by 45.0%, 50.5%, and 57.7% compared to the PMA-treated cells, respectively. Compared with TLs, the adhesion time of HT-29 cells treated with GLs I, II, and III decreased by 4.7%, 12.8%, and 19.3%; and the adhesion time of Bcap-37 cells was shortened by 8.1%, 13.6%, and 20.8%, respectively. The results suggested that GLs I, II, and III could reduce the adhesion of cancer cells, of which the effect was better than TLs, particularly for Bcap-37 cells. Effects of total lipids and glycolipids on the migration of HT-29 and Bcap-37 cells The effects of TLs on the migration of HT-29 and Bcap-37 cells are shown in Fig. 6.4. The cell migration of both HT-29 and Bcap-37 cells by adding PMA without TLs was significantly higher than those of the untreated cells and cells with TLs (P , .05). Among the cells treated with TLs, the cell migration decreased significantly with the increase of TLs concentration (100, 400, and 1000 μg/mL). For HT-29 cells, when the concentration of TLs was 1000 μg/mL, the cell migration of cells treated with TLs was 23.03%, which was reduced by 17.32% compared to

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Figure 6.4 Cell migration inhibition effects of sweet potato lipids on HT-29 (A) and Bcap-37 (B) cancer cells.

Figure 6.5 Cell adhesion inhibition effects of sweet potato lipids on HT-29 (A, GLs I; B, GLs II; C, GLs III) and Bcap-37 (D, GLs I; E, GLs II; F, GLs III) cancer cells.

PMA-treated cells (Fig. 6.4A). For Bcap-37 cells, when the concentration of TLs was 1000 μg/mL, the cell migration of cells treated with TLs was 35.83%, which was decreased by 18.36% compared with that of the PMA-treated cells (Fig. 6.4B). The effects of GLs I, II, and III on the migration of HT-29 and Bcap37 cells are shown in Fig. 6.5. The cell migration of HT-29 and Bcap-37 cells by adding PMA without GLs was significantly higher than those of the untreated cells and cells with GLs (P , .05). Among the cells treated with GLs, the cell migration decreased significantly with the increase of GLs concentration (100, 400, and 1000 μg/mL), of which GLs III showed the highest effect, followed by GLs II (P , .05). For HT-29 cells, when

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the concentration of GLs was 1000 μg/mL, the cell migrations of cells treated with GLs I, II, and III were 10.35%, 8.06%, and 6.04%, which reduced by 30%, 32.29%, and 34.31% compared to the PMA-treated cells, respectively. For Bcap-37 cells, when the concentration of GLs was 1000 μg/mL, the cell migrations of cells treated with GLs I, II, and III were 22.35%, 20.04%, and 16.72%, which reduced by 31.84%, 34.15%, and 37.47% compared to the PMA-treated cells, respectively.

Anticancer effects confirmation of sweet potato lipids To confirm the anticancer effects of TLs, GLs I, II, and III in sweet potato, the MGDG and DGDG contents were determined. Compared with TLs, GLs I, and II, GL III showed the highest level of MGDG, which was 35.84% (Fig. 6.6A, P , .05). There were significant differences

Figure 6.6 MGDG and DGDG in sweet potato lipids. (A) MGDG percentage; (B) DGDG percentage.

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in the percentages of DGDG in TLs and GLs (I, II, III), which followed the order GL III . TLs . GL I . GL II (Fig. 6.6B, P , .05). Thus GL III showed the highest percentage of DGDG compared with TLs, GLs I and II, which was 89.16% (Fig. 6.6B, P , .05). Napolitano et al. (2007) indicated that 16 novel and 10 known galactolipids identified from sweet potato leaves cultivated in Japan contained MGDG and DGDG. Roberts and Moreau (2016) showed that GLs in spinach leaves included MGDG, DGDG, and SQDG, which exhibited antiproliferative activity in many different cancer cell lines. MGDG from spinach also inhibited the proliferation of colon-26 cells, and oral administration suppressed colon tumor growth in mice (Maeda et al., 2013). In addition, further studies on the relationship between the molecular mechanisms and the structures of sweet potato lipids should be performed, as well as animal experiments.

Application prospect of sweet potato lipids Sweet potato lipids are rich in GLs and PLs, and could be a good source of essential GLs and PLs. The UFA content in sweet potato lipids is greater than the content of SFA, showing potential utilization in the prevention of cardiovascular diseases and the protection of the brain and nervous system. In addition, sweet potato lipids present certain anticancer effects, especially for GLs with the contribution of MGDG and DGDG. Thus sweet potato lipids could be potentially used in functional foods, health care products, and pharmaceuticals in the near future.

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CHAPTER 7

Sweet potato polyphenols Rie Kurata1, Hong-Nan Sun2, Tomoyuki Oki3, Shigenori Okuno4, Koji Ishiguro5 and Terumi Sugawara6 1

Division of Upland Farming Research, Kyusyu Okinawa Agricultural Research Center, NARO, Miyakonojo, Japan Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture, Beijing, People’s Republic of China 3 Graduate School of Health and Nutrition Sciences, Nakamura Gakuen University, Fukuoka, Japan 4 Department of Planning, Kyusyu Okinawa Agricultural Research Center, NARO, Kumamoto, Japan 5 Division of Field Crop Research and Development, Hokkaido Agricultural Research Center, NARO, Hokkaido, Japan 6 Crop Development and Agribusiness Research Division, Kyusyu Okinawa Agricultural Research Center, NARO, Kumamoto, Japan 2

Overview of sweet potato polyphenols A recent trend in the diet of most developed countries and some developing countries is the consumption of healthy food. To improve health the consumption of special dietary foods such as physiological functional foods, smoothies, and vegetable juices is increasing. The population in most developed countries and some developing countries is progressively aging and the desire to live as healthy for a long time has encouraged people to make efforts to maintain their health. Among the dietary strategies in some Asian countries, especially in China and Japan, sweet potatoes have recently become widely included in staple food, snack food, and vegetable juices. The Chinese Dietary Guidelines in 2016 recommend that 50
100 g sweet potato or potato should be consumed every day. With the implementation of the sweet potato and potato staple food strategy suggested by the Chinese Ministry of Agriculture, more and more staple food and snack food with high contents of sweet potato and potato are being researched and developed, and can be purchased in the market. Initially in Japan beverages popularly contained purple sweet potato pigment, but recently the whole purple sweet potato has been used to prepare vegetable juices containing anthocyanins, which are the purple pigment and the polyphenol content that are also available in the market. In addition, foods considered to have health functional effects because of their polyphenol content are also available as various products such as beverages, tablets, and jellies that are currently sold. The ingestion of Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00007-7

© 2019 Elsevier Inc. All rights reserved.

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polyphenols for health reasons is presently widespread in most developed countries, and the demand for polyphenols is increasing. Therefore more and more research institutes, especially the Potato and Sweet Potato Food Science Innovation Team, CAAS, China and Kyushu Okinawa Agricultural Research Center, NARO, Japan have been conducting research focusing on the high value of polyphenols in sweet potato. Polyphenolic compounds in sweet potato are separated into two main categories: flavonoids and phenolic acids. Flavonoids are mainly found in the tuberous root of sweet potato as pigments and can be classified by color. Purple, orange, and yellow sweet potatoes contain purple-colored anthocyanin, carotenoids (described in Chapter 8: Sweet potato carotenoids), and xanthophyll (as the main pigment), respectively. In addition, a patent reported that sweet potato tops contain quercetin glycosides, which are flavonoids (WO2006/014028). Phenolic acids in the sweet potato consist of a mixture of caffeic acid (CA) and caffeoylquinic acid (CQA) derivatives, and are commonly present in all parts of the leaves, petioles, stems, and tuberous roots. However, their content is most abundant in leaves, and the amount contained in the stems, petioles, and tubers is small. Therefore the main material source of polyphenols is the leaves. This chapter focuses on polyphenols, anthocyanins, and CQA derivatives, specifically their chemistry, functionality, and the processing required for their retention.

Chemistry Anthocyanins The anthocyanins (Greek anthos, flower, and kyanos, blue)—originally used to describe the blue pigment of the cornflower (Centaurea cyanus)— are the most important groups of water-soluble plant pigments visible to the human eye (Strack and Wray, 1989). They belong to the most widespread class of phenolic compounds collectively named flavonoids, and by the end of 1985, more than 4000 structures including 240 different naturally occurring anthocyanins had been found (Strack and Wray, 1989). They are present in a wide range of plant tissues, particularly in the flowers and fruits, but also in storage organs, roots, tubers, and stems. The basic anthocyanin structure consists of two or three chemical units, an aglycone base (also termed “anthocyanidin”), sugars, and organic acids in the case of acylated anthocyanins (Sui, 2017).

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In the last decade or two, sweet potato cultivars with purple flesh have been mainly grown in Japan, Korea, and New Zealand (Steed and Truong, 2008). Among them, the “Ayamurasaki” cultivar is the second generation of a local Japanese sweet potato variety “Yamagawamuarsaki” and accumulates high levels of anthocyanin pigments in the storage root, when cultivated in Japan. In “Yamagawamurasaki” and “Ayamuarsaki,” the major anthocyanins are cyanidin and peonidin 3-O-sophoroside-5-Oglucosides acylated with caffeic, ferulic, or p-hydroxybenzoic acids, abbreviated as YGM-1a, 1b, 2, 3, 4b, 5a, 5b, and 6 as listed in Table 7.1 (Odake et al., 1992, Goda et al., 1997, Terahara et al., 1999). China began to introduce purple sweet potato varieties in the 1980s, and then bred new varieties suitable for domestic cultivation, such as the purple sweet potato “Yan 176,” “135,” “Yan 337,” “Xushu 4,” “Jingshu 16,” “Yuzi 1,” and “Qunzi 1.” Currently, purple sweet potato are widely planted in northeastern China, including Shandong, Hebei, Guangxi, Jiangsu, and Guangdong provinces (Mu et al., 2017). With the increasing amount of purple sweet potato acreage in China, many researchers have studied the sweet potato anthocyanins structures because of their high photothermal stability. Yao (2009) found that the main anthocyanin of purple sweet potato (“Yan 176”) was 3-O-(6-Otranscaffeyl-2-O-β-glucopyranosyl-β-glucopyranoside)-5-O-β-glucoside peonidin. Qiu et al. (2009) found four anthocyanins from purple sweet potato, peonidin 3-O-(6-O-(E)-caffeoyl-2-O-β-D-glucopyranosyl-β-Dglucopyranoside)-5-O-β-D-glucoside, cyanidin 3-O-(6-O-p-coumaroyl)β-D-glucopyranoside, peonidin 3-O-(2-O-(6-O-(E)-caffeoyl-β-D-glucopyranosyl)-6-O-(E)-caffeoyl-β-D-glucopyranoside)-5-O-β-D-glucopyranoside, and peonidin 3-O-(2-O-(6-O-(E)-feruloyl-β-D-glucopyranosyl)6-O-(E)-caffeoyl-β-D-glucopyranoside)-5-O-β-D-glucopyranoside. Fig. 7.1 illustrates the chemical structure of anthocyanins isolated from purple-fleshed sweet potato. Other anthocyanins exist in the storage root, leaves, and cell lines of sweet potatoes as non-, mono, and diacylated forms with cyanidin, peonidin, or pelargonidin aglycone as listed in Table 7.1 (Islam et al., 2002a; Terahara et al., 2004; Tian et al., 2005; Truong et al., 2010; Kim et al., 2012). The anthocyanin composition affects the color of the paste, and those made from purple-fleshed sweet potatoes that are rich in peonidin and cyanidin anthocyanins exhibit a reddish- and bluish-purple color, respectively (Yoshinaga et al., 1999). Because commercial standards are not available, electrospray ionization mass

Table 7.1 Anthocyanins identified in storage root, leaf, and cell line of sweet potatoes. Anthocyanins

MW

Anthocyanins in Storage root

Leaf

3 3

3 3

3 3 3 3

3 3 3 3

3 3 3

3 3

Abbreviation Cell line

Cyanidin type

Cyanidin 3,5-diglucoside Cyanidin 3-sophoroside-5-glucoside Cyanidin 3-(60 -p-hydroxybenzoylsophoroside)-5-glucoside Cyanidin 3-(60 -p-coumarylsophoroside)-5-glucoside Cyanidin 3-(60 -caffeoylsophoroside)-5-glucoside Cyanidin 3-(6-caffeoylsophoroside)-5-glucoside Cyanidin 3-(60 -feruloylsophoroside)-5-glucoside Cyanidin 3-(6-caffeoyl-60 -p-hydroxybenzoylsophoroside)-5-glucoside Cyanidin 3-(6, 60 -dicoumarylsophoroside)-5-glucoside Cyanidin 3-(6-caffeoyl-60 -p-coumarylsophoroside)-5-glucoside Cyanidin 3-feruloyl-p-coumarylsophoroside-5-glucoside Cyanidin 3-(6, 60 -dicaffeoylsophoroside)-5-glucoside Cyanidin 3-(6-caffeoyl-60 -feruloylsophoroside)-5-glucoside

611 773 893 919 935 935 949 1055 1065 1081 1095 1097 1111

3 3 3 3 3 3 3 3 3 3 3 3

YGM-0a YGM-0c YGM-0d YGM-2 YGM-0g YGM-1a YGM-7a YGM-3' YGM-1b YGM-3

Peonidin type

Peonidin Peonidin Peonidin Peonidin Peonidin Peonidin Peonidin Peonidin Peonidin Peonidin Peonidin

3-sophoroside-5-glucoside 3-(60 -p-hydroxybenzoylsophoroside)-5-glucoside 3-(60 -p-coumarylsophoroside)-5-glucoside 3-(60 -caffeoylsophoroside)-5-glucoside 3-(6, 60 -caffeoylsophoroside)-5-glucoside 3-(60 -feruloylsophoroside)-5-glucoside 3-(6-caffeoyl-60 -p-hydroxybenzoylsophoroside)-5-glucoside 3-(6, 60 -dicoumarylsophoroside)-5-glucoside 3-feruloyl-p-coumarylsophoroside-5-glucoside 3-(6, 6'-dicaffeoylsophoroside)-5-glucoside 3-(6''-caffeoyl-6'''-feruloylsophoroside)-5-glucoside

787 907 933 949 949 963 1069 1079 1109 1111 1125

3 3

3

3 3 3 3

3 3 3

3 3 3

3

3 3 3 3 3 3 3 3 3

757 933

YGM-0f YGM-5b YGM-0i YGM-5a YGM-7b YGM-4b YGM-6

Pelargonidin type

Pelargonidin 3-sophoroside-5-glucoside Pelargonidin 3-feruloylsophoroside-5-glucoside

YGM-0b YGM-0e

3 3

Sweet potato polyphenols

181

R1 3′ 2′ HO

8

7

9

5

5′ 6′

O

4 O 6

OH HO

10

O

OH O

1′

3

6

HO

1 + O 2

OH 4′

1 O 1′

5 2 HO OR 2 4 O 5′ 3 6′ OR3 3′ HO 2′ HO OH OH 4′

Figure 7.1 Chemical structure of anthocyanins identified in purple-fleshed sweet potatoes: cyanidin, R1 5 OH; pelargonidin, R1 5 H; peonidin, R1 5 OCH3; R2, R3 5 H, caffeic acid; ferulic acid; p-hydroxybenzoic acid; and p-coumaric acid.

spectrometry and tandem MS have been used as powerful techniques for anthocyanin identification and characterization in purplefleshed sweet potatoes (Tian et al., 2005; Truong et al., 2010; Kim et al., 2012). The total anthocyanidin content was quantified using methods such as pH differential (Liu et al., 2013) and spectrometric (Oki et al., 2002b) methods. Individual anthocyanins were quantified using relatively stable standards such as cyanidin or peonidin 3,5-diglucoside (Kim et al., 2012; Lee et al., 2013) or using a relative response factor (Terahara et al., 2007; Oki et al., 2017a). Anthocyanin pigments in purple-fleshed sweet potatoes are highly stable against heat and ultraviolet irradiation owing to their acylated forms, which is an advantage when they are used in food additives as natural colorants (Hayashi et al., 1996). Anthocyanin colorants from purple-fleshed sweet potatoes are used as food additives such as E163 (E-number) and 73.260 (US CFR number). In addition to natural food colorants, purplefleshed sweet potato tubers are processed to produce juice concentrate, paste, and flour. Numerous kinds of processed foods such as noodles, bread, jams, chips, confections, beverages, and alcoholic beverages made from purple-fleshed sweet potato are currently available in Japanese stores (Suda et al., 2003). Major anthocyanins in purple-fleshed sweet potato remain intact in foods such as primary-processed foods, beverages, deepfried foods, and secondary-processed foods (Oki et al., 2010). In addition, with esthetic awareness increasing, many people, especially women, are

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paying more attention to skin care. The development of natural skin care products, characterized as safe, with an antiaging effect, has become an important research topic, and anthocyanins have these qualities, being classified as edible cosmetics and natural vitamins in Europe (Mu et al., 2017). They will have a vast market because of their applications in nontoxic makeup and their antioxidant capacity.

Caffeoylquinic acids Chlorogenic acids belong to the ester compound family and are formed by the condensation of quinic acid and transcinnamic acids, which include CA, p-coumaric acid, and ferulic acid (Marques and Farah, 2009; Michael et al., 2006). Because of the different positions, varieties, and quantities of the condensation between quinic and transcinnamic acids, there are a variety of chlorogenic acids (molecular structures are shown in Fig. 7.2 and Table 7.2). 3-O-Caffeoylquinic acid (3-CQA) is the most common of the chlorogenic acids, with the molecular formula C16H18O9, and a molar mass of 354.30 g/mol. Additionally, the esterified chlorogenic acids (esterification reactions occur at R1 of the molecular structure of quinic acid in Fig. 7.2) are also chlorogenic acids, such as methyl chlorogenate and chlorogenic ethyl ester (Mu et al., 2017). CQAs in sweet potato roots have been investigated for many years, especially in the United States, Japan, and China. Investigations of sweet potato respiration in the United States during the first half of the 20th century isolated chlorogenic acid (5-O-caffeoylquinic acid, 5-CQA) from roots, but did not quantify it (Rudkin and Nelson, 1947). CA, OR3 6

4

2 5 1

HOOC HO

OR2

3

HO

OR1

Quinic acid, Q

H C 3

O

Ferulic acid, F

OH

Caffeic acid, C

O

HO

O

HO

HO

O OH

OH

Coumaric acid, p-Co

Figure 7.2 Composition and molecular structure of chlorogenic acids.

Table 7.2 Molecular structure and plant sources of chlorogenic acids. Name

R1

R2

R3

Plant sources

3-O-caffeoylquinic acid 4-O-caffeoylquinic acid 5-O-caffeoylquinic acid 3-O-coumaroyl guinic acid 4-O-coumaroyl guinic acid 5-O-coumaroyl guinic acid 3-O-feruloylquinic acid 4-O-feruloylquinic acid 5-O-feruloylquinic acid 4,5-di-O-caffeoylquinic acid 3,4-di-O-caffeoylquinic acid 3,5-di-O-caffeoylquinic acid 3,4-di-O-coumaroyl guinic acid 3,5-di-O-coumaroyl guinic acid 4,5-di-O-coumaroyl guinic acid 3-O-feruloyl,4-O-caffeoylquinic 3-O-caffeoyl,4-O-feruloylquinic 3-O-feruloyl,5-O-caffeoylquinic 3-O-caffeoyl,5-O-feruloylquinic 3-O-feruloyl,5-O-caffeoylquinic 3-O-caffeoyl,5-O-feruloylquinic 3,4,5-tri-O-caffeoylquinic acid

C H H p-Co H H F H H H C C p-Co p-Co H F C F C H H C

H C H H p-Co H H F H C C H p-Co H p-Co C F H H F C C

H H C H H p-Co H H F C H C H p-Co p-Co H H C F C F C

Existing widely Eucommia ulmoides Sweet potato leaves, coffee Hemerocallis fulva Coffee, Hemerocallis fulva Hemerocallis fulva Eucommia ulmoides, honeysuckle Eucommia ulmoides, coffee Tea, coffee Eucommia ulmoides, tea Honeysuckle, sweet potato Honeysuckle, Eucommia ulmoides Coffee Coffee Coffee Sweet potato leaves, coffee Sweet potato leaves, coffee Sweet potato leaves Sweet potato leaves Sweet potato leaves Sweet potato leaves Sweet potato leaves, coffee

acid acid acid acid acid acid

Note: R1, R2, and R3 represent the three different sites in the molecular structure of quinic acid in Fig. 7.2, respectively. H represents hydrogen. C, F, and p-Co represent caffeic acid, ferulic acid, and p-coumaric acid, respectively, in Fig. 7.2.

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isochlorogenic acid (di-O-caffeoylquinic acid; diCQA), 5-CQA, “Band 510” (4-O-caffeoylquinic acid; 4-CQA), and neochlorogenic acid (3CQA) were later isolated and quantified from different kinds of plant sources including sweet potato peelings by open column chromatography, and isochlorogenic acid was found to be the most abundant CQA in sweet potatoes (Sondheimer, 1958). 5-CQA, two isochlorogenic acids, and tentative 4-CQA in the roots of seven sweet potato cultivars harvested in the United States, including Jewel, Centennial, and Julian, were quantified by the use of high-performance liquid chromatography (HPLC) (Walter et al., 1979). This investigation showed that CQA contents varied greatly between cultivars, and 5-CQA and one of the isochlorogenic acids were the most abundant. A study using paper chromatography and thin-layer chromatography isolated and identified four CQAs, 4-CQA, 5-CQA, isochlorogenic acid, and 3-CQA, from 14 cultivars, including Centennial, Jasper, and Jewel, but did not quantify their contents (Thompson, 1981). HPLC analysis of the roots from the Jewel cultivar revealed the tissue location of three isochlorogenic acids to be in the order of outer. skin. inner (Walter and Shadel, 1981). A series of investigations into black rot of sweet potatoes in Japan during the mid-1950s isolated and identified several polyphenols, including 5-CQA, from the roots of the Norin No. 1 cultivar affected by the disease (Uritani and Muramatsu, 1953). Six polyphenols were found in roots (cultivar name not shown), one of the six was isolated and identified as 5CQA, and four of them were identified as three di-CQAs; the remaining polyphenol was tentatively identified as 4-CQA (Hayase and Kato, 1984). This investigation also quantified the contents of the four polyphenols in the roots of the Kintoki and Kokei No. 14 cultivars. Three CQAs, namely, 3,4-di-O-caffeoylquinic acid (3,4-diCQA), 3,5-di-O-caffeoylquinic acid (3,5-diCQA), and 4,5-di-O-caffeoylquinic acid (4,5-diCQA), were isolated from the roots of the Beniotome cultivar (Shimozono et al., 1996). CA and four CQAs, namely, 5-CQA, 3,4-diCQA, 3,5-diCQA, and 4,5-diCQA, were isolated from the raw roots of the Beniazuma cultivar and two CQAs, namely, 3-CQA and 4-CQA, were from its steamed roots (Takenaka et al., 2006). The roots of some sweet potato cultivars are known to have purple flesh and contain anthocyanins. The contents of CA and four CQAs, namely, 5-CQA, 3,4-diCQA, 3,5-diCQA, and 4,5-diCQA, were quantified in the roots of the Benimasari, Koganesengan, J-Red, and Murasakimasari cultivars (Ishiguro et al., 2007a). The Beniazuma, Benimasari, and Koganesengan cultivars have

Sweet potato polyphenols

185

yellow-colored flesh. The J-Red cultivar has orange-colored flesh and is known to contain carotenoids (Ishiguro et al., 2010). The Murasakimasari cultivar has purple-colored flesh and is known to contain anthocyanins (Oki et al., 2002b). It is presumed that many types of CQAs are present in sweet potato roots, regardless of flesh color. The existence of an ester of CA and sugar in sweet potato roots has been reported. Beta-Dfructofuranosyl 6-O-caffeoyl-α-D-glucopyranoside (FCG), an ester of CA and sucrose, was isolated from the roots of the Beniazuma cultivar (Takenaka et al., 2006). FCG was also isolated from the roots of the JRed cultivar stored at 15°C (Ishiguro et al., 2007a). This investigation reported that FCG was undetectable in the nonstored roots of the J-red, Beniazuma, and Koganesengan cultivars, while it was present in those of the Murasakimasari cultivar, and the FCG content in these cultivars increased during storage in general. Sweet potato tops are also known to contain CQAs. Since the 2000s many studies on CQAs in sweet potato tops have been conducted in Japan, the United States, the United Kingdom, and China. The first isolation and identification of 3,4,5-tri-O-caffeoylquinic acid (3,4,5-triCQA) from sweet potato leaves and elucidation of their polyphenolic compositions (CA and five CQAs: 5-CQA, 3,4-diCQA, 3,5-diCQA, 4,5-diCQA, and 3,4,5-triCQA) was carried out in Japan (Islam et al., 2002b). This investigation of the leaves from 20 sweet potato genotypes by HPLC revealed that 3,5-diCQA was the most abundant among the six polyphenols in all genotypes tested. The polyphenol contents of sweet potato leaves reportedly vary depending on cultivation conditions, such as temperature and sunshine. In greenhouse experiments using the Simon No. 1, Kyushu No. 119, and Elegant Summer cultivars, the 3,5-diCQA content was higher after cultivation at 30°C than at 20°C or 25°C, and the contents of most CQAs (5-CQA, 3,4-diCQA, 3,5-diCQA, and 4,5diCQA) were decreased by shading (Islam et al., 2003a). Analyzing sweet potato leaf samples by HPLC using a conventional column with a length of 150 mm, an internal diameter of 4.6 mm, and a particle size of 5 μm and a methanol-based mobile phase proved very time consuming; the retention time for 3,4,5-triCQA was approximately 57 min (Islam et al., 2002b). HPLC analysis using short columns with a length of 75 mm and a particle size of 3 μm and an acetonitrile-based mobile phase shortened the retention time for 3,4,5-triCQA to approximately 15 min (Okuno et al., 2010). This investigation reported the contents of CA and five CQAs, namely, 5-CQA, 3,4-diCQA, 3,5-diCQA,

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Sweet Potato

4,5-diCQA, and 3,4,5-triCQA, in the leaves of 529 cultivars/lines as 0.08
3.09, 0.17
16.01, 0.50
34.08, 1.86
56.52, 0.69
20.73, and 0.25
13.81 mg/g of freeze-dried sample, respectively. The Suioh sweet potato cultivar was released for consumption of its leaves and petioles, and its leaves were shown to have a higher total polyphenol content and DPPH (1,1-dipheny-2-picrylhydrazyl) radical scavenging activity than were the leaves of other leafy vegetables (Ishiguro et al., 2004). The leaves of many cultivars have been shown to contain more CQAs than those of the Suioh cultivar (Okuno et al., 2010). However, the better taste of the leaves and petioles of Suioh than those of other cultivars is an important characteristic. Confirming the validity of polyphenol quantification methods will be increasingly important to consumers and manufacturers. In terms of CA and seven CQAs, namely, 3-CQA, 4-CQA, 5-CQA, 3,4-diCQA, 3,5-diCQA, 4,5-diCQA, and 3,4,5-triCQA, single-laboratory validation of quantification was conducted using Suioh leaves (Sasaki et al., 2014). In the validated quantification method, a column with a conventional size of 250 3 4.6 mm and a particle size of 3 μm and an acetonitrile-based mobile phase were used to separate the peaks of CA and CQAs from those of anthocyanins. The eight polyphenols mentioned above in the leaves of eight cultivars/lines (including Koganesengan, Suioh, Kokei No. 14, and others) were quantified by the validated method for two consecutive years (Sasaki et al., 2015b). Investigations of CQAs in sweet potato tops in the United States, the United Kingdom, and China were conducted as follows. The identification of CA, 5-CQA, 4,5-diCQA, 3,5-diCQA, and 3,4-diCQA in the leaves of the commercial cultivars Beauregard, Hernandez, and Covington in the United States was performed by HPLC with the aid of liquid chromatography-mass spectrometry (LC-MS) (Truong et al., 2007). This investigation showed that 3,5-diCQA and 4,5-diCQA were predominant in the leaves. Analysis by LC-MS3 identified phenolic compounds containing a feruloyl moiety in addition to CQAs in the stems of sweet potatoes (cultivar name not shown) cultivated in China (Zheng and Clifford, 2008). The compounds containing a feruloyl moiety were 3-Oferuloylquinic acid, 4-O-feruloylquinic acid, 5-O-feruloylquinic acid, 3-O-feruloyl-4-O-caffeoylquinic acid, 3-O-caffeoyl-4-O-feruloylquinic acid, 3-O-feruloyl-5-O-caffeoylquinic acid, and 3-O-caffeoyl-5-O-feruloylquinic acid, and the CQAs were 3-CQA, 4-CQA, 5-CQA, 3,5diCQA, and 4,5-diCQA. The four caffeoyl-feruloylquinic acids were presumed to be contained in smaller amounts than were the other

187

Sweet potato polyphenols

compounds. LC-MS2 analysis identified 37 compounds, including 20 phenolic acids and 12 flavonoids, in sweet potato leaves (cultivar name not shown) cultivated in China (Zhang et al., 2015). The 20 phenolic acids included CQAs, such as 3-CQA, 3,4-diCQA, 3,5-diCQA, 4,5-diCQA, and triCQAs. In the lab of the Potato and Sweet Potato Food Science Innovation Team, CAAS, China, sweet potato leaves from 40 cultivars were collected (Sun et al., 2014a). After determination of the nutritional and functional composition, two sweet potato cultivars, Yuzi No. 7 and Simon No. 1, were selected. The polyphenols from these two cultivars were extracted, purified, and identified. Results (Xi et al., 2015) showed that polyphenols from sweet potato leaves consisted mainly of seven CQAs and a small amount of CA (Fig. 7.3). The 3,5-diCQA content was highest, while the 3CQA content was lowest among the CQAs (Table 7.3). (A) 5.0

6

Yuzi No.7

4.0

mAU

3.0 2.0

5

7

1.0 1 23

8

4

0 0

2

4

6

8

10

12

14

16

Time (min) (B) 3.0

Simon No.1

6

2.0

mAU

3 5

1.0

1 2

7 4

8

0 0

2

4

6

8

10

12

14

16

Time (min)

Figure 7.3 The HPLC of sweet potato leaf polyphenols. (A) Yuzi No. 7; (B) Simon No. 1. Peak 1: 5-CQA; peak 2: 3-CQA; peak 3: 4-CQA; peak 4: CA; peak 5: 4,5-diCQA; peak 6: 3,5-diCQA; peak 7: 3,4-diCQA; peak 8: 3,4,5-triCQA.

Table 7.3 The constituents of polyphenols purified from two sweet potato leaf cultivars (Yuzi No. 7 and Simon No. 1). Peak No.

Retention time (min)

Identification

1 2 3 4 5 6 7 8

1.47 1.91 2.10 2.92 4.16 4.54 4.88 6.87

5-CQA 3-CQA 4-CQA CA 4,5-diCQA 3,5-diCQA 3,4-diCQA 3,4,5triCQA

Standard curve

y 5 11.372x 2 0.4279 y 5 9.9086x 1 0.2857 y 5 25.894x 2 17.128 y 5 28.183x 2 1.2114 y 5 9.2077x 2 7.244 y 5 18.056x 2 18.405 y 5 15.353x 2 12.021 y 5 6.2184x 2 5.1579

R2

0.9962 1.0000 0.9988 1.0000 0.9987 0.9981 0.9987 0.9949

Yuzi No. 7

Simon No. 1

Peak area

Content (%, DW)

Peak area

Content (%, DW)

55.55 6 0.92 17.45 6 0.07 35.90 6 0.57 3.80 6 0.14 378.7 6 57.00 1115.10 6 13.29 388.30 6 4.67 27.70 6 0.57

2.46 6 0.03 0.87 6 0.00 1.02 6 0.01 0.36 6 0.01 20.96 6 0.27 31.39 6 0.26 13.04 6 0.11 2.64 6 0.03

130.85 6 7.85 34.65 6 0.64 277.35 6 0.92 384.90 6 0.99 683.15 6 3.13 132.80 6 0.14 31.95 6 0.21 1.95 6 0.07

5.77 6 0.25 1.73 6 0.02 5.69 6 0.01 0.22 6 0.01 21.29 6 0.04 19.45 6 0.06 4.72 6 0.00 2.98 6 0.01

Sweet potato polyphenols

189

Physiological function Anthocyanins Purple-fleshed sweet potatoes exhibit a brilliant reddish-purple color with high levels of anthocyanins, total phenolics, and antioxidant activities (Yoshinaga et al., 1999; Oki et al., 2003; Steed and Truong, 2008). Recently sweet potato cultivars with deep purple flesh have been developed in China, Japan, Korea, New Zealand, and other countries to meet the growing demand for healthy food (Mu et al., 2017; Steed and Truong, 2008). The metabolism of ethanol induces reactive oxygen species (ROS) generation and depletion of the cell antioxidant activity, which leads to development of alcohol-related pathologies. The group members in the lab of the Potato and Sweet Potato Food Science Innovation Team, CAAS, China investigated the dealcoholic effect and preventive effect of purple sweet potato anthocyanins (PSPAs) (cultivar: Yan 176) on acute and subacute alcoholic liver damage (ALD) (Sun et al., 2014b). Sevenweek-old male inbred mice were grouped into five groups: control group (without PSPAs and ethanol treatments), model group (with ethanol treatment only), low-dose group (50 mg PSPAs/kg body weight), middle-dose group (125 mg PSPAs/kg body weight) and high-dose group (375 mg PSPAs/kg body weight), and the mice in all groups were administered intragastrically. Biochemical parameters of serum and liver were determined, and a histopathological analysis of liver tissue was also achieved. Results showed amelioration of all tested parameters following administration of PSPAs (Figs. 7.4
7.9). The abovementioned result suggested that PSPAs have a preventive effect on acute and subacute ALD, and could be used as a complementary reagent during prophylactic and therapeutic managements of ALD. In Japan the most notable example is the Ayamurasaki cultivar, which is the second generation of a local variety, Yamagawamuarsaki. The extracted anthocyanins from the Ayamurasaki storage root exhibited multiple physiological functions such as radical scavenging activity (RSA) (Furuta et al., 1998; Oki et al., 2002b; Kano et al., 2005), oxygen radical absorbance capacity (Oki et al., 2009), angiotensin I-converting enzyme inhibition (Suda et al., 2003), α-glucosidase inhibition (Matsui et al., 2001a, b), and antimutagenic activity (Yoshimoto et al., 1999; 2001). A cell line derived from the Ayamurasaki storage root exhibited potent antimutagenic activity against 3-amino-1,4-dimethyl-5H-pyrido[4,3-b] indole

190

Sweet Potato

(B)

200 180 160 140 120

Control Model Low-dose group Middel-dose group High-dose group

** ##

## ##

100 80 60 40

** ## ## ##

Activity of LDH (U/L)

Activity of ALT and AST (U/L)

(A)

ALT

##

350 300 250 200 150 100

Control

AST

(C)

(D)

450 400

160

Model

140

Low-dose group Middel-dose group High-dose group

100 80 60 #

40

Model

Low-dose Middel-dose High-dose group group group

**

Control

Activity of LDH (U/L)

Activity of ALT and AST (U/L)

**

0

0

120

**

400

50

20

180

**

450

# ##

350

##

300 250 200 150 100 50

20

0

0

ALT

AST

Control

Model

Low-dose Middel-dose High-dose group group group

Figure 7.4 Effect of purple sweet potato anthocyanins (PSPAs) on the activity of serum ALT, AST, and LDH of ALD mice. (A) Effect of PSPAs on the activity of serum ALT and AST of the acute ALD mice. (B) Effect of PSPAs on the activity of serum LDH of the acute ALD mice. (C) Effect of PSPAs on the activity of serum ALT and AST of the subacute ALD mice. (D) Effect of PSPAs on the activity of serum LDH of the subacute ALD mice. Compared with control,  P , .05,  P , .01; compared with model, # P , .05, ##P , .01. ALT, alanine aminotransferase; AST, aspartate aminotransferase; LDH, lactate dehydrogenase; TCH, total cholesterol; MDA, malondialdehyde; SOD, superoxide dismutase; GST, glutathione S-transferase

(Trp-P-1) and inhibition of HL-60 cell proliferation (Konczak-Islam et al., 2003). Despite the complex chemical structure of the anthocyanins in purple-fleshed sweet potatoes, studies have confirmed that following administration of an anthocyanin-rich extract of Ayamurasaki storage root to rats (Suda et al., 2002; Harada et al., 2004) or human consumption of a beverage prepared from purple-fleshed sweet potato (Harada et al., 2004; Kano et al., 2005; Oki et al., 2006), two anthocyanin components, YGM-2 and YGM-5b, were rapidly absorbed into the body (detectable in the blood). Furthermore, they were rapidly excreted

Sweet potato polyphenols

(A)

(B) 3

0.3

2.5

*#

2

Control Model Low-dose group Middle-dose group High-dose group

1.5

# # 1 0.5

Serum LDL_C levels (mmol/L)

Serum TCH and TG levels (mmol/L)

191

0.25

** **# **##

0.2

##

0.15 0.1 0.05 0 Control

0

TCH

Model

TG

Low-dose Middle-dose High-dose group group group

(D) 0.3

(C)

3

#

2.5

#

Control Model Low-dose group Middle-dose group High-dose group

2

* 1.5

# 1

## ##

Serum LDL_C levels (mmol/L)

Serum TCH and TG levels (mmol/L)

3.5 0.25 0.2 0.15 0.1 0.05

0.5 0 0

Control

TCH

TG

Model

Low-dose Middle-dose High-dose group group group

Figure 7.5 Effect of purple sweet potato anthocyanins (PSPAs) on the serum TCH, triglyceride (TG), and LDL-C levels of ALD mice. (A) Effect of PSPAs on the serum TCH and TG levels of the acute ALD mice. (B) Effect of PSPAs on the serum LDL-C level of the acute ALD mice. (C) Effect of PSPAs on the serum TG and TCH levels of the subacute ALD mice. (D) Effect of PSPAs on the serum LDL-C level of the subacute ALD mice. Compared with control,  P , .05,  P , .01; compared with model, #P , .05, ## P , .01.

together with nonacylated anthocyanins in the urine. In addition, it has been reported that the purple-fleshed sweet potato or derived beverages containing anthocyanins show pharmacological effects such as antihyperglycemic effects mediated by α-glucosidase inhibition (Matsui et al., 2002) and antiatherosclerotic effects (Miyazaki et al., 2008) in animal models. Moreover, beneficial effects on hypertension and hepatitis have been observed in both animal models (Suda et al., 2003; Kobayashi et al., 2005) and clinical trials (Suda et al., 2003, 2007; Oki et al., 2016, 2017b). Other pharmacological effects of anthocyanins in other purple-fleshed cultivars include the attenuation of oxidative stress and inflammatory responses induced by D-galactose in mouse liver (Zhang et al., 2009),

192

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*

Hepatic MDA level (mg/g liver)

(A) 30

#

25

## 20

##

15 10 5 0 Control

Model

Low-dose Middle-dose High-dose group group group

(B) Hepatic MDA level (mg/g liver)

30 25 20

## 15

##

##

10 5 0 Control

Model

Low-dose Middle-dose High-dose group group group

Figure 7.6 Effect of purple sweet potato anthocyanins (PSPAs) on the hepatic MDA level of the ALD mice. (A) Effect of PSPAs on the hepatic MDA level of the acute ALD mice. (B) Effect of PSPAs on the hepatic MDA level of the subacute ALD mice. Compared with control,  P , .05,  P , .01; compared with model, #P , .05, ##P , .01.

protection of mouse liver against D-galactose-induced apoptosis (Zhang et al., 2010), attenuation of hepatic lipid accumulation in high-fat diet-fed mice (Hwang et al., 2011a), attenuation of dimethylnitrosamine-induced liver injury in rats (Hwang et al., 2011b), attenuation of domoic acid
induced cognitive deficit in mice (Lu et al., 2012), improvement of fasting blood glucose level, glucose, and insulin tolerance in high-fat diet-treated mice (Zhang et al., 2013), beneficial effects on high-fat diet-induced kidney dysfunction and damage in high-fat diet mice (Shan et al., 2014), alleviation of high-fat diet-induced obesity in rats (Zhang et al., 2015), protective effects against high-fat diet-induced hepatic inflammation in

193

Sweet potato polyphenols

(B)

2 # 1.5 * 1

0.5

0 Control

Model

240 # *##

200 # 160 120 80 40 0

Low-dose Middle-dose High-dose group group group

(C)

Control

Model

Low-dose Middle-dose High-dose group group group

(D) 250

3.5

**##

**##

3

**##

2.5 2

*

1.5 1 0.5 0

Hepatic GST level (U mg/g protein)

Hepatic SOD level (U mg/g protein)

Hepatic GST level (U mg/g protein)

Hepatic SOD level (U mg/g protein)

(A)

200

*##

150

*##

**

100

50

0 Control

Model

Low-dose Middle-dose High-dose group group group

Control

Model

Low-dose Middle- High-dose group dose group group

Figure 7.7 Effect of purple sweet potato anthocyanins (PSPAs) on the hepatic SOD and GST levels of the ALD mice. (A) Effect of PSPAs on the hepatic SOD level of the acute ALD mice. (B) Effect of PSPAs on the hepatic GST level of the acute ALD mice. (C) Effect of PSPAs on the hepatic SOD level of the subacute ALD mice. (D) Effect of PSPAs on the hepatic GST level of the subacute ALD mice. Compared with control,  P , .05,  P , .01; compared with model, #P , .05, ##P , .01.

rats (Wang et al., 2017), protection of PC-12 cells against β-amyloidinduced injury (Ye et al., 2010), and beneficial effects on the growth of human retinal pigment epithelial cells (Sun et al., 2015).

Caffeoylquinic acids Currently quality of life and healthy life expectancy are considered important worldwide, and attention has been focused on strategies for living long and healthily. In working generations being overweight causes lifestyle diseases and increases medical expenses, and so health management with daily healthy meals is recommended. Sweet potato tops have a high content of polyphenolics that consist of CA and CQAs as described previously. This content is very high compared with that found in other

194

Sweet Potato

Hepatic ADH level (U mg/g protein)

(A) 50

40

30 ** 20

* **##

**

10

0 Control

Model

Low-dose Middle-dose High-dose group group group

Hepatic ADH level (U mg/g protein)

(B) 45 40 35 30

**##

**#

25 20

**

15

**

10 5 0 Control

Model

Low-dose Middle-dose High-dose group group group

Figure 7.8 Effect of purple sweet potato anthocyanins (PSPAs) on the hepatic ADH level of the ALD mice. (A) Effect of PSPAs on the hepatic ADH level of the acute ALD mice. (B) Effect of PSPAs on the hepatic ADH level of the subacute ALD mice. Compared with control,  P , .05,  P , .01; compared with model, #P , .05, ##P , .01.

vegetables (Ishiguro et al., 2004). In addition, the 2,2-diphenyl-1-picrylhydrazyl (DPPH) RSA and total polyphenolic content are very highly correlated, and most antioxidants contained in sweet potato tops are presumed to be CQA polyphenols (Islam et al., 2003b). Therefore the investigation of the functionality of CQAs was first considered based on its high associated antioxidant capacity. Antioxidant capacity and related effects ROS are a series of metabolic by-products involved in degenerative and pathological processes in the human body (Li et al., 2016).

Sweet potato polyphenols

195

Figure 7.9 Effect of purple sweet potato anthocyanins (PSPAs) on the histopathology of acute and subacute ALD mice. (A) The histopathology of mice in control group. (B) The histopathology of acute ALD mice in model group. (C) Low-dose PSPAs group. (D) Middle-dose PSPAs group. (E) High-dose PSPAs group. (F) The histopathology of subacute ALD mice in model group. (G) Low-dose PSPAs group. (H) Middledose PSPAs group. (I) High-dose PSPAs group.

Overproduction of ROS could disturb cellular redox balance, resulting in cell injury or apoptosis (Zhang et al., 2017), further triggering oxidative damage of tissues and organs, which accelerates the development of various diseases, such as cancer, atherosclerosis, diabetes, chronic inflammatory disease, cardiovascular disease, and Alzheimer’s disease (Babbar et al., 2011; Fiuza et al., 2004; Shoham et al., 2008; Valko et al., 2007). Although humans and other organisms have endogenous antioxidant defenses against ROS, these systems may sometimes not be sufficient to prevent the occurrence of cell damage (Rechner et al., 2002). In the lab of the Potato and Sweet Potato Food Science Innovation Team, CAAS, China, the antioxidant activity and inhibition of intracellular ROS of the total and individual phenolic compounds from Yuzi No. 7 sweet potato leaves were investigated. Results showed that sweet potato leaf polyphenols possessed significantly higher antioxidant activity than ascorbic acid,

196

Sweet Potato

tea polyphenols, and grape seed polyphenols (Table 7.4). Among the individual phenolic compounds CA showed the highest antioxidant activity, followed by mono-CQAs and di-CQAs, while 3,4,5-triCQA showed the lowest value (Table 7.5). Sweet potato leaf polyphenols could significantly decrease the level of intracellular ROS in a dose-dependent manner (Fig. 7.10). Among the individual phenolic compounds, CA and 3-CQA showed higher value than other individual phenolic compounds (Fig. 7.11) (Sun et al., 2018). Numerous reports have indicated the antimutagenic and anticancer effects of substances with high antioxidant capacity. Additionally, the inhibitory effect of CQAs was confirmed using the Ames method (Yoshimoto et al., 2002) which is used to measure antimutagenic activity and growth suppression of cultured human cancer cells (the stomach, large intestine, and leukemia) (Kurata et al., 2007). Furthermore, administering sweet potato green extract to a nude rat cancer model forcibly induced by the transplantation of cancer cells, suppressed the proliferation of cancer cells (Karna et al., 2011). Since both antimutagenic and growth inhibitory effects against cancer cells in vitro and in vivo were observed, it is likely that carcinogenesis-related processes leading to proliferation were suppressed. However, because there are no clear epidemiological results of the effect of dietary CQA intake on cancer, only possible cancer suppression has been shown. Therefore determining the further functionality of CQAs has mainly focused on those contained in sweet potato tops, which would be effective following consumption. Lifestyle-related disease prevention effect Lifestyle-related diseases include diabetes, hypertension, dyslipidemia, and hyperuricemia. In addition, obesity in association with one or more of these conditions is called metabolic syndrome. Dietary therapy has been the basis for the amelioration of obesity, and polyphenols also contribute to improving lifestyle diseases and metabolic syndrome. Therefore research on the effect of CQAs on lifestyle-related diseases targeted at improving these conditions by the dietary intake of sweet potato stems and leaves have been conducted in recent years. The white-skinned sweet potato appears to have been used in folk remedies for its diabetes improving effect (Kusano and Abe, 2000; Ludvik et al., 2004). Insulin secretion was also confirmed to be promoted in rat pancreatic cells (RIN-5F) treated with sweet potato foliage extract containing CQAs (Yoshimoto et al., 2006). Furthermore, STZ-induced

Table 7.4 Antioxidant activity of Yuzi No. 7 sweet potato leaf polyphenols, tea polyphenols, and grape seed polyphenols. Samples

Sample concentration (μg/mL) 5 UO2 2

SPLP TPP GPP

10

20

scavenging activity (μg ACE/mL)

14.57 6 0.31a 3.60 6 0.28b 3.02 6 0.11c

30.56 6 2.59a 7.29 6 0.31b 3.18 6 0.42c

5

10

20

Oxygen radical absorbance capacity (μg TE/mL)

62.71 6 2.99a 10.62 6 0.45b 6.73 6 0.12c

22.35 6 1.59a 16.67 6 2.98b 13.75 6 0.62b

SPLP, total polyphenols from sweet potato leaves; TPP, total polyphenols from tea; GPP total polyphenols from grape seeds. a
c Data in the same column that are significantly different are represented by different letter (P , .05).

33.72 6 2.61a 32.23 6 1.22a 29.21 6 1.68b

55.68 6 1.45a 43.53 6 0.59b 43.54 6 0.77b

198

Sweet Potato

Table 7.5 Antioxidant activity of individual phenolic compounds from sweet potato leaves. Samples

UO2 2 scavenging activity (μg ACE/mL)

Oxygen radical absorbance capacity (μg TE/mL)

SPLP CA 3-CQA 4-CQA 5-CQA 3,4-diCQA 3,5-diCQA 4,5-diCQA 3,4,5-triCQA

30.56 6 2.59b 51.12 6 5.35a 22.97 6 2.81c 19.36 6 1.45c 20.12 6 2.79c 20.68 6 1.55c 21.69 6 1.42c 22.14 6 2.15c 15.03 6 1.12d

33.72 6 2.61c 56.78 6 4.12a 41.23 6 1.06b 39.15 6 1.58bc 42.58 6 3.66b 39.91 6 8.37bc 35.21 6 2.11bc 42.16 6 3.89b 32.21 6 1.62c



The concentration of all tested samples was 10 μg/mL. SPLP, total polyphenols from sweet potato leaves; CA, caffeic acid; 3-CQA, 3-O-caffeoylquinic acid; 4-CQA, 4-O-caffeoylquinic acid; 5-CQA, 5-O-caffeoylquinic acid; 3,4-diCQA, 3,4-di-Ocaffeoylquinic acid; 3,5-diCQA, 3,5-di-O-caffeoylquinic acid; 4,5-diCQA, 4,5-di-O-caffeoylquinic acid; 3,4,5-triCQA, 3,4,5-tri-O-caffeoylquinic acid. a
d Data in the same column that are significantly different are represented by different letter (P , .05).

insulin-deficient diabetic rats fed foliar powder exhibited reduced blood glucose levels (Yoshimoto et al., 2006). Moreover, α-glucosidase and aldose reductase inhibitors used as oral antidiabetic agents were found in CQAs (Matsui et al., 2004; Kurata et al., 2011). It is interesting that both enzymes show strong inhibitory effects in the following order of increasing magnitude: mono , di , tri caffeoyl groups. In addition, the leaf and stem extracts of sweet potatoes containing CQAs have been reported to lower the blood glucose level of type 2 diabetes model AA-Ky mice after 4 weeks administration (Nagamine et al., 2014). The suppressive mechanisms of CQAs involve promoting the secretion of glucagon-like peptide-1 (GLP-1), which could be a type 2 diabetes remedy. As described previously, CQAs in sweet potato tops have been reported to have various effects in improving diabetes. Currently, reports of the effects on humans are pending. In addition, the inhibitory effect of CQAs on the angiotensin I-converting enzyme (ACE) and the subsequent amelioration of a hypertensive animal model were observed following the administration of foliar powder (Ishiguro et al., 2007b). Studies of obese rats administered foliage powder in combination with a high-fat diet revealed the reduction of body fat and adipose tissue, as well as serum triglyceride, total cholesterol, and liver total cholesterol levels. This result shows that sweet potato leaf powder improved obesity and dyslipidemia (Kurata et al, 2017). As a

Sweet potato polyphenols

199

Figure 7.10 Protective effect of total polyphenols from sweet potato leaves on human hepatocytes LO2 oxidative stress. (A) The effect of total polyphenols from sweet potato leaves on the cell viability of oxidative stress LO2 cells. (B) The effect of total polyphenols from sweet potato leaves on the level of intracellular reactive oxygen species (ROS). Note: Control was LO2 cells without H2O2 and antioxidants treatment; H2O2 control was LO2 oxidative stress model group which was treated by H2O2 of 100 μM; Trolox, ascorbic acid, TPP, and GPP were LO2 cells pretreated by 100 μg/mL Trolox, ascorbic acid, tea polyphenols, and grape seed polyphenols, respectively, and then treated by 100 μM H2O2; SPLP1
SPLP6 were LO2 cells pretreated by sweet potato leaf polyphenols of 25, 50, 100, 200, 400, and 800 μg/mL, respectively, and then treated by 100 μM H2O2; dotted line represents the values of blank control group, while solid line represents the values of LO2 oxidative stress model group. Values are means 6 SD of five determinations. Different letters above different bars mean the cell viability or the level of intracellular reactive oxygen species (ROS) are significantly different (P , .05).

200

Sweet Potato

Figure 7.11 Protective effect of individual phenolic compounds from sweet potato leaves on human hepatocytes LO2 oxidative stress. (A) The effect of individual phenolic compounds from sweet potato leaves on the cell viability of oxidative stress LO2 cells. (B) The effect of individual phenolic compounds from sweet potato leaves on the level of intracellular reactive oxygen species (ROS). Note: Control was LO2 cells without H2O2 and antioxidants treatment; H2O2 control was LO2 oxidative stress model group which was treated by H2O2 of 100 μM; SPLP, CA, 3-CQA, 4-CQA, 5-CQA, 3,4-diCQA, 3,5-diCQA, 4,5-diCQA, and 3,4,5-triCQA were LO2 cells pretreated by 100 μg/mL sweet potato leaf polyphenols, caffeic acid, 3-Ocaffeoylquinic acid, 4-O-caffeoylquinic acid, 5-O-caffeoylquinic acid, 3,4-di-O-caffeoylquinic acid, 3,5-di-O-caffeoylquinic acid, 4,5-di-O-caffeoylquinic acid, 3,4,5-tri-O-caffeoylquinic acid, respectively, and then treated by 100 μM H2O2; dotted line represents the values of blank control group, while solid line represents the values of LO2 oxidative stress model group. Values are means 6 SD of five determinations. Different letters above different bars mean the cell viability or the level of intracellular reactive oxygen species (ROS) are significantly different (P , .05).

Sweet potato polyphenols

201

related function, sweet potato foliage containing CQAs was reported to inhibit the oxidation of low-density lipoprotein (LDL), which causes arteriosclerosis (Nagai et al., 2011; Taira et al., 2013). Other functionality Several other effects that could be expected to be prevented by daily dietary intake of functional foods have also been reported. The antiosteoporotic effect of CQAs was revealed by the inhibition of osteoclast formation and suppression of bone destruction in adjuvant-induced arthritic rats (Tang et al., 2006). Moreover, anti-Alzheimer's disease effects were observed in human neuroblastoma clonal cells (SH-SY5Y) and senescence-accelerated-prone 8 mice, which exhibit age-related deterioration of memory and learning (Han et al., 2010; Sasaki et al., 2013). In conducting these tests, it was very interesting that CQAs promoted ATP production by SH-SY5Y cells (Miyamae et al., 2011), which is suggested to be attributed to the polyphenols. Some CQAs have been reported to possess properties such as antibacterial activity not suitable for consumption as food. In addition, only the characteristic polyphenol component 3,4,5-triCQA, which is the most functionally effective component in the leaves, has been reported to have anti-human immunodeficiency virus (HIV) effects (Tamura et al., 2006). No plants containing higher 3,4,5-triCQA levels than sweet potato leaves have been found to date. Therefore possible novel functional and medicinal effects of 3,4,5-triCQA are expected to be discovered. In Japan, there is a company that manufactures the dry powder of sweet potato foliage, which has been added to the class of processed foods. Furthermore, companies that extract polyphenols from the foliage of sweet potato and sell them as food materials have recently emerged. Therefore it appears that the variety of available sweet potato
based processed foods will expand further. Functional studies of sweet potato foliage are increasing in number, and its further development and widening application are expected.

Processing and utilization Sweet potato storage root Sweet potato is often used to produce processed foods after storage. The optimal storage temperature and relative humidity for sweet potato are 13°C
16°C and 80%
85%, respectively (Woolfe, 1992). Furthermore, prolonged exposure to lower temperatures induces irreversible

202

Sweet Potato

deterioration, and the product is more easily infected by nonpathogenic fungi (Uritani, 1999). Because sweet potato is a tropical root crop, it is susceptible to physiological damage during low-temperature storage. In addition, some biochemical changes, including in sugars and starch, occur in the roots during storage, especially at low temperature (Nakatani and Komeichi, 1991; Picha, 1987). However, sweet potato is stored at low temperature to enhance the sweetness before the preparation of “Hoshiimo,” which is made from sweet potato by steaming and then drying, in Japan. Ishiguro et al. (2007a) investigated changes in the polyphenolic content and RSA of four cultivars (Benimasari, Koganesengan, J-Red, and Murasakimasari) during storage at optimal and low temperatures. Storage time had a significant effect on all cultivars tested while temperature did not in the case of Koganesengan and Murasakimasari. Significant interactions were observed between storage time and temperature for Benimasari and J-Red using a two-way repeated measure analysis of variance (ANOVA). Levels of polyphenolics increased during storage except with Murasakimasari. The largest increase was observed in Benimasari after 13 days storage at 5°C, which was significantly greater than that observed at 15°C. After 37 days storage at 5°C, the polyphenolic content in Benimasari increased 3.7-fold relative to the initial sample. Lieberman et al. (1959) previously reported that 5-CQA increased more in sweet potato roots during storage at 7.5°C than at 15°C. The analysis of Benimasari showed consistent results with those of Lieberman et al., but with a cultivar-dependent pattern. Exposure to nonfreezing temperatures has been shown to stimulate an increase in unique phenolics in various fruits and vegetables (Blankenship and Richardson, 1985; Lattanzio and van Sumere, 1987; Lattanzio et al., 2001). Lattanzio et al. (1994) reported that there is a low, critical temperature below which an increase in phenylpropanoid metabolism, including that of phenylalanine ammonia lyase, is stimulated during the storage of plant tissues and this temperature varies between commodities. It has been suggested that the temperaturesensitivity of sweet potato roots at 5°C differs as a function of genotype. The RSA was found to increase during storage and was highly correlated with the polyphenolic content. Low-temperature storage may induce active oxygen species, radicals, or superoxide formation as a consequence of an imbalance in oxidative and reductive processes (Lattanzio et al., 1994). Benimasari and Murasakimasari, which have higher RSA because of inducible or preformed polyphenolics during storage, showed excellent

203

Sweet potato polyphenols

stability to low-temperature storage. This suggests that the cold resistance of sweet potato cultivars may, at least in part, be mediated by the RSA of polyphenolics. The main CQAs in all the cultivars at the beginning of storage were 5-CQA and 3,5-diCQA, which increased extensively during storage except in Murasakimasari (Fig. 7.12). The pattern of increase in 5-CQA (A)

(B) 10

10

BM

BM CA ChA

6

3,4-diCQA

4

3,5-diCQA

8

μmol/g DW

μmol/g DW

8

3,4-diCQA

4

4,5-diCQA

2

CA ChA

6

3,5-diCQA 4,5-diCQA

2 0

0 0

10

20

30

40

0

10

30

40

6

6

KS CA

4

ChA 3,4-diCQA 3,5-diCQA

2

μmol/g DW

KS μmol/g DW

20

Storage (days)

Storage (days)

CA

4

ChA 3,4-diCQA 3,5-diCQA

2

4,5-diCQA

4,5-diCQA

0

0 0

10

20

30

0

40

10

Storage (days) 10

10

JR

30

40

JR

8 CA ChA

6

3,4-diCQA

4

3,5-diCQA 4,5-diCQA

2

μmol/g DW

μmol/g DW

8

CA ChA

6

3,4-diCQA

4

3,5-diCQA 4,5-diCQA

2 0

0 0

10

20

30

0

40

10

Storage (days)

30

40

16

MM

MM

14

12

CA

10

ChA

8

3,4-diCQA

6

3,5-diCQA

4

4,5-diCQA

2

μmol/g DW

14

20

Storage (days)

16

μmol/g DW

20

Storage (days)

12

CA

10

ChA

8

3,4-diCQA

6

3,5-diCQA

4

4,5-diCQA

2

0

0 0

10

20

Storage (days)

30

40

0

10

20

30

40

Storage (days)

Figure 7.12 Changes in caffeoylquinic acids during storage 15°C (A) or 5°C (B). Data are represented as means 6 SD of five different roots. BM, Benimasari; KS, Koganesengan; JR, J-Red; MM, Murasakimasari.

204

Sweet Potato

and 3,5-diCQA levels differed as a function of storage temperature. The increase in 5-CQA was significantly greater with storage at 5°C than at 15°C in Benimasari and Koganesengan after 13 and 26 days of storage, respectively. Whereas, the 3,5-diCQA content was greater at 15°C than it was at 5°C in Koganesengan and J-Red after 13 days of storage. Kojima and Kondo (1985) reported that 5-CQA was enzymatically converted to 3,5-diCQA by esterification of CA with 5-CQA. Lower enzyme activity could explain the lower amounts of 3,5-diCQA generated at 5°C. More 3,5-diCQA might be produced from 5-CQA by the converting enzyme with higher activity at 15°C. The CQA content in Murasakimasari, except for CA, was higher than in other cultivars and the level and composition remained nearly constant. A component other than the five aforementioned CQAs was identified during storage (Fig. 7.13A) and was purified using successive chromatographic steps. This structure was identified as gluco-6-O-caffeoyl sucrose (CSu, 6-O-caffeoyl-[β-D-fructofuranosyl-{2-1}]-α-D-glucopyranoside)

Figure 7.13 HPLC of polyphenolics in “J-Red” sweet potato roots stored for 37 days at 15°C (A) and the structure of caffeoyl sucrose (CSu) (B).

Sweet potato polyphenols

205

based on nuclear magnetic resonance, infrared, and fast atom bombardment mass spectrometry (Fig. 7.13B) analyses. Generally, the CSu content significantly increased during storage, and the increase was greater at 15°C than it was at 5°C. This result is consistent with the higher levels of CSu found in sweet potato stored over the long term than in fresh sweet potato (Takenaka et al., 2006). The change was the greatest in J-Red stored at 15°C and after 37 days of storage, where the content increased to 0.646 μmol/g DW. CA might react with the sucrose generated during storage at the optimal storage temperature of 15°C. A variety of plant polyphenolics are currently the focus of considerable scientific attention because of their perceived beneficial pharmacological effects. Postharvest storage at a controlled temperature and storage duration is one possible approach for increasing the functional value of sweet potato roots. The sweet potato storage root is consumed after processing and cooking. Takenaka et al. (2006) monitored phenolic compounds in sweet potato during several model cooking and processing treatments. When blocks prepared from fresh sweet potatoes were boiled in water, CQAs, especially 5-CQA and 3,5-diCQA, decreased considerably. A part of these compounds was eluted into the boiling water as the boiling time increased. Furthermore, 3-CQA, 4-CQA, and 4,5-diCQA levels increased slightly as the boiling time increased, but the CA content hardly changed. Since their total amounts in boiling water and boiled sweet potato decreased, 5-CQA and 3,5-diCQA appeared to decompose during heating in boiling water. This decomposition was possibly mediated by the enzyme polyphenol oxidase (PPO) because purified 5-CQA and 3,5diCQA are stable under heating conditions. This possibility was supported by the observation that the decomposition was prevented in the presence of ascorbic acid, which is known to inhibit PPO. The findings are summarized as follows: when sweet potato is cooked whole or in larger pieces, heat is gradually transmitted from the surface to the interior, and PPO reacts with the phenolic compounds. The enzyme reaction progresses gradually, and the phenolic compounds are affected until the enzyme is deactivated at 60°C
80°C.

Sweet potato tops Sweet potato tops (mainly the leaves and petioles) are used as green leafy vegetables in tropical and subtropical regions such as Asia and Africa. In

206

Sweet Potato

Japan they are cultivated and used in some areas such as Okinawa. Since it has become clear that sweet potato leaves contain abundant nutrients such as vitamins and minerals, as well as functional ingredients such as polyphenols, the use of the tops has been examined, and some studies on the use of sweet potato will be introduced. Effect of cooking on polyphenols As mentioned previously, sweet potato tops are a good source of polyphenols, and they are usually cooked before consumption similar to other green vegetables. Cooking causes a number of changes in the physical characteristics and chemical composition of polyphenols. Therefore the effects of cooking methods on polyphenol concentration in sweet potato tops have been investigated. It is also important to estimate the appropriate amount of an ingredient that can be safely consumed. In the lab of the Potato and Sweet Potato Food Science Innovation Team, CAAS, China, the effects of boiling, steaming, microwaving, baking, and frying on proximate composition, total and individual polyphenol content, and antioxidant activity of sweet potato leaves (cultivar: Pushu 53) were investigated (Sun et al., 2014c). An increase of 9.44% in total polyphenol content was observed after steaming, whereas decreases of 30.51%, 25.70%, and 15.73% were noticed after boiling, microwaving, and frying, respectively (Fig. 7.14A). Decrease of 63.82% and 32.35% in antioxidant activity were observed after boiling and microwaving, respectively, whereas increases of 81.40%, 30.09%, and 85.82% in antioxidant

Figure 7.14 Effect of different domestic cooking methods on total polyphenol content (A) and antioxidant activity (B) of sweet potato leaves. Values are means 6 SD of three determinations. Cooking methods that were not significantly different were represented by same letter (P..05).

Sweet potato polyphenols

207

activity were observed after steaming, baking, and frying, respectively (Fig. 7.14B). The phenolic compounds in the sweet potato leaf extracts cooked by different methods were determined by the HPLC method. Eight phenolic compounds, 5-CQA, 3-CQA, 4-CQA, CA, 4,5-diCQA, 3,5-diCQA, 3,4-diCQA, and 3,4,5-triCQA, were identified (Fig. 7.15). In a comparison of the amounts of the eight phenolic compounds present in raw sweet potato leaves, the order was 4,5-diCQA. 3,5-diCQA. 3,4-diCQA. 5

5 6

Raw

4 4

3

mAU

mAU

3 2

5

2

7

1

1

2

4

1

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8

0 0

2

6

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Time (min)

8

10

Time (min)

6

5

5

Steaming

5

Microwave

4

4

6 3

5

3

mAU

3

mAU

6

3

7

2

1

Boiling

4

5

3

7 2

3

2 7

2

1

1

4

1

1

2

4 8

8 0

0 0

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6

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2

Time (min)

6

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Time (min)

5

5

4 5

6 5

4

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Frying

6

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4

3

mAU

mAU

3 2 7

2

1

7

3 2

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1

1

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1

8

0

8

0 0

2

4

6

Time (min)

8

10

0

2

4

6

8

10

Time (min)

Figure 7.15 Chromatograms of phenolic standards and sweet potato leaf extracts at 326 nm in HPLC.

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4-CQA. 3,4,5-triCQA. CA. 3-CQA. 5-CQA (Table 7.6) (Sun et al., 2014c). For 5-CQA, boiling, steaming, baking, and frying decreased its content significantly, while microwaving did not cause significant change. For 3-CQA, boiling, steaming, microwaving, and frying induced significant loss, while baking did not change its content significantly. 4-CQA was the most abundant mono-CQA in sweet potato leaves, and among the used cooking methods, only boiling decreased its content significantly. For CA, baking increased its content significantly, while the other cooking methods caused significant loss. 4,5-diCQA was the most abundant di-caffeoylquinic acid in sweet potato leaves, and only boiling induced significant loss of 4,5-diCQA. For 3,5-diCQA, only boiling decreased its content significantly. For 3,4-diCQA, steaming and frying increased its content significantly, while boiling decreased its content significantly. For 3,4,5-triCQA, steaming, baking, and frying induced significant increase, while there were no significant differences between raw and other treated sweet potato leaves (Table 7.6). The abovementioned results indicate that most of the individual phenolic compounds measured by HPLC in sweet potato leaves show the same general trend on their content, that is, a decrease after boiling and a better retention after other cooking methods. In particular, there is a common point for steamed and fried sweet potato leaves that the contents of both 3,4-diCQA and 3,4,5-triCQA increased significantly compared with raw samples, which is in accordance with the trend of antioxidant activity. This result suggests that steaming would be a preferred method for maintaining the polyphenols and antioxidant activity of sweet potato leaves (Sun et al., 2014c). Sugawara et al. (2011) examined the content of functional components such as lutein, total CA and CQA derivatives, and total polyphenols in cooked leaves of the Suioh cultivar (Sugawara et al., 2011). The leaf blades of Suioh were cooked by steaming, simmering, boiling, and stirfrying. The total CA and CQA derivative content was 244.7
428.2 mg/ 100 g in cooked products. The total polyphenol content of the leaf blades remained at a high value when steamed or simmered. Since the content of polyphenols in the leaves does not decrease significantly in usual cooking, it was evaluated that a considerable amount can be ingested from the cooked products. Sasaki et al. (2015a) also examined individual components such as CA and seven kinds of CQA derivatives in Suioh leaves before and after boiling treatment in detail. The result showed that the Suioh leaf’s contents

Table 7.6 Contents of individual phenolic compounds in sweet potato leaves treated by different domestic cooking methods (mg/g of DW). Peak No.

1 2 3 4 5 6 7 8

Identity

Cooking methods Raw

5-O-caffeoylquinic acid 3-O-caffeoylquinic acid 4-O-caffeoylquinic acid Caffeic acid 4,5-Di-O-caffeoylquinic acid 3,5-Di-O-caffeoylquinic acid 3,4-Di-O-caffeoylquinic acid 3,4,5-Tri-Ocaffeoylquinic acid

Boiling

Steaming

Microwave

Baking

Frying

2.58 6 0.16 3.06 6 0.31a 13.55 6 1.36ab 4.62 6 0.81b 27.23 6 2.28ab

1.55 6 0.11 1.16 6 0.35c 11.20 6 0.00c 0.62 6 0.00c 19.60 6 0.13c

1.93 6 0.16 1.63 6 0.25b 14.67 6 0.33a 0.75 6 0.04c 29.84 6 2.88a

2.63 6 0.22 1.14 6 0.12c 13.17 6 0.45b 0.74 6 0.02c 24.28 6 1.47b

2.25 6 0.05 3.06 6 0.06a 13.53 6 0.52b 5.80 6 0.59a 30.10 6 3.81a

1.76 6 0.00cd 1.32 6 0.06bc 12.86 6 0.19b 0.75 6 0.02c 30.04 6 0.54a

25.02 6 1.26ab

18.09 6 0.07c

26.96 6 2.53a

23.14 6 1.09b

26.01 6 2.80ab

25.90 6 0.38ab

14.18 6 0.44b

12.90 6 0.04c

17.71 6 1.16a

14.91 6 0.60b

14.38 6 1.04b

18.32 6 0.08a

10.68 6 0.00c

10.84 6 0.00bc

11.08 6 0.17a

10.81 6 0.03c

11.07 6 0.28ab

11.11 6 0.10a

a

d

c

a

b

Note: Values are means 6 SD of three determinations. Cooking methods that were not significantly different are represented by same letter (P..05). a-dValues within same lines with different letters are significantly different (p , 0.05).

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of CA, 3-CQA, 5-CQA, 3,5-diCQA, and 3,4,5-triCQA were significantly decreased, which was inferred to be attributable to the formation of structural isomers of mono-CQAs or di-CQAs, in addition to the outflow of CA and CQAs in the leaves. These results indicate that sweet potato leaves cooked using common household methods possibly produce products with a certain level of consumable polyphenols. Furthermore, the results showed that steaming rather than boiling would minimize the loss of polyphenols. In addition, it has been reported that polyphenols are generally sensitive to adverse environmental conditions, including unfavorable temperatures, light, pH, etc., and are therefore susceptible to degradative reactions during product processing and storage (Fang and Bhandari, 2011). In order to preliminarily clear the stability of sweet potato leaf polyphenols, and provide a theoretical basis for the application of sweet potato leaf polyphenols in food, medicine, and other fields, the group members in the lab of the Potato and Sweet Potato Food Science Innovation Team, CAAS, China investigated the effects of different pH values (3.0, 5.0, 7.0, and 8.0), temperatures (55°C, 65°C, 80°C, and 100°C), and light treatments on the total polyphenol content and antioxidant activity of polyphenols extracted from the leaves of two sweet potato cultivars, Jishu No. 04150 and Shangshu No. 19 (Sun et al., 2017). Results showed that sweet potato leaf polyphenols have higher stability in neutral and weakly acidic solutions (pH 5
7). There is no significant effect of light and lowtemperature heat treatment (50°C and 65°C) on total polyphenol content and antioxidant activity. High temperature heat treatment (80°C and 100°C) will cause the significant decrease of the antioxidant activity of sweet potato leaf polyphenols (Figs. 7.16
7.18). Application to food processing Oki et al. (2002a) reported the RSA and polyphenol contents of the hot water extraction of leaves of the sweet potato cultivar Simon-1 to investigate the use of the leaves as an ingredient for a tea-like beverage. The activity of hot water extraction on the freeze-dried powder at various temperatures (60°C, 80°C, and 100°C) was measured. The results indicated that higher extraction temperatures induced higher RSA in the extract. Furthermore, the hot water extract was confirmed to contain polyphenols such as CA and 5-CQA. Ishida et al. (2003) investigated the palatability and preservability of cookies containing sweet potato leaf powder by preparing samples in

Sweet potato polyphenols

Total polyphenol content (mg CAE/mL)

(A)

1.2

Jishu No. 04150

Shangshu No. 19

1.1 1

a

211

a

a

0.9

b

0.8 0.7

a

a

a

a 0.6

3

5

7

8

pH values

(B)

Jishu No. 04150

Shangshu No. 19

Antioxidant activity (mgTE/mL)

3.5

a 3 2.5

bc

b c

2 1.5

a

a ab b

1

3

5

7

8

pH values

Figure 7.16 Effect of different pH value on the total polyphenol content (A) and antioxidant activity (B) of sweet potato leaf polyphenols from Jishu No. 04150 and Shangshu No. 19. Values are means 6 SD of three determinations. Data on the same broken line that are not significantly different are represented by the same letter (P..05).

which 2% of the wheat flour was replaced with sweet potato leaf powder. The results showed that the sweet potato-containing cookies were more palatable and preferred to cookies prepared without the sweet potato, and had a similar texture (Ishida et al. 2003). Although some green discoloration was observed during storage, there was no significant decrease in quality. Suzuno et al. (2004) also examined the effects of adding sweet potato leaf powder to bread. Compared to the unsupplemented bread samples, the addition of sweet potato did not affect the specific volume, moisture content, and degree of gelatinization of the starch and bread hardness.

212

Sweet Potato

Figure 7.17 Effect of different heat treatment on the retention rates (%) of total polyphenol content and antioxidant activity of sweet potato leaf polyphenols from Jishu No. 04150 at 50°C (A), 65°C (B), 80°C (C), and 100°C (D), and that of Shangshu No. 19 at 50°C (E), 65°C (F), 80°C (G), and 100°C (H). Values are means 6 SD of three determinations. Data on the same broken line that are not significantly different are represented by the same letter (P..05).

Figure 7.18 Effect of different light treatment on the retention rates (%) of total polyphenol content of sweet potato leaf polyphenols from Jishu No. 04150 (A) and Shangshu No. 19 (B), and effect of different light treatment on the retention rates (%) of antioxidant activity of sweet potato leaf polyphenols from Jishu No. 04150 (C) and Shangshu No. 19 (D). Values are means 6 SD of three determinations. Data on the same broken line that were not significantly different are represented by same letter (P..05).

214

Sweet Potato

Specifically, 100 g of bread containing 2% sweet potato leaf powder had 2.8 g dietary fiber, 49 μg carotenes, 87 mg calcium, and 11 mg polyphenols, indicating that the bread functionality was improved. Dishes and products using new cultivar Suioh The new sweet potato cultivar Suioh was developed for the use of its tops by NARO, Japan, and its tops and leaves were shown to have higher total polyphenols content and DPPH RSA than those of other leafy vegetables (Ishiguro et al., 2004). The leaves are tasty and rich in nutrients (e.g., vitamins and minerals), and studies on cooking and processing methods were conducted to investigate possible additional uses. Because little is known about methods for preparing Suioh for dietary consumption, several recipes have been developed for home cooking. These sweet potato leaf recipes include “Ohitashi” (boiled greens seasoned with soy sauce), “Tempura” (deep fried after coated with batter mix), and stir-fried with garlic, as well as noodles, soups, and confections. In addition, processed items such as dietary supplements have been developed so that consumers can use the product more easily. Some Suioh products are commercially available in Japan, such as a green juice (called “Aojiru” in Japan) used as a nutritional food. It is commonly used as a dried powder. Kale and young barley leaves are mainly used as raw materials, and Suioh is used in some products. In addition to preparing this product as a beverage consumed after dissolving it in water, there are various other methods such as dissolving the powder in milk or soup or mixing it into yogurt, cookies, and pancake. The production process of green juice powder consists of washing, cutting, drying, and coarsely crushing, followed by finely grinding the material to obtain the final product.

Research and development trend of sweet potato polyphenols Research on polyphenols has increased all over the world, and with its increase applications are becoming more extensive. However, there is still room for improvement in some areas, such as (1) developing new raw materials for polyphenols: polyphenols could be extracted from agricultural by-products and food processing wastewater and residues. This comprehensive utilization of waste materials is of great environmental significance; (2) in-depth studies on the separation, purification, and the structure
activity relationships of polyphenols: polyphenols are composed

Sweet potato polyphenols

215

of a variety of components. At present, the separation and purification processes of individual components are limited, and the differences in activity levels among different components needs further study. Additionally, the mechanisms responsible for polyphenol activities, especially the in vivo mechanisms are still not clear, and as a result there is no reliable theoretical support for the utilization of polyphenol activities; and (3) expanding the application range, and improving the application depth: polyphenols have a variety of biological activities. At present the applications of their activities in food, medicine, and daily chemical fields are mainly focused on free radical scavenging, antisepsis, and antiinflammation. The development and application of activities, such as anticancer, cardiovascular protection, and diabetes prevention, need to be expanded. Thus there is still a lot of work to be done in the study of polyphenols.

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152.

CHAPTER 8

Sweet potato carotenoids Koji Ishiguro

Division of Field Crop Research and Development, Hokkaido Agricultural Research Center, NARO, Hokkaido, Japan

Overview of sweet potato carotenoids Carotenoids are widely distributed in various microorganisms, algae, plants, and animals that have acquired them through the food chain. Over 750 kinds of carotenoids have been reported so far. In plants, carotenoids are involved in light-harvesting for photosynthesis and also are essential for photoprotection against excess light (Ledford et al., 2004). Plant chlorophyll contains β-carotene, lutein, violaxanthin, and 90 -cis-neoxanthin as well as small amounts of α-carotene, β-cryptoxanthin, zeaxanthin, antheraxanthin, and lutein epoxide. In underground crops, such as carrot and pumpkin, β-carotene is abundant. Animals take in carotenoids in their food and absorb them directly or absorb their metabolites. In human serum β-carotene, α-carotene, lycopene, β-cryptoxanthin, zeaxanthin, and lutein occur. Carotenoids with β-ionone rings, such as β-carotene, α-carotene, and β-cryptoxanthin, are cleaved to retinol (vitamin A) in the intestinal mucosa of herbivore and omnivore animals. β-Carotene shows about twofold higher vitamin A activity than other provitamin A carotenoids. Carotenoids are a 40-carbon tetraterpenoid pigment having nine conjugated double bonds in the middle and end-groups. Carotenoids are divided into carotene consisting of carbon and hydrogen and xanthophyll having oxygen atoms (hydroxy group, ketone group, carboxy group, etc.). Carotenoids are important components for human health, and many physiological functions, as well as provitamin A activity, are attributed to carotenoids. They include antioxidant and anticancer activities and protection against coronary heart disease and eye diseases, such as age-related macular degeneration (AMD) and cataracts (Mayne, 1996). In this chapter, the composition and content of sweet potato carotenoids in roots and leaves, and the varieties with high content of carotenoids are reviewed,

Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00008-9

© 2019 Elsevier Inc. All rights reserved.

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along with the antioxidant activity of carotenoids and their retention and accessibility during processing. The study of an orange-fleshed sweet potato (OFSP) rich in β-carotene for vitamin A deficiency is also introduced.

Carotenoids in sweet potato Carotenoids in orange- and yellow-fleshed sweet potato roots Sweet potato flesh has various colors, including white, cream, yellow, orange, and purple. The storage roots with yellow or orange flesh contain some types of carotenoids. OFSPs contain β-carotene as a main carotenoid (approximately 90%) (Kimura et al., 2007). Takahata et al. (1993) investigated the β-carotene contents of 22 OFSP cultivars grown under possibly uniform conditions, and the content ranged from 1.1 to 26.5 mg/100 g flesh weight. The average retinol equivalent (RE) of representative cultivars (Resisto, Benihayato, Santo Amaro, Caromex, and Red Jewel) was 2.8. The highest β-carotene content was recognized in the US cultivar SPV-61 (26.5 mg/100 g), and US cultivars Resisto (20.3 mg/100 g), UC700 (18.9 mg/100 g), L-2-116 (18.5 mg/100 g), L4-89 (18.5 mg/100 g), and Japanese cultivar Benihayato (18.7 mg/100 g) had relatively high levels. The average RE value (2.8) is equal to the maximum value of the carrot cultivars. Donado-Pestana et al. (2012) demonstrated that all-trans-β-carotene content in four Brazilian sweet potatoes (CNPH 1007, CNPH 1194, CHPH 1202, and CNPH 1205), which are improved cultivars, ranged from 79.1 to 128.5 mg/100 g dry weight. Teow et al. (2007) reported that β-carotene content in US cultivars or breeding lines with yellow (breeding lines) and orange flesh (Beauregard, Hernandez, Covington, and breeding lines) ranged from 1.5 to 226 μg/g fresh weight. Tomlins et al. (2012) reported that the total carotenoid content in African cultivars with white (Dimbuka, Nakakande, New Kawogo, and Ndikirya N’omwami), yellow (Tanzania and Naspot 1), and orange flesh (Ejumula, Kakamega, SPK004/1, SPK004/6/6, and SPK004/1/1) ranged from 0.4 to 72.5 μg/g fresh weight. Islam et al. (2016) reported that the total carotenoid content in sweet potato with white- (Daulatpuri, BARI SP-7), cream- (BARI SP-6), yellow- (Tripti), and orange-flesh (Kamalasundari, BARI SP 4, BARI SP-5) cultivated in Bangladesh ranged from 1.02 to 61.94 mg/g fresh weight. They suggested

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that Kamalsundari and BARI SP-5 OFSPs have the potential to be used as food-based supplements to reduce vitamin A deficiency. Reports on the carotenoid composition of yellow-fleshed cultivars are scarce, although such cultivars are popular in Japan and other countries. Maoka et al. (2007) analyzed the components of the yellow pigment in the cv. Benimasari, which has deep yellow flesh. The high-performance liquid chromatography (HPLC) analysis of carotenoids in Benimasari sweet potato showed seven known carotenoids and four new carotenoids. That study identified a novel series of carotenoids with a 5,6-dihydro-5,6dihydroxy-β-end group, named ipomoeaxanthins A, B, C1, and C2 (Fig. 8.1A). Ishiguro et al. (2010) analyzed the total content and composition of carotenoids in Japanese yellow-fleshed cultivars/breeding lines as well as in orange-fleshed cultivars. The total carotenoid contents in eight sweet potato cultivars/breeding lines with yellow flesh were evaluated by HPLC and compared with those of four cultivars with orange flesh. The carotenoid contents ranged from 1.131 to 3.908 mg/100 g dry weight in yellowfleshed cultivars and from 14.791 to 46.187 mg/100 g dry weight in orange-fleshed cultivars (Table 8.1). Seventeen carotenoids were detected in yellow- and orange-fleshed sweet potatoes via HPLC analysis (Fig. 8.2). The main carotenoids were β-carotene-5,8;50 ,80 -diepoxide (A)

(B)

β-Carotene

Ipomoeaxanthin A

O O β-Carotene-5,8;5′,8′-diepoxide

Ipomoeaxanthin B

O

HO Ipomoeaxanthin C1

β-Cryptoxanthin-5′,8′-epoxide

Ipomoeaxanthin C2

Figure 8.1 Structures of the major carotenoids in sweet potato storage roots. (A) Ipomoeaxanthin A, B, C1, and C2 in yellow-fleshed sweet potatoes. (B) β-Carotene5,8;50 ,80 -diepoxide, and 9, β-cryptoxanthin-5,8-epoxide.

Table 8.1 Contents (mg/100 g of dry weight) and proportions of individual carotenoids (%) in cultivars/lines with yellow or orange flesh. Peak no.a

Beniharuka

Kokei 14

Kyushu 138

Kyukei 280

Benimasari

Kyushyu149

Kyushu 121

Tamaotome

J-Red

Benihayato

Sunny-Red

Hamakomachi

Mean SD % Mean SD % Mean SD % Mean SD % Mean SD % Mean SD % Mean SD % Mean SD % Mean SD % Mean SD % Mean SD % Mean SD %

b

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

Total

25.5 3.9 2.3 40.6 10.7 2.5 17.1 6.6 1.0 39.4 12.5 1.5 50.3 12.5 2.4 69.7 16.2 2.8 41.2 9.3 1.3 71.0 11.3 1.8 73.3 23.6 0.5 211.7 35.9 1.0 103.9 48.6 0.3 61.6 13.1 0.1

8.5 1.0 0.8 14.0 3.3 0.9 4.3 4.0 0.2 17.2 5.9 0.7 21.2 2.1 1.0 20.2 4.1 0.8 9.7 2.0 0.3 16.1 2.8 0.4 71.0 17.9 0.5 146.1 21.4 0.7 47.7 4.5 0.1 37.0 6.5 0.1

36.3 2.8 3.3 30.4 3.8 1.9 27.7 3.2 1.6 103.0 13.6 4.1 61.9 4.4 3.0 44.8 3.8 1.8 46.8 5.1 1.4 54.9 5.9 1.4 61.3 25.6 0.4 12.1 2.1 0.1 69.2 9.5 0.2 72.3 4.8 0.2

6.6 1.2 0.6 14.6 4.4 0.9 9.4 1.7 0.5 9.9 2.7 0.4 7.7 1.2 0.4 43.8 12.9 1.7 27.8 5.2 0.8 13.6 2.9 0.3 89.1 19.4 0.6 185.9 36.7 0.9 161.9 20.9 0.4 161.2 30.7 0.3

74.7 18.1 6.5 113.9 33.0 6.9 82.6 15.8 4.7 112.0 28.0 4.4 113.0 26.2 5.4 156.6 21.4 6.2 154.7 23.8 4.6 236.2 32.0 6.0 75.5 9.6 0.5 7.6 7.2 0.0 185.6 22.6 0.5 99.3 22.4 0.2

60.6 11.4 5.3 54.9 8.9 3.4 112.3 8.6 6.5 109.8 16.3 4.4 143.2 12.0 6.9 80.6 8.6 3.2 149.7 16.1 4.5 174.0 15.3 4.5 27.9 2.0 0.2 n.d.   71.9 10.8 0.2 41.7 8.9 0.1

28.6 4.8 2.5 30.9 4.8 1.9 38.4 2.0 2.2 37.5 5.5 1.5 94.0 55.3 4.4 32.1 3.7 1.3 48.9 6.6 1.5 77.2 6.7 2.0 15.4 1.2 0.1 n.d.   57.7 11.7 0.1 47.2 6.5 0.1

14.0 4.1 1.2 19.6 9.2 1.2 17.4 2.1 1.0 27.0 1.9 1.1 37.8 4.1 1.8 16.7 1.1 0.7 33.3 5.0 1.0 26.5 3.4 0.7 6.1 3.5 0.0 n.d.   23.2 5.1 0.1 7.7 7.0 0.0

205.8 15.0 18.5 199.3 14.5 12.4 244.7 19.8 14.1 754.8 97.9 30.0 388.1 32.7 18.7 284.9 29.9 11.3 525.0 43.5 15.8 422.1 50.2 10.8 173.2 10.5 1.2 52.5 30.8 0.2 338.5 35.2 0.9 265.7 24.3 0.6

73.0 17.6 6.4 156.9 68.6 9.4 62.8 9.1 3.6 52.4 18.7 2.0 57.3 15.9 2.7 241.0 44.1 9.4 122.3 23.5 3.6 287.9 79.2 7.3 491.9 23.6 3.3 68.6 48.4 0.3 969.2 76.6 2.5 701.6 114.6 1.5

217.9 52.4 19.0 344.7 128.9 20.8 296.1 69.9 16.8 226.4 79.8 8.8 231.3 57.8 11.0 584.2 84.7 22.9 634.3 128.4 18.8 835.4 173.8 21.2 238.3 23.9 1.6 59.9 45.6 0.3 963.3 137.6 2.5 471.5 86.2 1.0

102.7 20.0 9.0 162.9 16.1 10.1 199.7 27.7 11.4 158.0 25.6 6.3 222.4 28.8 10.6 184.7 20.0 7.3 323.4 67.7 9.6 365.1 35.5 9.4 66.3 6.0 0.4 8.5 6.9 0.0 243.9 22.8 0.6 95.9 16.1 0.2

132.9 28.8 11.7 212.2 31.3 13.2 404.7 49.0 23.2 436.7 76.8 17.3 404.0 52.1 19.3 387.8 42.6 15.3 744.1 141.4 22.0 694.5 67.8 17.8 66.1 7.2 0.4 24.7 10.7 0.1 365.9 197.8 1.0 175.3 23.6 0.4

23.8 5.3 2.1 49.4 14.7 3.0 33.5 2.4 1.9 50.5 6.9 2.0 36.7 5.4 1.8 116.0 31.5 4.5 68.1 35.3 2.1 131.3 29.3 3.3 1277.5 45.5 8.6 495.8 173.1 2.3 2671.6 225.2 6.8 3062.3 274.5 6.6

92.1 20.1 8.2 69.1 13.4 4.3 140.5 9.5 8.1 188.5 40.4 7.4 91.4 17.3 4.4 231.9 77.9 9.0 338.9 74.2 10.2 323.1 42.7 8.3 197.1 18.2 1.3 185.1 41.0 0.9 762.9 69.0 1.9 516.9 67.9 1.1

n.d.   n.d.   n.d.   n.d.   n.d.   n.d.   n.d.   n.d.   101.5 9.8 0.7 278.6 35.2 1.3 296.5 41.7 0.8 275.8 28.5 0.6

28.2 7.6 2.5 120.8 55.1 7.4 51.2 28.2 3.0 192.3 79.1 8.1 126.2 24.7 6.1 53.1 18.0 2.1 84.7 7.1 2.6 179.7 4.8 4.6 11,760 407.7 79.5 19,125 1864.5 91.7 32,276 5938.1 81.2 40,094 1679.4 86.8

1131 176 100 1634 253 100 1742 179 100 2516 329 100 2087 219 100 2548 301 100 3353 363 100 3908 477 100 14,791 470 100 20,863 2118 100 39,609 5977 100 46,187 2198 100

SD, standard deviation. a Peak 1: unknown, 2: unknown, 3: ipomoeaxanthin A, 4: unknown, 5: unknown, 6: ipomoeaxanthin C1, 7: ipomoeaxanthin C2, 8: β-cryptoxanthin-5,8;50 ,80 -diepoxide, 9: β-cryptoxanthin-5,8-epoxide, 10: unknown, 11: β-carotene-5,8;5 0 ,80 -diepoxide (cis isomer), 12, 13: β-carotene5,8;50 ,80 -diepoxide (diastereomer), 14: unknown, 15: β-carotene-5,8-epoxide, 16: unknown, 17: β-carotene. b The values represent the mean of five individual tubers.

Figure 8.2 HPLC chromatograms of carotenoids from (A) the yellow-fleshed cultivar Tamaotome and (B) the orange-fleshed cultivar Sunny-Red. Peak identifications: 1, unknown; 2, unknown; 3, ipomoeaxanthin A; 4, unknown; 5, unknown; 6, ipomoeaxanthin C1; 7, ipomoeaxanthin C2; 8, β-cryptoxanthin-5,8;5',8'-diepoxide; 9, β-cryptoxanthin-5,8-epoxide; 10, unknown; 11, β-carotene-5,8;50 ,80 -diepoxide (cis isomer); 12 and 13, β-carotene-5,8;50 ,80 -diepoxide (giastereomer); 14, unknown; 15, β-carotene-5,8-epoxide; 16, unknown; 17, β-carotene.

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(approx. 32%51%) and β-cryptoxanthin-5,8-epoxide (approx. 11% 30%) in the yellow-fleshed cultivars/lines, whereas β-carotene (approx. 80%92%) was dominant in orange-fleshed cultivars (Figs. 8.1B and 8.2). These results suggest that the content of each carotenoid differs according to the flesh color, although the carotenoid component in the yellow and orange flesh was almost identical. In the carotenoid pathway, β-cryptoxanthin is synthesized by adding a hydroxyl group to a β-ring of β-carotene (Burns et al., 2003). The balance of the synthesis of β-carotene and metabolization to the β-carotene epoxide or β-cryptoxanthin epoxide could be a determinant of the flesh color of the sweet potato root. In other words, a higher expression of β-carotene results in orange flesh, and a higher accumulation of β-carotene epoxide and β-cryptoxanthin epoxide leads to yellow flesh in sweet potato roots. Liu et al. (2009) measured the total carotenoid levels in Taiwanese orange- and yellow-fleshed sweet potato at various times. The carotenoid levels in both sweet potatoes at various times were in the order October . July . April . January. They proposed that weather may play the crucial factor to influence carotenoid content in the crop.

Carotenoids in sweet potato leaves Sweet potato leaves are seldom used, despite their high nutritional value. Some reports have shown that sweet potato leaves have high polyphenol content and exhibit many physiological functions, such as antioxidant activity, antimutagenicity, and improvement of lipid metabolism (Yoshimoto et al., 2003; Kurata et al., 2017). In Japan, a cultivar, Suioh, was developed for the use of its tops as a leafy vegetable (Ishiguro et al., 2004a) and also as an ingredient in vegetable juices and dietary supplements to provide polyphenols and better nutrition. Lutein is a member of the xanthophyll family of carotenoids and is found in vegetables and fruits (Fig. 8.3). Lutein is believed to mitigate eye diseases such as AMD and cataracts. Green leafy vegetables like spinach and kale have high lutein content (Alves-Rodrigues and Shao, 2004).

Figure 8.3 Structures of lutein in sweet potato leaves.

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Purified lutein has been used in dietary supplements and as a food ingredient for eye health in recent years. Chen and Chen (1993) demonstrated that various chlorophylls and carotenoids including lutein were present in sweet potato leaves acquired from a Taiwan market as analyzed by HPLC, and that the lutein content was 20.966 mg/100 g fresh weight. Ishiguro and Yoshimoto (2006) demonstrated that the lutein content was measured in constituent parts of Suioh tops over 2 years. The content in the leaves ranged from 31.5 to 42.6 mg/100 g fresh weight, and the average content was 36.8 mg/100 g fresh weight (median 37.7 mg/100 g fresh weight) (Table 8.2). The content was not so different from other reports. The lutein content in leaves of a Taiwanese cultivar was 20.966 mg/100 g fresh weight (Chen and Chen, 1993), those of US cultivars ranged from 38 to 58 mg/100 g fresh weight (Menelaou et al., 2006), and those of Korean cultivars ranged from 19.01 to 28.85 mg/100 g fresh weight (Li et al., 2017). The lutein contents in the stems and the petioles were much lower than that of the leaves (Table 8.2). The average content in leaves was higher than that of Ipomoea aquatica leaves (11.9 mg/100 g fresh weight), and exceeded those in other fruits and vegetables listed in a carotenoid database of 120 fruits and vegetables generated by Mangels et al. (1993) (Table 8.2). Table 8.2 Lutein content in constituent parts of sweet potato (Suioh) tops compared with other vegetables. Lutein content (mg/100 g) Mean (median)

Minmax

36.8 (37.7) 1.8 (2.2) 1.6 (2.0) 11.9 (12.4) (21.9) (10.2) (1.9) (1.8) (1.7)

31.542.6 0.63.0 0.42.8 8.514.5 14.739.6 4.415.9 1.82.1  1.12.4

Sweet potato cv. Suioh

Leaves Stems Petioles Ipomoea aquatica Kalea Spinacha Broccolia Lettucea Peas, greena

Sweet potato tops grown in the field were harvested in 2003 and 2004, while Ipomoea aquatica was harvested only in 2004. a The values for kale, spinach, broccoli, lettuce, and green peas are quoted from the database generated by Mangels et al. (1993).

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Changes in lutein content of sweet potato leaves grown in nursery beds throughout a growing season were analyzed using 13 cultivars or lines. Cultivar differences in the content were recognized among the tested cultivars or lines. The content tended to decline gradually throughout the growing season while showing cultivar differences. The average content progressively decreased from 21.8 to 10.0 mg/100 g fresh weight, while the content in Suioh leaves decreased from 25.7 to 12.9 mg/100 g. Weather conditions, including temperature, duration of sunshine, and rainfall level, or deficiencies in certain nutrients in the seed roots (due to repeated harvesting) might have influenced the change in lutein content. From these results, cultivar selection and harvesting time are important considerations in order to achieve higher lutein content in the leaves. β-Carotene and chlorophylls contents were also measured by HPLC analysis. Lutein content correlated strongly with these photosynthetic components. β-Carotene is another carotenoid that provides eye-protective nutrients. These results suggest that sweet potato leaves may be a good source of carotenoids which may help prevent or mitigate eye diseases. The lutein intake levels that have been associated with more than a 50% reduction in risk for AMD and cataracts are 614 mg per day (Alves-Rodrigues and Shao, 2004). This level will be achieved if 16.338.0 g of fresh leaves of Suioh grown in the field were eaten, as 100 g of fresh leaves will contain 36.8 mg lutein. Although the content declined gradually with harvesting times in nursery beds, the lutein content in sweet potato leaves, especially in Suioh, was higher than that in other vegetables and was equal to or exceeded that of kale (median 21.9 mg/100 g fresh weight), the highest in the database. Consumption of sweet potato leaves might help to improve visual functions in patients suffering from eye diseases. Currently most sweet potato leaves in Japan are discarded in the field and in the nursery beds. They could be used as a source for lutein extraction for use in nutritional supplements.

Development of sweet potato with high content of carotenoids and the rapid analysis method of total carotenoids Development of sweet potato with high content of carotenoids One objective of sweet potato breeders is to develop a variety with deep orange or yellow flesh. Sweet potato varieties with orange and yellow

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flesh have been developed recently in Japan (Takahata, 2014). For example, the following cultivars have been released: Sunny-Red (Yamakawa et al., 1999), J-Red (Yamakawa et al., 1997), Hamakomachi (Yoshinaga et al., 2006), and Ayakomachi (Kai et al., 2004) with orange flesh and Quick Sweet (Katayama et al., 2003), Tamaotome (Ishiguro et al., 2004b), Benimasari (Ishiguro et al., 2004c), Beniharuka (Kai et al., 2010), Himeayaka (Ohara-Takada et al., 2011), and Aikomachi (Ohara-Takada et al., 2016) with yellow flesh. Total carotenoid or β-carotene content of some of these varieties were shown in Table 8.1 and in the report by Oki et al. (2006). Many commercial products, such as chips, cakes, juice, distilled spirits, and “Hoshi-imo,” which is made from sweet potato through the processing of steaming and then drying, have been developed using orange and yellow cultivars (Komaki and Yamakawa, 2006). Increasing the carotene content in transgenic sweet potato is challenging in some reports. Kim et al. (2013) isolated a sweet potato orange (IbOr) gene and transformed it into nonembryogenic calli of a lightOFSP. The total carotenoid content in transgenic calli was higher than that in the control calli. The overexpression of ζ-carotene desaturase significantly increased β-carotene and lutein contents as well as enhancing salt tolerance in transgenic sweet potato (Li et al., 2017). Suppression of β-carotene hydroxylase gene increased β-carotene content in transgenic sweet potato (Kang et al., 2017). The development of sweet potatoes with higher contents of carotenoids is expected in future. The food products may be colored deep orange or yellow by the sweet potato, and the carotenoids may contribute to the prevention of some diseases.

Rapid analysis methods for total carotenoid content of sweet potato HPLC has been exclusively used for the measurements of total and individual carotenoid contents in sweet potato. In the case of the measurement of total carotenoid content, however, it can be evaluated by absorption spectrophotometry. Ishiguro et al. (2010) analyzed total carotenoid content of yellow- and orange-fleshed cultivars and breeding lines by absorption spectrophotometry using extinction coefficient of E 5 2500 at λmax (Schiedt and Liaaen-Jense, 1995). The content by this method showed a high correlation with the result by the HPLC method (Fig. 8.4), and there was no significant difference between the two methods. This result indicates that the total carotenoid content can be easily evaluated spectrophotometrically. The evaluation of yellow flesh

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Carotenoid content by spectrophotometrical analysis (mg/100 g DW)

60 R 2 = 0.9826

50 40 30 20 10 0 0

10

20

30

40

50

60

Carotenoid content by HPLC (mg/100 g DW)

Figure 8.4 Correlation between the carotenoid content (mg/100 g of dry weight) by HPLC and by spectrophotometric analysis.

color is sometimes uncertain because the results are based on visual determination. The results of our studies indicate that flesh color, from white to yellow, could be accurately determined by the total carotenoid content. For example, flesh color with a total carotenoid content of under 1.0 mg/100 g DW is white or pale cream, from 1.0 to 2.0 mg/100 g DW flesh is cream, from 2.0 to 3.0 mg/100 g DW flesh is pale yellow, and over 3.0 mg/100 g DW flesh is yellow. Takahata et al. (1993) analyzed flesh color using a color difference meter. They directly measured the color values of the cut surface. The correlation coefficient between the color value (L , a , b ) and β-carotene content were 0.885, 0.897, and 0.810 with a significant level at 1% (n 5 22), respectively. The color values of flesh, in particular, the a value, had the closest relationship to the content of β-carotene in OFSP cultivars/lines. This method could also be useful for the evaluation of β-carotene content in orange-fleshed cultivars as a rapid method.

Physiological function of sweet potato carotenoids Antioxidant activity of sweet potato carotenoids Ishiguro et al. (2010) analyzed the 2,20 -azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) radical-scavenging activity of sweet potato carotenoids. The IC50 values ranged from 6.6 μM

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(β-carotene) to 17.8 μM (β-cryptoxanthin-5,8-epoxide) (Table 8.3). The antioxidant activity of ipomoeaxanthin A was approximately the same as that of β-carotene, and those of the main carotenoids, β-carotene-5,8;50 ,80 -diepoxide and β-cryptoxanthin-5,8-epoxide, were lower than that of β-carotene. These carotenoids probably act as a radical scavenger, similarly to other carotenoids already reported. Previous reports indicated that the predominant contributor to the ABTS radicalscavenging activity in the lipophilic fraction in some orange-fleshed cultivars was β-carotene (Oki et al., 2006). However, the correlation between ABTS radical-scavenging activity and β-carotene content was not high (r 5 0.584, n 5 8). The contribution of β-carotene to the radical-scavenging activity ranged from 36.3% to 79.6%. Teow et al. (2007) evaluated antioxidant activities of sweet potatoes with distinctive color (white, cream, yellow, orange, and purple) by oxygen radical absorbance capacity (ORAC). The total polyphenol contents were highly correlated with the hydrophilicORAC values, but β-carotene contents were poorly correlated with the lipophilic-ORAC value. On the other hand, a large amount of p-coumaric acid esters, rather than yellow pigments, contributed significantly to the radical-scavenging activity in the lipophilic fraction of yellow-fleshed sweet potato (Ishiguro et al., 2008). A reason for the lack of correlation between the radical-scavenging activity and the carotenoid content in yellow-fleshed sweet potato could be that the carotenoid content is lower than that in OFSP. The lower antioxidant activities of the main carotenoids, β-carotene-5,8;50 ,80 -diepoxide and β-cryptoxanthin-5,8-epoxide, in yellow-fleshed sweet potato would be another reason for the lower contribution of the carotenoids to the antioxidant activities in yellow-fleshed sweet potato. Elevation of the carotenoid content in yellow-fleshed cultivars might lead to a higher contribution of carotenoids Table 8.3 ABTS radical-scavenging activity of carotenoids isolated from yellowfleshed sweet potato. Peak no.

Carotenoids

IC50 (μM)

3 6,7 9 11 12, 13 17

Ipomoeaxanthin A Ipomoeaxanthin C1 1 C2 β-Cryptoxanthin-5,8-epoxide β-Carotene-5,8;50 ,80 -diepoxide (cis) β-Carotene-5,8;50 ,80 -diepoxide β-Carotene

6.7 8.6 17.8 9.8 10.2 6.6

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to the antioxidant activity in the lipophilic fraction of sweet potato. Carotenoids as antioxidants have been reported to have a preventive effect against some diseases, such as allergies, arteriosclerosis, cancers, and diabetes, both in vitro and in animal models (Paiva and Russell, 1999). Epidemiological studies have demonstrated that dietary carotenoids result in lower risks for lifestyle-related diseases (Sugiura, 2015).

OFSP as a solution to vitamin A deficiency The OFSP has been popular in the United States and is now emerging as an important crop for supplying vitamin A. Many children and pregnant women in Africa and Southeast Asia suffer from vitamin A deficiency. Inadequate intake of vitamin A by infants and preschool children leads to vitamin A deficiency that in turn may cause night blindness and undermine growth and immune function. Vitamin A deficiency in preschool children and pregnant women is most likely attributable to diets deficient in vitamin A and/or β-carotene (Islam et al., 2016). The largest contribution of vitamin A intake comes from the provitamin A carotenoids in plant foods, which may contribute up to 82% of the total vitamin A intake, whereas the contribution from fish and meat is of minor importance, because these foods are expensive and/or are not accessible (van den Berg et al., 2000). The OFSP is expected to combat vitamin A deficiency because it is rich in β-carotene, which is a precursor of vitamin A. OFSP consumption increased the vitamin A intake of Kenyan women and children (Hagenimana et al., 1999). A diet containing β-carotene mostly derived from the OFSP increased serum retinol concentrations in Indonesian children with vitamin A deficiency (Jalal et al., 1998). A 125 g serving of boiled or mashed OFSP improved vitamin A in liver stores (van Jaarsveld et al., 2005). A 100150 g serving of boiled OFSP roots can supply the daily requirement of vitamin A for young children, which can protect them from night blindness (Mitra, 2012). In Ethiopia, vitamin A-enriched bread was developed using OFSP flour and whole wheat flour (Kurabachew, 2015). Bread containing 30% OFSP flour contributed 75%85% of the required daily vitamin A for children aged 36 years. Flat bread composed of maize and OFSP flours has also been developed, and an increase in the vitamin A level was observed with an increase in the proportion of OFSP flour. This could contribute to the daily intake of vitamin A for children and lactating women. β-Carotene-rich cookies, juice, dried chips, porridge, etc., made from the OFSP have also been

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developed to help reduce vitamin A deficiency in African countries. Along with β-carotene, the OFSP can provide adequate calories, vitamin C, minerals, and vitamins for people in developing countries. Therefore OFSP is a staple food that can supply energy and nutrition as well as vitamin A.

Retention and bioaccessibility/bioavailability of carotenoids in sweet potato during processing Retention and bioaccessibility/bioavailability of carotenoids in roots during processing Boy and Miloff (2009) summarized the data on carotenoid retention during processing of the OFSP. Seven different processing techniques were explored for the retention of carotenoids (boiling, steaming, frying, roasting, microwaving, baking, and drying). Average retentions were 84% (range: 50%130%) for boiling, 77% (range: 48%95%) for steaming, 79% (range: 67%95%) for frying, 74% (range: 40%110%) for roasting, 69% for baking, 67% (range: 34%92%) for microwaving, 59% (range: 54%92%) for sun drying, and 87% (range: 79%96%) for oven drying. The average of the isomerization from trans-β-carotene to cis isomer was 11% for boiling, 6.4% for steaming, 7% for frying, 12% for roasting, 30% for baking, 17% for microwaving, and 6% for sun drying, while it was 1.5% in fresh roots. All-trans-β-carotene contributes twice to the amount of retinol activity equivalents compared to the cis isomers. DonadoPestana et al. (2012) also reported that the heat processing decreased carotenoids in OFSP. Namely, flour presented the greatest losses of major carotenoids, likely because of the longer exposure to heat and to air circulation, which promote the degradation and oxidation of carotenoids. Boiling and steaming of roots seemed to result in better retention of alltrans-β-carotene than roasting and flour. Similar results were obtained by Islam et al. (2016). Total carotenoid content was higher in all raw OFSPs compared with corresponding boiled samples, and the percentage of cisβ-carotene was higher in the boiled sample than in the raw sample. In fried sweet potato 13-cis-β-carotene comprised the highest proportion of the cis isomer of β-carotene, whereas 15-cis- and 9-cis-β-carotene were formed in minor amounts (Kidmose et al., 2006). The retention and isomerization depended on the time, depth of immersion in water, shape of sample, power of apparatus, etc. The cis isomers of β-carotene possess low provitamin A activity when compared with the trans configurations,

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however, research on the cis configurations attributes them with chemopreventive activities during the initial phases of hepatocarcinogenesis, the inhibition of cellular proliferation, and antimutagenic properties (DonadoPestana et al., 2012). Although the reduction in β-carotene content was recognized during processing, heat processing improves β-carotene bioaccessibility from OFSPs. Tumuhimbise et al. (2009) demonstrated that the bioaccessibility of the all-trans-β-carotene of the heated OFSP was significantly higher than raw samples. The reason for this is probably due to the flow of β-carotene out of the cell when the wall is ruptured during heat processing (Bengtsson et al., 2010). The order in the bioaccessibility of β-carotene was raw , baking , steaming/boiling , deep frying. The low β-carotene accessibility of baked sweet potato could be due to the hardening of the surface, which limited the extent of matrix disruption at the center of the sweet potatoes. The micrograph of baked OFSP showed thick cell walls compared to those of boiled, steamed, and deepfried samples. The reason for the highest bioaccessibility in deep frying might be attributed to the presence of fat in the diet since fat in the diet is known to improve the bioaccessibility of β-carotene. Microwave or steam pretreatment of raw sweet potato flour also showed the higher bioaccessibility of β-carotene (Trancoso-Reyes et al., 2016). The bioavailability of β-carotene was 37% when men were fed with sweet potato balls (Huang et al., 2000). The bioavailability of β-carotene in vitro was different between the foods, for example, mango was 10.1%, papaya was 5.3%, tomato was 3.1%, and carrot was 0.5%, probably due to the physical form of β-carotene within the food, that is, whether it was in liquid or crystalline form. Thermal processing also improved the bioavailability of β-carotene in other vegetables (Donhowe and Kong, 2014). In vivo β-carotene plasma concentrations increased over threefold when raw carrots and spinach were heated at 121°C for 40 min after canning and sterilization (Rock et al., 1998). Commercial carrot purees, which went through retort processing in addition to cooking, were shown by Edwards et al. (2002) to have double the plasma β-carotene concentration compared to carrots merely boiled for 40 min and mashed. A reduction in particle size, which increases the surface area exposed to the oil phase and increases the partitioning of β-carotene into the oil phase, has an effect on the solubilization of β-carotene (Riche et al., 2003). Dietary fiber has been shown to inhibit the bioavailability of β-carotene (Ridedl et al., 1999).

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Spray-drying is a representative method for improving the bioavailability of β-carotene. Spray drying is a popular method because of cheap, fast, and high reproducibility (de Vos et al., 2010). The wall material selection (algae, carbohydrate, etc.), temperature and flow rate during spray drying are important for encapsulation and bioavailability. Microencapsulation by gelation methods (alginate, chitosan, etc.) could be another representative method for improving the bioavailability of β-carotene. These processes to enhance bioavailability could be effective methods for increasing the intake of β-carotene from sweet potato roots.

Retention of carotenoids in leaves during cooking Chen and Chen (1993) showed the effect of microwave heating on the carotenoids and chlorophyll contents in sweet potato leaves. The lutein content decreased along with the increase of heating time and the retention rate was 44% at 700 W for 8 min. Two lutein dehydration products, 3,4-dehydroβ,ε-caroten-30 -ol and 30 ,40 -didehydro-β,β-caroten-3-ol, were formed instead. Sugawara et al. (2011) demonstrated the effects of different cooking methods on the physiologically functional components in Suioh leaves. The leaf blades and petioles were cooked by steaming, simmering, boiling, and stir-frying. The retention rate in the lutein content in leaves was 91.7% in steaming, 79.7% in simmering, 75.9% in boiling, and 118.9% in stir-frying. The retention in petioles was 62.5% in steaming, 75.0% in simmering, 75.0% in boiling, and 100.0% in stir-frying. The lutein content of the cooked leaf blades was highest when stir-fried. These results can be used to design recipes with high contents of physiologically functional components in cooked foods.

Research and development trends of sweet potato carotenoids Research on carotenoids in sweet potato is significantly less than for other bioactive components, such as anthocyanins and caffeoylquinic acids, although orange- or yellow-fleshed cultivars are grown all over the world. At present the research into the health benefits of orange- or yellowfleshed sweet potato is limited, and therefore human intervention studies on life-related diseases such as arteriosclerosis and diabetes are needed. The physiological functions other than the antioxidant activity of individual carotenoids in yellow- or OFSP should also be researched. The preventive effect of sweet potato tops on eye disease should be also

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confirmed in human intervention or epidemiological studies. Additionally, the metabolism of carotenoids in yellow- or orange-fleshed sweet potato roots is still not clear, and research into metabolism is needed for reliable theoretical support for the utilization of carotenoid activities. Purified lutein from marigold petals has been used in dietary supplements for eye health. The sweet potato tops can be harvested repeatedly, and the yield would be higher than marigold petal. The development of the harvesting and processing methods for sweet potato tops is needed for lutein isolation. The development of new cultivars with higher carotenoids has been going on in Japan and other countries. Deep yellow- or orange-fleshed cultivars are decorative foods and could contribute to human health. Effective processing methods, such as thermal treatments and spray-drying, are needed to combat vitamin A deficiency and to obtain the health benefits, because carotenoids have limited bioavailability. Taking advantage of the color or bioactivity of carotenoids and the further utilization of yellow- or orange-fleshed sweet potatoes can be expected in the future.

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Kurabachew, H., 2015. The role of orange fleshed sweet potato (Ipomoea batatas) for combating vitamin A deficiency in Ethiopia: A review. Int. J. Food Sci. Nutr. Eng. 5 (3), 141146. Kurata, R., Kobayashi, T., Ishii, T., Niimi, H., Niisaka, S., Kubo, M., et al., 2017. Influence of sweet potato (Ipomoea batatas L.) leaf consumption of rat lipid metabolism. Food Sci. Technol. Res. 23, 5762. Ledford, H.K., Baroli, I., Shin, J.W., Fischer, B.B., Eggen, R.I.L., Niyogi, K.K., 2004. Comparative profiling of lipid-soluble antioxidants and transcripts reveals two phases of photo-oxidative stress in a xanthophyll-deficient mutant of Chlamydomonas reinhardtii. Mol. Genet. Genomics 272 (4), 470479. Li, M., Jang, G.Y., Lee, S.H., Kim, M.Y., Hwang, S.G., Sin, H.M., et al., 2017. Comparison of functional components in various sweet potato leaves and stalks. Food Sci. Biotechnol. 26, 97103. Liu, S.-C., Lin, J.-T., Yang, D.-J., 2009. Determination of cis- and trans- α- and β-carotenoids in Taiwanese sweet potatoes (Ipomoea batatas (L.) Lam.) harvested at various times. Food Chem. 116, 605610. Mangels, A.R., Holden, J.M., Breecher, G.R., Forman, M.R., Lanza, E., 1993. Carotenoid content of fruits and vegetables: An evaluation of analytic data. J. Am. Diet. Assoc. 93, 284296. Maoka, T., Akimoto, N., Ishiguro, K., Yoshinaga, M., Yoshimoto, M., 2007. Carotenoids with a 5,6-dihydro-5,6-β-end group, from yellow sweet potato “Benimasari“, Ipomoea batatas Lam. Phytochemistry 68, 17401745. Mayne, S.T., 1996. Beta-carotene, carotenoids, and disease prevention in humans. FASEB J. 10 (7), 690701. Menelaou, E., Kachatryan, A., Losso, J.N., Cavalier, M., La Bonte, D., 2006. Lutein contetn in sweetpotato leaves. Hort Science 41, 12691271. Mitra, S., 2012. Nutritional status of orange-fleshed sweet potatoes in alleviating vitamin A malnutrition through a food-based approach. Nutrition & Food 2 (8), 1000160. Ohara-Takada, A., Kuranouchi, T., Nakamura, Y., Katayama, K., Nakatani, M., Tamiya, S., et al., 2011. “Himeayaka,” a new sweet potato cultivar with good taste and compact size storage root. Bull. Natl. Inst. Crop Sci. 12, 103122. Ohara-Takada, A., Kumagai, T., Kuranouchi, T., Nakamura, Y., Fujita, T., Nakatani, M., et al., 2016. “Aikomachi,” a new sweet potato cultivar with good appearance and high confectionery quality. Bull. Natl. Inst. Crop Sci. 16, 3556. Oki, T., Nagai, S., Yoshinaga, M., Nishiba, Y., Suda, I., 2006. Contribution of beta-carotene to radical scavenging capacity varies among orange-fleshed sweet potato cultivars. Food Sci. Technol. Res. 12 (2), 156160. Paiva, S.A.R., Russell, R.M., 1999. Beta-carotene and other carotenoids as antioxidants. J. Am. Coll. Nutr. 18 (5), 426433. Rock, C.L., Lovalvo, J.L., Emenhiser, C., Ruffin, M.T., Flatt, S.W., Schwartz, S.J., 1998. Bioavailability of β-carotene is lower in raw than in processed carrots and spinach in women. J. Nutr. 128 (5), 913916. Riche, G.T., Bailey, A.L., Faulks, R.M., Parker, M.L., Wickham, M.S., Fillery-Travis, A., 2003. Solubilization of carotenoids from carrot juice and spinach in lipid phases: I. Modeling the gastric lumen. Lipids 38 (9), 933945. Ridedl, J., Linscisen, J., Holfmann, J., Wolfram, G., 1999. Some dietary fibers reduce the absorption of carotenoids in women. J. Nutr. 129 (12), 21702176. Schiedt, K., Liaaen-Jense, S., 1995. Isolation and analysis. In: Britton, G., Liaaen-Jensen, S., Pfander, H. (Eds.), Carotenoids, 1B. Birkhäuser, Verlag, Basel, pp. 81108. Sugawara, T., Negishi, Y., Yumi, K., Ishiguro, K., Oki, T., Suda, I., 2011. Effect of cooking method on the lutein and polyphenol contents in edible leaves of the Suioh sweet potato cultivar. J. Cookery Sci. Jpn. 44, 291298.

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Sugiura, M., 2015. Beta-cryptoxanthin and the risk for lifestyle-related disease: findings from recent nutritional epidemiologic studies. Yakugaku Zasshi—J. Pharm. Soc. Jpn. 135 (1), 6776. Takahata, Y., Noda, T., Nagata, T., 1993. HPLC determination of β-carotene content of sweet potato cultivars and its relationship with color values. Jpn. J. Breed. 43, 421427. Takahata, Y., 2014. Sweetpotato in Japan: Past and future. In: Proceedings of NARO International Symposium (6th Japan-China-Korea Sweetpotato Workshop), November 2830, Kagoshima, Japan, pp. 67. Teow, C.C., Truong, V.-D., McFeeters, R.F., Thompson, R.L., Pecota, K.V., Yencho, G.C., 2007. Antioxidant activities, phenolic and b-carotene contents of sweet potato genotypes with varying flesh colours. Food Chem. 103, 829838. Tomlins, K., Owori, C., Bechoff, A., Menya, G., Westby, A., 2012. Relationship among the carotenoid content, dry matter content and sensory attributes of sweet potato. Food Chem. 131, 1421. Trancoso-Reyes, N., Ochoa-Martinez, L.A., Bello-Pérez, L.A., Morales-Castro, J., Estévez-Santiago, R., Olmedilla-Alonso, B., 2016. Effect of pre-treatment on physicohemical and structural properties, and the bioaccessibility of β-carotene in sweet potato flour. Food Chem. 200, 199205. Tumuhimbise, G.A., Namutebi, A., Muyonga, J.H., 2009. Microstructure and in vitro beta carotene bioaccessibility of heat processed orange fleshed sweet potato. Plant Foods Hum. Nutr. 64, 312318. van den Berg, H., Faulks, R., Granado, H.F., Hirschberg, J., Olmedilla, B., Sandmann, G., et al., 2000. The potential for the improvement of carotenoids levels in foods and the likely systemic effects. J. Sci. Food Agric. 80, 880912. van Jaarsveld, P.J., Faber, M., Tanumihardjo, S.A., Nestel, P., Lombard, C.J., Benade, A.J. S., 2005. Beta-carotene-rich orange-fleshed sweet potato improves the vitamin A status of primary school children assessed with the modified-relative-dose-response test. Am. J. Clin. Nutr. 81 (5), 10801087. Yamakawa, O., Kumagai, T., Yoshinaga, M., Ishiguro, K., Hidaka, M., Komaki, K., et al., 1999. “Sunny-Red”: a new sweetpotato (Ipomoea batatas) cultivar for powder. Bull. Natl. Agric. Res. Center Kyushu Okinawa Reg. 35, 1940. Yamakawa, O., Yoshinaga, M., Kumagai, T., Hidaka, M., Komaki, M., Kukimura, H., et al., 1997. “J-Red”: a new sweetpotato cultivar. Bull. Natl. Agric. Res. Center Kyushu Okinawa Reg. 33, 4972. Yoshimoto, M., Okuno, S., Islam, M.S., Kurata, R.A., Yamakawa, O., 2003. Polyphenolic content and antimutagenicity of sweetpotato leaves in relation to commercial vegetables. Acta Hortic. 628, 677685. Yoshinaga, M., Kai, Y., Katayama, K., Sakai, T., 2006. New varieties for dried sweetpotato products Hamakomachi and Kyushu No. 137. Sweetpotato Res. Front 20, 3.

CHAPTER 9

Sweet potato microstructure, starch digestion, and glycemic index Sunantha Ketnawa1, Lovedeep Kaur2, Yukiharu Ogawa1 and Jaspreet Singh2 1

Graduate School of Horticulture, Chiba University, Matsudo, Japan School of Food and Advanced Technology and Riddet Institute, Massey University, Palmerston North, New Zealand

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Introduction The sweet potato is attracting a lot of attention globally, which can be attributed to its distinctive nutritional and functional properties. Its starch composition varies between 50% and 80% on a dry weight basis, depending on the type of cultivar (Abegunde et al., 2013; Lee and Lee, 2016; Zhu and Wang, 2014). Its complex composition also includes bioactive carbohydrates, proteins, lipids, carotenoids, anthocyanins, conjugated phenolic acids, and minerals, which have been reported to contribute to the various health benefits associated with sweet potato consumption, including antidiabetic and antiobesity effects (Wang et al., 2016; Zhu and Wang, 2014). Regardless of the variety, the sweet potato has been reported to be beneficial to people with type 2 diabetes, owing to its high fiber and manganese contents. Manganese and fiber are associated with blood sugar stabilization and a reduction of insulin resistance (Bahado-Singh et al., 2011). As the sweet potato root is a starchy vegetable, it is essential to understand the physicochemical and functional characteristics of its starch as they may influence its digestive properties (Ek et al., 2014). In the past few years, many studies have been carried out to gain an understanding of the properties of sweet potato starch, including the granule morphology (Lee and Lee, 2016) and size (Tian et al., 1991; Zhang et al., 2018); amylose content and granule crystalline structure (Chen et al., 2006a,b; Zhang et al., 2018); starch hydrolysis and digestion properties (Chen and Sopade, 2012; Huang et al., 2015, 2016; Liu et al., 2006; Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00009-0

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Mennah-Govela and Bornhorst, 2016a; Odenigbo et al., 2012; Remya et al., 2018; Senanayake et al., 2014a; Sun et al., 2012; Trung et al., 2017; Vieira Fabiana and Sarmento Silene, 2008); and glycemic index (GI) (Allen et al., 2012; Bahado-Singh et al., 2011; Chen et al., 2012, 2013). These studies have indicated that the kinetics of sweet potato starch digestion and GI are affected by the type of cultivar and the processing methods, and may also be improved by various physical and chemical starch modifications (Li et al., 2014; Lv et al., 2018). This chapter reviews these studies and also discusses the influence of sweet potato microstructure on starch digestion and glycemic properties.

Sweet potato microstructure The cultivars (white-, orange-, cream-, and purple-fleshed), have been observed to be obovate, oblong, elliptic, curved, irregular in shape, and usually thin-cortexed (1 2 mm) (Waramboi et al., 2011). Sweet potatoes have starch as the main component (58% 76% on a dry basis), which reportedly has similar physical properties to maize and potato starches (Chen et al., 2006a,b; Zhao et al., 2012; Zhu and Wang, 2014; Zhu and Xie, 2018). To evaluate the starch digestibility of sweet potato, not only the characteristics of the starch and/or starch granule but also the structural cell and tissue attributes need to be considered. Ultrastructural characteristics of cellular components of vegetables including sweet potato have been reported to be key elements in determining the nutrient bioaccessibility. The edible parenchyma cells of sweet potato are moderate to small in size, and starch granules are a prominent feature (Jeffery et al., 2012). Such cell- and/or tissue-scale microstructure has been reported to be influenced by thermal processing (Bengtsson et al., 2010) and cooking procedures (Valetudie et al., 2000), ultimately affecting the nutrient bioaccessibility (Tumuhimbise et al., 2009). Mennah-Govela and Bornhorst (2016a) reported that hardness, acid, and moisture uptake during simulated gastric digestion of sweet potatoes were influenced by cooking method and digestion time. This showed that the relationship between food processing and digestive behavior is important in determining the optimal food processing methods for specific food functional properties. Commercially grown sweet potatoes are either “moist” or “yam” types, with a soft, syrupy texture, or “dry” types, with a firm, mealy texture (Walter et al., 2000). These two types have been reported to differ in

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their alcohol insoluble solid content, total starch amount, and starch structure (Kim et al., 2013). Sweet potato starch content differs greatly among genotypes, but is quite stable among different planting years or growing environments within a genotype (Lu et al., 2015). However, the granule size of the starches has been reported to be influenced by the type of cultivar, growing conditions, and plant physiology (Aina Adebisola et al., 2009); Waramboi et al. (2011). Granular morphology and crystallinity have been shown to differ slightly across the cultivars (Zhang et al., 2018). The starch granules reportedly have smooth surfaces and shapes vary from polygonal, oval, or semioval or round to bell shaped, with diameters ranging from around 2 to 45 μm for the different cultivars (Fig. 9.1; Zhang et al., 2018). Although the volume distributions of the different purple sweet potato starches varied significantly, they all showed bimodal

Figure 9.1 The photos of purple sweet potato root tubers, the morphologies of starch granules under normal light microscope (NLM), polarized light microscope (PLM) and scanning electron microscope (SEM), and the granule size distribution of starches. Scale bar 1/4 20 mm. Reproduced with permission from Zhang, L., Zhao, L., Bian, X., Guo, K., Zhou, L., Wei, C., 2018. Characterization and comparative study of starches from seven purple sweet potatoes. Food Hydrocoll. 80, 168 176. doi:10.1016/j. foodhyd.2018.02.006.

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size distributions. Lee and Lee (2016) reported that the starch granules from purple sweet potato are smaller in size compared with those from white and orange sweet potato. Starch granule size plays an important role in influencing the starch pasting characteristics. Small starch granules possess fat-like mouthfeel, making them a good candidate to be used as a fat substitute in low-fat versions of high-fat foods (Abegunde et al., 2013). X-ray diffraction studies have showed sweet potato starch granules to be composed of alternating, 100- to 400-nm-thick, amorphous and semicrystalline shells as “growth rings.” Crystalline and amorphous lamellae found within the semicrystalline shells, have a periodicity of about 9 10 nm (Zhu and Wang, 2014). The wide-angle X-ray diffraction pattern for starch granules has been reported to be either A or C. C is a mixture of both A and B types (Zhu and Wang, 2014). Lee and Lee (2016) compared structural properties of starch from orange- and purplefleshed sweet potatoes to those of typical white-fleshed sweet potatoes and reported that the granules were a Ca-type (type C near A-type) and had no significant differences in their amylose content. The starch from the orange- and purple-fleshed sweet potatoes had higher swelling power when heated at temperatures above 70°C, showing a lesser degree of intermolecular associative forces in the former. This was in agreement with the observed higher peak gelatinization temperature and enthalpy for starch from white sweet potatoes that indicated their more ordered and thermally stable granular structure (Lee and Lee, 2016). Sweet potato starch is comprised of 20% 30% linear and slightly branched amylose and 70% 80% highly branched amylopectin (Chen et al., 2005). Abegunde et al. (2013) reported that amylose and amylopectin contents vary among the sweet potato starches from different cultivars, with their values ranging from 13.33% to 26.83% and 73.17% to 86.67%, respectively. Kim et al. (2013) studied the structural properties of starch isolated from different Korean varieties: purple-fleshed Shinjami and Borami; orange-fleshed Juwhangmi and Shinwhangmi; and white/creamfleshed Shinyulmi, Shinchunmi, Yeowhangmi, and Jeungmi. Their amylose contents ranged from 14.66% (Juwhangmi) to 30.51% (Shinchunmi). Differences in amylose content of sweet potato starches have been attributed to differences in genotype, growing environment, and starch extraction method (Abegunde et al., 2013). Low- and high-amylose starches provide distinctly different physicochemical and functional properties during their various food applications (Schirmer et al., 2013). Amylose content has also been correlated to mean starch granule size (Chen et al., 2005).

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Starch digestibility and glycemic index of sweet potato Sweet potato is considered a superfood and therefore there is an increasing interest in understanding its health benefits and its utilization for the development of novel functional foods. Being a starch-rich vegetable, part of this interest will be influenced by how its starch is digested in comparison to starches from other starchy vegetables such as potato and what are the factors influencing its starch digestibility. Sweet potato starch has attracted much attention for novel food development as a new functional food component in recent years. Starch can be grouped into three major fractions according to the rate and extent of starch digestion: rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) (Englyst et al., 1992). RDS is rapidly digested and leads to a quick elevation of blood glucose compared to SDS, which is digested slowly through the small intestine to provide sustained glucose release (Lehmann and Robin, 2007). The benefit of consuming SDS-rich food is its moderate impact on the GI. RS is a starch fraction that cannot be digested in the small intestine (Englyst et al., 1992; Jo et al., 2016). A low-GI diet has been shown to be linked to a reduced risk of diabetes and cardiovascular disease (Chen et al., 2012, 2013; Jenkins et al., 2002). Starch-rich foods are usually considered high GI and have been associated with the development of type 2 diabetes (Chen et al., 2012). Interestingly, sweet potato starch has been reported to belong in the lowGI category due to its low GI values—55.07 compared to 85.46 for the high-GI potato starch (Chen et al., 2012). It has been reported that the feeding of low-GI sweet potato starch for 4 weeks could improve the postprandial glycemic response in hyperglycemic rats compared to the feeding of high-GI potato starch ( Chen et al., 2012, 2013). These results also indicated that sweet potato starch diet can improve insulin sensitivity in insulin-resistant rats, possibly by improving the adipocytokine levels, proinflammatory cytokines, and insulin signaling. The molecular structure of starch is thought to be the key determinant for its functionality and nutritional properties (Zhang and Hamaker, 2009). Thus many attempts have been made to improve the functionality and to increase the SDS and RS contents of sweet potato starches through the use of enzymatic (Jo et al., 2016), chemical (cross-linking and substitution) (Das et al., 2010; Remya et al., 2018; Zhao et al., 2012), and physical modifications, including processing (heating/postprocessing storage),

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heat moisture treatment (HMT), annealing (ANN) (Chen et al., 2017; Hung et al., 2014, 2015, 2016; Lv et al., 2018; Shin et al., 2005; Trung et al., 2017), and starch granular size reduction (Chen and Sopade, 2012; Chen et al., 2004, 2005; Liu and Sopade, 2011).

Effects of cultivar In order to study the influence of the type of cultivar on the digestive properties of sweet potato starch, Zhang et al. (2018) isolated starches from a purple sweet potato variety (Ningzi) and six advanced breeding lines with different genotypes (X66-1, X83-1, X85-15, Y79-4, Y16-5, and Y46-4) and employed porcine pancreatic α-amylase (PPA) and Aspergillus niger amyloglucosidase (AAG) to simulate small intestinal digestion of native, gelatinized, and retrograded starches from those sweet potatoes. The result showed that the seven different genotypes’ starches exhibited different hydrolysis degrees when subjected to hydrolysis by PPA and AAG. The native starches from all the genotypes showed higher resistance to the in vitro digestion process (RS 84.9% 88.0%) followed by retrograded (RS 14.3% 22.4%) and freshly gelatinized (RS 10.2% 14.7%) starches. During gelatinization, inter- and intramolecular hydrogen bonds between starch chains are disrupted, leading to improved accessibility and starch degradation by the digestive enzymes. The amylose chains associate to form the double helical structure and the amylopectin recrystallizes to form the crystallites during the retrogradation process, which enhances the resistance toward digestive enzymes (Chung et al., 2006; Zhang et al., 2018). This could be the reason for the observed lower RDS and higher RS for the native raw starches compared to their gelatinized and retrograded counterparts. Also gelatinized sweet potato starches had higher RDS and lower RS than retrograded ones (Zhang et al., 2018). Senanayake et al. (2013) studied the nutritional attributes of uncooked flours obtained from five sweet potato cultivars that were commonly available in Sri Lanka (SWP1, Wariyapola red; SWP3, Wariyapola white; SWP4, Pallepola variety; SWP5, Malaysian variety; SWP7, CARI 273). The RS for flours from these cultivars (RS) ranged from 14.2% to 17.2% while the pancreatic starch digestibility was in the range of 36% 55%. Starch from Wariyapola red (SWP1) cultivar had higher amounts of RS which also led to its lower pancreatic digestibility in vitro. Cultivars with higher amounts of total starch and lower amounts of RS showed higher

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Figure 9.2 Enzyme digestibility of starches from different sweet potato cultivars. Error bars represent standard deviations. Columns with the same letters are not significantly different at P , .05. Reproduced with permission from Abegunde, O.K., Mu, T.-H., Chen, J.-W., Deng, F.-M., 2013. Physicochemical characterization of sweet potato starches popularly used in Chinese starch industry. Food Hydrocoll. 33(2), 169 177. doi:10.1016/j.foodhyd.2013.03.005.

starch digestibility values. Moreover, cultivars containing larger granules were associated with lower digestibility values while the cultivars with higher amylose content showed higher digestibility (Senanayake et al., 2013). The variability in the digestibility among sweet potato starches from various cultivars has also been related to environmental conditions associated with the crop growth location, such as temperature, precipitation, and soil (Abegunde et al., 2013). Abegunde et al. (2013) studied the enzyme digestibility of raw sweet potato starches from 11 cultivars using pancreatin hydrolysis. The digestibility of the raw sweet potato starches varied from 10.35% to 15.15% among the studied cultivars (Fig. 9.2).

Effects of starch modification Starch digestion has been reported to be controlled by multiple factors such as amylose content (Das et al., 2010; Senanayake et al., 2013; Zhu et al., 2011), granule shape and size (Chen and Sopade, 2012), structural

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characteristics, crystallinity (Waramboi et al., 2011), presence of nonstarch components (Liu et al., 2010), and interactions of starch with nonstarch components, that is, chemical modification (Das et al., 2010; Liu et al., 2010; Liu and Sopade, 2011; Lv et al., 2018; Remya et al., 2018). Changes in the composition and structure of starch upon chemical modification reportedly lead to resistance to the action of digestive enzymes (Remya et al., 2018). Enzymatic modification In order to improve the nutritional attributes of sweet potato starch, enzymatic modification is applied, which is considered safer compared to chemical modification (Jo et al., 2016). Also it has fewer by-products and more specific reactions. As an example, 1,4-α-glucan branching enzyme (BE; EC 2.4.1.18) catalyzes transglycosylation to form α-1,6 branching points in both amylopectin and amylose, and leads to the production of highly branched glucans while amylosucrase (AS; EC 2.4.1.4) catalyzes transglycosylation to form an insoluble α-1,4 glucan (Jo et al., 2016) have reported an enhancement of SDS content of sweet potato Daeyumi (a newly developed Korean cultivar) starch by changing its branch density and chain length using dual enzymatic modification (using BE from Streptococcus mutans and AS from Neisseria polysaccharea). The modification with BE led to an increase in short side chains (degree of polymerization (DP) # 12), whereas AS treatment decreased the proportion of short side chains and increased the proportion of long side chains (DP $ 25) and molecular mass of the native starches. The combined enzymatic treatment led to the formation of new glucan polymers that showed enhanced retrogradation and had higher SDS and RS. Chemical modification Chemical modifications including substitution and cross-linking are widely studied and have proven to be a good way of increasing RS in sweet potato starches (Chen et al., 2005; Das et al., 2010; Lv et al., 2018; Remya et al., 2018). Octenyl succinylation (OSA) is one of the chemical modifications to reportedly impair the binding of starch granules with α-amylase, thus decreasing the extent of sweet potato starch digestion in vitro (Remya et al., 2018). The modified sweet potato starch has been reported to show enhanced SDS and RS contents and a decrease in RDS content. Besides, OSA-esterified starch provides hydrophobic domains that improve the starch emulsifying capacity (Heacock et al., 2004).

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The RS content of unmodified sweet potato cooked starch has been reported to be around 3.3%, whereas the OSA-esterified starches showed RS content in the range of 24.8% 37.1%, depending on the degree of substitution (3%, 5%, and 10% substitution) (Remya et al., 2018). For the modified starches, the GI was considerably lower than that of their corresponding native starches. The native sweet potato starch, exhibited high GI of 86.3, whereas all the OSA-modified starches belonged to the medium GI category with their GIs in the range of 66.3 63.9 (Remya et al., 2018). An in vitro study on raw sweet potato starches by Lv et al. (2018) also showed that as OSA concentration increased from 2% to 10%, RDS decreased from 28.29% to 21.48% and RS increased significantly from 69.07% to 76.96%. The changes in starch digestive properties have been attributed to the introduction of OSA groups that acted in the same way as an uncompetitive enzyme inhibitor (for both amyloglucosidase and pancreatic α-amylase) and the release of enzymes from the substrate was delayed due to the physical hindrance or the hydrophobic microenvironment caused by the bulky OSA groups (Lv et al., 2018; Zhang et al., 2017). Acetylation (esterification) is also a method for chemical modification of sweet potato starch that has been associated with the improvement of its digestion properties. Chen et al. (2004) fractionated acetylated sweet potato starches according to their granule size and determined their degradability with different digestive enzymes, including α-amylase, β-amylase, and amyloglucosidase. They reported that the action of all these enzymes was hindered by acetyl groups. Also the acetylated amylose (isolated from starches) originating from small-sized granule fractions showed less digestion than that formed from larger sized granule fractions. This was reported to be probably due to variations in acetyl group distribution among the different granule sizes. The enzyme-resistant residues contained at least one acetyl group, thereby strongly linking the enzyme (α-amylase, β-amylases, and amyloglucosidase) resistance with the acetyl group. Zhao et al. (2012) investigated the distribution of substituents within cross-linked (using POCl3) and hydroxypropylated sweet potato starches and related that to their digestibility behaviors. The native and modified sweet potato starches were gelatinized and hydrolyzed using a combination of pullulanase, α-amylase, and amyloglucosidase. Some resistant oligosaccharides were reported to be present in the hydrolysates of both hydroxypropylated and dual-modified (cross-linked hydroxypropylated)

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starches, while the native and cross-linked starches were found to be completely digested to glucose. The glucose levels in cross-linked starch digests (84%, w/w) were similar to those of native sweet potato starch digests (82%, w/w). On the contrary, the glucose levels in hydroxypropylated and cross-linked hydroxypropylated sweet potato starch digests were only half of those of native starches, at 39% and 40% (w/w), respectively (Zhao et al., 2012). This study reported that cross-linking with phosphorus oxychloride had only a slight effect on the enzymatic hydrolysis, whereas hydroxypropylation evidently limited the enzymatic hydrolysis. In a similar study on the esterification of sweet potato starch using sodium trimetaphosphate, a large drop in digestible starch content was observed, from 63.4 for the unmodified sweet potato starch to 15.8% for the modified starch. The RS content was reported to increase from 14.5% to 58.7% after modification. The predicted GI of sweet potato starch phosphodiester was 66.31, proving that esterification greatly reduced the digestion ability and improved the nutritional value of sweet potato starch (Li et al., 2014). It is important to mention here that the modified starches displayed gelatinization profiles similar to their unmodified counterpart. Physical modification Physical modification is considered to be a safe, nontoxic, and costeffective way of enhancing digestibility and the physicochemical and structural properties of tuber starches. Several researchers have reported that physical modification, such as HMT, could enhance the SDS content of sweet potato starch considerably (Huang et al., 2015, 2016; Senanayake et al., 2014a,b,c; Shin et al., 2005; Trung et al., 2017; Vieira Fabiana and Sarmento Silene, 2008). HMT involves the treatment of starches at low moisture level (,35%, w/w) and is usually carried in the temperature range of 84°C 120°C for an appropriate time (0.25 16 h) (Huang et al., 2016). It can lead to structural changes in starch amorphous and crystalline domains to variable extents, thereby inducing changes in the X-ray patterns and crystallinity, granule swelling and pasting, and gelatinization and retrogradation properties (Zavareze and Dias, 2011). These changes in physicochemical and structural properties incurred during HMT have also been reported to affect the susceptibility of starch to enzymatic hydrolysis, probably due to the rearrangement of disrupted crystallites increasing the areas accessible to α-amylase and glucoamylase attack (Vieira Fabiana and Sarmento Silene, 2008). Huang et al. (2016) modified sweet potato starch by repeated heat moisture treatments (RHMT) and studied the effects of

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RHMT on the structure and in vitro digestibility of starches. They reported that the RHMT changed the starch crystalline pattern from Ca to A type. It also changed the starch digestive properties, with the SDS content reaching a maximum of 19.61% after three cycles. Sweet potato starch was modified at a moisture content of 30% and heating temperature of 100°C for 2 h (one cycle). It was reported that the RS content of the cooked starch significantly decreased after the first treatment cycle but increased to the maximum amount of 20.99% after five treating cycles (Trung et al., 2017). Vieira Fabiana and Sarmento Silene (2008) studied the effects of HMT (27% moisture, 100°C, 16 h) with enzymatic digestion (α-amylase and glucoamylase) on the properties of sweet potato starches. The structural modification with HMT affected enzyme susceptibility. In another study, the contents of RDS, SDS, and RS of native sweet potato starch were 32.01%, 1.89%, and 66.10%, respectively. A combination of HMT (30% moisture, 100°C, 4 h) and OSA increased both RDS and SDS by 2.86% and 1.35%, respectively, but the content of RS decreased. Partial disruption of organized chain structures by HMT could have facilitated the attack by α-amylase, thereby increasing the RDS and SDS contents and decreasing the RS content (Lv et al., 2018). In a study on starches from Sri Lankan sweet potato cultivars, different heat moisture levels (20%, 25%, and 30% moisture, 85°C and 120°C for 6 h) were applied in order to study their effects on starch digestibility with porcine pancreatin (Senanayake et al., 2014a,b,c). The native starches from different cultivars showed no significant difference in their digestibility values. However, the HMT-treated starches showed significant changes in their digestibility values. Overall, HMT at 85°C led to an increase in starch digestibility, and a temperature beyond 85°C decreased digestibility. No significant change in digestibility was observed at 20% or 25% moisture levels at 85°C whereas increased levels were seen at 85°C and 30% moisture. Trung et al. (2017) investigated the change in physicochemical properties and digestibility of starches isolated from sweet potato varieties after HMT or ANN treatment. After modification (HMT or ANN), a decrease in RDS and an increase in SDS and RS contents compared to the native starches was observed. Concentrations of RDS, SDS, and RS of native sweet potato starches were in the ranges of 73.7% 75.3%, 0.7% 1.0%, and 24.0% 25.3%, respectively. The concentrations of SDS, RS, and RS of the heat moisture treated starches were in the ranges of 3.7% 4.9%, 30.6% 39.3%, and 30.6% 39.3%, respectively.

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Hung et al. (2014) investigated the formation of SDS and RS and the changes in physicochemical properties of sweet potato starches using a combination of different acids including citric acid and HMT at 110°C for 8 h. In the native form sweet potato starch contained higher amounts of RDS and SDS, but a lower amount of RS. The SDS content was not affected by HMT, whereas the RS content significantly increased after the treatment. However, both SDS and RS significantly increased from 6.6% and 14.7% in native starch to 8.7% 13.2% and 37.5% 42.1% in acid and HMT-treated starches, respectively. The RS content in the acid and HMT-treated sweet potato starches was also higher than that of the HMT-starches without acid hydrolysis. The citric acid had the highest impact on RS formation and starch physicochemical properties, followed by lactic acid and acetic acid. Therefore a combination of appropriate acid and HMT may be used to enhance the amounts of SDS and RS in sweet potato starches (Hung et al., 2014). Hung et al. (2014) concluded that the formation of RS after HMT is due to the enhancement of either the order or the proportion of the crystalline fraction in starch granules. Moreover, the partial acid hydrolysis of starches before HMT can enhance the molecular mobility and allow more efficient rearrangement, and therefore an improved RS yield over the HMT without acid hydrolysis. Yadav et al. (2009) reported the effect of multiple heating/cooling treatments on the RS content of pressure-cooked (15 psi, 121°C for 15 min; sample to water ratio of 1:2) sweet potato. Samples were cooled at 4°C for 24 h and reheated by heating in a boiling water bath for 10 min before bringing the samples to room temperature. Fresh sweet potato contains enough water to allow complete starch gelatinization during heating, as shown by its low RS content. The rapid expansion of sweet potato starch by gelatinization loosens the cell matrix structure of most potato products, which allows easy access for starch-degrading enzymes. In the thrice-heating/cooling treatments, sweet potato reported a minimum increase of 62.1% in its RS content (Yadav et al., 2009).

Other factors Apart from the abovementioned factors, starch granule size also plays an important role in determining the digestion outcome. Starch granules are known to vary in size (small to large), even within a starch source, and small granules are more easily digested than big granules (Das et al., 2010; Li et al., 2014; Lv et al., 2018; Remya et al., 2018; Zhao et al., 2012).

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Other influencing factors may include milling conditions. Significant effects of mill type and/or milling conditions on the starch digestion have been reported for sweet potato flours (Chen and Sopade, 2012), with greater starch digestion reported when the particle size was reduced through the use of cryo- and hammer-mills. After particle size reduction, the sweet potato flour was found to be more susceptible to in vitro pancreatic digestion than flours obtained from sorghum and cowpea. Thus along with granule size, it is important to understand the optimum particle size distribution of the flours required to design specialized low-GI foods from sweet potato. Also several other factors, including amylose lipid complexes, the amylose to amylopectin ratio, the composition of the food matrix, antidigestibility factors, or conditions such as starch encapsulation, may affect the digestion of sweet potato starch in humans. Samples that are high in amylose swell less, are resistant to digestion, gelatinize at high temperatures, and have low pasting properties (Waramboi et al., 2011). Zhu et al. (2011) showed that apparent amylose contents of 11 genotypes with diverse geographic origins in China ranged from 23.3% to 26.5%. In vitro digestibility by pancreatic enzyme varied from 29.5% to 41.2%. Amylose contents were highly correlated to digestibility. Nonstarch components, that is, mineral in the flours, have been reported to have an influence on starch digestibility (Waramboi et al., 2011). Minerals, bound or unbound, can influence starch digestibility directly or indirectly, for example, by changing the liquid phase flow properties and/or by affecting the mobility of enzymes in the liquid phase (Liu et al., 2010; Liu and Sopade, 2011). It is important to mention here that most of the studies reviewed in this chapter have used different methods and digestive enzymes to analyze sweet potato starch digestion in vitro, and therefore their results are not directly comparable. However, this knowledge would be useful for the processing of sweet potato due to the growing interest in its utilization worldwide in view of its health and nutritional advantages.

Influence of processing on sweet potato starch digestibility Sweet potato is a major staple food in several countries and it is a highly nutritious root vegetable (Bahado-Singh et al., 2011). The type of starch source, cultivar, amylose and amylopectin levels, starch structure, starch granule size and shape, minerals, lipids, and fiber content are the significant factors determining starch digestibility and thus the GI of foods

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(Allen et al., 2012; Odenigbo et al., 2012). Other important factors include the processing method and food particle size, and also other components of the food matrix could alter food textural and rheological properties, thus affecting digestibility as well as the GI (Al Dhaheri et al., 2017; Bahado-Singh et al., 2011). Like other tuber starches, raw sweet potato starch is mostly resistant to digestion in the human gastrointestinal tract. Raw starches are known to be more resistant to enzymatic hydrolysis compared with their gelatinized counterparts (Annison and Topping, 1994). However, very little starch is consumed in its raw form and usually foods are subjected to heat or moisture processing for varying times before consumption. The cooking (heating) process causes structural changes in the starch and causes hydrogen bonding sites involved in intermolecular starch bonding to engage more water, releasing individual molecules (Vosloo, 2005). Incomplete cooking processes followed by cooling (e.g., during postcooking refrigerated storage) result in starch becoming resistant to digestion, and thereby lowering its glycemic response (Asp, 1995; Yadav et al., 2009). As processing can modify the structure and nature of the starches and has significant effects on their postprandial blood glucose responses (Allen et al., 2012; Englyst et al., 1992), understanding the effect of different cooking methods on glucose availability during digestion will not only help in providing guidance to consumers for choosing the right processing method but also will help in developing sweet potato-based low-GI foods. Heat processing has been reported to soften plant tissues through cell wall swelling and cell separation. The expansion of the cells results from a combined effect of starch swelling pressure and middle lamella degradation (Tumuhimbise et al., 2009). The various physical modifications that starch molecules undergo during processing that may lead to the formation of RS depend on the starch type, the processing parameters, and the other components of the food matrix (Singh et al., 2010). Therefore the change in starch structure during heating and cooling has a strong impact on how it is digested in the gastrointestinal tract. Heating breaks down starch granules to allow amylopectin and amylose to be more readily digested by pancreatic amylase, which theoretically should increase the sweet potato GI (Allen et al., 2012). The retrograded or recrystallized starch is formed in high moisture cooked, baked, canned, or autoclaved foods (Sagum and Arcot, 2000; Tharanathan and Tharanathan, 2001). Retrograded amylose has been observed to be indigestible due to the presence of stronger hydrogen bonding in comparison with retrograded amylopectin (Englyst et al., 1992). Studies have shown that high

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temperatures during processing similar to those used in canning enhance the effect of heating and cooling (Yadav et al., 2009). Precooking or allowing the food to cool and then reheating before consumption has also been reported to lower its glycemic response compared with consumption immediately after cooking (Fernandes et al., 2005; Kinnear et al., 2011; Bahado-Singh et al., 2011). The human digestive system consists of the mouth, stomach, and intestine. In the mouth, ingested food is broken down and mixed with saliva during mastication, and a bolus is formed which is transported through the esophagus to the stomach. In the stomach, mechanical and chemical breakdown occurs due to both stomach contractions and gastric secretions, that is, gastric digestion (Bornhorst and Singh, 2014). Gastric secretions contain enzymes (i.e., pepsin and lipase), electrolytes (i.e., sodium chloride), and are acidic (pH  2). These gastric conditions affect the structure and property of food matrices. The rate of diffusion of gastric fluids into food matrices in the gastric environment may have implications for the overall gastric breakdown as well as the absorption of nutrients in the small intestine (Bornhorst and Singh, 2014). Aside from the digestion fluid composition, there are other factors that may influence the gastric acid diffusion rate, which include food composition, food properties, and processing of food (Mennah-Govela and Bornhorst, 2016a). Mennah-Govela and Bornhorst (2016a) determined the macro- and microstructural changes, moisture uptake, and acid uptake into sweet potatoes during simulated gastric digestion induced by different cooking methods such as boiling, steaming, microwave steaming, or frying. The microstructural changes of cooked and digested samples were observed using a light microscope. The results showed that acid and moisture uptakes were significantly influenced by cooking method and digestion time. Hardness was also significantly influenced by cooking method, digestion time, and their interaction. Microstructural changes were observed both as a result of cooking and after in vitro gastric digestion. In contrast, moisture uptake was not statistically significant among boiled and steamed but was greater than in microwave-steamed and fried. These trends indicated that the quantity of water absorption or loss during cooking may have an impact on the volume change of the sweet potato cubes during cooking and the moisture uptake during digestion. Microwave-steamed cubes showed the greatest volume decrease (i.e., the smallest volume) whist boiled ones showed the smallest decrease after cooking (i.e., the largest volume). Light microscopy images of variations

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Figure 9.3 Light microscopy images of sweet potato samples stained with Toluidine Blue for steamed (A and B) and fried (C and D) before (A and C) and after 240 min of simulated gastric digestion (B and D). The images are shown with a magnification of 3 10 (1) and 3 20 (2). B, breakage of cell walls; G, gaps between cells. Reproduced with permission from Mennah-Govela, Y.A., Bornhorst, G.M., 2016b. Mass transport processes in orange-fleshed sweet potatoes leading to structural changes during in vitro gastric digestion. J. Food Eng. 191, 48 57. doi: 10.1016/j.jfoodeng.2016.07.004.

in sweet potato tissue cooked by steaming and frying, and digested for 240 min using a simulated gastric digestion method are shown in Fig. 9.3. These tissues were stained by toluidine blue to enhance the cell matrix structure (Mennah-Govela and Bornhorst, 2016b). The microstructures of similarly treated tissues were stained by periodic acid-Schiff to detect starch and some complex polysaccharide. In these images from Mennah-Govela and Bornhorst (2016b), the cells are complete and there is no cell wall breakage before digestion (Figs. 9.3A and C and 9.4A and C), however cell wall breakage can be observed after 240 min of digestion (Figs. 9.3B and D and 9.4B and D). Fried sweet potato tissues did not show cell wall breakdown, however, gaps between cells can be observed in the digested tissue sample (Fig. 9.3C and D, marked with arrows). The fried sample showed a largest decrease in hardness (59%) during simulated digestion, and the decrease was considered to be due to the rapid softening of the crust formed during frying. Particularly, the hardness decreased drastically from 11.2 6 0.9 to 5.2 6 0.3 N after 15 min of digestion. Similar rapid decreases in hardness were not observed in the sweet potatoes from the other cooking methods. The hardness in all cooking methods was not significantly different (average 3.8 6 0.2 N). In contrast, the hardness at different digestion time was different across cooking treatments. These trends may be related to the microstructural changes (Mennah-Govela and Bornhorst, 2016b).

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Figure 9.4 Light microscopy images of sweet potatoes stained with periodic acidSchiff for steamed (A and B) and fried (C and D) sweet potatoes before (A and C) and after 240 min of digestion (B and D). The images are shown with a magnification of 3 10 (1) and 3 20 (2). SD, starch degradation; G, gaps between cells; B, breakage of cell walls. Reproduced with permission from Mennah-Govela, Y.A., Bornhorst, G.M., 2016b. Mass transport processes in orange-fleshed sweet potatoes leading to structural changes during in vitro gastric digestion. J. Food Eng. 191, 48 57. doi: 10.1016/j. jfoodeng.2016.07.004.

Changes in the macrostructure of sweet potato cubes (approx. 0.012 3 0.012 3 0.012 m3) during simulated digestion are shown in Fig. 9.5. The microstructural changes after 240 min of simulated digestion were observed at the center of the cube, however, the macrostructure remained its shape, or at least it was not completely disintegrated. This was linked to the intracellular space of the fried sample microstructure after digestion. This was linked to the fried samples showing the lowest change in texture and the fastest effective diffusivity of acid among the four cooking methods. The increase in effective diffusivity in the fried samples was assumed to be by the formation of intercellular spaces, as shown in Fig. 9.4D1, that allowed acid to flow around the cell walls more easily. But the fried sample microstructure did not show any rupture of cell walls which might be due to the absorption of oil during frying. The oil used for the frying was taken up by the surroundings of the cell wall in the cube and may inhibit their breakdown by gastric acid. In addition, the cells in fried samples were more spread out after cooking in the steamed sweet potatoes (Figs. 9.4A1 and C1). Mennah-Govela and Bornhorst (2016b) also described that gastric acid would enter into the food matrix but not go inside the cells in the fried samples because the effective diffusivity of the acid in the fried samples was greater than that of the steamed samples. These results were consistent with the decrease of hardness in fried sweet potatoes, which had the least

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Figure 9.5 Examples of sweet potato cubes after boiled and fried, and after 60 and 240 min of simulated gastric digestion. Reproduced with permission from MennahGovela, Y.A., Bornhorst, G.M., 2016b. Mass transport processes in orange-fleshed sweet potatoes leading to structural changes during in vitro gastric digestion. J. Food Eng. 191, 48 57. doi: 10.1016/j.jfoodeng.2016.07.004.

hardness decrease compared to the other cooking methods, without taking the crust layer into account,. It was also suggested that the effects of cooking were evident in the microstructure and texture at certain digestion times, although it was important to recognize that the microscopy could obtain the information from only a very small area. These findings indicated that the effective diffusivity of gastric acid would not follow the same trend as the moisture effective diffusivity during digestion and should be estimated separately. Such mass transport processes were influenced by the cooking method and digestive conditions. Therefore it was concluded that phenomena regarding structural changes in the sweet potato tissue during cooking and digestion were important in order to develop a better understanding of the physical and chemical changes in foods during digestion (Mennah-Govela and Bornhorst, 2016a,b). Dehydrated sweet potatoes are usually consumed as chips for snacks. Allen et al. (2012) determined the effect of different cooking methods on the GI of sweet potato skin and flesh and investigated the physiological impact and causes of low glycemic effect of raw sweet potato and its components. Twelve volunteers consumed 25 g of available carbohydrate from Beauregard sweet potato skin and flesh separately that were cooked through conventional cooking methods: baking at 163°C for 1 h; microwaving for 5 min in a 1000 W microwave; dehydrating at 60°C for 16 h;

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and steaming at 100°C for 45 min. Blood glucose levels were measured at 0, 30, 60, 90, and 120 min after consumption. The calculated GI values for steamed, baked, and microwaved sweet potato flesh were 63 6 3.6, 64 6 4.3, and 66 6 5.7, respectively, indicating sweet potato flesh to be a moderate GI food. On the other hand, dehydrated and raw sweet potato flesh was found to have low GI values (41 6 4.0 and 32 6 3.0, respectively). It is worth mentioning that the GI values for steamed or baked skin and dehydrated flesh were not statistically different from those of the raw sweet potatoes. The skin and flesh of Beauregard sweet potato could therefore be considered as low and medium GI foods, irrespective of the cooking method. Allen et al. (2012) also showed that a commercial extract of the sweet potato cortex, named Caiapo, had a GI value similar to raw sweet potato peel. Sweet potato is often consumed in a fried form, termed as “sweet potato wedges,” a popular alternative to French fries from potato (Bahado-Singh et al., 2011). Odenigbo et al. (2012) compared French fries prepared from five sweet potato cultivars (Ginseng Red, Beauregard, White Travis, Georgia Jet clone #2010, and Georgia Jet) for their glycemic impact. Interestingly, the GI values varied from low to moderate among fries from different cultivars, suggesting their varying nutritionally important starch compositions. The starch digestibility was also significantly affected by the cultivar. Among the cultivars that were studied, French fries from White Travis and Ginseng Red showed higher proportions of RS and SDS, which are the nutritionally important starch fractions. These cultivars also exhibited low predicted GI and starch digestion index. The lower GIs after frying compared to baking or roasting have been suggested to be due to an increase in fat content, slowing down starch degradation, and ultimately delaying gastric emptying and glycemic response (BahadoSingh et al., 2011). Several studies have reported the formation of amylose lipid complexes that reduce the rate of amylolysis, resulting in lower glycemic impact and GI values (Holm et al., 1983; BahadoSingh et al., 2011; Singh et al., 2010). Baked sweet potato elicited a high GI of 94 when compared with 14 West Indian carbohydrate-rich foods (Bahado-Singh et al., 2006). Similarly all tubers cooked through roasting also showed high GI (82 for sweet yam). However, boiled sweet potatoes possessed a low GI of 46 6 5. This study showed that the blood glucose response curves of boiled, roasted, and fried sweet potatoes were similar to the curves shown

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for low (,55), intermediate (56 69), and high (70) GI foods, respectively. Bahado-Singh et al. (2011) reported that the processing of Jamaican sweet potatoes by boiling led to lower glycemic response in comparison to frying, baking, and roasting. Mature tubers were cooked by different traditional cooking methods and immediately consumed by nondiabetic test subjects (five males and five females; mean age of 27 6 2 years). The results showed that the GI values varied between 41 6 5 and 93 6 5. Samples prepared by boiling had the lowest GI values (41 6 5 50 6 3), while those processed by baking (82 6 3 94 6 3) and roasting (79 6 4 93 6 2) showed the highest GI values. The study concluded that the GI of Jamaican sweet potatoes was influenced strongly by the method of processing and to a lesser extent by intervarietal differences. The study also indicated the importance of the consumption of boiled, instead of baked or roasted sweet potato by diabetics or prediabetics (Bahado-Singh et al., 2011). Boiling was suggested to retain larger amounts of RSs (RS1, RS2, and RS3) in starch-rich foods. The R3-RS content is further increased with starch retrogradation during the cooling process. Other RSs (R1 and R2) present in the foods after the leaching of free sugars during the boiling process have also been reported to play a role in slowing down the enzymatic starch degradation, thus reducing their glycemic response (Bahado-Singh et al., 2011). In another study, Englyst and Cummings (1987) reported that more than double the amount of starch from reheated boiled potatoes (Solanum tuberosum sp.) escaped ileal digestion in comparison to that from freshly cooked potato. The absence of a water-rich environment similar to that of boiling, during baking or roasting, would have resulted in lower starch gelatinization and the production of lower levels of retrograded/RS3 starch (Bahado-Singh et al., 2011). Table 9.1 shows the GI values of the different sweet potato varieties calculated relative to the reference food (glucose GI 5 100) and classified as high (70 100), intermediate (55 69), or low (,55) (Wolever et al., 1994). The GI values were significantly lower for foods processed by boiling when compared to the other processing methods. Foods baked and roasted had high GI values, while those fried (sweet potato wedges) had intermediate to moderately high GI values (63 6 2 77 6 4). Fig. 9.6 shows the mean glycemic responses of all the sweet potato cultivars processed by the different cooking methods. The GI values of these sweet potato cultivars could be substantiated by correlating the texture of the sweet potato cultivars.

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Table 9.1 Glycemic indices of selected Jamaican sweet potato (Ipomoea batatas) varieties determined by different cooking methods. Sweet potato varieties

Dor Quarter Million Yellow Belly Ganja Watson Clarendon Minda Ms Mac Eustace Fire on Land

GI Boiled

Fried

Baked

Roasted

Mean

SE

Mean

SE

a

3 4

b

47 49a

76 70b

50a

3

41a 43a 46a 49a 45a 49a 46a

5 4 5 4 3 5 4

Mean

4 6

c

83 94c

72b

4

69b 67b 73b 68b 63b 77b 75b

3 4 3 3 2 4 3

SE

Mean

SE

6 3

c

86 91c

4 2

86c

2

85c

2

82c 85c 83c 91c 87c 93c 87c

3 2 3 3 4 5 4

79c 87c 81c 89c 85c 93c 90c

4 2 4 3 4 2 3

Superscripts in rows sharing different letters are significantly different. Values are means 6 SEM for subjects. Glycemic index for each sample was calculated by expressing the IAUC as a percentage of the mean response area of glucose as outlined by Wolever et al. (1994).

Source: Reproduced with permission from Wolever, T.M., Katzman-Relle, L., Jenkins, A.L., Vuksan, V., Josse, R.G., Jenkins, D.J., 1994. Glycemic index of 102 complex carbohydrate foods in patients with diabetes. Nutr. Res. 14(5), 651 669.

Table 9.1 shows the GIs of differently processed sweet potato varieties when compared to a reference food (glucose GI 5 100), whereas their mean glycemic responses are presented in Fig. 9.6. The GI values of sweet potatoes have been reported to range from 44 (low) to 78 (high), and varied between countries, regions, manufacturers, variety, maturity, cooking method, cutting method, cooling process, and/or storage conditions (Foster-Powell et al., 2002). Jenkins et al. (1981) found that sweet potato from Canada and Australia had GI values of 48 and 44 compared to a GI value of 77 reported by Perry et al. (2000) for Kumara sweet potato from New Zealand. Wolever et al. (1994), however, reported a GI of 59 for the peeled, cubed, and boiled (in salted water for 15 min) sweet potatoes. According to the 2008 international tables of GI and GL (glycemic load) values, the mean GI value for boiled sweet potato is 63 (Atkinson et al., 2008). Thus it is recommended that the sweet potato should be precooked and consumed as a cold food like potato salad or reheated (Fernandes et al., 2005; Tahvonen et al., 2006) to lower the glycemic response (Al Dhaheri et al., 2017). In this regard the consumption

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Figure 9.6 Mean glycemic responses elicited by 50 g available carbohydrate portions of 10 sweet potato cultivars processed by boiling (solid diamond), baking ( 3 ), roasting (solid square), and frying (gray triangle). From Bahado-Singh, P.S., Riley, C.K., Wheatley, A.O., Lowe, H.I., 2011. Relationship between processing method and the glycemic indices of ten sweet potato (Ipomoea batatas) cultivars commonly consumed in Jamaica. J. Nutr. Metab. 2011, 584832.

with other ingredients such as vinegar (Liljeberg and Björck, 1998), vinaigrette dressing (Leeman et al., 2005), or topping baked potatoes with cheddar cheese (Henry et al., 2006) is also recommended.

Sweet potato products and their digestibility Sweet potatoes are consumed as fresh tubers or processed into dry or powdered flour, starch, and fermented products containing additives such as monosodium glutamate, citric acid, soy sauce, vinegar, and “shochu” (an alcoholic beverage made in Japan and Korea) (El Sheikha and Ray, 2017; Senanayake et al., 2014b). The use of sweet potato starch in starchbased baked products, noodles, and soup has been reported to be more nutritious and healthier than using wheat, corn, and rice starch (BovellBenjamin, 2007; Chen et al., 2006a,b; Ho and Noomhorm, 2011; Mu et al., 2016; Senanayake et al., 2014a,b,c; Van Toan, 2018). The use of sweet potato starch in surimi seafood has also been suggested to be advantageous due to its easy dispersibility during chopping and for providing

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viscosity due to its better water absorption and swelling during thermal treatment (Jia et al., 2018). Furthermore, improvements in gel strength and freeze thaw stability of surimi have also been observed which also reduce processing costs. The effects of the replacement of wheat starch (up to 20%) with acetylated sweet potato starches on the quality of white salted noodles (WSN) was studied by Chen et al. (2006a,b). A considerable reduction in the cooking losses along with a significant increase in the softness, stretchability, and slipperiness of WSN was observed. Nandutu and Howell (2009) studied two sweet potato-based infant food recipes for their nutritional properties and starch digestibility. The in vitro starch digestibility of both the recipes were observed to be comparable with commercial baby foods (Heinz and Cerelac), which reflects the potential of sweet potato for the development of infant foods. The application of modified sweet potato starches as a substitute thickener for corn starch was studied by Senanayake et al. (2014a,b,c). It was also found that the starch hydrolysis levels of these native sweet potato starches were significantly reduced after physical modification (HMT) (Table 9.2). The hydrothermal treatment of sweet potato starches resulted in an increase in their thickening capacity and a lowering of digestibility. Van Toan (2018) studied the replacement of wheat flour with purple sweet potato flour at 10%, 20%, 30%, 40%, and 50% levels for biscuit making. The physicochemical analysis showed that biscuits containing 40% and 50% sweet potato flour had significantly higher ash, fiber, and total flavonoid content and a similar sensory score when compared with Table 9.2 Digestibility of sweet potato starches. Digestibility (%) Sample

Native starch

HMT-starch

SWP1 SWP3 SWP4 SWP5 SWP7

21.7 6 0.7b 21.9 6 1.5b 23.5 6 0.9b 23.5 6 0.4b 19.3 6 0.3c

13.2 6 0.3d 23.5 6 0.9b 26.6 6 0.7a 19.5 6 0.5c 13.7 6 0.2d

 HMT refer of modified (heat moisture treated, 20% moisture, 85°C for 6 h) starches, values are means of triplicate determinations 6 standard deviation and the values denoted by different superscripts in each column are significantly different at P , .05. Source: From Senanayake, S.A., Ranaweera, K., Gunaratne, A., Bamunuarachchi, A., 2014b. Formulation of vegetable soup mixture using physically modified sweet potato starch as a thickener. J. Food Process. Technol. 5(4), 1.

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control samples. Even though the starch digestibility has not been studied, the high fiber content may help in lowering starch hydrolysis during oral and gastrointestinal digestion.

Conclusion Sweet potatoes have gained attention in recent years due to their superior nutritional and functional components. The main carbohydrate of sweet potato is starch, which accounts for nearly 80% of the dry matter. Therefore a number of studies on sweet potato starch composition, and its physicochemical properties and granule and molecular structures have been reported. The morphology of starch granules has been observed to be round, polygonal, and oval or semioval. Sweet potato starch granules vary in size from 2 to 45 μm with a crystalline structure of either A type or C type polymorph (a mixture of both A and B types). The structural characteristics of sweet potato starches have been correlated with their physicochemical properties and product applications. To alter the functional properties of native sweet potato starch, several enzymatic, chemical, and physical modifications have been performed. Several studies have reported that the starch digestibility differs among sweet potato cultivars. However, the processing methods have been observed to strongly influence the glycemic features of sweet potatoes, with boiled sweet potatoes showing low GI values and fried and baked or roasted sweet potatoes have been classified as having intermediate and high GI values, respectively. Further research on this important botanical resource may help to gain a greater understanding of the mechanisms of starch digestion and the utilization of sweet potatoes for the development of new products with low glycemic features.

Acknowledgments The authors would like to thank Elsevier for giving permission to reproduce parts of the article by Mennah-Govela and Bornhorst (2016b).

References Abegunde, O.K., Mu, T.-H., Chen, J.-W., Deng, F.-M., 2013. Physicochemical characterization of sweet potato starches popularly used in Chinese starch industry. Food Hydrocoll. 33 (2), 169 177. Available from: https://doi.org/10.1016/j.foodhyd.2013.03.005. Aina Adebisola, J., Falade Kolawole, O., Akingbala John, O., Titus, P., 2009. Physicochemical properties of twenty-one Caribbean sweet potato cultivars. Int. J.

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Lu, H., Tang, D., Wu, Z., Luo, K., Han, X., Jing, F., 2015. Genotypic variation and environmental effects on yield, quality and agronomic traits of sweet potato. Chin. J. Eco Agric. 23, 1158 1168. Lv, Q.-Q., Li, G.-Y., Xie, Q.-T., Zhang, B., Li, X.-M., Pan, Y., et al., 2018. Evaluation studies on the combined effect of hydrothermal treatment and octenyl succinylation on the physic-chemical, structural and digestibility characteristics of sweet potato starch. Food Chem. 256, 413 418. Available from: https://doi.org/10.1016/j. foodchem.2018.02.147. Mennah-Govela, Y.A., Bornhorst, G.M., 2016a. Acid and moisture uptake in steamed and boiled sweet potatoes and associated structural changes during in vitro gastric digestion. Food Res. Int. 88, 247 255. Available from: https://doi.org/10.1016/j. foodres.2015.12.012. Mennah-Govela, Y.A., Bornhorst, G.M., 2016b. Mass transport processes in orange-fleshed sweet potatoes leading to structural changes during in vitro gastric digestion. J. Food Eng. 191, 48 57. Available from: https://doi.org/10.1016/j.jfoodeng.2016.07.004. Mu, T.H., Li, P.G., Sun, H.N., 2016. Bakery products and snacks based on sweet potato. Tropical Roots and Tubers: Production, Processing and Technology. Wiley, pp. 507 531. Nandutu, A., Howell, N., 2009. Nutritional and rheological properties of sweet potato based infant food and its preservation using antioxidants. Afr. J. Food Agric. Nutr. Dev. 9 (4), 1076 1090. Odenigbo, A., Rahimi, J., Ngadi, M., Amer, S., Mustafa, A., 2012. Starch digestibility and predicted glycemic index of fried sweet potato cultivars. Funct. Foods Health Dis. 2 (7), 280 289. Perry, T., Mann, J., Mehalski, K., Gayya, C., Wilson, J., Thompson, C., 2000. Glycemic index of New Zealand foods. N. Zeal. Med. J. 113 (1108), 140. Remya, R., Jyothi, A.N., Sreekumar, J., 2018. Morphological, structural and digestibility properties of RS4 enriched octenyl succinylated sweet potato, banana and lentil starches. Food Hydrocoll. 82, 219 229. Available from: https://doi.org/10.1016/j. foodhyd.2018.04.009. Sagum, R., Arcot, J., 2000. Effect of domestic processing methods on the starch, nonstarch polysaccharides and in vitro starch and protein digestibility of three varieties of rice with varying levels of amylose. Food Chem. 70 (1), 107 111. Schirmer, M., Höchstötter, A., Jekle, M., Arendt, E., Becker, T., 2013. Physicochemical and morphological characterization of different starches with variable amylose/amylopectin ratio. Food Hydrocoll. 32 (1), 52 63. Available from: https://doi.org/ 10.1016/j.foodhyd.2012.11.032. Senanayake, S.A., Ranaweera, K.K.D.S., Gunaratne, A., Bamunuarachchi, A., 2013. Comparative analysis of nutritional quality of five different cultivars of sweet potatoes (Ipomea batatas (L) Lam) in Sri Lanka. Food Sci. Nutr. 1 (4), 284 291. Available from: https://doi.org/10.1002/fsn3.38. Senanayake, S., Gunaratne, A., Ranaweera, K., Bamunuarachchi, A., 2014a. Effect of heat moisture treatment on digestibility of different cultivars of sweet potato (Ipomea batatas (L.) Lam) starch. Food Sci. Nutr. 2 (4), 398 402. Senanayake, S.A., Ranaweera, K., Gunaratne, A., Bamunuarachchi, A., 2014b. Formulation of vegetable soup mixture using physically modified sweet potato starch as a thickener. J. Food Process. Technol. 5 (4), 1. Senanayake, S.A., Ranaweera, K.K.D.S., Gunaratne, A., Bamunuarachchi, A., 2014c. Application of hydrothermally modified sweet potato starch as a substitute additive for soup mixture. J. Food Process. 2014, 5. Available from: https://doi.org/10.1155/ 2014/904125.

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Shin, S.I., Kim, H.J., Ha, H.J., Lee, S.H., Moon, T.W., 2005. Effect of hydrothermal treatment on formation and structural characteristics of slowly digestible non-pasted granular sweet potato starch. Starch—Stärke 57 (9), 421 430. Available from: https:// doi.org/10.1002/star.200400377. Singh, J., Dartois, A., Kaur, L., 2010. Starch digestibility in food matrix: a review. Trends in Food Science & Technology 21, 168 180. Available from: https://doi.org/ 10.1016/j.tifs.2009.12.001. Sun, M., Mu, T., Zhang, M., Arogundade, L.A., 2012. Nutritional assessment and effects of heat processing on digestibility of Chinese sweet potato protein. J. Food Compos. Anal. 26 (1), 104 110. Available from: https://doi.org/10.1016/j. jfca.2012.03.008. Tahvonen, R., Hietanen, R., Sihvonen, J., Salminen, E., 2006. Influence of different processing methods on the glycemic index of potato (Nicola). J. Food Compos. Anal. 19 (4), 372 378. Tharanathan, M., Tharanathan, R., 2001. Resistant starch in wheat-based products: isolation and characterisation. J. Cereal Sci. 34 (1), 73 84. Tian, S., Rickard, J., Blanshard, J., 1991. Physicochemical properties of sweet potato starch. J. Sci. Food Agric. 57 (4), 459 491. Trung, P.T.B., Ngoc, L.B.B., Hoa, P.N., Tien, N.N.T., Hung, P.V., 2017. Impact of heat moisture and annealing treatments on physicochemical properties and digestibility of starches from different colored sweet potato varieties. Int. J. Biol. Macromol. 105, 1071 1078. Available from: https://doi.org/10.1016/j.ijbiomac.2017.07.131. Tumuhimbise, G.A., Namutebi, A., Muyonga, J.H., 2009. Microstructure and in vitro beta carotene bioaccessibility of heat processed orange fleshed sweet potato. Plant Foods Hum. Nutr. 64 (4), 312. Available from: https://doi.org/10.1007/s11130-0090142-z. Valetudie, J.-C., Gallant, D.J., Bouchet, B., Colonna, P., Champ, M., 2000. Influence of cooking procedures on structure and biochemical changes in sweet potato. Starch— Stärke 51 (11 12), 389 397. Available from: https://doi.org/10.1002/(SICI)1521379X(199912)51:11/12 , 389::AID-STAR389 . 3.0.CO;2-H. Van Toan, N., 2018. Preparation and improved quality production of flour and the made biscuits from purple sweet potato. J. Food Nutr. 4, 1 14. Vieira Fabiana, C., Sarmento Silene, B.S., 2008. Heat-moisture treatment and enzymatic digestibility of Peruvian carrot, sweet potato and ginger starches. Starch—Stärke 60 (5), 223 232. Available from: https://doi.org/10.1002/star.200700690. Vosloo, M.C., 2005. Some factors affecting the digestion of glycemic carbohydrates and the blood glucose response. J. Consum. Sci. 33 (1), 1 9. Walter, W.M., Truong, V.D., Wiesenborn, D.P., Carvajal, P., 2000. Rheological and physicochemical properties of starches from moist- and dry-type sweetpotatoes. J. Agric. Food Chem 48 (7), 2937 2942. Available from: https://doi.org/10.1021/ jf990963l. Wang, S., Nie, S., Zhu, F., 2016. Chemical constituents and health effects of sweet potato. Food Res. Int. 89, 90 116. Available from: https://doi.org/10.1016/j. foodres.2016.08.032. Waramboi, J.G., Dennien, S., Gidley, M.J., Sopade, P.A., 2011. Characterisation of sweet potato from Papua New Guinea and Australia: physicochemical, pasting and gelatinisation properties. Food Chem. 126 (4), 1759 1770. Available from: https://doi.org/ 10.1016/j.foodchem.2010.12.077. Wolever, T.M., Katzman-Relle, L., Jenkins, A.L., Vuksan, V., Josse, R.G., Jenkins, D.J., 1994. Glycemic index of 102 complex carbohydrate foods in patients with diabetes. Nutr. Res. 14 (5), 651 669.

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Yadav, B.S., Sharma, A., Yadav, R.B., 2009. Studies on effect of multiple heating/cooling cycles on the resistant starch formation in cereals, legumes and tubers. Int. J. Food Sci. Nutr. 60 (Suppl. 4), 258 272. Available from: https://doi.org/10.1080/ 09637480902970975. Zavareze, E. d R., Dias, A.R.G., 2011. Impact of heat moisture treatment and annealing in starches: a review. Carbohydr. Polym. 83 (2), 317 328. Available from: https:// doi.org/10.1016/j.carbpol.2010.08.064. Zhang, G., Hamaker, B.R., 2009. Slowly digestible starch: concept, mechanism, and proposed extended glycemic index. Crit. Rev. Food Sci. Nutr. 49 (10), 852 867. Available from: https://doi.org/10.1080/10408390903372466. Zhang, B., Mei, J.-Q., Chen, B., Chen, H.-Q., 2017. Digestibility, physicochemical and structural properties of octenyl succinic anhydride-modified cassava starches with different degree of substitution. Food Chem. 229, 136 141. Available from: https://doi. org/10.1016/j.foodchem.2017.02.061. Zhang, L., Zhao, L., Bian, X., Guo, K., Zhou, L., Wei, C., 2018. Characterization and comparative study of starches from seven purple sweet potatoes. Food Hydrocoll. 80, 168 176. Available from: https://doi.org/10.1016/j.foodhyd.2018.02.006. Zhao, J., Schols, H.A., Chen, Z., Jin, Z., Buwalda, P., Gruppen, H., 2012. Substituent distribution within cross-linked and hydroxypropylated sweet potato starch and potato starch. Food Chem. 133 (4), 1333 1340. Available from: https://doi.org/10.1016/j. foodchem.2012.02.021. Zhu, F., Wang, S., 2014. Physicochemical properties, molecular structure, and uses of sweet potato starch. Trends Food Sci. Technol. 36 (2), 68 78. Available from: https://doi.org/10.1016/j.tifs.2014.01.008. Zhu, F., Xie, Q., 2018. Rheological and thermal properties in relation to molecular structure of New Zealand sweet potato starch. Food Hydrocoll. 83, 165 172. Available from: https://doi.org/10.1016/j.foodhyd.2018.05.004. Zhu, F., Yang, X., Cai, Y.-Z., Bertoft, E., Corke, H., 2011. Physicochemical properties of sweet potato starch. Starch—Stärke 63 (5), 249 259. Available from: https://doi. org/10.1002/star.201000134.

Further reading Mennah-Govela, Y.A., Bornhorst, G.M., Singh, R.P., 2015. Acid diffusion into rice boluses is influenced by rice type, variety, and presence of α-amylase. J. Food Sci. 80 (2), E316 E325. Available from: https://doi.org/10.1111/1750-3841.12750.

CHAPTER 10

Sweet potato staple foods Tai-Hua Mu1, Miao Zhang1, Hong-Nan Sun1 and Isela Carballo Pérez1,2 1

Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China Institute of Food Research, Havana, Cuba

2

Overview of sweet potato staple foods Definition and types of sweet potato staple foods The sweet potato is an economical and healthful food crop and is the fifth most important food crop in the world after rice, wheat, maize, and potato (FAOSTAT, 2016). The sweet potato is rich in protein, dietary fiber, vitamins, minerals, and many other healthy ingredients (Woolfe, 1992), and sweet potato protein has a high content of essential amino acids and a balanced amino acid composition that is superior to those of cereal proteins. Based on its good nutritional value, sweet potato has been processed into flour, flakes, granules, paste, purees, chips, canned products, beverages, and various snack foods. Nowadays, sweet potato is also used as an important supplement for different staple products in the food industry, such as sweet potato steamed breads, baked breads, noodles, and pancakes, etc. Thus sweet potato staple foods refer to the staple foods made from sweet potato or supplemented with sweet potato components. Sweet potato steamed breads As a traditional Chinese staple food, steamed bread has been consumed for at least 2000 years in China. Nowadays steamed bread is gaining considerably more popularity in many countries due to its lower acrylamide content and the lower loss of soluble amino acids compared with baked bread (Sui et al., 2016). Mu et al. (2014a) prepared gluten-free sweet potato steamed bread with sweet potato flour (SPF), sweet potato starch and modified starch with the addition of protein, pectin, gum arabic, sugars, and yeast. Mu et al. (2014b) also prepared gluten-free sweet potato steamed bread rich in dietary fiber with sweet potato dietary fiber, sweet potato starch, different hydrocolloids, protein, sugars, salt, and yeast. Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00010-7

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Sweet potato breads Bakery products, especially breads, are one of excellent sources of energy foods, which can be used in vehicle, planetary and ground-based food systems (Greene and Bovell-Benjamin, 2004). To improve the nutritive value of bakery products, studies have been conducted on the use of composite flours by blending wheat flour (WF) with flours from other cereals, oilseeds, legumes, or tubers (Trejo-González et al., 2014). Breads supplemented with 50%
65% SPF showed higher β-carotene contents, lower protein contents, and different appearance, texture, and flavor compared with whole wheat breads (Greene and Bovell-Benjamin, 2004). TrejoGonzález et al. (2014) replaced WF with 5%
20% SPF prepared from sun-dried slices of an orange-fleshed Mexican cultivar, and indicated that SPF addition to the extent of 5% yielded acceptable doughs and breads from the perspective of physical dough and bread properties, and WF replacement with 10% SPF yielded good quality breads based on sensory properties. Sweet potato noodles The popularity of noodles is increasing, particularly in Asian countries, due to their simple preparation, long shelf life, desirable sensory attributes, product diversity and nutritive value. With the expansion of the world market, research in the development and improvement of the quality of noodles in order to satisfy consumer’s demands is of immense importance. WF is the main ingredient for the manufacturing of noodles, and the demand to use novel sources as substitutes for WF has increased in recent years. To add variety and functionality to the noodle products, flours from alternative sources such as sweet potato, colocasia, water chestnut, and other tubers are being used as potential WF substitutes for noodle making (Yadav et al., 2014). Collado and Corke (1996) prepared Chinese-style yellow alkaline noodles and Japanese-style white salted noodles from a standard brand of hard red winter WF and from composite flours containing 25% SPF, and indicated that the addition of ascorbic acid tended to increase the firmness of noodles with wheat
sweet potato composite flours, while inducing a higher degree of browning after storage. Collins and Pangloli (1997) studied the chemical, physical, and sensory attributes of noodles supplemented with sweet potato and soy flour, and found that SPF increased color acceptability with no change in flavor or overall acceptability. Pangloli et al. (2000) indicated that noodles supplemented with 10% defatted soy flour and 10% SPF or 15% sweet potato

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puree could be stored successfully under air at 4.4°C, which showed greater quality retention than WF noodles. Other types of sweet potato staple foods Cake is a kind of bread or bread-like food, typically a baked sweet dessert, which has enjoyed a relatively constant place in our diet for a long time and its continuing popularity has encouraged the development of more attractive cake products that are available in the market today (EkeEjiofor, 2013). Cake produced from sweet potato
wheat flour presents improved nutritional and sensory properties when 20% of SPF is substituted, while in the case of biscuit, up to 30% of SPF is substituted (EkeEjiofor, 2013). Okorie and Onyeneke (2012) indicated that sweet potato cakes were at their best for volume increase, softness, and overall acceptability at 20% sweet potato substitution. In addition, Shih et al. (2006) prepared gluten-free pancakes using rice flour and rice flour replaced with 10%
40% of SPF, and indicated that the nutritional properties of the rice
sweet potato pancakes, including protein, dietary fiber, total carbohydrate contents, and calories, were generally comparable with those of their wheat counterpart. Saeed et al. (2012) indicated that the addition of 10%
20% of SPF lowered the width and thickness of cookies, but improved their flavor, taste, and overall acceptability.

Main raw ingredients for sweet potato staple foods Many different types of sweet potato raw ingredients can be used to produce sweet potato staple foods, and the main raw ingredients used for sweet potato staple foods now include fresh sweet potato, mashed sweet potato, and SPF, etc. Fresh sweet potato The fresh sweet potato can be directly used as a processing ingredient for sweet potato staple foods. For example, the fresh sweet potato can be peeled, steamed, pounded, and then mixed with WF to prepare steamed bread, noodles, and other foods. The direct use of fresh sweet potato to make sweet potato staple foods has low processing costs, while sweet potato has high moisture content, short shelf life, and difficulty in storage and transportation, and the processing process is not easy to handle. Therefore the production of sweet potato staple foods using fresh potatoes has certain limitations, and it is more suitable for home-made processing

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and not suitable for large-scale industrial production. In addition, the final quality of staple foods is highly dependent on the quality of the raw ingredients used. Thus if sweet potato is to be incorporated into staple foods, it must be of high quality. Mashed sweet potato Mashed sweet potato is a kind of puree-like product made from fresh sweet potato after being processed and matured. Generally, it can be eaten directly, eaten after being adjusted with different flavors, or made into various kinds of snack foods. However, the high starch gelatinization degree of mashed sweet potato limits the proportion of mashed sweet potato in staple foods. Wu et al. (2009) investigated the effects of 5%
30% sweet potato pastes from different varieties on the physicochemical properties of dough and bread, and found that toast of bread supplemented with sweet potato paste was more favorable than that of the control, while loaf volume slightly decreased with the addition of more than 20% of sweet potato paste. Bhosale et al. (2011) added 0%
15% of mashed sweet potato to chicken meat nuggets to improve their nutritional value and present some beneficial effects due to the presence of dietary fibers and β-carotene, and indicated that chicken meat nuggets with 10% of mashed sweet potato sustained the desired cooking yield and emulsion stability, and showed higher overall acceptability scores. Sweet potato flour Flour is the powder made from cereals—most commonly wheat—which is the key ingredient in bread production and constitutes a staple diet in many countries. Therefore the availability of an adequate supply of flour has often been a major economic and political issue. Flour can be also made from legumes and nuts, roots, and tubers such as sweet potato, yam, cassava, etc. Flour produced from nonwheat sources is known as composite flour (Adeleke and Odedeji, 2010). Sweet potato can be processed into raw flour by being peeled, cut into slices, and dried in an oven at different temperatures (55°C
65°C). The flour can be used as a thickener in soup, gravy, fabricated snacks, and bakery products (Ahmed et al., 2010). SPF can also be processed into cooked flour, in a process where sweet potatoes are peeled, cut into slices, color protected, steamed, and air-dried. Nowadays, to improve the nutrition value of stable food, SPF is partly substituted for cereals flours, for example, WF, which shows benefits for individuals diagnosed with celiac

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disease (Sukhcharn et al., 2008). A study of Ahmed et al. (2007) showed that the dough and bread characteristics were essentially unchanged when 5%
10% of SPF is substituted for WF bread. In addition, SPF and flakes can partly substitute for wheat and other cereals flours in order to enrich the β-carotene content in bakery products and pancakes (Woolfe, 1992). It was found that the incorporation of SPF in the formulation of rice pancakes improved the flow behavior of the batter and the physicochemical properties of the product (Shih et al., 2006).

Development of sweet potato steamed bread As a main staple food in China, steamed bread comprises almost 40% of wheat consumption (Wu et al., 2010). There is less acrylamide content and lower soluble amino acids loss in steamed bread when compared with baked bread (Becalski et al., 2003). However, for only-wheat steamed bread, there is not enough lysine, vitamins, and mineral elements for human nutritional balance, so it is necessary to supplement some functional components or other flours to improve its nutritional values and provide choices for consumers, such as steamed breads with sweet potato, potato, yam, oat, barely, buckwheat, corn, and wheat germ flours, as well as those with fiber and polyphenols (Liu et al., 2016). Some research on the development of sweet potato steamed bread is introduced here.

Sweet potato steamed bread prepared with sweet potato flour and other food components Mu et al. (2014a) prepared sweet potato steamed bread with 20%
40% raw SPF, 30%
50% cooked SPF, 10%
18% sweet potato starch, 6%
13% sweet potato modified starch, 3%
8% protein, 0.5%
2.5% pectin, 0.1%
0.6% gum arabic, 0.1%
2% sugar, and 2%
3% yeast. When producing the sweet potato steamed bread prepared with SPF and other food components, the ratio of the mixed powder to water is 100:70
90 (w/w). The sweet potato steamed bread produced by the method above is suitable for people with allergies caused by gluten and presents a relatively comprehensive nutrient composition.

Sweet potato steamed bread prepared with sweet potato fiber and other food components Mu et al. (2014b) prepared sweet potato steamed bread with 10%
40% sweet potato crude dietary fiber, 10%
40% sweet potato ultrafine dietary

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fiber, 10%
20% sweet potato nano dietary fiber, 4%
12% sweet potato modified starch, 5%
12% extruded puffed sweet potato dietary fiber, 5%
13%, microwave-treated sweet potato crude dietary fiber, 2%
6% sweet potato starch, 0.5%
3% hydrocolloids, 0.5%
3% protein, 0.1%
3% sugar, 0.1%
1% salt, and 1%
2% yeast. When producing the sweet potato steamed bread prepared with sweet potato fiber and other food components, the ratio of the mix powder to water is 100:50
70 (w/w). The sweet potato steamed bread produced by the method above is yellowish, presents the unique flavor of sweet potato, and has good nutritional values.

Development of sweet potato
wheat bread Different food processing methods could have different effects on the quality of food. As the main ingredients of sweet potato staple food, the properties of SPF are important in order to develop sweet potato staple foods with acceptable quality. Take sweet potato
wheat bread as an example: the effects of heat and high hydrostatic pressure treatment of SPF on dough properties and bread characteristics were researched by the author team and are introduced here.

Effect of heat treatment of sweet potato flour on dough properties and bread characteristics The heat treatment (HT) of flour can be used in many applications in food processing and has been suggested to be a viable method of improving bread quality, particularly for weak and substandard flour (Marston et al., 2016). The mechanism by which HT improves the flour is well known. During the HT process, protein denaturation and the partial gelatinization of starch granules occurs, as well as an increase in batter viscosity (Neill et al., 2012). HT of WF at 100°C for 12 min enhanced the dough stability of bread (Bucsella et al., 2016). HT of wheat and the resulting changes in rheological properties were of considerable importance to the characteristics of the final baked products (Lagrain et al., 2005). Nakamura et al. (2008) observed an increase in the volume of Kasutera cake by the heating of WF at 120°C for 30 min. Marston et al. (2016) developed the bread from sorghum flour treated at 125°C for 30 min with good characteristics in the structure of bread. In addition, Puncha-Arnon and Uttapap (2013) found that a significant increase in paste and greater effects on

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279

thermal parameters of gelatinization and gel hardness of flours were observed when rice flour was treated at 100°C for 16 h. To develop sweet potato
wheat bread with acceptable quality, the author team investigated the effect of HT on color, particle size, thermal properties, and microstructure of SPF, as well as the effect of HT of SPF on dough rheology, fermentation, and texture of sweet potato
wheat bread, which had a partial substitution of WF by SPF. Characteristics of wheat flour and sweet potato flour Table 10.1 shows the characteristics of WF and SPF. WF showed higher moisture content (9.01%) than SPF (5.76%). WF had higher protein (11.41%) than SPF (5.62%), which was in accordance with the results of Vallons and Arendt (2010) and Adeyeye et al. (2014), who reported that SPF had 7.4% and 5.8% of protein, respectively, while Dewaest et al. (2017) found that protein content was 8.6% in WF. Starch content of WF (70.41%) was greater than SPF (59.02%), which was in accordance with the results reported before that starch values ranged from 60% to 75% in WF and from 38.6% to 62.29% in SPF (Sukhcharn et al., 2008). Fat content analysis showed that WF contains higher percentages of fat (1.06%) than SPF (0.90%). The ash contents of WF and SPF were 0.26% and 2.91%, separately. Sukhcharn et al. (2008) found the ash content of WF to be 0.65%. Akonor et al. (2017) found the ash content of SPF to be in Table 10.1 Characteristics of wheat and sweet potato flour. Basic components (%)

Wheat flour (WF)

Sweet potato flour (SPF)

Moisture Protein Starch fiber Fat Ash Vitamins (mg/100 g) B1 B2 B3 B6 B9 C

9.01 6 2.16a 11.41 6 0.06a 70.41 6 0.12a 0.54b 1.06 6 0.12a 0.26 6 0.05b

5.76 6 0.01b 5.62 6 0.01b 59.01 6 0.01b 1.88a 0.90 6 0.04b 2.91 6 0.04a

0.08 6 0.01b 0.075 6 0.001b 0.82 6 0.01b 0.06 6 0.01b 11.90 6 0.14b 7.48 6 0.12b

0.14 6 0.04a 0.28 6 0.01a 4.85 6 0.02a 0.37 6 0.01a 12.75 6 0.01a 38.15 6 0.15a

The values denoted by different letters in the same column are significantly different (P , .05).

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the range of 2.25%
2.54%, which was possibly due to a higher content of minerals and inorganic salts. In addition, SPF contained higher proportions of vitamins than WF, especially vitamins B3 and C, which were about six and five times than those in WF, respectively. Many researchers have studied the importance of vitamins in adult and child nutrition as alternatives to food fortification (Prasad and Kochhar, 2015; Low et al., 2015; Laurie et al., 2015), and SPF could be a good source of vitamins. Characteristic of sweet potato flour after heat treatment Color The color results of SPF after HT showed an increase in terms of lightness (L ), with respect to the control (83.9), except for at 120°C (83.71) (Table 10.2). Significant differences appeared in all treatments. There was an increase in a with HT and the highest value was observed at 110°C. In the case of b , there was an increase with HT and the highest value was 16.57 at 120°C. Particle size The middle particle size of SPF without HT (Control) was 33.52 μm, which decreased to 26.23 μm after HT at 90°C (Table 10.2). Volume mean diameter and area mean diameter also decreased significantly after HT compared to the control. Particle size, including size distribution, was one of the characteristics that most markedly affected the functional properties. Kim and Yao (2014) indicated that as the temperature increased from 30°C to 90°C, the peak particle size of waxy WF increased in different ways. Scanning electron microscopy After HT a slight change in the structure was shown, compared to the control, where the rupture of the granule occurred, which increased as the temperature increased from 90°C to 120°C (Fig. 10.1). Depending on the presence of moisture, HT can change the granular and molecular structure of starch (Chung et al., 2007). Sun et al. (2014) performed 130° C of HT to proso millet flour for 4 h and observed that the structure of the gel became more compact compared with the control, which could be due to the nonstarch compositions interacting with the starch granules and adhering to the surface of the granule during HT.

Table 10.2 Color, particle size, volume mean diameter, and area mean diameter of SPF after HT. Samples

L

a

b

Middle particle size (µm)

Volume mean diameter (µm)

Area mean diameter (µm)

Control 90°C 100°C 110°C 120°C

83.9 6 0.0b 84.13 6 0.01a 84.14 6 0.01a 83.96 6 0.03b 83.71 6 0.00c

0.3 6 0.0e 0.35 6 0.01d 0.41 6 0.00c 0.60 6 0.02a 0.46 6 0.05b

15.1 6 0.0e 15.58 6 0.00d 16.13 6 0.00c 16.31 6 0.01b 16.57 6 0.05a

33.52 6 0.39a 26.23 6 0.99b 26.98 6 0.52b 26.68 6 0.63b 27.46 6 0.98b

38.97 6 0.61a 29.98 6 1.19b 30.83 6 0.77b 30.43 6 0.79b 31.31 6 1.19b

20.13 6 0.97a 16.06 6 0.46b 16.54 6 0.34b 16.85 6 0.30b 17.16 6 0.28b

The values denoted by different letters in the same column are significantly different (P , .05).

282

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Figure 10.1 Scanning electron micrograph (magnification: 3 1000) of SPF after HT. (A) SPF without HT (control); (B) SPF at 90°C; (C) SPF at 100°C; (D) SPF at 110°C; (E) SPF at 120°C.

Differential scanning calorimetry for sweet potato flour It is important to understand thermal properties, such as gelatinization behavior temperature and enthalpy changes, during the baking process. Significant differences were observed with respect to endothermic peak temperatures (TP) of SPF after HT (Table 10.3). The lowest value was observed at 110°C, while there was a slight increase in values of TP in the flour treated at 90°C, 100°C, and 120°C with respect to control. The gelatinization enthalpy change (ΔH) of SPF without HT was 5.64 J/g, which decreased after HT, and the lowest value was observed at 120°C (3.68 J/g). Sun et al. (2014) found slightly higher values of TP and reduction on ΔH in HT millet flour, which might be related to the presence of nonstarch components such as proteins. Dough properties Dough fermentation Different factors could influence dough fermentation and modify the development of the final product, such as yeast, temperature, and the use of new materials. Dough height (Hm) was influenced by HT (Table 10.4). The greatest height of the dough was observed in the control. With regard to dought formation time (T1), there were significant differences in all treatments compared with the control. The longest time was observed in the treatment at 90°C and the shortest was at 110°C. Rosell et al. (2001) found different values of T1 on WF dough which could be attributed to the treatments of HT and the type of yeast. Regarding the gas 0 behavior, the time of maximum gas formation (T1 ) was the shortest in the treatment at 110°C and was much higher at 90°C. After HT the gas retention of the dough with SPF increased significantly from 1199 mL without HT to 1214 mL at 90°C. Liu et al. (2016) studied the steamed breads with potato flour and WF, and showed values of Hm from 17 to 36 mm and gas volume from 1572 to 2100 mL, respectively.

Table 10.3 Differential scanning calorimetry (DSC) of SPF and dough after HT. Samples

SPF TP (°C)

Control 90°C 100°C 110°C 120°C

Dough ΔH (J/g)

78.59 6 0.08 78.68 6 0.02ba 79.00 6 0.15a 77.94 6 0.34c 78.66 6 0.1ba b

TP1 (°C)

5.64 6 0.00 4.94 6 0.35b 5.08 6 0.04b 3.67 6 0.05c 3.68 6 0.22c a

ΔH1 (J/g)

69.63 6 0.44 69.62 6 0.41a 69.44 6 0.01a 69.43 6 0.03a 69.21 6 0.42a a

The values denoted by different letters in the same column are significantly different (P , .05).

TP2 (°C)

0.79 6 0.56 0.56 6 0.14b 0.68 6 0.06ba 0.81 6 0.24a 0.55 6 0.06b a

ΔH2 (J/g)

98.33 6 0.11 98.71 6 0.02b 98.87 6 0.43ba 97.74 6 0.12d 99.17 6 0.28a c

0.32 6 0.02b 0.28 6 0.02c 0.28 6 0.01c 0.34 6 0.02ba 0.36 6 0.01a

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Table 10.4 Analysis of fermentation on dough treated with HT by rheofermentometer. Samples

Dough Development Hm (mm)

Control 90°C 100°C 110°C 120°C

Gas behavior 0

T1 (min)

39.4 6 0.4 37.5 6 0.0b 31.9 6 0.0c 16.1 6 0.1e 29.3 6 0.0d a

Gas retention (mL)

T1 (min)

103 6 0.0 110 6 0.0a 78.5 6 0.0b 65 6 0.03c 91 6 0.03d a

174 6 1.41 176 6 1.42a 76.5 6 0.7c 67 6 2.82d 155.5 6 2.1b a

1199 6 4.94b 1214 6 2.12a 487 6 1.41e 984 6 2.12d 1136 6 2.82c 0

Hm, maximum dough height; T1, time at which dough reaches the maximum height; T1 , time of maximum gas formation; gas retention, volume of the gas retained in the dough at the end of the assay. The values denoted by different letters in the same column are significantly different (P , .05).

Differential scanning calorimetry for dough For DSC two peaks were shown in all the dough (Table 10.3). TP1 was from 69.21°C to 69.63°C and TP2 was from 97.74°C to 99.17°C. It was observed that as the temperature increased, TP1 of SPF after HT decreased compared to the control, but with no significant differences. The values of ΔH1 were 0.55 to 0.81 J/g. In the second phase, the ΔH2 was 0.28 to 0.36 J/g, and the lower values were at 90°C and 100°C. Zaidul et al. (2008) suggested that TP2 of the mixtures of WF and sweet potato starch were lower compared to WF. Sun et al. (2014) explained that the differences of thermal properties might be attributed to the different varieties of flour and the influence of nonstarch components of the flour such as proteins.

Bread-making process and quality evaluation Crust and crumb color The greatest brightness (L ) of crust was observed in sweet potato
wheat bread with SPF at 100°C, which showed lower a and b (Table 10.5). The color of the sweet potato
wheat bread crust that exceeded 100°C was caused by Maillard reactions and the caramelization of sugars, which depended on the distribution of water and the presence of reducing sugars as well as amino acids and their types (Purlis, 2010). The crumb of sweet potato
wheat bread with SPF at 120°C showed the lowest brightness, while the crumb at 90°C showed the lowest a and the control had the lowest b followed by that at 90°C.

Table 10.5 Color of crust and crumb and specific volume (cm3/g) of sweet potato
wheat breads. Samples

Control 90 °C 100 °C 110 °C 120 °C

Crust

Crumb

L

a

b

L

a

b

54.94 6 0.04a 56.44 6 0.33d 58.38 6 0.40b 57.68 6 0.31c 56.41 6 0.38d

15.5 6 0.1a 15.68 6 0.16a 11.96 6 0.03c 12.60 6 0.24b 12.37 6 0.21b

34.9 6 0.1a 35.31 6 0.09a 35.29 6 0.30a 35.72 6 0.08a 32.72 6 0.98b

67.53 6 0.16a 67.77 6 0.27a 64.80 6 0.26b 63.49 6 0.39c 62.71 6 0.26d

2.04 6 0.04c 1.92 6 0.00c 2.77 6 0.01a 2.55 6 0.28b 2.85 6 0.19a

18.48 6 0.31c 18.68 6 0.10c 19.47 6 0.17a 18.73 6 0.06c 20.72 6 0.24a

The values denoted by different letters in the same column are significantly different (P , .05).

Specific volume (cm3/g)

2.3 6 0.0c 2.53 6 0.03a 2.35 6 0.03c 2.47 6 0.04b 2.43 6 0.02b

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Specific volume The highest volume was obtained in the sweet potato
wheat breads with SPF at 90°C, which was 2.53 cm3/g (Table 10.5). Infinite loaf volume is not so desirable, but consumers associate a certain amount of lightness and high loaf volume with certain breads, and low loaf volumes with others (Hathorn et al., 2008). The results of the specific volume in breads might be related to dough development, gas retention, middle particle size, volume mean diameter, and area mean diameter. Texture analysis Texture analysis results of springiness, hardness, cohesiveness, and chewiness of crust and crumb of sweet potato
wheat breads are shown in Fig. 10.2. Springiness was defined as the speed at which a deformed material returned to the initial condition after the force causing the deformation was removed. This was greatly affected by moisture content, moisture redistribution, and retrogradation of starch (Osella et al., 2005; Lazaridou and Biliaderis, 2009). The springiness of the crust and crumb

Figure 10.2 Springiness, hardness, cohesiveness, and chewiness of crust and crumb of sweet potato
wheat bread.

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287

were high in the control, followed by sweet potato
wheat bread with SPF at 90°C. High-quality bread with a good degree of freshness was related to high springiness values, while a loaf with low springiness values tended to crumble when it was sliced (McCarthy et al., 2005). The force required to achieve a given deformation (hardness) was the highest in the crumb of sweet potato
wheat bread with SPF at 110°C, while that of the control was the lowest. In composite breads the lower volume and harder texture had been mainly attributed to the “dilution” of the gluten matrix in the mixture and to protein network disruption (Pérez et al., 2008). Chewiness was expressed as the intensity of chewing needed before swallowing (Pasqualone et al., 2017), and cohesiveness was an indicator of the internal cohesion of the material: generally, breads with low cohesiveness were susceptible to fracture and crumble (Onyango et al., 2010) and were not desirable. In the crust the cohesiveness increased as the temperature increased, while there was a slight decrease in crumb. In addition, chewiness showed significant differences in different sweet potato
wheat breads, and the lowest value was found for the sweet potato
wheat crumb with SPF at 100°C.

Effect of high hydrostatic pressure to sweet potato flour on dough properties and bread characteristics In the last decades the development of nonconventional methods for food processing, like high hydrostatic pressure (HHP), has attracted much attention. HHP is applicable to food and raw material processing for obtaining innovative sensorial and functional properties (Huang et al., 2017). During HHP processing, different pressure and temperature combinations can be utilized to achieve the desired effects on texture, color, and flavor of foods. The quality of HHP processed food can, however, change during storage due to coexisting chemical reactions, such as oxidation and biochemical reactions (Nunes et al., 2017). HHP modifies the microstructure and rheological properties in a different way than thermal treatment (Cappa et al., 2016a), and it is highly dependent on the type of pressure level and time of treatment. Cappa et al. (2016b) indicated that breads with rice flour treated with HHP (600 MPa, 5 min, 40°C) showed high specific volumes and good crumb softness. To develop novel raw material for sweet potato
wheat bread, the author team investigated the effect of HHP on color, particle size, thermal properties, and microstructure of SPF, as well as the effect of HHP to

288

Sweet Potato

SPF on dough fermentation, specific volume, texture, and flavor of sweet potato
wheat bread, which had a partial substitution of WF by SPF. Characteristic of sweet potato flour after high hydrostatic pressure Color After HHP, the L value of SPF significantly decreased from 83.9 at 0.1 MPa to 80.01 at 400 MPa, while the a value increased from 0.3 at 0.1 MPa to 1.12 at 400 MPa (Table 10.6). In the case of b , the smallest value was 12.38 at 100 MPa, while the highest value was 16.73 at 300 MPa. Ahmed et al. (2017a, b) indicated that the L value of the rice flour treated with HHP at 300
400 MPa with a flour to water ratio of 1:4 decreased significantly compared to that without HHP treatment. Particle size Compared to SPF at 0.1 MPa, the median particle size, volume mean diameter, and area mean diameter of SPF after HHP at 100
400 MPa were decreased significantly (Table 10.6). The lower values were observed in SPF at 100, 200, and 400 MPa, followed by that at 300 MPa. Ahmed et al. (2017a) proved the effect of HHP (300
600 MPa) on whole WF and observed a significant decrease in particle size at 10% (Dv10), 50% (Dv50), and 90% (Dv90) of the volume distribution. Ahmed et al. (2017a, b) indicated that the rice flour showed a bimodal particle size distribution, of which the particle size significantly decreased after HHP treatment. Scanning electron microscopy Compared to SPF without HHP (0.1 MPa), no significant differences were observed in the granules of SPF treated at 100 and 400 MPa (Fig. 10.3 A, B, and C), while the rupture of the granules occurred at 300 and 400 MPa (Fig. 10.1D and E). The surfaces of starch granules in HHP-treated chestnut flour dispersions were smooth and showed a minor crack when pressure was raised up to 600 MPa (Ahmed and Al-Attar, 2017). Zhu et al. (2016) found that the majority of starch granules in brown rice flours after HHP treatment kept their integrity, while granule size increased slightly for the flours treated at 400 and 500 MPa. Differential scanning calorimetry Compared with SPF at 0.1 MPa, the TP of SPF after HHP treatment at 100
300 MPa decreased significantly, while that at 400 MPa showed no significant difference (Table 10.7). Among all the treatments, the ΔH of

Table 10.6 Color, median particle size, volume mean diameter, and area mean diameter of sweet potato flour after high hydrostatic pressure (HHP). HHP (MPa)

L

a

0.1 100 200 300 400

83.93 6 0.11 80.35 6 0.01c 80.92 6 0.02b 80.13 6 0.02d 80.01 6 0.00e a

b

0.30 6 0.00 0.65 6 0.07c 0.61 6 0.04c 0.84 6 0.05b 1.12 6 0.05a d

Size (μm)

15.11 6 0.01 12.38 6 0.01e 12.86 6 0.02d 16.73 6 1.70a 15.90 6 0.02b c

The values denoted by different letters in the same column are significantly different (P , .05).

Volume (μm)

34.86 6 2.56 18.79 6 3.69bc 14.95 6 0.80c 21.65 6 2.19b 18.71 6 1.53bc a

Area (μm)

40.63 6 3.27 20.93 6 2.28bc 16.66 6 0.96c 24.36 6 0.87b 21.09 6 1.85bc a

20.13 6 0.97a 12.37 6 2.19b 9.77 6 0.45c 13.80 6 1.30b 11.90 6 0.86bc

290

Sweet Potato

Figure 10.3 Scanning electron micrograph of sweet potato flour (SPF) after high hydrostatic pressure (HHP) (magnification: 3 1000). (A) 0.1 MPa; (B) 100 MPa; (C) 200 MPa; (D) 300 MPa; (E) 400 MPa.

SPF after HHP treatment at 100 MPa was the highest, followed by that at 400 MPa (Table 10.7). Ahmed and Al-Attar (2017) found that there was no significant increase in the TP of chestnut flour after HHP treatment. McCann et al. (2013) indicated that the TP of WF with 56% moisture increased with an increase in the pressure level, particularly at 500 and 600 MPa. Dough properties Thermal properties TP of dough with SPF after HHP was shown by two peaks, of which TP1 ranged from 69.01°C to 70.16°C and TP2 ranged from 97.85°C to 98.66° C (Table 10.7). There were no significant differences in TP1 and TP2 of dough with SPF after HHP with respect to that with SPF at 0.1 MPa. Zaidul et al. (2008) suggested that the interaction between wheat and sweet potato starch could bring anomalies in the gelatinization temperature and enthalpy. The values of the enthalpy of gelatinization (ΔH) in the first phase were from 0.76 to 0.88 J/g. In the second phase, ΔH2 was from 0.26 to 0.38 J/g, and the lower values were shown in dough with SPF at 0.1, 100, and 400 MPa. Ahmed et al. (2017a) noted that whole WF dough treated by HHP exhibited two endothermic peaks: TP1 : (63° C
66°C) and TP2 (107°C
117°C), which decreased with the pressure increased. The TP1 was ascribed to the gelatinization of starch, whereas the TP2 could be the fusion of amylose
lipid complexes formed in the

Table 10.7 Differential scanning calorimetric (DSC) of SPF and dough after HHP. HHP (MPa)

0.1 100 200 300 400

SPF

Dough

TP (°C)

ΔH (J/g)

TP1 (°C)

ΔH1 (J/g)

TP2 (°C)

ΔH2 (J/g)

78.59 6 0.08a 78.16 6 0.41b 77.32 6 0.18c 78.00 6 0.12b 78.60 6 0.26a

5.64 6 0.00c 11.86 6 0.48a 4.11 6 0.12d 5.85 6 1.56c 8.66 6 0.90b

69.35 6 0.44ab 69.01 6 0.16b 69.01 6 0.69b 69.30 6 0.51b 70.16 6 0.79a

0.79 6 0.56a 0.76 6 0.14a 0.84 6 0.01a 0.88 6 0.02a 0.82 6 0.43a

98.33 6 0.12a 97.85 6 0.38a 98.66 6 1.54a 98.22 6 0.25a 98.03 6 0.43a

0.32 6 0.02ab 0.31 6 0.00ab 0.38 6 0.00a 0.36 6 0.00a 0.26 6 0.05b

The values denoted by different letters in the same column are significantly different (P , .05).

292

Sweet Potato

course of the starch gelatinization or due to protein denaturation. Liu et al. (2009) explained that the pressurization treatment caused the destruction of the granular structure of starch followed by hydration of the amorphous phase, resulting in the decrease of the enthalpy of gelatinization. Zaidul et al. (2008) reported that the apparent shifting of slightly higher temperatures resulted in a more prominent biphasic gelatinization behavior of the mixture due to the influence of the wheat gluten. Dough fermentation Dough height (Hm) was in the range from 36.74 to 38.85 mm when treated with HHP at 100
400 MPa (Table 10.8). With regard to dough formation time (T1), there were significant differences in all treatments compared with the 0.1 MPa, which significantly decreased after HHP at 100
400 MPa. Regarding the gas behavior, the time to reach the maxi0 mum gas formation rate (T1 ) of SPF was less in the treatment of 100 MPa, while there were no significant differences at 200 and 300 MPa compared with 0.1 MPa. Gas retention of dough with SPF at 100 MPa was the highest, at 1246.5 mL. The relation between gas production and retention is given as a percentage of gas retained in the dough. Gas retention was related to dough’s ability to be stretched in a thin membrane, which was attributed to the quality of the protein network (Renzetti and Rosell, 2016). Favorable gas production and retention were essential to obtain good product quality. Thus dough with SPF after proper HHP treatment could be used in the production of bread (Table 10.8). Table 10.8 The effect of HHP on dough fermentation by rheofermentometer. HHP (MPa)

0.1 100 200 300 400

Dough development

Gas behavior 0

Hm (mm)

T1 (min)

T1 (min)

Gas retention (mL)

39.40 6 0.41a 38.85 6 0.07b 36.74 6 0.06d 36.85 6 0.07d 38.05 6 0.07c

108.00 6 4.24a 97.50 6 3.53b 95.50 6 4.94b 97.50 6 3.53b 97.50 6 2.10b

174.0 6 1.7a 148.5 6 1.4c 173.5 6 2.1a 174.02 6 1.44a 164.6 6 0.7b

1199.0 6 4.9c 1246.5 6 3.5a 1189.5 6 0.7d 1221.5 6 2.1b 1133.0 6 2.8e

Hm, maximum dough height; T1, dough formation time at which dough reaches the maximum 0 height; T1 , time to reach the maximum gas formation rate; gas retention, volume of the gas retained in the dough at the end of dough fermentation. The values denoted by different letters in the same column are significantly different (P , .05).

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Bread-making process and quality evaluation Crust and crumb color Compared with that at 0.1 MPa (59.93), the lightness of crust significantly decreased from 55.38 to 44.96 as the pressure increased from 100 to 400 MPa (Table 10.9). After HHP the a value of the crust increased and was the highest at 400 MPa. The b values decreased with the increase of HHP, which decreased from 34.88 (0.1 MPa) to 30.49 (400 MPa). It was observed that the L value of the crumb decreased from 67.53 (0.1 MPa) to 57.32 (400 MPa). The a value of the crumb at 100 MPa was the highest, which was 3.06. The b of the crumb showed the highest value in SPF at 0.1 MPa (18.48) and the lowest value at 100 MPa (16.57). Bárcenas et al. (2010) indicated that the L , a , and b values in the crust and crumb of wheat bread decreased as HHP increased from 50 to 500 MPa. Cappa et al. (2016b) found that no significant differences were observed for L and b values, whereas an increase of a value was evidenced in gluten-free bread with rice flour treated at 600 MPa. Specific loaf volume The highest specific loaf volume was obtained in bread with SPF at 400 MPa, which was 2.62 cm3/g (Table 10.9). The lowest specific loaf volume was presented in the bread with SPF at 100 MPa, which might be due to the disruption of the protein network in the dough during baking. Hüttner et al. (2010) researched the bread made with oat flour treated at 200, 350, and 500 MPa, and indicated that the specific loaf volume of bread significantly increased when 10% of oat flour treated at 200 MPa was added, while incorporation of oat batters treated at 350 or 500 MPa resulted in reduced bread quality with low specific loaf volumes and uneven gas cell distribution. Appropriate HHP treatment could provide proper dough elasticity and protein network formation to obtain a high specific loaf volume (Hüttner et al., 2009). In the case of SPF, HHP at 400 MPa might be suitable to reach specific loaf volume for sweet potato
wheat bread. Texture analysis Springiness is defined as the speed at which a deformed material returns to its initial condition after the force causing deformation is removed. There were significant differences in springiness of crust and crumb at 300 and 400 MPa with respect to that at 0.1 MPa (Table 10.10). Springiness is greatly affected by moisture content, moisture redistribution, and starch

Table 10.9 Color of crust and crumb and specific loaf volume of bread with different treatments. HHP (MPa)

Crust L

0.1 100 200 300 400



59.93 6 0.35a 55.38 6 0.21b 54.94 6 0.20b 52.53 6 0.49c 44.96 6 0.70d

a



15.46 6 0.33d 16.70 6 0.30b 15.50 6 0.10c 15.63 6 0.35c 18.40 6 0.25a

Crumb b



34.88 6 0.11a 34.80 6 0.16a 33.42 6 0.20b 33.94 6 0.69b 30.49 6 0.21c

L



67.53 6 0.16a 63.50 6 0.29b 61.26 6 1.96c 58.67 6 0.26d 57.32 6 0.33d

The values denoted by different letters in the same column are significantly different (P , .05).

a



2.04 6 0.04c 3.06 6 0.02a 2.23 6 0.19c 2.58 6 0.08b 2.77 6 0.12b

b



18.48 6 0.31a 16.57 6 0.03d 17.57 6 0.07bc 17.72 6 0.13b 17.29 6 0.03c

Specific loaf volume (cm3/g)

2.30 6 0.00c 1.46 6 0.03e 2.00 6 0.02d 2.51 6 0.03b 2.62 6 0.02a

Table 10.10 Springiness, hardness, cohesiveness, and chewiness of the crust and crumbs of sweet potato
wheat bread with sweet potato flour (SPF) after high hydrostatic pressure (HHP). HHP (MPa)

0.1 100 200 300 400

Bread samples

Springness

Hardness (N)

Crust Crumb Crust Crumb Crust Crumb Crust Crumb Crust Crumb

0.92 6 0.01 0.93 6 0.01a 0.90 6 0.01a 0.93 6 0.01a 0.78 6 0.01c 0.92 6 0.01a 0.85 6 0.02bc 0.85 6 0.02bc 0.86 6 0.01b 0.90 6 0.01b a

2278.16 6 1.96 1840.30 6 0.05e 3134.08 6 1.58a 3295.95 6 6.16b 2350.83 6 1.54b 3326.62 6 2.95a 1977.40 6 1.09d 2540.66 6 0.73c 1356.14 6 1.45e 2071.41 6 0.63d

The values denoted by different letters in the same column are significantly different (P , .05).

c

Cohesiveness

Chewiness (N)

0.48 6 0.01 0.65 6 0.01a 0.53 6 0.01a 0.58 6 0.01c 0.53 6 0.01a 0.59 6 0.01c 0.53 6 0.02a 0.62 6 0.01b 0.52 6 0.02a 0.64 6 0.01a

968.21 6 0.19b 1120.15 6 0.04e 1485.65 6 0.57a 1779.82 6 0.27a 920.19 6 1.01c 1752.71 6 0.89b 903.28 6 0.28d 1406.28 6 0.31c 600.07 6 0.99e 1193.32 6 0.97d

b

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retrogradation (Osella et al., 2005). The hardness was the force required to achieve a given deformation, which was the highest in the crust at 100 MPa and crumb at 200 MPa, and the lowest in the crust at 400 MPa and in crumb of SPF at 0.1 MPa. The hardness of gluten-free bread with rice flour treated with HHP at 600 MPa decreased significantly (Cappa et al., 2016b). However, HHP treatment on doughs at 50
200 MPa increased the hardness of wheat bread, which might be due to the protein network modification induced by HHP (Bárcenas et al., 2010). Cohesiveness is the extent to which a material can be deformed before breaking. In the crust, no significant differences were observed among all treatments, while there were significant differences in the crumb, being firstly decreased and then increased as the pressure increased. The changes in cohesiveness of sweet potato
wheat bread were not consistent with the report by Angioloni and Collar (2012), who demonstrated that the incorporation of pressured flours from wheat, oat, millet, and sorghum into bread formulations provoked a significant general decrease in crumb cohesiveness. Considering that dough cohesiveness has been reported as a good predictive parameter of fresh bread quality, the maximization of dough cohesiveness is a recommended trend for providing good breadmaking performance. Chewiness is the energy required to chew solid food until it is in the appropriate state to be swallowed. The lowest value was at 400 MPa in crust, and the chewiness in the crust and crumb decreased as the pressure increased. It was reported that the bread quality could be significantly improved when the specific volume was increased and hardness and chewiness was reduced (Renzetti et al., 2010). Thus SPF treated at 400 MPa could be potentially used in the production of breads with acceptable texture.

Development of sweet potato noodles Noodles are one of the Chinese’s favorite foods due to their acceptable taste to almost all age groups, availability at affordable prices, and the fact they can be produced by small-, medium-, or large-scale industries. The main ingredient of noodles is WF (Krishnan et al., 2012). In recent years, more and more studies have been carried out to improve the nutritional properties of noodles by adding other flours or functional components, such as sweet potato, yam, oat, corn, potato, wheat germ, barely, buckwheat, dietary fiber, polyphenols, etc. (Chillo et al., 2008; Gelencsér et al., 2008; Iafelice et al., 2008). Sweet potatoes have been

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reported to contain high amounts of dietary fiber, β-carotene, phenolic compounds, etc., and possess high antioxidant activity (Bovellbenjamin, 2007; Teow et al., 2007). Several researchers have tried the addition of sweet potato in making noodles (including traditional Chinese noodles, pasta, etc.) (Ginting and Yulifianti, 2015; Krishnan et al., 2012). Some of them are introduced here.

Traditional Chinese noodles prepared with sweet potato mash and wheat flour Ginting and Yulifianti (2015) prepared traditional Chinese noodles with sweet potato mash (40%) and WF. The processing method of sweet potato mash was that the fresh roots of sweet potatoes were steamed, the skins were removed, and then they were mashed to obtain a paste/mash. The physicochemical and sensory properties of the abovementioned noodle samples were analyzed. Results showed that a blend of 60% domestic WF with 40% sweet potato mash could improve the noodle color acceptance. The noodles prepared from 100% WFs and the blend with 40% sweet potato mash both met the national standard quality for moisture and protein content, which suggested that sweet potato mash shows promise as a WF substitute in noodles.

Pasta prepared with sweet potato flour and wheat flour Pasta has its origin in Italy and has gained wide popularity as a convenient and nutritionally palatable food (Petitot et al., 2009). Although traditionally pasta is made from durum wheat semolina which provides the desired texture and cooking quality to the product, wheat semolina proteins are deficient in lysine and threonine leading to low biological value for the product (Stephenson, 1983). Gopalakrishnan et al. (2011) prepared pasta with SPF and WF. The processing method of SPF was as follows: sweet potato roots were peeled and sliced to 0.5 cm thickness. The slices were soaked in acetic acid (1.0% w/v; 1.0 kg sweet potato slices per 5.0 L water) for 1 h to eliminate the browning problem, after which they were washed in running water and dried in sunlight for 36 h. Dried chips were powdered in a blender and sieved (mesh: 355 μm) to obtain fine SPF. All the formulations had 27% refined WF and 3% gelatinized cassava starch. The content of SPF was 50%
70%, and the rest was whey protein concentrate, defatted soy flour, or fish powder. Pasta was extruded at room temperature (30°C 6 1°C)

298

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using the round die (No. 62) and cut to short pieces of length 3.0 cm. The freshly extruded pasta tubes had an internal diameter of 0.5 cm and were dried at 50°C in an air oven for 18 h to get a product with ,12.0% moisture content. The hydration level, swelling index, cooking loss, protein nutritional quality, in vitro starch digestibility, and the size, shape, and arrangement of particles in the pasta matrix were analyzed. Results showed that all samples exhibited high swelling index and significantly high lysine and threonine contents. Whey protein concentrate-fortified sweet potato pasta had high values for essential amino acid index, biological value, nutritional index, and protein efficiency ratio. In vitro starch digestibility progressed slowly over a period of 2 h for all samples, with the lowest values for the whey protein concentrate-fortified pasta.

Prospect of sweet potato staple foods Sweet potato staple foods have the special flavor of sweet potato, and could be a good source of dietary fiber, minerals, and vitamins. However, the current processing technologies of staple foods are not suitable for making sweet potato staple foods, and sweet potato lacks the gluten protein, thus making it difficult to form into a stable dough structure. Thus a series of key production techniques and technical problems during sweet potato staple foods processing is one of the important means to successfully prepare sweet potato staple foods. In addition, the addition of different food components, such as starch, protein, polysaccharide, and hydrocolloid from other food sources, is necessary to form a stable dough structure and to obtain an acceptable quality of sweet potato staple foods. Meanwhile, the storage condition also has a significant effect on the quality of sweet potato staple foods. Thus studying the effects of sweet potato type/amount, packaging atmosphere, storage temperature, and storage period on color, β-carotene concentration, and sensory attributes of sweet potato staple foods is also necessary.

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1853.

CHAPTER 11

Sweet potato snack foods Tai-Hua Mu, Hong-Nan Sun and Meng-Mei Ma

Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China

Introduction Sweet potatoes have become a research focus in recent years due to their unique nutritional and functional properties (Wang et al., 2016). Bioactive carbohydrates, proteins, lipids, carotenoids, anthocyanins, conjugated phenolic acids, and minerals represent versatile nutrients in different parts (tubers, leaves, stems, and stalks) of the sweet potato. Sweet potato root starch with its unique physicochemical properties is particularly valued as a functional food ingredient (Zhu and Wang, 2014). Yellowand orange-fleshed sweet potatoes contain a blend of phenolic acids (i.e., hydroxycinnamic acids) and have relatively high levels of carotenoids (i.e., β-carotene). Purple-fleshed sweet potato has high levels of acylated anthocyanins and other phenolics with antioxidant and antiinflammatory activities (Grace et al., 2014). Anthocyanins of purple sweet potatoes possess aromatic acylated glycosyl groups, and exhibit relatively high pH tolerance and thermostability (Kim et al., 2012). The unique composition of sweet potato contributes to their various health benefits, such as antioxidative, hepatoprotective, antimicrobial, antiobesity, antiinflammatory, antidiabetic, antitumor, and antiaging effects (Wang et al., 2016). Considering the many varieties of sweet potato processing products, this chapter will mainly introduce a series of common sweet potato snacks, for example, sweet potato chips, roasted sweet potatoes, sweet potato biscuit, dried sweet potato slices, sweet potato cakes, doughnuts, and extruded sweet potato snacks.

Roast sweet potato Roast sweet potatoes have become a famous street snack in China, and some other Asian countries, such as Japan and Korea, because of their sweet and delicious taste and the tender texture in the outer coke. Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00011-9

© 2019 Elsevier Inc. All rights reserved.

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Recently, yellow-, orange-, and purple-fleshed varieties have been used to make roast sweet potatoes owing to their healthful components, and many researchers have studied how these components change during the roasting process.

Effects of home-processed methods on the content of carotenoid and anthocyanin from different sweet potato varieties Kim et al. (2015) collected nine freshly harvested sweet potato cultivars, and studied the effects of different cooking methods, such as baking, boiling, frying, steaming, and pressure-cooking, on the different kinds of carotenoids, anthocyanins, and antioxidant capacity of sweet potatoes. For the yellow-fleshed sweet potatoes, Juhwangmi and Sinhwangmi showed the highest levels of total carotenoids, which were 665 and 500 mg/g dry weight (DW), respectively. For the purple-fleshed sweet potato, Sinjami and Yeonjami exhibited the highest anthocyanin contents, which were 7.14 and 2.98 mg/g DW, respectively. The total carotenoids, β-cryptoxanthin, and all-trans-β-carotene of Sinhwangmi were decreased after all of the different cooking methods, and the contents of these components in roasted, boiled, and steamed sweet potatoes were higher than that of other cooking methods. However, the contents of 13Z-β-carotene and 9Z-β-carotene were increased after cooking by all methods. The anthocyanin content of sweet potatoes was decreased after cooking by all the home-processed methods, however, the anthocyanin content in roast, boiled, and steamed bread was higher than that of the other methods. The color results showed that the lightness (L ), redness (a ), and yellowness (b ) values were decreased after different cooking methods. Kim et al. (2012) compared the effects of roasting and steaming on the anthocyanin changes in the purple-fleshed sweet potato (cultivar Shinzami) using liquid chromatography-diode array detector-electrospray ionization/mass spectrometry (LC-DAD-ESI/MS). Before roasting and steaming, the total anthocyanin content was 1342 mg/100 g DW in the raw sweet potatoes, and 15 kinds of individual anthocyanin were detected. The amounts of peonidin 3-caffeoyl-p-hydroxybenzoyl sophoroside-5-glucoside and di-acylated cyanidin 3-caffeoyl-p-hydroxybenzoyl sophoroside-5-glucoside were the highest, at 566 and 137 mg/100 g DW, respectively. After roasting and steaming, the main two individual anthocyanins were same as that of the raw materials, however, the contents of each individual and total anthocyanin were significantly decreased

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after steaming, while roasting could protect from the loss of anthocyanin. For example, the contents of total anthocyanin were 751 and 1086 mg/100 g DW after steaming and roasting.

Influence of roasting treatment on the sugar composition and starch morphology of sweet potatoes Apart from nutritional and functional components, the physicochemical and sensory characteristics are also related to the quality of roast sweet potatoes, and any change of these properties has to be carefully monitored during the food processing. The sugar content is the key factor for determining the sensory evaluation of roast sweet potatoes, however, some previous studies only focused on the carbohydrates. In addition, some previous studies showed that thermal treatment could change the textural properties of sweet potatoes, for example, the starch granules, which gelatinize at high temperatures, have a significant influence on textural properties (Koehler and Kays, 1991; Lindeboom et al., 2010). Therefore the study of starch morphology will be useful for understanding the changes of the textural properties of roast sweet potatoes. Lai et al. (2013) investigated the changes of sugars, such as fructose, sucrose, maltose, glucose, and total sugars during the process of roasting, and observed the electronic micrographs. Results showed that the total sugar content in the raw fresh sweet potatoes ranged from 5.08% to 8.41%, and the main sugars were sucrose (2.52% 7.77%) and glucose (0.38% 2.02%), followed by fructose (0.24% 1.06%). In the roasted sweet potatoes, the content of sucrose and glucose was significantly decreased, in the range of 1.53% 7.45%, and 0.31% 1.37%, respectively. However, the maltose content was increased, and was between 8.81% and 13.97%. Electronic micrographs of fresh sweet potato samples showed that the starch granules were oval-shaped, and the size of them was generally less than 20 μm. After the roasting treatment, starch granules completely gelatinized.

Aroma components of roast sweet potatoes Roast sweet potatoes are a snack food with a long history in China, and their rich and attractive flavor is greatly popular with large customers. Many researchers have focused on the nutritional and functional composition changes during the process of roasting. Besides this, the aroma components and their mechanisms of formation should also be known. Some researchers have analyzed the aroma components of roast sweet

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potato by gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS), gas chromatography olfactometry (GCO), and aroma extract dilution method combined with GCO method. The Maillard reaction and caramel reaction occur during the process of sweet potato roasting, which play an important roles in the aroma of sweet potatoes. In addition, the synthesis of the sweet potatoes’ characteristic aroma also involves enzymatic reactions. Different kinds of aroma components were detected, and the variety of aroma substances depended on the testing instruments and the varieties of sweet potatoes. For example, 8, 48, and 75 kinds of aroma components were detected from roast sweet potatoes using gas chromatography-headspace (GC-HS), gas chromatography-flame ionization detector (GC-FID), and GC-MS (Tiu et al., 1985; Nakamura et al., 2013). Furthermore, the most important aroma component is maltol, which can be endowed with the sweet smell of roast sweet potatoes, and maltose is the main precursor of many volatile compounds in the roast sweet potato. The flavors of cooked sweet potatoes were significantly different as a result of roasting, boiling, and microwave cooking. Results determined by GCO showed that the kinds of aroma components were 37, 20, and 32 from roasted, boiled, and microwave-cooked sweet potatoes, respectively. The main reasons for the faint scent of boiled and microwaved sweet potato might be as follows: (1) lower heating temperature could inhibit Maillard reaction; (2) the leaching of water-soluble substances and the loss of polar compounds near the surface of sweet potato could lead to the loss of water phase, thus leading to the decrease of the volatile compounds in the roasted sweet potatoes; (3) because of the high thermal conductivity of water, boiling water can easily inactivate the key enzymes involved in the release of aroma precursors, such as maltose, and the release of bound aromatic substances, such as glycoside release; (4) the lack of terpene aroma compounds, a series of microwave baking and sweet potato aroma terpenes’ great contribution (such as linalool, geraniol, copaene) does not appear in the cooked sweet potato; (5) low temperature and isothermal treatment conditions are not conducive to the caramelization reaction and Maillard reaction, and thus under the condition of microwave radiation are very unlikely to happen; and (6) the rapid heating system inactivates the amylase, maltose is one of the main precursors of many baked sweet potato volatile compounds, the rapid heating and microwave treatment of the passivation system is essential to produce amylase hydrolysis of starch and maltose, hampering the production of maltose (Sun et al., 1994; Wang and Kays, 2001).

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Pollution analysis and safety evaluation of roasted sweet potatoes Nowadays, there are many problems in the food safety of cooked food, and many more people pay attention to them. In recent years, many reports of polycyclic aromatic hydrocarbons, such as polycyclic aromatic hydrocarbons, have been reported in the roasted food. Roast sweet potato is one of the most common products in sweet potato processing products and for the analysis of the food safety of roast sweet potato, the acrylamide content needed to be analyzed and detected. Zhang et al. (2012) determined the acrylamide content of roast sweet potato bought from supermarket and roasted in the oven. The results showed that the acrylamide content in the skin of roast sweet potato was 4.73 μg/kg, and there was no determination in the central layer of flesh. Lu et al. (2011) collected roast sweet potato from the supermarket, and determined the content of S, Cu, Pb, Hg, and As, and found that the skin, subcutaneous flesh, and central layer flesh of roasted sweet potato were highly polluted by S and heavy metals with comprehensive pollutant indexes of 209.1, 24.0, and 3.9, respectively, which indicated that roast sweet potato made by coal is very dangerous, and it should be forbidden in terms of food safety.

Frozen roasted sweet potatoes The traditional roasted sweet potato has short shelf life, is not easy to store, has a small circulation radius, and is easily restricted by season, so it cannot meet the daily needs of consumers. Therefore it is urgent to develop a product with a long shelf life, good taste, high nutritional value, and open bag (or heated) ready-to-eat roasted sweet potato, so as to improve the fresh food proportion of sweet potato and promote the healthy development of the sweet potato industry. Houqin (2013) made a kind of quick-frozen sweet potato conditioning product by adding many other ingredients besides roasted sweet potato flesh, such as sugar, milk powder, custard powder. Xianyao (2014) and some other researchers from South Korea and Japan studied a method for processing frozen roasted sweet potato without any food additives, the technological flow was as follows: sweet potato was collected, washed, and roasted for 200°C 280°C for 60 120 min (depending on the size of sweet potato), and then was frozen for 2 4 h under 16 to 21°C.

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However, there are many sweet potato cultivars in the world, and the quality of frozen roasted sweet potato cultivars may not all be excellent under the condition of slow freezing. Thus further studies might focus on the screening of special varieties of roasted sweet potato, and explore the optimum freezing conditions.

Sweet potato chips or French fries The classification of sweet potato chips or French fries In general, the term “chips” or “French fries” means fried slender pieces of potatoes in the United States and most of Canada. While in Australia, Ireland, New Zealand, and the United Kingdom, the fried potatoes which are cut thinly are called skinny fries or shoestring fries to distinguish from chips, which are cut thicker. French fries sometimes can be made from sweet potatoes instead of potatoes, and are usually eaten together with salt, tomato sauce, vinegar, mayonnaise, etc. There are two groups of chips, that is, traditional or general chips and simulation chips. General chips are made from the processes of cutting and cleaning, thin cutting, and frying; while simulation chips are made from flour which undergoes the process of mixing, thin layer forming, molding, and frying. Compared to general chips, the simulation chips have some advantages such as (1) form and size can be molded as preferred and in a uniform manner; (2) seasoning can be applied easily; and (3) higher yield (Elisabeth, 2015).

The traditional or general sweet potato chips or French fries Bovell-Benjamin (2007) prepared a French fries-type product from Jewel and Centennial sweet potatoes. The sweet potato roots were washed, lye peeled, sliced into strips, and blanched in hot water containing 1% sodium acid pyrophosphate. The blanched strips were partially dried at 121°C. The dehydrated strips were frozen until fried at 175°C. Panelists evaluated the color, flavor, and texture of the products on a five-point scale. The flavor and texture results indicated that a good product could be prepared from both cultivars of sweet potato. French fries primarily contain carbohydrates (mostly in the form of starch) and protein from the sweet potato, fat absorbed during the deepfrying process, and some of the highest levels of acrylamides of any foodstuff. According to the American Cancer Society, it is not clear as of 2013

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whether acrylamide consumption affects people's risk of getting cancer. A meta-analysis indicated that dietary acrylamide is not related to the risk of most common cancers, but could not exclude a modest association for kidney, endometrial, or ovarian cancers (Pelucchi et al., 2014). Vacuum frying is an alternative method to produce fried foods with superior product quality attributes (Da Silva and Moreira, 2008; Nunes and Moreira, 2009) and low acrylamide content (Granda and Moreira, 2005). Ravli et al. (2013) carried out a study on a two-stage (TS) frying process for high-quality sweet potato chips. In their study, vacuum frying (1.33 kPa), with the aid of a deoiling mechanism, was used to produce low-fat sweet potato chips. The kinetics of oil absorption and oil distribution in the chips (total, internal, and surface oil content) was studied so that effectiveness of the deoiling system could be established. Product quality attributes (PQAs) such as moisture content, oil content, diameter shrinkage, and thickness expansion, as well as, color, texture, bulk density, true density, and porosity of chips fried at different temperatures (120°C, 130°C, and 140°C) was performed to evaluate the effect of process temperature on the product. The final oil content of the vacuum fried chips was 60% lower than those found in traditionally fried sweet potato chip, which indicates that the deoiling mechanism is crucial in the vacuum frying process. The rate of change in PQAs is greatly affected by temperature; however, the final values of bulk density, true density, porosity, diameter shrinkage, and thickness expansion were not affected by temperature. The structure of the chips settled faster when fried at 130°C 140°C. Color b values were not affected by the range of temperature used in this study. The product fried in a TS frying process (1 min fried at atmospheric pressure and 2 min under vacuum) had better appearance and texture compared to the ones that were only fried under vacuum or single-stage (SS) conditions. The samples were lighter and more yellow (less compact) than the chips fried under the SS process. The atmospheric frying prior to vacuum frying helped the starch to gelatinize, thus producing a better product in terms of texture, oil content, and flavor. The final oil content of the TS fried chips was 15% lower than those fried by the SS process, showing that the structure of the chips formed during the process affected the oil absorption during frying.

The simulation sweet potato chips Sweet potato chips are deeply loved by consumers, and different consumers have different requirements for the flavor of sweet potato chips, so

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the simulation sweet potato chips came into being. More recently consumers are increasingly demanding products that are additive-free (Oke and Workneh, 2013). Oh et al. (2017) studied the effects of pulsed infrared (IR) radiation followed by hot-press (HP) drying on the properties of mashed sweet potato chips to develop an additive-free dried sweet potato snack. To provide a crispy texture, sweet potatoes were dried by a TS process. At the first drying stage, steamed sweet potatoes were semidried for 6 h using hot-air convection or pulsed IR radiation, and the drying rate was compared under varying sample thicknesses and drying temperatures. The IR exhibited enhanced drying speed, in particular the IR radiation at 60°C was favorable for application to the drying of sweet potatoes with large thickness. For the secondary drying, the IR-dried sweet potatoes with varying moisture content were applied to HP drying at 180°C for 2 s. The quality of final products indicated that the crispy texture of the products was generated when the semidried sample had a moisture content lower than 0.5 kg/kg dry base (d.b.). When the moisture content of samples prior to the HP process was lower than 0.3 kg/kg d.b., the final product was easily broken and had a discoloration to dark-brown. Considering the entire processing procedure, they demonstrated that IR radiation at 60°C for 5 h followed by HP was an effective combination for the mass production of a crispy sweet potato snack.

Dried sweet potato slices Besides roast sweet potato, sweet potato bread, biscuits, and chips, and dried sweet potato slices can also be regarded as a kind of healthy food. What is more, the dried sweet potato slices can provide various advantages, such as a long shelf life, convenient application, and increased concentration of nutrients (Kim and Chin, 2016). The conventional processing method of dried sweet potato slices is with sunshine, which takes more than 50 hours and the obtained products may exhibit inferior quality with high microbial contamination (Silayo et al., 2003). In order to reduce the drying time, to obtain better quality products, to avoid the dependency on the weather, and to reduce microbial contamination of the product, some mechanical dryers have been applied to process dried sweet potato slices (Jayaraman and Gupta, 2006). Besides the above advantages, much more water could be removed by mechanical dryers, thus reducing the mass and volume, and improving the efficiency of packaging, storing, and transportation

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(Jayaraman and Gupta, 2006). Therefore some researchers investigated some kinds of mechanical dryers, such as high-velocity cross-flow hot-air dryer, pulsed IR radiation followed by HP drying, osmotic dewatering combined with oven drying, etc., on the qualities properties of dried sweet potato slices.

High-velocity cross-flow hot-air dryer The high-velocity cross-flow hot-air dryer is one of the most popular dryers at present. It is characterized by the vertical flow of hot air and the flow of samples. During the process of a traditional hot-air dryer, the sample near the side of the hot-air chamber is always in contact with the high-temperature drying medium, and the water loss is faster. The sample near the exhaust chamber is always in contact with the medium with low temperature and humidity, and the water loss is slower, so the sample drying is very uneven. However, the high-velocity cross-flow hot-air dryer could resolve the problem of drying inhomogeneity. Singh et al. (2012) investigated the effect of drying temperature (50°C 90°C), air-flow rate (1.5 55 m/s), and sample thickness (5 12 mm) on the moisture ratio, drying rate, and diffusion coefficients of sweet potato slices, and confirmed model equations. The results showed that during the initial period of drying, the drying rate was similar in all the drying conditions. After the removal of surface moisture, increasing the drying temperature caused an important increase in the drying rate, thus the drying time was decreased. For the sweet potato thickness, as the thickness of sample increased, the drying time increased due to an increased diffusion path. During the process of drying, the activation energy for moisture diffusion was 11.38 kJ/mol.

Pulsed infrared radiation followed by hot-press drying As we know that, for some ready-to-eat dried sweet potato slices, sweet potato should be steamed completely prior to drying, leading to higher starch gelatinization and increased sweetness generated during the steaming process. A higher content of sugar in the steamed sweet potato can lower the drying rate and make a hard texture when the drying process is completely finished (Oke and Workneh, 2013). Therefore some alternative techniques should be applied to accelerate the drying rate and improve the final quality of dried sweet potato slices.

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IR drying is an effective method compared to hot-air drying, and some researchers reported that IR drying could decrease the drying time of vegetables and fruits and improve their quality (Nowak and Lewicki, 2004; Sharma et al., 2005). However, this method with intensive heat may cause some adverse influence, such as surface overheating, lipid oxidation, and some nutrients damage (Doymaz, 2012; Oh et al., 2017). Therefore IR combined with HP is a good method to produce high quality, and crispy textured dried sweet potato slices. Oh et al. (2017) showed that IR radiation at 60°C for 5 h was favorable for application for the drying of sweet potatoes with large thickness, and 180°C for 2 s was applied for HP drying; the above combination was effective for the mass production of dried sweet potato slices.

Osmotic dewatering combined with oven drying In order to improve nutritional, sensorial, and functional properties of dried sweet potato slices without changing their integrity, different concentrations of NaCl (10% 30%, w/v) were used to pretreat sweet potato slices for different durations (20 100 min) and then the sweet potato slices were dried by oven drying. Results showed that the amount of water removed by pretreatment with NaCl solution decreased significantly as the concentration and the time soaking increased, NaCl solution pretreatment had no effect on the content of β-carotene, but subsequent oven drying degraded the β-carotene content (Clifford et al., 2014).

Effect of different drying methods on the quality properties of dried sweet potato slices Zhao et al. (2013) investigated the effects of different drying methods, such as oven drying, microwave drying, and oven combined with microwave drying, on the drying time, water retention capacity, oil holding capacity, and sensory evaluation of dried sweet potato slices. Microwave drying exhibited the shortest time for drying the same quality of sweet potato slices, while oven drying exhibited the longest time, this might be because sweet potato is a bad conductor, the heat transfer is slow, and the temperature gradient is not consistent with the direction of the humidity gradient, so the drying time is longer. The dried sweet potato slices obtained by microwave drying had the lowest water retention capacity and oil retention capacity, followed by oven drying and oven combined with microwave drying. This might be

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because the temperature gradient of wet material is consistent with the moisture gradient, which is conducive to the migration of moisture in the material, thus increasing the diffusion rate of moisture and speeding up the drying process during the process of microwave drying, so the structure of sweet potato was damaged to some extent, and decreased the immersion of water and oil. However, the combined drying by hot air and microwave can avoid the rapid escape of water during the drying process, causing little damage to the material structure and is conducive to the immersion in external moisture and grease. Sensory evaluation consists of color, surface shrinkage, and sense of taste. Results showed that oven combined with microwave drying had the best sensory evaluation, followed by microwave drying and oven drying. For example, the color of dried sweet potato had become yellowish brown, and with a small amount of brown spots after being dried by the oven. Microwave drying can inhibit the enzyme activity of sweet potato, which can prevent the occurrence of browning. In addition, microwave drying can penetrate sweet potato slices, accelerating the mass transfer rate, thereby reducing the drying shrinkage of the slices, however, it is very difficult to control the drying rate and time, thus affecting the sense of taste and color. Oven combined with microwave drying can combine the advantages of oven and microwave drying.

Sweet potato cakes The concept and classification of cakes Cake is a kind of sweet dessert which is typically baked. A long time ago, cakes were modifications of breads, but now cakes cover a lot of preparation methods which can be simple or complicated, and share features with other snacks such as meringues, custards, pastries, and pies. The ingredients in typical cakes include flour, eggs, butter, oil, sugar, baking soda, etc. Sometimes additional ingredients including nuts, fruits, cocoa, and so on will also be used. Cakes are broadly divided into several categories, based primarily on their ingredients and mixing techniques. Butter cakes are made from creamed butter, sugar, eggs, and flour. They rely on the combination of butter and sugar beaten for an extended time to incorporate air into the batter (Robbins, 2018). A classic pound cake is made with a pound each of butter, sugar, eggs, and flour. Baking powder is in many butter cakes, such as a Victoria sponge (Cloake, 2018). The ingredients are sometimes

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mixed without creaming the butter, using recipes for simple and quick cakes. Sponge cakes (or foam cakes) are made from whipped eggs, sugar, and flour. They rely primarily on trapped air in a protein matrix (generally of beaten eggs) to provide leavening, sometimes with a bit of baking powder or other chemical leaven added as insurance. Sponge cakes are thought to be the oldest cakes made without yeast. An angel food cake is a white sponge cake that uses only the whites of the eggs and is traditionally baked in a tube pan. The French Génoise is a sponge cake that includes clarified butter. Highly decorated sponge cakes with lavish toppings are sometimes called gateau, the French word for cake. Chiffon cakes are sponge cakes with vegetable oil, which adds moistness (Medrich, 1997). Chocolate cakes are butter cakes, sponge cakes, or other cakes flavored with melted chocolate or cocoa powder (e.g., brownies) (Berry, 2018). German chocolate cake is a variety of chocolate cake. Fudge cakes are chocolate cakes which contain fudge. Coffee cake is generally thought of as a cake to serve with coffee or tea at breakfast or at a coffee break. Some types use yeast as a leavening agent while others use baking soda or baking powder. These cakes often have a crumb topping called streusel or a light glaze drizzle. Baked flourless cakes include baked cheesecakes and flourless chocolate cakes. Cheesecakes, despite their name, are not really cakes at all. Cheesecakes are in fact custard pies, with a filling made mostly of some form of cheese (often cream cheese, mascarpone, ricotta, or the like), and have very little flour added, although a flour-based or graham cracker crust may be used. Cheesecakes are also very old, with evidence of honey-sweetened cakes dating back to ancient Greece. Butter or oil layer cakes include most of the traditional cakes used as birthday cakes, etc., and those sold as packaged cakes. Baking powder or bicarbonate of soda is used to provide both lift and a moist texture. Many flavorings and ingredients may be added, examples include devil's food cake, carrot cake, and banana bread. Yeast cakes are the oldest and are very similar to yeast breads. Such cakes are often very traditional in form, and include such pastries as babka and stollen.

Effect of adding different proportions of sweet potato on the quality of cakes Selvakumaran et al. (2017) determined the physicochemical properties of orange-fleshed sweet potato-enriched brownies and investigated the consumer’s acceptance level on different formulations of orange-fleshed sweet potato-enriched brownies. The raw orange-fleshed sweet potatoes were

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washed with tap water, manually peeled, and sliced to 10 mm thickness. Then they were steamed with a commercial steamer for 20 min and mashed into puree. A total of four brownie formulations were prepared. Wheat flour was substituted by 25%, 50%, and 75% orange-fleshed sweet potato puree. First, butter and chocolate chips were melted using a double boiler method and cooled to room temperature. Then, eggs, oil, brown sugar, orange-fleshed sweet potato puree, and vanilla extract were mixed in a bowl. The mixture was then combined with the cooled chocolate mixture. Wheat flour, cocoa powder, and baking powder were then added into the mixture. The final mixture was placed in a baking tin and was oven-baked at 180°C for 23 min. After baking, it was removed from the baking tin and left to cool for 1 h at room temperature. Results showed that the substitution of wheat flour with orange-fleshed sweet potato puree in brownies formulations significantly increased total dietary fiber in the order: 75% orange-fleshed sweet potato (6.41%) . 50% orange-fleshed sweet potato (5.13%) . 25% orange-fleshed sweet potato (3.24%) . 0% (1.70%). Additionally, moisture, fat, and specific volume were elevated with increased amount of orange-fleshed sweet potato puree. The color of brownies was affected with decreases of L , a , and most prominently b . Hardness, adhesiveness, gumminess, cohesiveness, and chewiness of brownies reduced significantly with the incorporation of up to 50% orange-fleshed sweet potato puree, but 75% orange-fleshed sweet potato puree brownies were not significantly different to 50% orange-fleshed sweet potato puree brownies. Springiness and resilience had more prominent impacts with the increased amount of orangefleshed sweet potato puree. Sensory scores of appearance and color of enriched fiber did not differ significantly with control brownies. However, texture, flavor, and overall acceptability of the brownies were most preferred for 50% and 75% orange-fleshed sweet potato puree substitution. The study suggests that orange-fleshed sweet potato substitutions at 50% and 75% are suitable to increase the fiber in brownies with little effects on the appearance and were able to improve sensory attributes.

Sweet potato biscuits The etymology of the biscuit is “the bread which is toasted two times,” from French bis (again) and cuit (bake). Biscuits are baked with some kind of flour, water, or milk, and some kinds of biscuits are also processed by adding yeast. It can be used as a storage food for travel, navigation, and

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mountaineering. It is also very convenient for military personnel’s spare food during wars. At the beginning the biscuit industry was mainly concerned with long-term sea travel and war, in order to provide nutritional and emergency food with a long shelf life. The production of biscuits was based on handmade-type (manual transmission form), but after the industrial revolution due to mechanical technology, the rapid development of production equipment and technology of biscuits, spread across the whole world. Biscuits include hard biscuits, crisp biscuits, and fermented biscuits. Traditionally, biscuits are made of wheat flour, corn flour, and some other cereal flours. As we already know, sweet potato is rich in protein, dietary fiber, vitamins, minerals, and some other nutrients—it is an allround nutritious food which is recognized worldwide. In recent years, with the rapid development of China’s economy and the improvement of the living standard of its residents, the demand for nutritious and healthy food is increasing. Under this background, using sweet potato as a raw material for making biscuits can not only enrich the types of sweet potato food and increase the consumption of sweet potato, but can also improve on the disadvantages of single nutritional components of the existing biscuit products. This could be of great significance for improving the dietary nutrition and structure of Chinese residents.

The concept and classification of sweet potato biscuit Sweet potato biscuit is a kind of delicious food, of which the main ingredients are sweet potato, wheat flour, and sugar. To obtain the sweet potato product which is used in biscuit-making, there are mainly four types or process: (1) sweet potato is made into mash after being steamed completely; (2) sweet potato is made into paste directly; (3) sweet potato can be made into flour after steaming (or not steaming), drying, and smashing; and (4) sweet potato residue obtained after starch extraction can also be used in biscuit-making. According to the different processing methods, sweet potato biscuits can be divided into the following categories: 1. Hard sweet potato biscuit: less oil and sugar are used during the processing of hard biscuit; when making the dough, it is easy to form gluten; the dough is rolled and extended to form a thin slice, which is then baked in the roaster. 2. Crisp sweet potato biscuit: much more oil and sugar, less water are used; not much more gluten is formed during the processing of the crisp biscuit.

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3. Fermented sweet potato biscuit: with sugar, oil, yeast, and some other auxiliary material, yeast as a loose agent; then processed by fermentation, molding, and baking.

Dietary fiber-enriched biscuits Sweet potato residue is generated in the processing of starch, and is rich in dietary fiber. Dietary fiber cannot be digested by the enzymes in the intestine, but can promote intestinal peristalsis, chelate cholesterol, and inhibit the hypertension, hyperlipidemia, hyperglycemia, and obesity, thus it is regarded as “the seventh nutrient.” Therefore sweet potato dietary fiber can be used to process biscuit, so as to increase the variety of high dietary fiber food, and meet the demands of the market. Ni et al. (2011) and Wang et al. (2015) extracted dietary fiber from sweet potato residue, and determined the best formula and processing technology of biscuits. Results showed that the hardness increased with the increase of dietary fiber addition (10% 40%), sensory evaluation and crispness was increased when the dietary fiber was increased from 10% to 30%, and then decreased when the dietary fiber addition further increased to 40%, thus 30% dietary fiber addition was chosen as the best addition amount. The best formula and baking conditions were obtained by single factor and orthogonal test, the results were as follows: shortening 30%, ammonium bicarbonate 0.6%, sweet potato dietary fiber 30%, sugar 30%, baking soda 0.4%, eggs 5%, salt 0.6%. The best baking condition is 180°C for 8 min.

Some other kinds of biscuits enriched with sweet potato Iron deficiency (anemia) and vitamin A deficiency are the most common nutritional disorders worldwide, and mainly affect preschool children and pregnant woman, contributing to 20% of all maternal mortality, and are considered public health conditions of epidemic proportions (Infante et al., 2017). Infante et al. (2017) prepared sorghum biscuits with biofortified sweet potato carotenoids, and evaluated their acceptance, nutritional composition, and iron bioavailability. They prepared four kinds of biscuits: (1) sorghum biscuit with 100% dry sorghum flour; (2) extruded sorghum biscuit with 100% extruded sorghum flour; (3) enriched biscuit with 50% dry sorghum flour and 50% sweet potato flour; and (4) enriched extruded sorghum biscuit with 50% extruded sorghum flour and 50% sweet potato

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flour. Nutritional analysis showed that the carotenoids (from 0 to 28.09 mg/100 g) and total polyphenol content (from 0.19 to 33.48 mg EGA/100 g) were significantly increased with the addition of sweet potato flour. Animal experiments showed that the iron bioavailability of biscuits which were made of extruded sorghum and sweet potato flour groups were similar to that of the ferrous sulfate control. In addition, high total antioxidant capacity and adequate expression of the intestinal proteins were related to the absorption of iron. The above results showed that biofortified sweet potato carotenoids can increase nutritional and sensory quality of the biscuits, allowing their potential as a functional food to reduce the risk of iron deficiency.

Sweet potato doughnuts The concept and classification of doughnuts Doughnuts are a kind of ring-shaped snack food popular in many countries, which are usually deep fried from flour doughs. After being fried, doughnuts can be spread with chocolate or icing on top, covered with powdered sugar or fruit, or glazed with sugar icing. Generally, people like to enjoy doughnuts together with a cup of coffee or milk.

Effect of adding different proportions of sweet potato on the quality of doughnuts Collins and Aziz (1982) used the Jewel cultivar of sweet potato as raw material. This has orange-colored flesh and is classified as a “moist” type of sweet potato. Firstly, they processed the sweet potato roots to flour, obtained puree from baked roots, and obtained puree from steam-cooked flesh. The following ingredients were used in the recipe for preparing yeast-raised doughnuts: wheat flour (all purpose) 425.4 g, cane sugar 56.8 g, salt 7.1 g, nonfat milk solids 21.3 g, margarine 42.6 g, water 180.0 g, whole egg 46.4 g, vanilla extract 2.0 g, mace 1.8 g, active dry yeast (suspended in water) 28.4 g, and sodium stearoyl-2-lactylate 3.8 g. Doughnuts were prepared according to the recipe presented above, but a portion of the flour was replaced with an equivalent portion of sweet potato ingredient (0, 7%, 14%, and 21% of the wheat flour), and water was adjusted to give the desired dough consistency. Then the cut pieces of dough were fried in soybean oil at 191°C using a small commercialtype fryer. The doughnuts were heated on each side for 50 s. The proximate composition, color, firmness, volume, and sensory of the doughnuts

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samples were evaluated. Results showed that, sweet potato as flour and puree can be used as an ingredient of yeast-raised doughnuts. Measurements and analyses of samples which contained sweet potato up to 21% showed that certain attributes and their components were altered by the addition of sweet potato. From a quality standpoint baked sweet potato puree was probably the most desirable form; doughnuts did not undergo any adverse changes and, in fact, some of the changes may be desirable. From the standpoint of composition, amount of fat, and caloric content, steamed sweet potato would seem to be the least desirable form to use. Comparatively, doughs with steamed sweet potato required a greater amount of moisture to develop a consistency comparable to that of the control of dough. The higher level of moisture resulted in a greater uptake of fat when the doughnuts were fried. Concomitant to the increase in fat uptake was an increase in caloric content. Generally, the use of EEC and baked sweet potato resulted in a calorie reduction in the doughnuts. Sweet potato flour might be most desirable from the production standpoint since it is a dry material and could be handled and stored more easily and inexpensively than the other forms.

Extruded sweet potato snacks Extrusion cooking is a high-temperature, short-time process in which starchy and/or proteinaceous food materials are plasticized and cooked in a tube by a combination of moisture, pressure, temperature, and mechanical shear (Singh et al., 2007). Extrusion is a relatively easy process that is widely used to produce a variety of textured and shaped convenience products including breakfast cereals, baby foods, soups, and ready-to-eat snacks (Brennan et al., 2013; Singh et al., 2007). Extruded snacks are, however, predominantly prepared from high carbohydrate-containing ingredients such as corn, rice, wheat, potato, and oats (Brennan et al., 2013). The snacks are thus energy dense with a limited content of protein and other nutrients, such as vitamin A (Brennan et al., 2013; Riaz et al., 2009). Consumers are, however, increasingly demanding more nutritious snacks that are low in fat but rich in protein, fiber, minerals, and vitamins (Brennan et al., 2013). Honi et al. (2018) investigated the proximate composition, provitamin A retention, and shelf life of extruded orange-fleshed sweet potato and bambara groundnut-based snacks. In their study, six formulations of orange-fleshed sweet potato and bambara groundnut were extruded at a

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feed rate of 10.15 kg/h, screw speed of 30 rpm, and at 100°C and 130°C in the first and second zones, respectively. Proximate composition was determined using standard methods. Provitamin A was determined using high-performance liquid chromatography. An untrained panel (n 5 73) was used to determine consumer acceptability. Shelf life was predicted by using peroxide values. Results showed that the concentration of orangefleshed sweet potato or bambara groundnut significantly (P , .05) affected proximate composition of the snacks. Moisture (4.79 8.34 g/100 g), carbohydrates (55.53 78.99 g/100 g), and provitamin (0.54 17.33 mg/100 g) contents increased with the increasing proportion of orange-fleshed sweet potato. Extrusion significantly reduced provitamin A content from 0.90 20.73 to 0.54 17.33 mg/100 g (P , .05). The presence of orangefleshed sweet potato improved provitamin A retention and consumer acceptability. Predicted shelf life (ranging from 118 to 150 days at room temperature) was inversely proportional to the concentration of bambara groundnut.

Packaging of sweet potato snack food Generally, sweet potato snack foods are carotenoids-rich products. The carotenoids are susceptible to degradation during packaging, which is influenced by oxygen availability in the head space of the package, oxygen dissolved in the product, oxygen permeability through the packaging material, light transmission, faults in the hermiticity of the seal, and the storage time and temperature (Lesková et al., 2006; RodriguesAmaya, 1999). Thus care must be taken during packaging. Júnior et al. (2018) evaluated the influence of the packaging material and packaging system on the stability of dehydrated carotenoid-rich sweet potato chips, and evaluated which packaging system provided the longest shelf life. In their study, the sweet potato chips were processed and packaged with nitrogen in polyester (PET)/aluminum foil (Al)/low density polyethylene (LDPE), metallized PET/LDPE, biaxially oriented polypropylene (BOPP)/metallized BOPP, and BOPP/metallized BOPP with an oxygen scavenger; and also without nitrogen in BOPP/metallized BOPP, and stored at 25°C and 75% relative humidity. The shelf life of the chips packed in BOPP/met BOPP without nitrogen was 153 days, losing 61% of the β-carotene, and leading to sensory alterations in the flavor, odor, and color. The shelf life of the chips packaged with nitrogen in PET

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met/LDPE was defined as 184 days due to sensory alterations involving the loss of crispness. The chips packaged with nitrogen in PET/Al/LDPE, BOPP/met BOPP, and BOPP/met BOPP with an oxygen scavenger retained 90%, 83%, and 80% of the β-carotene, respectively, and showed no significant sensory alterations during 207 days of storage. One can use the packaging systems with nitrogen with the structures of PET/Al/ LDPE, BOPP/met BOPP, or BOPP/met BOPP with oxygen scavenger to obtain a shelf life of 7 months at 25°C/75% relative humidity, the packaging system of BOPP/met BOPP with nitrogen showing the greatest cost benefit due to the lower cost of the packaging material, which is also the material most used on the market for chips in general.

Trends and prospects Snack foods occupy an extremely important position in the food consumption market. They meet the consumption habits and consumption fashion of modern people for their good color, fragrance, and taste, they are convenient for eating, and offer a variety of nutrition. Therefore the snack foods can be seen in supermarkets, large food stores, hotels, restaurants, and so on, and the market space is huge. According to the longterm development plan of the food industry in China, the output value of the instant food manufacturing industry will have grown at an average annual rate of 30% by 2020, and the value of the snack food industry will reach hundreds of billions of Chinese yuan. As a kind of snack food, sweet potato snack foods have huge market potential and broad development space. In recent years, sweet potato processing enterprises have introduced new technology and equipment to improve the quality of their products. At the same time, new ideas in raw materials, ingredients, flavors, texture, production, and packaging have been brought forth, and many new type sweet potato snack foods with various shapes and tastes have developed to meet the diversified demand of market consumption. Meanwhile, sweet potato snack foods are developing toward health, nutrition, and safety. For example, crisp fruit and vegetable chips made from dozens of fresh fruits and vegetables, such as sweet potato, compound apple, carrot, and edible fungus, are a kind of pure natural snack food with a good taste and rich in nutrition, which sell well in the market.

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References Berry, M., 2018. Chocolate sponge cake. Food: Recipes. BBC (accessed 28.04.18). Bovell-Benjamin, A.C., 2007. Sweet potato: a review of its past, present, and future role in human nutrition. Adv. Food Nutr. Res. 52, 1 59. Brennan, M.A., Derbyshire, E., Tiwari, B.K., Brennan, C.S., 2013. Ready-to-eat snack products: the role of extrusion technology in developing consumer acceptable and nutritious snacks. Int. J. Food Sci. Technol. 48, 893 902. Clifford, I.O., Kingsley, E., Chika, C.O., Chinyere, I.I., 2014. Effects of osmotic dewatering and oven drying on β-carotene content of sliced light yellow-fleshed sweet potato (Ipomea batatas L.). Niger. Food J. 32, 25 32. Cloake, F., 2018. How to make the perfect Victoria sponge cake. Guardian (accessed 28.04.18). Collins, J.L., Aziz, N.A.A., 1982. Sweet potato as an ingredient of yeast-raised doughnuts. J. Food Sci. 47, 1133 1139. Da Silva, P.F., Moreira, R.G., 2008. Vacuum frying of high-quality fruit and vegetable based snacks. LWT Food Sci. Technol. 41, 1758 1767. Doymaz, I., 2012. Infrared drying of sweet potato (Ipomoea batatas L.) slices. J. Food Sci. Technol. 49, 760 766. Elisabeth, D.A.A., 2015. Added value improvement of taro and sweet potato commodities by doing snack processing activity. Procedia Food Sci. 3, 262 273. Grace, M.H., Yousef, G.G., Gustafson, S.J., Truong, V.D., Yencho, G.C., Lila, M.A., 2014. Phytochemical changes in phenolics, anthocyanins, ascorbic acid, and carotenoids associated with sweet potato storage and impacts on bioactive properties. Food Chem. 145, 717 724. Granda, C., Moreira, R.G., 2005. Kinetics of acrylamide formation during traditional and vacuum frying of potato chips. J. Food Process. Eng. 28, 478 493. Honi, B., Mukisa, I.M., Mongi, R.J., 2018. Proximate composition, provitamin A retention, and shelf life of extruded orange-fleshed sweet potato and bambara groundnutbased snacks. J. Food Process. Preserv. 42, e13415. Houqin, D., 2013. A quick-frozen processing product of roasted sweet potato and its processing method. Infante, R.A., Natal, D.I.G., Moreira, M.E.D.C., Bastiani, M.I.D., Chagas, C.G.O., Nutti, M.R., et al., 2017. Enriched sorghum cookies with biofortified sweet potato carotenoids have good acceptance and high iron bioavailability. J. Funct. Foods 38, 89 99. Jayaraman, K.S., Gupta, D.K., 2006. Dehydration of fruits and vegetables—recent developments in principles and techniques. Dry. Technol. 24, 1487 1494. Júnior, L.M., Ito, D., Ribeiro, S.M.L., Silva, M.G.D., Alves, R.M.V., 2018. Stability of β-carotene rich sweet potato chips packed in different packaging systems. LWT Food Sci. Technol. 92, 442 450. Kim, H.J., Park, W.S., Bae, J.Y., Kang, S.Y., Yang, M.H., Lee, S., et al., 2015. Variations in the carotenoid and anthocyanin contents of Korean cultural varieties and homeprocessed sweet potatoes. J. Food Compos. Anal. 41, 188 193. Kim, H.S., Chin, K.B., 2016. Effects of drying temperature on antioxidant activities of tomato powder and storage stability of pork patties. Korean J. Food Sci. Anim. Resour. 36, 51 60. Kim, H.W., Kim, J.B., Cho, S.M., Chung, M.N., Lee, Y.M., Chu, S.M., et al., 2012. Anthocyanin changes in the Korean purple-fleshed sweet potato, Shinzami, as affected by steaming and baking. Food Chem. 130, 966 972. Koehler, P.E., Kays, S.J., 1991. Sweet potato flavor: quantitative and qualitative assessment of optimum sweetness. J. Food Qual. 14, 241 249.

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Lai, Y.C., Huang, C.L., Chan, C.F., Lien, C.Y., Liao, W.C., 2013. Studies of sugar composition and starch morphology of baked sweet potatoes (Ipomoea batatas (L.) Lam). J. Food Sci. Technol. 50, 1193 1199. Lesková, E., Kubíková, J., Kováciková, E., Kosická, M., Porubská, J., Holcíková, K., 2006. Vitamin losses: retention during heat treatment and continual changes expressed by mathematical models. J. Food Compos. Anal. 19, 252 276. Lindeboom, N., Chang, P.R., Tyler, R.T., 2010. Analytical, biochemical and physicochemical aspects of starch granule size, with emphasis on small granule starches: a review. Starch—Stärke 56, 89 99. Lu, H.B., Zhang, L., Yin, L.F., Chi, M.L., Chen, C.Q., Gui-Fu, M.A., 2011. Pollution analysis and safety evaluation of roasted sweet potato. Food Sci. 32, 229 231. Medrich, A., 1997. Joy of Cooking. Scribner, New York, p. 949, ISBN 0-684-81870-1. Nakamura, A., Ono, T., Yagi, N., Miyazawa, M., 2013. Volatile compounds with characteristic aroma of boiled sweet potato (Ipomoea batatas L. cv Ayamurasaki, I. batatas L. cv Beniazuma and I. batatas L. cv Simon 1). J. Essent. Oil Res. 25, 497 505. Ni, W.X., Wang, S.Y., Wang, H.X., Huang, Z.Y., 2011. Study on the application of modified sweet potato residue in biscuits. Food Eng. 32, 104 107. Nowak, D., Lewicki, P.P., 2004. Infrared drying of apple slices. Innov. Food Sci. Emerg. Technol. 5, 353 360. Nunes, Y., Moreira, R.G., 2009. Effect of osmotic dehydration and vacuum-frying parameters to produce high-quality mango chips. J. Food Sci. 74, 355 361. Oh, S., Ramachandraiah, K., Hong, G.P., 2017. Effects of pulsed infra-red radiation followed by hot-press drying on the properties of mashed sweet potato chips. LWT Food Sci. Technol. 82, 66 71. Oke, M.O., Workneh, T.S., 2013. A review on sweet potato postharvest processing and preservation technology. Int. J. Agric. Res. Rev. 1, 1 14. Pelucchi, C., Bosetti, C., Galeone, C., La, V.C., 2014. Dietary acrylamide and cancer risk: an updated meta-analysis. Int. J. Cancer 136, 2912 2922. Ravli, Y., Da, S.P., Moreira, R.G., 2013. Two-stage frying process for high-quality sweet-potato chips. J. Food Eng. 118, 31 40. Riaz, M.N., Asif, M., Ali, R., 2009. Stability of vitamins during extrusion. Crit. Rev. Food Sci. 49, 361 368. Robbins, M.J., 2018. Creaming butter and sugar. King Arthur Flour (accessed 28.04.18). Rodrigues-Amaya, D.B., 1999. Changes in carotenoids during processing and storage of foods. Arch. Latinoam. Nutr. Venez. 49, 38 47. Selvakumaran, L., Shukri, R., Ramli, N.S., Dek, M.S.P., Ibadullah, W.Z.W., 2017. Orange sweet potato (Ipomoea batatas) puree improved physicochemical properties and sensory acceptance of brownies. J. Saudi Soc. Agric. Sci. Available from: https://doi. org/10.1016/j.jssas.2017.09.006. Sharma, G.P., Verma, R.C., Pathare, P.B., 2005. Thin-layer infrared radiation drying of onion slices. J. Food Eng. 67, 361 366. Silayo, V.C.K., Laswai, H.S., Mkuchu, J., Mpagalile, J.J., 2003. Effect of sun-drying on some quality characteristics of sweet potato chips. Afr. J. Food Agric. Nutr. Dev. 3 (2). Singh, S., Gamlath, S., Wakeling, L., 2007. Nutritional aspects of food extrusion: a review. Int. J. Food Sci. Technol. 42, 916 929. Singh, N.J., Pandey, R.K., 2012. Convective air drying characteristics of sweet potato cube (Ipomoea batatas L.). Food Bioprod. Process. 90, 317 322. Sun, J.B., Severson, R.F., Kays, S.J., 1994. Effect of heating temperature and microwave pretreatment on the formation of sugars and volatiles in Jewel sweet potato. J. Food Qual. 17, 447 456. Tiu, C.S., Purcell, A.E., Collins, W.W., 1985. Contribution of some volatile compounds to sweet potato aroma. J. Agric. Food Chem. 33, 223 226.

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Wang, Y., Kays, S.J., 2001. Effects of cooking method on the aroma constituents of sweet potato [Ipomoea batatas (L.) Lam.]. J. Food Qual. 24, 67 78. Wang, Y., Wang, C., Guo, P., 2015. Preparation and optimization of dietary fiber biscuits from sweet potato residue. Hubei Agric. Sci. 54, 5700 5706. Wang, S.N., Nie, S.P., Zhu, F., 2016. Chemical constituents and health effects of sweet potato. Food Res. Int. 89, 90 116. Xianyao, L., 2014. A roasted sweet potato and its processing method. Zhang, S.S., Gao, G.T., Sun, X.Y., Cheng-Cheng, F.U., Wei, T., Bing, L.I., 2012. Determination of acrylamide in candied yams by high performance liquid chromatography. Acad. Period. Farm Prod. Process. 9, 115 120. Zhao, G.H., Zhen-Jiao, M.A., Chen, Z.L., 2013. Effect of different drying methods on quality of sweet potato slices. Jiangsu Condiment Subsid. Food 4, 17 19. Zhu, F., Wang, S., 2014. Physicochemical properties, molecular structure, and uses of sweet potato starch. Trends Food Sci. Technol. 36, 68 78.

Further reading Cureton, P., Fasano, A., 2009. The increasing incidence of celiac disease and the range of gluten-free products in the marketplace. In: Gallagher, E. (Ed.), Glutenfree Food Science and Technology. Wiley-Blackwell, Oxford, pp. 1 15. Hill, I.D., Dirks, M.H., Liptak, G.S., Colletti, R.B., Fasano, A., Guandalini, S., 2005. Guideline for the diagnosis and treatment of coeliac disease in children: recommendations of the North American Society for pediatric gastroenterology, hepatology and nutrition. J. Pediatr. Gastroenterol. Nutr. 40, 1 19. Okorie, S., Onyeneke, E., 2012. Production and quality evaluation of baked cake from blend of sweet potatoes and wheat flour. Acad. Res. Int. 3, 171 177. Rosell, C.M., Foegeding, A.E., 2007. Interaction of HPMC with gluten proteins: small deformation properties during thermal treatment. Food Hydrocoll. 21, 1092 1100. Stokes, A.M., Tidwell, D.K., Briley, C.A., Burney, S.L., Schilling, M.W., 2014. Consumer acceptability of gluten-free sweet potato cookies. J. Acad. Nutr. Diet. 114, A49-A49.

CHAPTER 12

Sweet potato fermentation food (sweet potato shochu) Kazunori Takamine

Division of Shochu Fermentation Technology, Education and Research Center for Fermentation Studies, Faculty of Agriculture, Kagoshima University, Kagoshima, Japan

Introduction Shochu is a distilled alcoholic beverage that is produced in Japan, and sweet potato shochu, a shochu made from sweet potatoes, is a classic example of a fermented product that uses sweet potato as its base ingredient. Sweet potato shochu is made using a production process that is unique to Japan. Water is first added to koji (Aspergillus malt), which is produced by culturing koji fungi, at a water:koji ratio of 120:100 (v/w), and fermentation is allowed to proceed. After this, sweet potato is added at a ratio of 500:100 (v/w) with respect to the koji, and water is then added at a ratio of 280:100 (v/w) with respect to the koji, then further fermentation is performed. Distillation of this fermented liquid yields a sweet potato shochu with about 38% (v/v) ethanol. This shochu is diluted in water to give an alcohol content of 25% and then bottled (in Japan, one bottle of liquor is generally 900 mL or 1.8 L) and shipped. The overall manufacturing process can be broken down into a number of smaller processes as shown in Fig. 12.1: raw materials processing, a koji manufacturing process, a primary preparation process, a secondary preparation process, a distillation process, a purification process, and an aging process. Sweet potato shochu has an aroma that is derived from its raw materials, including the koji used for fermentation, yeast, and the distillation process itself. It is widely believed that drinking sweet potato shochu has health benefits. This chapter introduces the sweet potato shochu manufacturing methods, the factors affecting shochu aroma, and briefly discusses its potential health benefits.

Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00012-0

© 2019 Elsevier Inc. All rights reserved.

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First stage moromi preparation process

Koji production process Traditional method

Raw material for koji (rice, barley, etc

Koji, water, yeast

automatic koji production

method Fermentation for 5 to 6 days

Raw material (sweet potato, brown sugar, rice, barley, etc)

Second stage moromi preparation process Raw material

Water

First stage moromi

Fermentation for 8–15 days

Distillation process

Purification & aging process

Bottling process

Figure 12.1 Manufacturing process for shochu.

Raw material Raw sweet potato About 860,700 tons of sweet potato are harvested in Japan annually, with 322,800 tons (38%) being produced in Kagoshima prefecture, which is the primary production location for sweet potato shochu in Japan. Approximately 50% of sweet potatoes produced in Kagoshima are used in making shochu, 40% are used for starch production, and 10% are used for fresh consumption. In contrast, in other Japanese sweet potato production regions, such as Tochigi prefecture (172,000 tons) and Chiba prefecture (103,500 tons), 90% of sweet potatoes are harvested for fresh consumption (Crop yield of sweet potato, 2016). There are many types of sweet potatoes cultivated in Japan, including sweet potatoes used for fresh consumption, such as the “Beni-Satsuma,” which have reddish-purple skin and pale yellow flesh; sweet potatoes that contain anthocyanins, such as the “Aya-Murasaki,” which have purple flesh; and sweet potatoes that contain beta-carotene, such as the “Beni-Hayato,” which have yellow flesh. The sweet potato primarily used in the production of shochu is the “Kogane-Sengan,” which has been specifically bred to improve its starch production.

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Cultivation period (days)

Sweet potatoes are highly nutritious with a well-balanced nutrient profile, making them a semicomplete nutritional food. The principal component in sweet potato is the storage carbohydrate starch, which is generally present at 25% 35%. Other sweet potato components include dietary fiber and minerals. Sweet potatoes also contain glucose, fructose, and sucrose at approximately 0.7%, 0.5%, and 3.0%, respectively. When sweet potatoes are steamed, a proportion of starch is converted into maltose as a result of beta-amylase activity inside. Because steamed sweet potatoes contain approximately 10% of maltose, they can also be considered to be a sugar source in addition to being a starch source. An interesting characteristic of sweet potatoes is that their water content does not change significantly after steaming. The following conditions must be met for sweet potatoes to be used in manufacturing sweet potato shochu: 1. They cannot be infected with black rot or soft rot. 2. They cannot be damaged by larvae of Scarabaeidae and other insects. 3. They must be as fresh as possible, since bruising progresses easily after harvesting. 4. They should have a high starch content. 5. They should have an appropriate size of 300 500 g. Fig. 12.2 shows the relationship between the cultivation period and size of sweet potato. In general, the percentage of medium-sized sweet potatoes (between 300 and 500 g in weight) and extra-large sized sweet potatoes (800 g or more) increases as the cultivation period increases.

120 XS S M L XL

150

180 0

20

40 60 Ratio (%)

80

100

Figure 12.2 Weight distributions used for the size categorization of sweet potatoes harvested 120, 150, and 180 days after planting. The sweet potatoes were divided into size categories of XS ( . 49 g), S (50 299 g), M (300 549 g), L (550 799 g), and XL ( . 799 g).

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Table 12.1 Starch value of sweet potatoes (%). Cultivation period

120 days 150 days 180 days

Sweet potato XS

S

M

L

XL

25.7 28.3 28.1

27.9 30.9 27.2

28.4 27.2 27.5

24.6 27.5

23.0

, L and XL size sweet potatoes that harvested 120 days after being planted were absent in this study. XL size sweet potatoes that harvested 150 days after being planted could not be checked owing to sample shortage.

Larger sized sweet potatoes are, however, not as desirable in the manufacture of sweet potato shochu. They require significantly more time and effort in order to process them, since the steaming process takes longer time to heat the center of sweet potato and in addition the larger size means that the sweet potato needs to be physically cut into smaller sized pieces. Table 12.1 shows the influence of cultivation period and size of sweet potato on its starch value. The starch value of short-cultivated sweet potato (120 days) increases with its size, whereas that of long-cultivated sweet potato (150 or 180 days) is much lower with larger sweet potatoes (L or XL). Thus the relationships between the size and starch value of sweet potato are inconsistent. The crop yields are actually increased with longer cultivation, but the proportion of large sweet potato with lower starch value is increased. Therefore the moderate culture time for sweet potato production is approximately 150 days (Okutsu et al., 2016).

Raw material processing Like a lot of products sweet potatoes begin to deteriorate immediately after harvesting. They are initially susceptible to damage that can occur during transportation from the field to the factory as a result of physical interaction with the container and other sweet potatoes, as well as physical damage that can occur during unloading. Therefore the sweet potatoes must be processed as quickly as possible. After delivery, the sweet potatoes are thoroughly washed with water, and any defective portions, such as parts with black rot or soft rot, or with insect damage caused by Scarabaeidae, are removed. Since sweet potatoes are not uniformly shaped, raw material processing cannot be mechanized easily, so this relies on humans.

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When the defective parts of sweet potatoes are removed, and also when the skin is peeled, the sweet potatoes will soon begin to dry and become bruised. As a result, they must be steamed as soon as possible. In general, a steaming duration of approximately 60 min is sufficient, assuming that appropriately sized sweet potatoes have been selected. After steaming, they are either air-cooled immediately and then prepared, or left to naturally cool overnight and then prepared the following morning. Preparation involves pulverizing them by machine to pieces with sizes of approximately 2 cm or less.

Koji production process Koji fungi Koji is the rice or barley that has been cultured with koji fungi, and it is broadly categorized into Chinese koji or Japanese koji. In the production of Chinese koji, barley, wheat, or peas are used as raw materials. The immersed raw materials are pulverized and solidified into either a brick or a ball shape. Fungi of the Rhizopus or Mucor genus, which are present in the raw materials or in the natural environment, are used in culture to produce koji which is referred to as Mochikoji, with the brick-shaped koji being referred to as Daqu and the ball-shaped koji being referred to as Xiaoqu. In contrast, Japanese koji uses steamed rice or barley cultured with koji fungi by inoculation, and it is referred to as “Bara-koji.” The color of a colony of Aspergillus is derived from the color of its conidia. When different species of Aspergillus genus are inoculated onto an agar medium there are numerous colors of the mature colony observed. Generally, black koji fungi (Aspergillus luchuensis) appears black-brown, yellow koji (Aspergillus oryzae) fungi appears green with some yellow coloration, and white koji fungi (A. luchuensis mut. kawachii) appears brown with some yellow coloration. However, it is important to be aware that even though a type of koji fungi may be referred to as yellow koji fungi, various different strains of yellow koji fungi exist, such as those used for sake, soy sauce, and miso, and the color of the conidia may be also different. For example, there is a white mutant strain of yellow koji fungi used for manufacturing miso, and it is even whiter in color than white koji fungi. Also black koji fungi is broadly classified into two different types: A. luchuensis var. awamori and A. luschuensis var. saitoi. In taxonomy, “var.” represents a variety. The former variety has a strong saccharifying power, and a low capacity of citric acid production, whereas the latter variant has

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a low saccharifying power, and a high citric acid production capacity. In the brewing of Awamori, two strains of koji fungi with different properties are generally mixed so that they are well-balanced, and often used in this combined state. Shochu koji is produced from a black koji fungi or white koji fungi. Yellow koji used for sake had been used for shochu making, but it was shifted to black koji in around 1919. Since black koji produces a considerable amount of citric acid compared to yellow koji, the pH of the moromi is decreased to 3 3.5. This pH decrement promotes the dominant growth of shochu yeast, which has acid tolerance without contamination. In 1945 White koji has started to be used for shochu making, because shochu made with white koji has a light and floral flavor compared to that with black koji. Meanwhile, black koji was revived for shochu making in the late 1980s.

Role of koji Koji has many roles, including the production of enzymes that degrade starch, protein, and other polymers in the raw material into small molecules, and the production of citric acid required for lowering the pH of the moromi (the liquid produced after adding water and yeast to the raw material and performing fermentation). In addition, the koji imparts an aroma to the shochu. Production of enzymes When koji fungi grows on rice or barley, it produces a wide variety of enzymes. The primary enzymes include amylolytic enzymes, proteolytic enzymes, and lipolytic enzymes. The pH of the moromi is low, usually between 3 and 3.5, so it is necessary for the enzymes produced by koji fungi to be acid-tolerant. The enzymes produced by the white and black koji used for shochu production have an optimal pH in the acidic range, and also have stable activities at low pH, providing them with excellent acid tolerance. In addition, the enzymatic activities of α-amylase in rice koji and barley koji are very small compared to those in yellow koji, the koji used for sake. The α-amylase activities in rice koji and barley koji are 1/8 and 1/14 of that in yellow koji, respectively. Despite this, the starch in the raw material dissolves and breaks down well in shochu moromi, so the enzymatic activity is sufficient for the production of shochu. The acidic proteases in rice koji and barley koji have very high activities compared to yellow koji being approximately 13-fold and 6-fold higher, respectively.

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Shochu koji has an important characteristic, a raw starch degrading enzyme, that yellow koji does not have it (Iwano et al., 1986). Amylolytic enzymes Amylolytic enzymes are critical for producing shochu using raw materials in which starch is the main ingredient, because glucose is produced by the hydrolysis of starch in the raw material and then metabolized by yeast to produce alcohol. Amylolytic enzymes produced by koji fungi that are important in degrading starch include α-amylase, glucoamylase, and α-glucosidase. α-Amylase Starch has a structure whereby glucose molecules are linked together with α-1,4 bonds. α-Amylase is an endoenzyme that can randomly hydrolyze these bonds. As it can rapidly reduce the viscosity of starch solutions, α-amylase is also referred to as a liquefying enzyme. Glucoamylase Glucoamylase is an exoenzyme that hydrolyzes the α-1,4 bonds of the starch backbone into glucose units starting from the nonreducing end, and because it produces glucose, it is also referred to as a saccharifying enzyme. This enzyme can also hydrolyze α-1,6 bonds, which are associated with the branches present in starch. Proteolytic enzymes Proteolytic enzymes hydrolyze protein and peptides to produce amino acids. The resulting amino acids are further metabolized by yeast to produce aromatic components such as higher alcohols, as well as their esters and aldehydes. In addition, some of the amino acids are involved in forming aromatic compounds, such as aldehydes, by undergoing nonenzymatic thermal reactions (e.g., the Maillard reaction, Strecker degradation, etc.) in the acidic moromi environment during distillation. Acidic protease Acidic protease is an endoenzyme that can randomly hydrolyze peptide bonds in proteins and peptides. The proteases present in white koji, black koji, and sake yellow koji are called acidic protease, because their optimal pH is 3.0. This enzyme is principally responsible for producing the peptides and amino acids present in the moromi. Acidic carboxypeptidase Acidic carboxypeptidase is an exoenzyme that hydrolyzes carboxy-terminal peptide bonds in proteins and peptides into amino acids. The optimal pH of the carboxypeptidases in white koji, black koji, or sake yellow koji are approximately 3.0 3.5, so for this reason

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these enzymes are referred to as acidic carboxypeptidases. This enzyme is also important in producing the peptides and amino acids present in the moromi. Lipolytic enzymes Lipolytic enzymes produce fatty acids by hydrolyzing the ester bonds present in lipids. Among the fatty acids that are produced, palmitic acid, stearic acid, oleic acid, linoleic acid, and their ethyl ester compounds contribute to the physical taste of shochu, giving it a roundness and a mellow sharp flavor. These fatty acids are the principal lipid components in shochu. β-Glucosidase β-Glucosidase is an enzyme that hydrolyzes linked structures containing β-1,4 bonds, such as cellulose. The monoterpene alcohol that gives sweet potato shochu its characteristic aroma is present in sweet potato as a monoterpene glycoside linked to glucose through a β-1,4 bond. During fermentation, β-glucosidase acts on monoterpene glycoside, and produces monoterpene alcohols such as geraniol, which provide the characteristic aroma of sweet potato shochu. For further details, please refer to the section titled “Shochu aroma.” Production of citric acid The citric acid produced by koji fungi is responsible for decreasing the pH of the moromi to between 3 and 3.5, and in doing so, prevents the moromi from rotting. Citric acid is a nonvolatile organic acid that can be separated during the distillation step, so that it does not become an ingredient in shochu. The quantity of citric acid contained in the koji requires a koji acidity of 5 7. Koji acidity is defined as the quantity of 0.1 N NaOH required to neutralize 10 mL of filtrate after 100 mL of distilled water has been added to 20 g of koji which has been allowed to leach over a 3-h period at room temperature with periodic stirring. Citric acid is produced from the metabolism of glucose, since glucose is required to produce alcohol. The higher the amount of citric acid produced, the lower the yield of shochu. The citric acid contained in 100 kg of koji is calculated to be 320 g (1.67 mol) per koji acidity level.

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Flavor contributions Koji fungi significantly affects the flavor of shochu. For example, sweet potato shochu manufactured using yellow koji has the splendid characteristic aroma of yellow koji. Shochu manufactured using white koji and black koji shares a fruity aroma and a sweet burnt aroma, whereas shochu produced with white koji is light and elegant, and shochu produced with black koji is rich and mellow.

Koji production Koji is produced either by a traditional method that uses a koji board or by an automatic koji production method that uses mechanized equipment. The conventional methods include the board koji method, the box koji method, and the floor koji method. The automatic koji production methods include a ventilated koji production device that uses a rotating drum and a triangular shelf, a fully-automated koji production device that uses a rotating drum and a disk-type automatic koji production device. All of these devices require approximately 43 h to produce koji. The raw materials, rice and barley are first steamed, and the starter koji is then used to inoculate them at 38°C. Approximately after 27 h, the temperature is reduced to approximately 35°C and maintained at that level for the remainder of the process. This reduction in temperature and its maintenance enables the koji fungi to produce citric acid. Starter koji is generally rice koji, in which koji fungi has been cultured on rice and the growth of spores is allowed to occur. The starter koji is used for inoculating spores into the raw material being used for koji production. There are two types of starter koji, namely, granular starter koji and powder starter koji (Fig. 12.3). The amount of starter koji dispersed during inoculation is 1:1000 with respect to the weight of the koji raw material in the case of granular starter koji, and 1:500 with respect to the weight of the koji raw material in the case of powder starter koji. The raw materials for the production of koji are generally rice and barley. This is because a water content of 40% or less is desirable as the koji fungi culture environment. However, the water content of sweet potatoes is very high, typically from 60% to 70%, which makes culturing koji fungi difficult. Also the specific surface area of sweet potato is very small compared with rice and other cereals, which decreases the quantity of enzymes and citric acid produced. These problems were resolved by developing modern technologies in which sweet potatoes are cut into dice shapes

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Figure 12.3 Two types of starter koji.

with each side being 5 mm in length, which are then either immediately dried, or steamed and then dried. These dried, dice-shaped sweet potatoes are then allowed to absorb water until the water content is approximately 40%. After this, they are steamed and the koji is produced. The sweet potato shochu manufactured using this koji has a sweeter sweet potato aroma, and a more refreshing taste than that made with rice koji.

Preparation process The shochu preparation method is a unique method, consisting of a primary preparation process and a secondary preparation process. First, the primary preparation process is performed using koji, water, and yeast, allowing the fermentation to proceed for 5 days to obtain a primary moromi. Then steamed/pulverized sweet potato and water are added to this primary moromi, and a second preparation process is performed.

Yeast From one molecule of glucose, yeast produces two molecules of ethanol and two molecules of carbon dioxide. This phenomenon is referred to as ethanol fermentation, and it is carried out by yeast under anaerobic conditions. It has also been discovered that various aromatic compounds are produced as a result of yeast metabolism. For further details, please refer to the section, “Shochu aroma.”

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The characteristics of shochu yeast (Saccharomyces cerevisiae) are as follows: 1. It can grow even at high temperatures, and has a high fermentation capacity. 2. It can grow even at a low moromi pH (3 3.5). 3. It has a capacity for growth/fermentation even in the presence of ethanol (3% 5%). 4. It can survive even in high ethanol concentrations (18% 20%). 5. It contributes to the desirable flavors in the shochu. 6. It can bring out the properties of the base material in each shochu. Several types of yeast are used for shochu making, such as Miyazaki yeast, Kumamoto yeast, Awamori No. 1, Shochu yeast Kyokai No.2 and No.3, and Kagoshima yeast (Ko). In addition, Kagoshima No. 2 (K2) was selected from Ko in the 1970s, and Kagoshima No. 4 (C4) and No. 5 (H5) were isolated from shochu mash in 1995. These yeast strains are also used for shochu making. Sweet potato shochu, which uses C4, produces a high quantity of higher alcohols and their esters, and is prized as being “splendid” and having a “soft flavor.” The use of H5 gives an approximately 3% improvement in alcohol yield compared to the use of K2 (Takamine et al., 1994).

Primary preparation process In the primary preparation process, water is added to the koji base material at a ratio of 120:100 (water:koji). Cultured yeast solution (200 mL) per 100 kg of koji is then added to this mixture. Following this the moromi is fermented by controlling the temperature to prevent it from exceeding 32°C. The pH of the primary moromi is reduced to pH 3 3.5 by the citric acid produced by the koji, which then inhibits the proliferation of bacteria. Meanwhile, the shochu yeast with superior acid resistance grow preferentially, and the moromi is fermented safely without the risk of bacterial contamination. The primary moromi fermented over a period of approximately 5 days is then used in the secondary preparation process. Yeast concentration The yeast concentration immediately after preparation is approximately 1 to 2 3 105 cells/mL, however after 2 days of preparation, the yeast proliferates to a concentration of 2 to 4 3 108 cells/mL. The level of viable yeast is the highest from 3 to 4 days, but it is desirable to use the moromi for the secondary preparation process after 5 days have passed. One reason

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for this is that, immediately after initiating the second preparation process, any young yeast present that have high fermentation capacity initiate a burst of fermentation, which can cause a spike in the temperature of the secondary moromi that then makes it impossible to suppress the temperature of the moromi. This leads to the death of the yeast, which can lead to problems with fermentation. Acidity The acidity is approximately the koji acidity value multiplied by four, and the standard range is 20 27. Therefore if the moromi acidity falls below 5, acid supplementation is required. The maximum value occurs 2 days after preparation, and it falls by about one to two levels after 5 days. Alcohol concentration The alcohol concentration rises to 6% 8% after 2 days of preparation, and is 15% 17% by 5 days. Excess alcohol production inhibits yeast proliferation, leading to death and affecting the fermentation capacity of the secondary moromi. Therefore strategic measures are required to suppress alcohol production, such as adjusting the temperature of the moromi to be between 20°C and 25°C, beginning 4 days after preparation. Temperature The standard preparation temperature is 20°C 25°C, which is adjusted according to the time it takes to reach ambient temperature, or for the moromi to begin fermenting, as well as other factors. When the ambient temperature is expected to be getting cool, measures such as wrapping insulating material around the tank are usually undertaken. The maximum temperature of the moromi after 2 days of preparation is controlled to be between 28°C and 32°C. The temperature of the moromi immediately before the secondary preparation step is controlled from 20°C to 25°C.

Secondary preparation process The secondary preparation process is the operation of adding the base ingredient, sweet potato, and water to the primary moromi. The primary moromi used in the secondary preparation process has an acidity of approximately 25, a pH of approximately 3 3.2, an alcohol content of approximately 16%, a sugar concentration of 6% 10%, and a yeast concentration of 2 to 4 3 108 cells/mL. The largest factor in safely fermenting the primary moromi without bacterial contamination is keeping the pH of the

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moromi low enough to suppress bacterial proliferation while still being able to preferentially grow only the shochu yeast that have superior acid tolerance. Sweet potato that has been steamed, cooled, and pulverized, and water are then added to the koji raw material with stirring. The sweet potato is added at a ratio of 500 100, and the water is added at a ratio of 280 100, each with respect to the koji raw material. Stirring, either mechanical stirring or air stirring, is then performed to prevent clumping of the pulverized sweet potato. The inside of any clump does not come in contact with the primary moromi, which contains alcohol and citric acid, so it is easy for bacteria, especially lactic acid bacteria, to proliferate there. Therefore attention must be paid to ensure that clumping does not occur. A pulverizer is used to pulverize the sweet potato to a particle size of 2 cm or less. Sweet potato with a high starch value, such as “KoganeSengan,” has a low water content, so it can be pulverized easily. However, sweet potatoes that are used for raw food have high water content and therefore are sticky, so particular attention must be paid in order to prevent clumping when pulverizing. The β-amylase activity in the sweet potatoes causes approximately one-third of the starch to convert to maltose when steamed, so as a result it becomes sweet. When water and steamed sweet potato containing approximately 10% maltose are added to the primary moromi, fermentation is rapidly promoted immediately after preparation. As sweet potato shochu has a high viscosity, the carbon dioxide gas generated during fermentation builds up in the secondary moromi, causing it to swell, and when the inside of the moromi becomes saturated with carbon dioxide, the gas violently erupts, causing natural agitation to occur. Generally, the initial preparation temperature is approximately 25°C, so after 2 3 days of preparation, it rises to 32°C. The rate at which yeast die increases at 35°C, causing not only poor fermentation, but also the production of acetic acid, which can decrease the quality of the shochu. The moromi alcohol concentration typically reaches 14% 15%.

Distillation process Atmospheric distillation Methods of heating the moromi include direct heating, direct steaming, indirect steaming, and a combination of direct and indirect steaming. In addition to ethanol and water, shochu contains trace components, such as higher alcohols, fatty acid esters, organic acids, and minerals, and despite

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the fact that their total content is only approximately 0.2%, they are very important in the flavor of shochu. In fact, the important differences in flavor between sweet potato shochu base materials and sweet potato shochu from each manufacturer are derived from these trace components, which affect the base material properties and the shochu quality. In addition to originating from the base materials, or being produced during the fermentation process, these components are also produced by thermal reactions that occur during the distillation process, meaning that the distillation operations can also affect the quality of the shochu. With respect to the distillation time, it takes about 30 min from steam being blown onto the moromi to the shochu beginning to distill (begins dripping), and the quantity of steam is adjusted so that the distillation will finish approximately 180 min after it begins. The end point of the distillation is generally at the time that the alcohol content of the distillate reaches 8% 10%. If it is set lower, then it is easier for the “final distillate aroma,” the characteristic scent of the distillate near the end of distillation, to be expressed. However, if there is only a small amount of distillate near the end of the distillation, the flavor tends to be light.

Vacuum distillation In vacuum distillation, the moromi is distilled at a reduced pressure of approximately 100 Torr at 40°C 50°C, so it is difficult for thermal reactions to occur. As a result, a light and soft shochu is produced, which is completely different from the shochu obtained with atmospheric distillation. The shochu quality is therefore intimately related to the moromi temperature, so each company must set the distillation conditions required for the shochu quality of their own shochu brand. The moromi is not heated by steam blowing, but rather by indirect heating using a coiled tube-type or a jacket-type pot still. During distillation, the quantity of the moromi will gradually decrease. If the liquid surface of the moromi is lower than the height of the steam-heated surface, the moromi will burn, so the amount of moromi added to the pot still must be carefully adjusted.

Purification process The properties of shochu obtained by atmospheric distillation and vacuum distillation vary greatly, so the purification methods are also different. In the case of shochu obtained by atmospheric distillation, the unique flavor of the base ingredient is strong, the scent of strong-smelling gases, such as

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aldehydes or sulfur compounds, is present, and lipid components, comprising primarily higher fatty acids and their ethyl esters, make the shochu cloudy and they float to the surface. In contrast, shochu obtained by vacuum distillation has hardly any strong-smelling gas odors, and few lipid components. The strong-smelling gas odors and lipid components, if present, are primarily removed during the purification process.

Gas components The gas produced consists of a mixture of aldehydes and sulfur compounds. Typically, shochu distilled under atmospheric pressure contains a significant amount of gas, while shochu distilled under a vacuum contains relatively little gas. The alcohol steam that is produced during distillation is cooled and becomes the shochu. At this time, the alcohol steam is cooled gradually, and it is necessary to adjust the cooling water so that the temperature of the shochu that comes out of the condenser is approximately 30°C. By slowly cooling, it is possible to minimize the amounts of strong-smelling gas components that dissolve in the shochu. If it is cooled too rapidly, it is easy for strong-smelling gas components, such as aldehydes, to dissolve in the shochu because they have a low boiling point, and therefore it takes time to remove the strong-smelling gas components contained in the shochu. The dissolved strong-smelling gas components will naturally evaporate during storage, but several methods can be used to force gas removal including by causing agitation by transferring shochu between tanks, stirring with a paddle, or using an air pump or circulation pump. Regardless, the strong-smelling gas will eventually be removed and the product quality stabilizes in approximately 2 4 months. However, larger tanks may require more time to become stabilized. In the case of sweet potato shochu, these gas components still remain after about 1 month of short-term storage following distillation, but this product may be shipped and sold as “new sake,” which has a rich sweet potato aroma.

Lipid components Lipid components serve to smooth out the physical taste of shochu, giving it a roundness and a tempering of sharp flavors. However, to some extent, the lipid components must also be removed, owing primarily to two negative effects. The first negative effect is that they precipitate during storage and form white, thread-like aggregates that float around in the shochu. The white, floating material is a precipitate/aggregate formed when lipid

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components in the shochu bind to the mineral (metal) components, such as calcium, magnesium, copper, and iron, present in the water used to dilute the raw alcohol to 25% (v/v) ethanol. The second negative effect is that the lipid components produce an oily odor when they are oxidized. Therefore successful retention of the ideal amount of lipids in the shochu reflects the fine balancing act between their positive attributes (i.e., roundness of flavor and a tempering of sharp flavors) and their negative attributes (i.e., precipitation and oxidation). Filtration is used to appropriately eliminate the oily component ethyl linoleate, in particular. Ethyl linoleate, itself, possesses only a weak oily smell, so it is not directly responsible of the oily odor. However, upon decomposition ethyl linoleate breakdown products, namely alzeic acid semialdehyde monoethyl ester, n-hexanal, 2,4-nonadienal, and pimelic acid semialdehyde ethyl ester, are produced, which are responsible for the oily smell (Nishiya and Sugama, 1978). There are two principal methods used for removing oily components. The first is the skimming method. Oily components have a low specific gravity and therefore float to the surface of the tank following cooling during storage. Utilizing this property, the oily components on the surface of the shochu, can be skimmed away using filter paper or a flannel cloth. Although it is difficult to skim off transparent oily components, they can also be removed by causing them to adhere to the food wrap used for food storage. The second method is the cold filtration method. The temperature of the shochu affects the solubility of ethyl linoleate, the main precursor substances responsible for the oily aroma in shochu. The lower the temperature, the lower the solubility of shochu. After using a cooling apparatus to decrease the temperature of the shochu, the ethyl linoleate can be easily removed by filtration using filter paper.

Shochu aroma In addition to ethanol and water, shochu contains trace components, such as higher alcohols, fatty acid esters, organic acids, and minerals, but their total content is low, being approximately 0.2% 0.5%. However, these trace components play important roles in shochu. The differences in the flavor for each base material (such as sweet potato shochu or brown sugar shochu) from each manufacturer are due to these trace components. The oily components temper the sharp taste of shochu and give it a

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rounded flavor, so they are an essential ingredient of shochu that contribute to its physical taste. Higher alcohols include isoamyl alcohol, activated amyl alcohol, isobutyl alcohol, normalpropyl alcohol, β-phenethyl alcohol, etc. These components primarily exhibit an alcohol-like aroma, whereas β-phenethyl alcohol has a rose-like aroma. The presence of higher alcohols is intimately related to amino acid metabolism in yeast, and they are produced from intermediates in the pathways by which yeast conduct the biosynthesis or degradation (Ehrlich pathway) of amino acids. Specifically, isoamyl alcohol is produced from leucine, activated amyl alcohol is produced from isoleucine, isobutyl alcohol is produced from valine, n-propyl alcohol is produced from threonine, and β-phenylalanine is produced from phenylalanine. Whether or not alcohols are generated by any of these biosynthetic pathways depends on the amino acid content of the moromi. When the amino acid content in the moromi is low, amino acids must by biosynthesized using the nitrogen sources taken up by the yeast, and higher alcohols are produced as by-products (amino acid biosynthetic pathway). However, when the amino acid content in the moromi is high, the yeast acquires the nitrogen components from the amino acids that are taken up, converts the remaining keto acids to higher alcohols containing one less carbon atom, and releases them outside the fungal body (Ehrlich pathway). Isoamyl acetate, which has an apple-like aroma, and β-phenyl acetate, which has a rose-like aroma, are also produced by yeast, and the amount produced varies depending on the type of yeast. As a result, shochu with various alcohol qualities can be produced by yeast. The characteristic aroma of sweet potato shochu is reported to be caused by the presence of monoterpene alcohols, such as linalool, α-terpineol, citronellol, geraniol, and isoeugenol, as well as rose oxide and β-damascenone (Kamiwatari et al., 2006; Kuriyama et al., 2005; Ota, 1991; Takamine et al., 2011). In comparison, these components are barely present in rice shochu and barley shochu. Monoterpene alcohols exist as monoterpene glycosides in sweet potatoes, and they are hydrolyzed and liberated by β-glucosidase derived from koji during fermentation. In addition, some of the geraniol and nerol are converted to citronellol by yeast during fermentation and to linalool and α-terpineol by acid and heat during distillation. β-Damascenone is an important characteristic aromatic component that contributes to the sweet scent of sweet potato shochu. Both monoterpene alcohols and β-damascenone are said to have a soothing effect, and these sweet potato shochu-specific components are believed

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% of each total content

70 60 50 40 30 20 10 0 Nerol

:Top,

Geraniol

:Bottom,

:Skin,

Linalool

α-Terpineol

:Cambium,

:Center

Figure 12.4 Content distribution of monoterpene alcohol in various parts of sweet potato.

to play an important role in the “relief” that is felt with an evening drink of sweet potato shochu. In particular, drinking diluted shochu with hot water allows the aroma to stand out more easily, and is believed to bring out the sweetness of the shochu and its soothing effects. The monoterpene alcohol that gives sweet potato shochu its characteristic aroma is derived from sweet potatoes. However, these are not free molecules since they all exist as glycosides. Fig. 12.4 shows the distribution of monoterpene glycosides in “Kogane-Sengan,” a sweet potato variety used as the base ingredient for sweet potato shochu. In this analysis, the “Kogane-Sengan” sweet potato was divided into five distinct parts as follows. The upper and lower 10% of the length were cut off and defined as the upper and lower part, respectively. The remaining parts were subsequently divided into three parts: central part (defined within cambium layer), cambium part, and skin part (outside of cambium layer). The monoterpene glycoside fraction was extracted from each of these sweet potato parts. The parent monoterpene alcohols were then liberated by the actions of β-glucosidase and β-primeverosidase on these monoterpene glycoside fractions. The liberated monoterpene alcohols were then measured using GC-MS, and the values were used as the basis for calculating the monoterpene glycoside distribution shown in Fig. 12.4. Nerol was found to be present at 8.4% at the top part, and was found at the highest percentage, 38.6%, in the central part. Geraniol, linalool, and α-terpineol were found at the highest concentration in the skin, being 37.4%, 65.9%,

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and 60.3%, respectively. Linalool was not detected in the central part, but α-terpineol was found, although at a low percentage of 1.7% (Takamine et al., 2012).

Sweet potato varieties and shochu quality Joy white The first sweet potato variety bred as the base ingredient for shochu was the “Joy White.” When sweet potato shochu was produced using this variety, the monoterpene alcohol content was high, and a characteristic fruity and strong citrus-like shochu quality was provided. Linalool in shochu with sweet potato variety “Joy White,” in particular, was present at an approximately fivefold higher concentration compared with shochu made with the sweet potato variety “Kogane-Sengan,” which is generally used in sweet potato shochu manufacturing. This led to the discovery that monoterpene alcohols are important compounds that affect the quality of sweet potato shochu.

Colored sweet potatoes Some sweet potato shochus use sweet potato varieties that have bright orange or purple flesh. Sweet potato shochu manufactured with orangecolored sweet potatoes are prized for having a “heated carrot aroma” or “a steamed squash scent” and β-ionone compounds, which have the aroma of violets, and have been specifically detected in this sweet potato shochu. In addition, sweet potato shochu manufactured with purple sweet potatoes is said to have a “yogurt aroma” or a “red wine aroma,” and diacetyl compounds have been found to contribute to these aromas (Kamiwatari et al., 2006).

Health properties of shochu J-curve effect of alcohol It is said that if alcohol is drunk well it is the best of all medicines. Moderate alcohol consumption has mental and physical effects, such as increasing appetite, promoting sleep, and eliminating stress, and it is socially useful for easing human interactions, maintaining cultural practices, and social customs. Those who have been drinking small quantities of alcohol for many years are said to have a lower rate of death from heart disease, cancer, and other diseases as compared with people who do not drink any alcohol, or

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1.8

Relative risk

1.6 1.4 1.2 1.0 Male

0.8

Female

0.6 0

0–9

10–19 20–29 30–39 40–49 50–59

60–

Drinking volume (g/d) Figure 12.5 J-curve effect of alcohol.

who drink large quantities of alcohol. Relative risk of all-cause mortality in male drinkers compared with abstainers fell to 0.84 of alcohol at 10 19 g of alcohol per day, in female drinkers the lowest relative risk of 0.88 was at 0 9 g of alcohol per day (Fig. 12.5) (Holman et al., 1996). This is known as the “J-curve effect of alcohol.” This theory was first proposed in 1981 by Dr. Marmot in England in an epidemiological study.

Thrombolytic effects of shochu An alcohol consumption study was conducted, over 15 years, in healthy subjects using shochu, sake, wine, beer, and whiskey, each containing 60 mL of ethanol, and a very large amount of hematological data was collected (Sumi et al., 1988; Sumi, 2001). After measuring the fibrinolysin activity in the blood 1 h after drinking alcohol, the consumption of alcoholic beverages was found to increase this activity, with shochu increasing it the most. It was notable that shochu did not suppress the formation of blood clots, but only degraded blood clots that had already been formed. In other words, the consumption of shochu does not affect “hemostasis,” which is a process essential for life.

Blood glucose lowering effect of shochu In several epidemiological studies a moderate quantity of alcohol was effective in preventing diabetes mellitus (Mackenzie et al., 2006). As to the relationship between alcohol and blood glucose, it has been reported

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that drinking large amounts of alcohol, under fasting conditions, causes serious hypoglycemia, and that alcohol consumption inhibits gluconeogenesis in the liver and suppresses the secretion of insulin (Steiner et al., 2015). However, these studies only examined the effect of alcohol alone, they did not examine the effect of alcohol on blood sugar elevation following the consumption of food. As a result, Kido et al. (2014) researched how blood glucose values and insulin secretion changes when alcohol is consumed with a meal. In addition, they showed how those effects differed according to alcohol type. In this study, there were five healthy subjects (three men and two women) who were asked to drink beer, sake, shochu, or water with a meal, and blood glucose values and blood insulin concentration were measured immediately before eating and at 1, 2, and 12 h after eating. Since alcohol concentration varies for each beverage, the quantity of each drink was adjusted so that each volunteer consumed 40 g of alcohol. The results showed that blood glucose values and insulin concentrations differed depending on the type of alcohol (Fig. 12.6). The alcohol associated with the highest blood glucose quantity was beer. In contrast, the shochu or sake drinking groups resulted in a lower blood glucose level than in those groups drinking water and beer. In addition, shochu resulted in lower insulin levels compared with the other groups. One conceivable reason for this is that the carbohydrate content differs depending on the type of alcohol. The carbohydrate content of shochu is 0 g; whereas in beer it is 31 g, and in sake it is 13.3 g. These carbohydrate values are lower than that of carbohydrates derived from food (approximately 100 g), but carbohydrates in alcoholic beverages have been shown to potentially affect postprandial blood glucose levels. Another interesting result in this study was the fact that when shochu was consumed, postprandial blood glucose levels and blood insulin concentration were lower than they were after the consumption of water, which similarly contains no carbohydrates. There are two mechanisms that could be responsible for this. The first is the possibility that components in shochu increase the effects of insulin (i.e., they may act as insulin sensitizers to improve glucose tolerance), thereby blunting the increases in blood glucose. The other possibility is that the components in shochu might inhibit the absorption of carbohydrates. Indeed, alcohol has been reported to inhibit the motility of the gastrointestinal tract, which would inhibit the absorption of carbohydrates. However, in this study, the same quantity of alcohol was consumed, so it cannot be concluded that alcohol alone in shochu

Blood alcohol (mg/dL)

(A)

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200

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*** *** ***

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Water Beer Shochu Sake

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60 40 20 0

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1 2 Time (h)

12

Figure 12.6 Blood glucose and insulin levels after drinking four types of beverages in healthy subjects. Blood alcohol levels (A), blood glucose levels (B), blood insulin levels (C) after drinking four types of beverages in healthy subjects. Data are expressed as mean standard error (n 6 5). Two-way repeated measures analysis of variance and post hoc analysis using least significant differences were used to compare clinical data between the beverages. P , .05, P , .01, P , .001, compared with water; #P , .05, ##P , .01, ###P , .001, compared with beer.

suppressed gastrointestinal motility. Other components besides alcohol might be involved. In the future, we will anticipate discovering the identity of the components that contribute to this effect and the mechanisms underlying their effect.

References Crop yield of sweet potato, 2016. ,http://www.maff.go.jp/j/tokei/sokuhou/sakumotu/ sakkyou_kome/kansyo/h28/index.html. (accessed 30.01.18). Holman, C.D., English, D.R., Mine, E., Winter, M.G., 1996. Meta-analysis of alcohol and all-cause mortality: a validation of NHMRC recommendations. Med. J. Aust. 164, 141 145.

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Iwano, K., Mikami, S., Fukuda, K., Shiinoki, S., Shimada, T., Obata, T., et al., 1986. Distribution of enzyme activities of shochu koji. J. Inst. Brewing 81, 495 498. Kamiwatari, T., Setoguchi, S., Kanda, J., Setoguchi, T., Ogata, S., 2006. Effects of a sweetpotato cultivar on the quality of Imo-shochu with references to the characteristic flavor. J. Brewing Soc. Jpn 101, 437 445. Kido, M., Asakawa, A., Koyama, K.K., Takaoka, T., Tajima, A., Takaoka, S., et al., 2014. Acute effect of traditional Japanese alcohol beverages on blood glucose and polysomnography levels in healthy subjects. Peer J. 1853. Available from: https://doi.org/ 10.7717/peerj. Kuriyama, K., Nagatomo, M., Yamanaka, H., Yoshihama, Y., Watanabe, Y., 2005. Relationships between panel preference and volatile compounds in various types of shochu. J. Brewing Soc. Jpn 100, 817 823. Mackenzie, T., Brooks, B., O’connor, G., 2006. Beverage intake, diabetes, and glucose control of adults in America. Ann. Epidemiol. 16, 688 691. Nishiya, N., Sugama, S., 1978. Oil flavor developed during aging process of honkaku shochu, the traditional distilled liquor. J. Soc. Brewing Jpn 73, 844 849. Okutsu, K., Yoshizaki, Y., Kojima, M., Yoshitake, K., Tamaki, T., Takamine, K., 2016. Effects of the cultivation period of sweet potato on the sensory quality of imo-shochu, a Japanese traditional spirit. J. Brewing Soc. Jpn. 122, 168 174. Ota, T., 1991. Characteristic flavor of kansho-shochu (Sweet potato shochu). J. Brewing Soc. Jpn. 86, 250 254. Steiner, J.L., Crowell, K.T., Lang, C.H., 2015. Impact of alcohol on glycemic control and insulin action. Biomolecules 5, 2223 2246. Sumi, H., 2001. Physiological functions of traditional shochu and awamori. J. Brewing Soc. Jpn. 96, 513 519. Sumi, H., Hamada, H., Tsushima, H., Mihara, H., 1988. Urokinase-like plasminogen activator increase plasma after alcohol drinking. Alcohol Alcoholism 23, 33 43. Takamine, K., Setoguchi, S., Kamesawa, H., Hamasaki, Y., 1994. Study on screening of shochu yeast. Annu. Rep. Kagoshima Inst. Ind. Technol. Cent. 8, 1 6. Takamine, K., Yoshizaki, Y., Shimada, S., Takaya, S., Tamaki, H., Ito, K., et al., 2011. Estimation of the mechanism for cis and trans rose oxides formation in sweet potato shochu. J. Brewing Soc. Jpn. 106, 50 57. Takamine, K., Yoshizaki, Y., Yamamoto, Y., Yoshitake, K., Hashimoto, F., Tamaki, H., et al., 2012. Distribution of monoterpene glycosides in sweet potato. J. Brewing Soc. Jpn. 107, 782 787.

CHAPTER 13

Quality evaluation of sweet potato products Yoshiyuki Nakamura

Division of Field Crop Research, Institute of Crop Science, National Agriculture and Food Research Organization (NARO), Tsukuba, Japan

Sweet potato and its production and utilization in Japan Sweet potato is the 10th most important crop, the 7th most important crop for food, in terms of production in the world. The larger producing regions are now Asian and African countries, and China is the largest producing country with about 70% of the world’s annual production (FAOSTAT, 2016). Sweet potato is important not only as a food resource but also as industrial materials in these Asian and African countries (Woolfe, 1992). This crop was introduced into Japan from China about 400 years ago and has been now cultivated all over the country except in the north in Hokkaido. Its cultivation area is about 37,000 ha, and its production is about 860,000 tons, ranking it 13th in the world in recent years (FAOSTAT, 2016). Sweet potato was cultivated first in the Okinawa Islands and southern Kyushu areas, where people have been often struck by natural disasters, for example, typhoon and drought. Sweet potato is one of the most suitable crops in these regions since it is relatively tolerant to such disasters and can be grown in severe agricultural environments. Sweet potato was also sometimes utilized as an emergency crop to compensate for drastic decreases in rice production in the Edo era (1603 1867). In this era sweet potato production was propagated in the northeastern part of Japan (Kanto and Tohoku area) because the Japanese government recommended the production of sweet potato as an emergency crop in these areas where many people often suffered from famine (Kobayashi, 2010). The public sweet potato breeding system in Japan was established by the Japanese government in 1937, although the first cross-breeding of sweet potato was conducted in Okinawa prefecture in 1914 (Takahata, 2014).

Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00013-2

© 2019 Elsevier Inc. All rights reserved.

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At that time the Japanese government recommended the production of sweet potato for fuel materials as well as a staple food. According to the national statistical data, the cultivation area and the production of sweet potato in 1940 were about 270,000 ha and about 3.5 million tons, respectively. The cultivation area and production did not change much from 1920 to 1940. After World War II, the sweet potato production continued increasing together with the development of the starch industries and peaked at about 720 million tons in 1955. In this year about 30% of the production was taken up by the starch industries, which was the secondary to the largest consumption for food (about 38%). However, the consumption by the starch industries dramatically decreased when starch from the sugar industries could be inexpensively imported from foreign countries in 1960s. On the other hand, the consumption for food has not shown any drastic change for the last 70 years, and consequently it has made certain contribution to the sweet potato utilization in Japan. Although the larger part of the harvest was selfconsumed by producers in the 1950s and 1960s, the consumption through fresh markets gradually came to occupy a major part from the 1970s. At present, approximately 85% of the sweet potato production for food is consumed through fresh markets. Other uses for sweet potato in Japan are feed for livestock, materials for processed food, and materials for liquor (shochu) industries. The present percentages of the use for these in terms of the total consumption are around 0.3%, 6%, and 28%, respectively. Consequently, sweet potato is mostly consumed for food, including processed food (about 53%), alcohol (about 28%), and starch (about 15%) in Japan today (Katayama et al., 2017). As for food, Japanese people prefer to consume sweet potato after a simple cooking method, such as roasting, steaming, and frying. The roast sweet potato is called as “yaki-imo” and it is the most popular eating style of this crop. The utilization of sweet potato for processed food is performed mainly with two elements, namely its starch and its storage root. The starch isolated from sweet potato storage root was traditionally utilized as a material for confectioneries, cooking gel, and binding materials for mashed fish meat, and now it is mainly provided by the sugar industries. However, domestic sweet potato starch comprises only about 1.7% of the total consumption of starch in Japan. The processed foods made from sweet potato storage root include chips, steamed and cured sweet potato (called “hoshi-imo”), fried sweet potato coated with sugar syrup (called “daigaku-imo”), and soft cake made from sweet potato

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paste (called “imo-yokan”). In addition, sweet potato has been also available in the form of dehydrated powders and purees as the materials for processed food such as pigments and nutritional ingredients (Baba, 1990; Komaki and Yamakawa, 2006). The cultivars with orange and purple flesh are mostly suitable for such utilization because of their high content of physiologically functional components such as β-carotene and anthocyanins (Yamakawa and Yoshimoto, 2001).

Japanese sweet potato varieties for food and processed food Since sweet potato was first introduced into the southern Kyushu region via the Okinawa Islands about 400 years ago, many varieties of sweet potato have been produced by natural hybridization and mutation, or introduced from foreign countries. There were consequently more than 300 varieties in Japan at the beginning of the 1900s. For example, “Beniaka” was selected by farmers in Saitama prefecture from spontaneous mutants of “Yatsufusa” in 1898, and extended its production area to about 30,000 ha in the 1930s. “Genji” (“Genki”) was introduced into Hiroshima prefecture from Australia in 1895 and prevailed in 100,000 ha between 1940 and 1943. In addition to these two varieties, “Shichifuku,” “Taihaku,” and “Oiran” were also generally cultivated t that time. These five major native varieties were mainly used for food and their production area covered about 70% of the total production area in 1940s. “Shichifuku” was introduced from the United States in 1900 and cultivated mainly in western regions of Japan including Kyushu, Shikoku, and Setouchi areas. The production area of this variety was more than 25,000 ha in 1942. “Taihaku” was cultivated mainly in the Kanto area in eastern Japan. This variety was said to be domestically introduced from the Kyushu area—the place of origin—to the Kanto area in the 1910s, and became the leading variety in 1945 with 55,000 ha. This variety was also called “Yoshida,” and it played important roles as a parent in the breeding of early national varieties such as “Norin 2,” “Norin 3,” and “Norin 4.” “Oiran” originated in the Kyushu area, and was introduced into the Kanto area via Shizuoka prefecture at the beginning of the Meiji era in the 1870s. This variety has another name, “Iigoh,” and prevailed mainly in the Kanto area with 25,000 ha in 1943. “Oiran” (“Iigoh”) has a unique purple pattern in the center of the cross-section of its storage root, and was widely used as the material of “hoshi-imo” (local traditional

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processed food made from steamed sweet potato) because of its soft texture after steaming. Since a systematic cross-breeding of sweet potato started in Okinawa in 1914, several practical varieties were bred by the crossing of indigenous and/or introduced varieties. “Okinawa 100” was derived from a cross between “Shichifuku” and “Choshu,” and released in 1934 as a high yield variety. This variety was extensively cultivated during the postwar period as a food and energy supply, and its production area peaked at 81,000 ha (about 20% of the total production area of the crop in Japan) in 1946. “Gokoku-imo” was derived from a cross between “Genji” (“Genki”) and “Shichifuku,” and released in 1937. This variety was also called “Kokei 4,” and mostly cultivated in Japan in the latter half of the 1940s. Its maximum production area was recorded in 1949 at about 100,000 ha, which corresponded to about 25% of the total production area of sweet potatoes. In 1942, the first products of the Norin number varieties, “Norin 1” and “Norin 2,” were released. They occupied 25% 30% of the total sweet potato production area during the 1950s and 1960s, and their maximum production areas were recorded at about 100,000 ha in 1955 and 80,000 ha in 1962, respectively. These four varieties described earlier were used for food and starch materials. The most famous and important variety bred by crossing practiced in Okinawa is “Kokei 14,” which was derived from a crossing between “Nancy Hall” and “Siam” conducted in 1935 and released in 1945. This variety possessed a wide regional adaptability and thus produced many local derivative lines in many prefectures in Japan. The production area of the variety including its derivative lines was about 18,000 ha in 1973, which was equivalent to about 25% of the total area, and about 25,000 ha in 1985, equivalent to about 32% of the total area. Its production area was the largest for 20 years from 1973 to 1992, and was still the third largest in 2015. This variety can be adapted to the materials for various processed products, for example, “yaki-imo” (roast sweet potato), snacks, confectionaries, and paste because of the annual and local stability for the size and the starch content of the storage root. This variety is also used as the standard variety for quality evaluation in the current Japanese breeding system of sweet potatoes. After World War II, a new breeding system of sweet potato was started in 1947. In this system, a crossing was totally conducted in Kagoshima and then, a local selection was conducted in Chiba (mainly for the eastern region of the Japanese mainland), Kurashiki (mainly for the western region of the Japanese mainland), and Kumamoto

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(mainly for the southern region of Japan). This breeding system produced several major varieties, most of which are still available (Katayama et al., 2014). “Tamayutaka” was released in 1960 for starch production at first (Onoda et al., 1970), but today it has been utilized mainly as the material for “hoshi-imo” (Kuranouchi et al., 2006, 2010a,b, Nakamura et al., 2006, 2007). “Koganesengan” was also released initially for starch production in 1966 because of its high starch content (Sakai et al., 1967), and recently utilized mainly for the production of shochu, a liquor made from sweet potatoes. “Benikomachi,” derived from a cross between “Kokei 14” and “Koganesengan,” was released in 1975 (Sakai et al., 1978). This variety has nice shape, beautiful deep purple-red skincolor, and good taste, but was susceptible to some kinds of diseases such as soil rot and stem rot. In 1984 an improved variety, “Beniazuma,” was released (Shiga et al., 1985). This was also derived from a cross with “Koganesengan” with a father plant similar to “Benikomachi,” but is relatively resistant to soil rot and stem rot diseases. It also has beautiful red skin-color and good eating quality. The superiority of “Beniazuma,” that is, the practical resistance to diseases and good eating quality, accelerated the spread of this variety mainly in Kanto region, and in 1993 “Beniazuma” replaced “Kokei 14” as the number one variety in terms of production area in Japan. This variety had been the most popular sweet potato variety in Japan for about 30 years until 2010. Although the high sweetness is one of the great appeals for the eating quality of sweet potato, it may obstruct this crop in enlarging its utilization as a dietary food menu like as potato. Therefore sweet potato varieties with extremely low sweetness were developed from the end of the 20th century to the beginning of the 21st century. The varieties “Satsumahikari” released in 1987 (Kukimura, 1988; Kukimura et al., 1989) and “Okikogane” released in 2000 (Yoshinaga, 2010) produce almost no maltose during heat-cooking due to their lack or mere trace of β-amylase activity. In the 21st century other many new varieties of sweet potato having unique properties were also developed in Japan. “Quick Sweet,” released in 2002, was the first sweet potato variety containing starch with lower pasting temperature (Katayama et al., 2002). The temperature at which starch in sweet potato is gelatinized is ordinarily 70°C 75°C but the starch of this variety is able to become gelatinized at about 55°C. Thus the storage roots of the variety are able to produce maltose earlier during heat-cooking than other cultivars. In addition, the gelatinized gel made from such sweet potato starch with a lower pasting temperature was able

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to stay soft because it was hardly able to be retrogelatinized. Recently another variety containing a high content of such types of starch, “Konamizuki,” was developed (Katayama et al., 2012). Varieties with unique flesh colors have also been developed. The varieties with purple flesh color contain high anthocyanins in their flesh and are utilized mainly for food pigments. The varieties with orange flesh color contain high carotenoids and they are utilized for processed food, in particular, for “hoshi-imo” because of their soft texture. These varieties with purple and/ or orange flesh color have attracted the interest of health-conscious consumers in Japan (Tanaka et al., 2017). In 2007 a new variety “Beniharuka” with ordinary flesh color (yellow) was released. This variety is higher in sweetness and less mealy in texture compared to the current leading varieties such as “Kokei 14” and “Beniazuma” after cooking (Kai et al., 2017). During the last 10 years the production of this variety has been increasing in Japan due to the novelty of eating quality and the wide adaptability for utilization of the variety (Komaki and Yamakawa, 2006).

Free sugar components in relation to the sweetness of sweet potato products The sweetness is of great appeal in the eating quality of sweet potato, and is mainly due to the free sugars. The free sugars predominantly existing in the storage root of sweet potatoes are fructose, glucose, sucrose, and maltose (Picha, 1986a). Among them, maltose is hardly detected in raw fresh storage roots, while the other three sugars are present both in raw and in heat-cooked storage roots. It has been reported that these four free sugars were different for sweetness at the same temperature; the sweetness of fructose, glucose, and maltose are about 1.0, 0.55, and 0.35 times of that of sucrose at 40°C, respectively (Yoshizumi et al., 1986), and the total sweetness of the four free sugars, namely the summation of the products of the content and the relative sweetness of each free sugars were reported to be fairly correlated with sensory evaluation of the steamed sweet potato (Takahata et al., 1993a). Sweet potato cultivars and breeding lines are classified into three genotype groups based on the free sugar composition of their steamed storage roots (Takahata et al., 1992). Group 1 consists of cultivars and lines which contain large amounts of maltose in their steamed roots. Group 2 consists of cultivars and lines that generate small amounts of maltose by steaming. The cultivars and breeding lines which contain relatively large amounts of fructose and glucose are classified as group 3.

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The cultivars and lines belonging to group 1, which possess β-amylase activity, are the majority of currently grown sweet potato varieties, while group 2, which are completely or mostly lacking in this enzyme activity, contains only two varieties, “Satsumahikari” and “Okikogane,” that are currently available. The varieties belonging to group 3 that are used for food are relatively old ones, such as “Kokei 14” and “Tamayutaka.” The storage roots of ordinary sweet potato cultivars produce a large amount of maltose during heating due to the hydrolysis of their inner starch by β-amylase, and the maltose produced is largely responsible for the sweetness of heat-cooked sweet potato (Ito et al., 1968; Picha, 1986a; Takahata et al., 1992). Nakamura et al. (2014b, 2018) demonstrated that the sweetness of steamed storage roots, which is given in Brix% value of homogenate of the roots measured by a refractometer, increased linearly with maltose concentrations (wt.%) in the roots within 3 months after harvesting using the current Japanese sweet potato cultivars (Fig. 13.1). It was also reported that the maltose concentrations (wt.%) in the roots of the cultivars investigated ranged from approximately 0 wt.% to 15 wt.%. Older varieties such as “Kokei 14” and “Tamayutaka” contained maltose at concentrations less than 10 wt.% in their steamed storage roots, whereas recently developed varieties such as “Beniharuka” and “Himeayaka” contained maltose at concentrations higher than 12 wt.% (Nakamura et al., 2014b). Such new varieties 30 r = 0.855 ***

Sweetness (Brix%)

25 20 15 10 5 0 0

5 10 Maltose concentration (wt.%)

15

Figure 13.1 Relationships between the maltose concentrations and the sweetness values in steamed storage roots of Japanese cultivars and breeding resources of sweet potato (Nakamura et al., 2014b) (n 5 221, 2012, and 2013). The experiments were conducted within 3 months of harvest.  Significant at P , .001.

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therefore exhibited higher sweetness in their steamed roots than the older varieties (Katayama et al., 2014) because they exhibited higher β-amylase activity in their fresh storage roots than older varieties (Nakamura et al., 2014b, 2017). In contrast, varieties with extremely low sweetness in their steamed roots have also been developed over the past 30 years, such as “Satsumahikari” and “Okikogane.” The sweetness of their steamed storage roots was very low (about 7 9 Brix%) as almost no maltose was produced during heating due to their extremely low β-amylase activity (Baba et al., 1987b; Kukimura, 1988; Kukimura et al., 1989; Kumagai et al., 1990). Kumagai et al. (1990) reported that a variant lacking or having only traces of β-amylase in sweet potato storage roots was controlled by a single recessive allele and inherited in a hexasomic or tetradisomic manner. They also described that a new type of sweet potato with or without extremely low β-amylase activity could easily be developed as the allele was frequently detected in cultivated germplasm of the genetic resources of sweet potato. Fig. 13.2 shows the effect of β-amylase activity in fresh storage root on the maltose concentration of its steamed storage root, which was 16

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Figure 13.2 Relationship between the β-amylase activities of fresh storage roots and the maltose concentrations in steamed storage roots of Japanese cultivars and breeding resources of sweet potato (Nakamura et al., 2014b) (n 5 221, 2012, and 2013).

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determined by quantification of the reducing sugar produced via starch hydrolysis by β-amylase isolated from fresh roots of the tested cultivars investigated. The maltose concentrations of the steamed storage roots increased with increasing β-amylase activity up to about 0.2 mmol maltose/(min mg) protein in enzyme solution, while the maltose concentrations did not clearly increase with increasing activity even if the activity increased over this level. The results suggested that maltose generation in sweet potato storage roots could be regulated not only by β-amylase but also by other factors regarding the roots, particularly those with higher [higher than 0.2 mmol maltose/(min mg) protein] β-amylase activity. Another factor could be starch gelatinization that is required prior to maltose generation by β-amylase in sweet potato, which is not able to digest raw starch (Kiribuchi and Kubota, 1976). The maltose concentration exhibited a negative correlation (r 5 20.53 , n 5 221) with the pasting temperature of starch isolated from the fresh storage roots of cultivars investigated (Fig. 13.3). This negative correlation between maltose concentration and starch pasting temperature was stronger (r 5 20.69 , n 5 111) for roots with higher β-amylase activity (Nakamura et al., 2014b). The starch gelatinization characteristics such as pasting temperature would be also important factors for maltose generation in sweet potato storage roots during heat-cooking as well as β-amylase activity of the root. The starch pasting temperature of sweet potato starch is closely related to the molecular structure of its amylopectin (Noda et al., 1998), and greatly affected by the soil temperature during the growth period of sweet potato (Noda et al., 2001). The storage roots of “Beniazuma” and “Beniharuka” cultivated in Hokkaido, the northernmost prefecture of Japan, generated higher amounts of maltose than those of the same varieties cultivated in Ibaraki located about 700 km south of Hokkaido, despite lower β-amylase activity in the roots cultivated in Hokkaido (Nakamura et al., 2014b). The pasting temperatures of starch isolated from the roots harvested in Hokkaido was 5°C 7°C lower than those harvested in Ibaraki, where the average temperature during the summer season was about 5°C higher than Hokkaido for both varieties. Thus the gelatinization of intracellular starch in the storage roots harvested in Hokkaido was practically recognized at a lower temperature than in those harvested in Ibaraki for variety “Beniazuma.” Sweet potato cultivars containing starch with a lower pasting temperature could have the potential for higher sweetness. In the 21st century some new varieties with lower starch pasting temperature have been developed for expanding the use of sweet potato

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16 r = – 0.53***

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65

70

75

80

Starch pasting temperature (°C) Beniazuma

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Figure 13.3 Relationship between the pasting temperatures of starch isolated from fresh storage roots and the maltose concentrations in steamed storage roots of Japanese cultivars and resources of sweet potato (Nakamura et al., 2014b) (n 5 221, 2012, and 2013). The starch pasting temperatures were estimated from the Rapid Visco Analyzer profiles of 7 wt.% suspensions of starch isolated from fresh storage roots of cultivars.  Significant at P , .001.

and its starch. “Quick Sweet” was the first of these new varieties (Katayama et al., 2002, 2004), and it possesses the lowest starch pasting temperature, at about 53°C determined by the Rapid Visco Analyzer, among the current cultivars (Katayama et al., 2015). This unique variety was able to produce maltose earlier during heat-cooking than other traditional popular varieties with higher pasting temperatures (70°C 75° C). The decrease in starch pasting temperature is effective for an increase in maltose generation because it will prolong the duration of maltose generation (Fig. 13.4). In addition, Nakamura et al. (2014a) reported that the activity of β-amylase isolated from “Quick Sweet” storage roots heated at 80°C almost maintained its original level in the fresh roots, whereas the

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Slow heating

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50 Time Starch gelatinization and maltose generation occur at the temperature from about 70°C – 80°C(white zone) for "Beniazuma" having ordinary starch pasting temperature. Slow heating prolongs the period( ) the sweet potato stays under the effective temperature zone, and thus increase maltose generation.

Time Starch gelatinization and maltose generation occur at the temperature from about 50°C – 80°C (White zone) for "Quick Sweet" due to its lower starch pasting temperature. The storage root of this variety stays under the effective temperature zone for long time even by rapid heating.

Figure 13.4 Illustration of starch gelatinization and maltose generation during rapid or slow heating in the storage roots of sweet potato varieties containing starch with different pasting temperatures: “Beniazuma” and “Quick Sweet.”

activity of “Beniazuma” (with a starch pasting temperature of about 75° C) was severely inhibited at the same temperature. Takahata et al. (1994) indicated the importance of β-amylase stability during heat-cooking as well as starch gelatinization for maltose generation in sweet potato. “Quick Sweet” has an advantage for maltose generation during heating because β-amylase in its storage roots could maintain its activity at a higher temperature than the enzyme in “Beniazuma” storage roots. However, the activity of the enzyme isolated from the fresh roots of both varieties exhibited similar responses to temperature. This indicated that β-amylase in the heated storage roots of “Quick Sweet” remained stable due to starch gelatinization at lower temperatures before its inactivation during heating. Therefore maltose generation in “Quick Sweet” storage roots started at lower temperatures and continued at higher temperatures than that in “Beniazuma” during heating. In addition, such varieties as “Quick Sweet” and “Hoshikirari” that contain starch with lower pasting temperatures in their storage roots exhibited another useful function of being able to maintain a larger percentage of total ascorbic acid content in the fresh roots after heat-cooking than other older

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varieties (Nakamura et al., 2016). The earlier generation of maltose in heated storage roots of “Quick Sweet” and “Hoshikirari” containing starch with lower pasting temperature may also have a protective effect against the heat breakdown of ascorbic acid, a well-known antioxidant compound. Although maltose is one of the key components for the sweetness of steamed sweet potato storage roots as described earlier, it becomes less important for the sweetness of roots stored for longer periods (3 6 months) after harvesting. The long-term storage of sweet potato storage roots induces an increase in the content of sucrose together with a decrease in starch content (Picha, 1986b). Conversely, maltose concentration in steamed roots did not increase, but actually decreased during storage because of the decrease in β-amylase activity. The sweetness of sucrose is two to three times higher than that of maltose, and the sucrose concentration does not change significantly via heat-cooking. Thus sucrose instead of maltose played an important role in the sweetness of steamed sweet potato roots after long-term storage. Takahata et al. (1995) demonstrated that changes in sucrose-synthesizing enzymes, such as sucrose synthase (SUS) and sucrose-6-phosphate synthase (SPS), were possibly associated with sucrose accumulation in fresh sweet potato roots during storage. Masuda et al. (2007) reported that storage of sweet potato “Kokei 14” at 5°C or 10°C promoted sucrose accumulation due to the stimulation of SPS activity accompanying the suppression of β-amylase activity. Sucrose accumulation in sweet potato storage roots was also induced under stressful treatment, such as gamma irradiation (Hayashi et al., 1984). Although the enzymatic properties of SUS and SPS in higher plant species including sweet potato were extensively studied (Murata, 1971a,b; Ono and Ishimaru, 2006), the mechanism for sucrose accumulation in sweet potato storage roots still remains to be completely elucidated because the sucrose metabolisms are regulated by other metabolic enzymes such as acid invertases other than SUS and SPS. It had been reported that sucrose accumulation was induced in potato tubers stored at 5°C 7 °C due to the decrease in this enzyme activity together with the increase in SPS activity (Hironaka et al., 2004, 2005a,b). An acid invertase plays an important role in the conversion of sucrose into fructose and glucose, and a strong relationship between the hexose (fructose and glucose) content and the enzyme activity in various sweet potato cultivars (Takahata et al., 1996).

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Fructose and glucose are the predominant monosaccharides detected in sweet potato storage roots (Picha, 1986a). The concentrations of these monosaccharides in its fresh storage roots are up to about 1.5 wt.% in Japanese current commercial varieties. Among them, the older varieties with lower sucrose content such as “Kokei 14” and “Tamayutaka” contain the two kinds of monosaccharides at higher concentrations compared to current higher sucrose varieties such as “Beniazuma” and “Beniharuka” because these older varieties are higher in acid invertase activity among the current Japanese varieties. Although the concentrations of fructose and glucose in heat-cooked storage roots, which are little different from those in the fresh roots, are considerably lower than those of maltose and sucrose, the former two kinds of monosaccharides could play an important role for characterizing the sweetness of heat-cooked sweet potatoes. For example, fructose is the highest in sweetness among the four free sugars that exist in the sweet potato root, and its sweetness is about 1.2 times and three times that of sucrose and maltose at the same temperature, respectively. Thus fructose has a strong impact on the sweetness of heat-cooked sweet potato. On the other hand, glucose makes a soft impact on the sweetness due to its weak sweetness. Fructose and glucose therefore have complicating effects for the sweetness of sweet potato, and these effects still remain to be elucidated. In summary, the free sugars concerned with the sweetness of heat-cooked sweet potato are fructose, glucose, sucrose, and maltose. Among them, maltose generated by starch hydrolysis by β-amylase during heat-cooking is the most abundant constituent despite a gradual decrease in its content during storage even under normal conditions. The concentration of maltose in steamed sweet potato storage roots increases together with the enzyme activity up to about 0.2 mmol maltose/(min mg) protein of the enzyme solution but, however, does not increase in proportion to the activity when it increases beyond this level. The concentration of maltose exhibits a negative correlation with the pasting temperature of starch isolated from the fresh storage root. On the other hand, sucrose can be detected in fresh storage roots as well as heat-cooked ones at the same concentrations. Although its concentration is about one-third of the concentration of maltose, its sweetness is about 2.5 times that of maltose. Therefore sucrose contributes to the sweetness of heat-cooked sweet potato to a similar extent as maltose. Furthermore, sucrose plays a more important role for the sweetness of sweet potato roots instead of maltose

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when the root is stored for long period (more than 3 months) because its concentration practically increases after the long-term storage. Fructose and glucose are minor constituents of free sugars contained in sweet potato storage root, and are detected at lower concentrations in recently developed cultivars than in older cultivars in Japan.

Chemical factors in relation to the textural properties of steamed storage roots of Sweet potatoes The textural character of heat-cooked storage root is another key factor for the eating quality of heat-cooked sweet potato storage root, and it also plays an important role in the processing of sweet potato (Nara, 1951). In the current Japanese official breeding system of sweet potatoes for food, the breeding resources and lines were evaluated for texture as well as sweetness (Kitahara et al., 2017). The textural evaluation of breeding resources and lines tested is conducted by a comparison of their texture with the texture of the standard variety (“Kokei 14”) after eating their steamed storage roots together. The texture of the tested sample was sorted into five categories: mealy, slightly mealy, intermediate, slightly soggy, and soggy (Nakamura et al., 2010, 2015; Yoshinaga, 2014). The “intermediate” means that the sensory textural evaluation for the tested sample is almost equal to that of the standard variety. The “mealy” and “soggy” mean that the texture of the tested sample is mealy and soggy compared with the texture of the standard variety, respectively. However, such a sensory evaluation system has some problems, for example, the texture of the standard variety often differs between harvest locations or from year to year, and the tasting peculiarities of panel members are liable to influence the result of evaluation. Therefore it is necessary to determine the relationships between the texture of steamed sweet potato storage roots and physicochemical properties of their constituents using an invariant evaluating system of the textural properties of sweet potatoes. The most primary factor associated with the texture of sweet potato storage roots after cooking is starch content. Although the fresh storage roots with higher and lower starch contents tend to exhibit mealy and soggy texture in their steamed roots, respectively (Nara, 1957a), the textural differences in the steamed roots could not be definitely determined by starch content in the fresh roots. Nakamura et al. (2015) demonstrated that the groups of cultivars and the breeding resources having the five kinds of texture of their steamed storage roots were sorted into four

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b

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c

d

1 10

15

20

25

30

Starch content in fresh storage root (%) Texture index:1; mealy, 2; slightly mealy, 3; intermediate, 4; slightly soggy, 5; soggy

Figure 13.5 The relationship between the starch content in fresh storage roots and the texture indices of steamed storage roots of Japanese sweet potato cultivars (Nakamura et al., 2015). Horizontal bars represent standard deviations in the starch contents of the root samples (n 5 60 65) for each texture index. Different letters indicate significant differences (P , .05) among the starch content by Tukey’s HSD test.

groups with significant differences (P , .05) based on the starch content in their fresh roots (Fig. 13.5). The starch content in the fresh storage root of sweet potato will change during heat-cooking via starch digestion by amylolytic enzymes, such as β-amylase, and therefore it is easily assumed that the content of starch remaining in the heat-cooked storage root must be much closely related to the texture of the cooked roots. However, there is little information about starch digestion regarding the texture of sweet potato storage roots (Nara, 1957b; Walter et al., 1975; Nakamura et al., 2017, 2018). The amount of starch digested by β-amylase during steaming was calculated from the maltose concentration in the steamed roots, and the starch contents of both in fresh and steamed roots were quantified after the complete digestion of starch into glucose (Nakamura et al., 2017). The differences in starch content between the fresh and the steamed roots were highly (r 5 0.94 , n 5 40) consistent with the amount of digested starch for the six varieties of sweet potato with three different levels of

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β-amylase. The starch digestion rate in steamed roots (i.e., ratio of starch content in steamed roots against that in fresh roots) was practically correlated to β-amylase activity in the fresh roots. The maltose concentration and the degradation rate of starch in steamed storage roots of three different groups for the levels of β-amylase activity in sweet potato varieties are shown in Table 13.1. The starch content in fresh storage roots of “Beniharuka” and “Himeayaka,” which have higher β-amylase activity, decreased remarkably to less than 50% during steaming. In contrast, older varieties such as “Kokei 14” and “Tamayutaka,” which have 50% 60% of the β-amylase activity in the former varieties, maintained approximately 70% of the starch content existing in their fresh roots after steaming due to lower starch digestion. Furthermore, in such varieties as “Okikogane” and “Satsumahikari” that lack or have extremely low levels of β-amylase activity, the starch content of their fresh roots hardly decreased during steaming. The steamed roots of these two varieties consequently contained the highest content of starch among the six varieties described earlier, and thus showed a mealy texture even though their fresh roots had the lowest starch content. The texture of the steamed roots of sweet potato could be closely correlated with the remaining content of starch after steaming rather than that prior to steaming. The starch content in steamed roots could be predicted by the starch content and β-amylase activity of fresh roots before steaming. Therefore β-amylase activity was thought to be an important factor in determining texture as well as the sweetness of steamed sweet potato. Table 13.1 β-Amylase activities, maltose concentrations, and starch digestion rates in three groups of sweet potato varieties with different levels of β-amylase activity (Nakamura et al., 2017). Activity level of sample

β-Amylase† activity [mmol maltose/(min mg)]

Maltose concentration† (wt.%)

Starch digestion rate , † (%)

High (n 5 12) Middle (n 5 11) Extremely low (n 5 11)

0.283 6 0.073a 0.139 6 0.037b 0.0126 6 0.022c

11.29 6 2.70a 7.25 6 2.20b 0.13 6 0.037c

51.19 6 8.45a 41.31 6 8.89b 0.92 6 2.43c

 High active varieties: “Beniharuka” (n 5 7), “Himeayaka” (n 5 5). Middle active varieties: “Kokei 14” (n 5 6), “Tamayutaka” (n 5 5). Extremely low active varieties: “Okikogane” (n 5 7), “Satsumahikari” (n 5 4). The number of samples examined is enclosed in parentheses.  Percentage of the amount of digested starch, which can be calculated as maltose content 3 0.95, against the starch content in fresh root. †Different alphabets in a column indicate significant differences at P , .05 as determined by Tukey’s HSD test among the three sample groups.

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The starch content of sweet potato storage roots also changed even in fresh roots during storage via starch digestion, and consequently textural change could occur after the heat-cooking of the root (Picha, 1986b; Walter, 1987). Although the starch degradation in sweet potato roots during storage is revealed to be concerned with α-amylase (Morrison et al., 1993; Takahata et al., 1995), there is little information about the enzymatic mechanism for starch degradation by α-amylase in sweet potato during the storage period (Hagenimana et al., 1994). Because sweet potato is normally high in β-amylase activity, the measurement of its α-amylase activity required the elimination of β-amylase activity. Baba et al. (1987a) measured the α-amylase activity from dehydrated sweet potato flour using β-limit dextrin, which is not susceptible to β-amylase digestion, as a substrate for the enzymatic reaction. It was reported that the activity of α-amylase in storage roots was extremely low (less than 0.1% of that of β-amylase) just after harvest, and still low (about 1% of β-amylase activity) even after storage for 8 months despite a prominent increase of the activity during storage (Baba, 1990). Although α-amylase showing very low activity compared to β-amylase, α-amylase could remarkably affect the texture of heat-processed sweet potato products, such as fried chips and granules, via the digestion of starch into free sugars. Although there have been some reports about the varietal difference in α-amylase activity of sweet potato cultivars in relation to textural differences (Baba, 1990; Morrison et al., 1993; Takahata et al., 1995), further investigation is still needed for the relationships between starch metabolism by α-amylase and the texture of heat-cooked sweet potato after storage under different conditions. It has been well known that the apparent amylose content in starch of rice grains shows a wide range of variation and significantly affects the texture of cooked rice. It was reported that the difference in amylose content in sweet potato starch had little effect on the texture of steamed storage roots (Nara, 1957c), because the diversity of the amylose content in the starch of Japanese sweet potato cultivars is relatively lower than that in rice starch (Kitahara et al., 1996; Tokimura et al., 2002; Katayama et al., 2004). Recently, amylose-free starch has been obtained by genetic transformation techniques (Kimura et al., 2001; Otani et al., 2007) in sweet potato as well as in other tuberous and root crops (Hovenkamp-Hermelink et al., 1987; Ceballos et al., 2007). However, the texture of the storage roots containing those starches has not been elucidated.

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In addition to the difference in starch properties, there are some other differences in cellular microstructure and water status between the steamed storage roots with different textures (Nakamura et al., 2010). In steamed roots having a mealy-type texture, the cells in the root tissues exist independently of each other for keeping their structures. On the other hand, in steamed roots having a soggy-type texture the cells in the root tissue melt together to lose their original structures (Fig. 13.6). The cells in the steamed roots with mealy-type texture are completely filled with starch gel and thus expand. On the other hand, the cells in the steamed roots with soggy-type texture shrink because of a decrease in expanding pressure due to a lower content of starch gel. The analysis of the molecular status of water using the magnetic resonance imaging technique revealed that the root tissue with mealy-type texture showed a smaller content of free water molecules with heterogeneous distribution compared to the root with soggy-type texture. The heterogeneous distribution of free water molecules was also concerned with some kind of deterioration in quality for “hoshi-imo” (Nakamura et al., 2007). “Hoshi-imo” is a local and traditional processed food made from steamed sweet potato roots and sometimes suffer from a deterioration in quality, known as “Shirota” (Nakamura et al., 2007; Kuranouchi et al., 2010a,b). Nakamura et al. (2007) observed that the portion of steamed storage roots showing a lower distribution of free water molecules will suffer from the “Shirota” damage after processing the storage roots into “hoshi-imo” product. Such a decrease in free water molecules in the “Shirota” part of the steamed storage roots could be caused by insufficient starch

Figure 13.6 The tissue microstructure of steamed storage root of sweet potato varieties showing mealy-type and soggy-type texture observed by scanning electron microscopy (magnification: 3 500). (A) Variety showing mealy-type texture; (B) variety showing soggy-type texture.

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gelatinization during steaming due to a rise in starch pasting temperature (Nakamura et al., 2006, 2007). The new varieties with lower starch pasting temperature, for example, “Quick Sweet” (Katayama et al., 2002) and “Hoshikirari” (Kuranouchi et al., 2012), are suitable materials for making “hoshi-imo” without the “Shirota” defect. In summary, the differences in cellular microstructure and molecular status of water in steamed storage root tissue seem to be concerned with the textural differences of steamed sweet potato storage roots. The tissue disintegration of storage roots after heat-cooking is also an important factor for eating quality and adaptability for cooking and manufacturing of sweet potato. The heat-cooked sweet potato storage roots exhibiting low tissue disintegration generally tend to have a mealy texture and are suitable for cooking and manufacturing. The degree of tissue disintegration of steamed storage roots of sweet potato can be evaluated by measuring the decreasing rate in their weights after shaking the root sections in water (Nakamura et al., 2015). The degree of disintegration was correlated with texture, and the textural differences could be accounted for by the differences in the degree of disintegration as well as by the differences in starch content. In particular, the textures of the roots with moderate starch content (16% 22%) in their fresh roots showed closer correlation to the degree of tissue disintegration because their textures were less affected by starch content compared to the roots with high and low starch content. The degree of tissue disintegration could be a different factor to the starch content affecting the texture of steamed sweet potato storage root. Matsuura-Endo et al. (2002a) observed the differences in the texture of steamed potato tubers with same starch content. They demonstrated that such differences in texture could be related to the degree of tissue disintegration caused by cell separation which was influenced by calcium and cell wall polysaccharides contents (Matsuura-Endo et al., 2002b). In sweet potato the disintegration rate of steamed storage root tissue exhibited a negative correlation (r 5 20.403) with the calcium content per fresh weight of the roots, which did not account for the textural differences, and also showed weak negative correlation (r 5 20.214) with the content of chelating agent soluble fraction in pectic substances extracted from starch residue (Nakamura et al., 2015). The chelating agent soluble ones play important roles together with divalent cations in cell adhesion in plant tissue (Grant et al., 1973). The treatment of storage root tissues of sweet potato with chelating agent decreases their firmness

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along with histological changes in the microstructure of cell wall (Fuchigami et al., 2002), and divalent cations such as calcium ions are reported to have a prevention effect against the decrease in firmness in the potato (Ross et al., 2011). Although the chemical compositions of other cell wall polysaccharides such as cellulose and hemicellulose in sweet potato storage roots are analyzed (Noda et al., 1994; Tsukui, 1988; Tsukui et al., 1994; Salvador et al., 2000), little information about their functions regarding the texture of cooked sweet potato roots is available (Nara 1958; Shen and Sterling, 1981) and further investigations are still needed. In summary, disintegration rates of steamed root tissues were associated with texture, which largely depended on starch content, and the rate related to calcium content and equally to starch content via interactions with chelating agent soluble pectic substances.

Carotenoids and anthocyanins—their association with discoloration in sweet potato storage root Another important factor for the quality of food is color. In the current Japanese breeding system of sweet potatoes for food, the colors of the skin and the flesh of the storage root of progeny strain are important items of investigation. The colors of the top parts, such as stem, petiole, and leaf blade, are also investigated mainly as markers of genetic characterization. The skin colors of sweet potato storage roots are caused by pigments accumulated in its periderm, and are categorized into eight groups: white, yellow, dark, orange, scarlet, red, purple (violet), and other. In Japan the cultivars with scarlet or red skin colors, for example, “Beniazuma” and “Beniharuka,” are thought to be suitable for fresh utilization, for example, for “yaki-imo.” On the other hand, the pale-colored cultivars, that is, with white and yellow skin colors, for example, “Konahomare” and “Kogenesengan,” are used for the production of starch and liquor (shochu) in order to avoid color-contamination from their skin. The major types of pigment associated with the colors of sweet potato storage root are carotenoids and anthocyanins (Woolfe, 1992), and these chemical components possess various health-promoting functions (Yoshimoto, 2010). In Japan many sweet potato varieties containing these components at high levels have been developed over the last 20 years (Takahata, 2014). The varieties containing high levels of carotenoids and anthocyanins in their storage roots have orange and purple flesh, respectively.

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The main carotenoid in these orange-fleshed varieties is β-carotene, which is the dominant component (more than 80% of total carotenoids). On the other hand, the carotenoids mainly detected in yellow-fleshed varieties are not β-carotene but β-carotene diepoxide and β-cryptoxanthin epoxide, which are produced from β-carotene via metabolic pathways (Maoka et al., 2007). The content of β-carotene in the storage roots have been extensively investigated for various sweet potato cultivars with yellow and orange flesh (Takahata et al., 1993b; Kimura et al., 2007). The carotenoid contents in the orange-fleshed cultivars were about 10 times that of the ordinary yellow-fleshed cultivars (Ishiguro et al., 2010) ranging from 13.5 to 39.9 mg/100 g DW. The high β-carotene varieties such as “Hitachi Red” (Tarumoto et al., 1995) and “J-Red” (Yamakawa et al., 1998) are suitable for processed food, for example, “hoshi-imo” and juices, because they have soft and moist textures due to their lower starch content. The deep and bright yellow-fleshed varieties are preferable for food such as “yaki-imo” and “hoshi-imo.” The deep yellow-fleshed variety “Beniazuma” replaced “Kokei 14” with pale yellow flesh as a leading variety for “yaki-imo,” and “Beniharuka,” a bright yellow-fleshed variety, became a leading variety for “hoshi-imo” (steamed and cured sweet potato) instead of “Tamayutaka” with its white yellow flesh color. Over the last 20 years various sweet potato varieties with purple flesh were also developed. They contain anthocyanins at high levels in their storage roots, and most of them are utilized as food colorants in powder or puree forms. Eight major peaks of anthocyanins were detected by investigations using HPLC for the crude pigments extracted from the storage root of “Yamagawamurasaki,” a local cultivar in the southern Kyushu area, and were abbreviated as YGM-1a, -1b, -2, -3, -4b, -5a, -5b, and -6 after the name of cultivar “Yamagawamurasaki” (Goda et al., 1997). The molecular identification of the eight peaks demonstrated that the basic molecular structures of the former four major anthocyanins (YGM-1a, -1b, -2, and -3) are acylated forms of cyanidin, and those of the later four anthocyanins (YGM-4b, -5a, -5b, and -6) are acylated forms of peonidin (Terahara et al., 1999). The sweet potato cultivars containing cyanidin-type anthocyanins predominantly have a bluish purple color, and those containing peonidin-type anthocyanins predominantly have reddish purple color in the flesh of their storage roots, respectively (Yoshinaga et al., 1999). In addition, the cyanidin-type anthocyanins in purplefleshed sweet potatoes were revealed to have higher antioxidative activity

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estimated by radical-scavenging activity of DPPH (Ishiguro et al., 2007). Takahata et al. (2011) reported that cyanidin-based anthocyanins are closely related to DPPH radical-scavenging activity in sweet potato storage roots. Besides the antioxidant activity, various physiological functionalities of anthocyanins isolated from purple-fleshed sweet potatoes have been extensively investigated in the recent decades in Japan (Suda et al., 2003, 2008; Yoshimoto, 2010). The anthocyanins contained in sweet potato storage roots have high potential as natural colorants for processed foods (Oki et al., 2010) because of their advantages such as high yield of pigment (Yamakawa et al., 1997), color stability against heating and light irradiation, and bright tone (Hayashi et al., 1996; Tsukui et al., 1999). In Japan most of the sweet potato varieties with purple flesh were first utilized as the source of food colorants, while a few official varieties became available for table use. “Purple Sweet Lord” (Tamiya et al., 2003) was the first national variety for table use and “Kyushu No. 137” (Yoshinaga et al., 2006) was the second variety. These two varieties have lower anthocyanin content (about 20% 35%) and better eating quality (higher sweetness and softer texture), compared to the special varieties for food colorants such as “Murasakimasari” and “Akemurasaki.” “Kyushu No. 137” also possesses a high suitability for “hoshi-imo” manufacturing because this variety has a soggy texture and a light color tone in its steamed storage root. Soggy texture in the steamed storage root is highly required for “hoshi-imo” manufacturing, and light color due to its lower content of phenolic compounds including anthocyanins is effective in preventing the product’s color from becoming dark. The “hoshi-imo” made from “Kyushu No. 137” is excellent in color quality because of its lower content of phenolic compounds. The darkening discoloration of storage roots during processing is also one of the serious problems for the quality of sweet potato products. The darkening of the flesh color of sweet potato storage roots is caused through enzymatic or nonenzymatic mechanisms. The enzymatic darkening occurs in the outer portion of fresh or heat-cooked storage roots at a not sufficiently high temperature in the presence of oxygen, and is caused by oxidation of phenolic compounds via polyphenol oxidase (Walter and Schadel, 1981; Ma et al., 1992). The degree of this enzymatic discoloration is closely correlated to the content of phenolic compounds (Walter and Purcell, 1980) and the activity of polyphenol oxidase (Ma et al., 1992).

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On the other hand, the nonenzymatic darkening of the flesh color can occur even in the storage root after being cooked at a temperature high enough to inhibit polyphenol oxidase. It is caused by the binding of ferric ions, which are transformed from ferrous ions by oxidation during heating, to phenolic compounds, which are predominately chlorogenic acids and their isomers in sweet potato (Nakabayashi, 1970). Reduction of the contents of ferric ions or chlorogenic acids, which are active factors for the formation of the nonenzymatic darkening in heat-cooked sweet potato storage roots, was necessary to prevent the nonenzymatic darkening in heat-cooked sweet potato roots. Shimozono et al. (2000) reported that a treatment of sweet potato paste prepared from the varieties with high chlorogenic acid contents with chlorogenic acid esterase decomposed the isomers of chlorogenic acid presented in the paste into low active isomers such as caffeic and quinic acid, and thus effectively suppressed the darkening of the paste. Baba (1992) reported that reducing or chelating the ferric ions by some kind of chemical agent, for example, ascorbic acid and calcium chloride, was also effective for the prevention of the darkening in peeled sweet potato storage roots. The content of chlorogenic acids and the activity of polyphenol oxidase in sweet potato storage roots are the important factors for the darkening discoloration of sweet potato storage roots, whether it is caused by enzymatic mechanisms or not, and therefore a cultivar with lower chlorogenic acids content and polyphenol oxidase activity has a great potential for the development of sweet potato products with excellent colors. It has been reported that there are cultivar differences for the contents and the enzyme activity, and hence the contents of phenolic compounds and polyphenol oxidase activity for breeding materials are roughly checked in the present sweet potato breeding system in Japan (Komaki et al., 1992). “Aikomachi,” a recently developed variety in which hardly darkening discoloration in its flesh color occurs, was suitable for paste and confectioneries due to its light yellow flesh color (Ohara-Takada et al., 2016).

Conclusion The sweetness and texture of steamed sweet potato storage roots, which were thought to be major factors regarding the eating quality and the suitability for processing of sweet potatoes, were investigated for various varieties in Japan. The sweetness of steamed storage roots stored for not

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longer than 3 months after harvesting were correlated with the concentration of maltose in the roots. The maltose concentration increased to a peak of approximately 10 wt.%, with β-amylase activity in the fresh roots increasing up to about 0.2 mmol maltose/(min mg) protein of the enzyme solution. The maltose concentration, however, did not increase along with increased β-amylase activity even if the activity exceeded these levels. The maltose concentration also exhibited a negative correlation with the pasting temperature of starch isolated from the fresh roots. The recently developed varieties with high sweetness, such as “Quick Sweet” and “Beniharuka,” produced a larger amount of maltose due to their higher β-amylase activity and/or lower starch pasting temperature than older varieties. The textures of steamed sweet potato storage roots were correlated to the contents of starch remaining in the steamed roots after digestion into maltose by β-amylase during steaming. The digestion rates of starch were significantly correlated to the activities of β-amylase in the fresh roots. The disintegration of steamed storage root tissues is also an important factor for eating quality and suitability for processing of sweet potato. In general, the steamed storage roots with a lower degree of tissue disintegration showed a mealy texture regardless of their starch content. The texture of steamed roots is highly correlated to the degree of tissue disintegration, particularly in the roots with moderate starch contents (16% 22%). The disintegration rate of steamed root tissue showed negative correlations with the contents of calcium (r 5 20.403) and chelating agent soluble pectin fraction from starch residue (r 5 20.214) per the fresh weight of the root. From these results the texture of steamed sweet potato storage root is largely affected by the content of starch remaining after the digestion by β-amylase during steaming and the degree of tissue disintegration, which is correlated with the contents of calcium and pectin probably existing in cell wall of the root tissue. The color of the skin and the flesh of the storage root are another important factors for the quality of sweet potato products, such as foods and food materials. In Japan there are many varieties of sweet potato with different colored skin and flesh. Among them purple- and orange- fleshed varieties have high contents of anthocyanins and β-carotene, respectively, and the functionality of these compounds has been extensively investigated in recent decades. Such sweet potato varieties therefore have been of great interest to health-conscious people, thus resulting in the promotion of the production of sweet potato products. In contrast, a negative

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aspect of sweet potato flesh color is the darkening discoloration caused by the chemical reactions of phenolic compounds that are plentiful in sweet potato storage roots. The discoloration appearing in flesh storage roots is primarily induced by enzymatic oxidation of phenolic compounds, and when it occurs after heat-cooking and processing it is caused by nonenzymatic binding of phenolic compounds to ferric ions transformed from ferrous ions by heating. The darkening discoloration clearly decreases the quality of sweet potato food products, including food materials such as starch and powder, and thus the degree of the darkening discoloration is an important characteristic to check when breeding sweet potatoes as a food crop.

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Hironaka, K., Ishibashi, K., Koaze, H., Umezaki, K., Mori, M., Tsuda, S., et al., 2004. Relationship between invertase activity and reducing sugar content of cold-stored Japanese processing potatoes. Food Preserv. Sci. 30, 295 299. Hironaka, K., Ishibashi, K., Koaze, H., Kobayashi, S., Mori, M., Tsuda, S., et al., 2005a. Changes in invertase, sucrose-6-phosphate synthase and UDP-glucose pyrophosphorylase activities, and their relations to reducing sugar content in Japanese processing potato varieties stored at low temperature. Food Preserv. Sci. 31, 9 14. Hironaka, K., Ishibashi, K., Koaze, H., Miyashita, H., Mori, M., Tsuda, S., et al., 2005b. Effect of storage temperature on invertase, sucrose-6-phophate- synthase and UDP-glucose pyrophosphorylase activities of Japanese processing potatoes. Food Preserv. Sci. 31, 67 74. Hovenkamp-Hermelink, J.H.M., Jacobsen, E., Ponstein, A.S., Visser, R.G.F., Vos-Scheperkeuter, G.H., Bijmolt, E.W., et al., 1987. Isolation of an amylose-free starch mutant of the potato (Solanum tuberosum L.). Theor. Appl. Genet. 75, 217 221. Ishiguro, K., Yahara, S., Yoshimoto, M., 2007. Changes in polyphenolic content and radical scavenging activity of sweet potato (Ipomoea batatas L.) during storage at optimal and low temperatures. J. Agric. Food Chem. 55, 10773 10778. Ishiguro, K., Yoshinaga, M., Kai, Y., Maoka, T., Yoshimoto, M., 2010. Composition, content and antioxidative activity of the carotenoids in yellow-fleshed sweetpotato (Ipomaea batatas L.). Breed. Sci. 60, 324 329. Ito, T., Ando, T., Ichikawa, K., 1968. Effects of cooking processes on the saccharification of sweet potato (part 1): the relation between heating temperature and increase of sugar contents. J. Home Econ. Jpn. 19, 170 173 [in Japanese]. Kai, Y., Sakai, T., Katayama, K., Kumagai, T., Ishiguro, K., Nakazawa, Y., et al., 2017. “Beniharuka”: a new sweetpotato cultivar for table use. Bull. NARO Kyushu Okinawa Agric. Res. Cent. 66, 87 119 [in Japanese with English summary]. Katayama, K., Komae, K., Kohyama, K., Kato, T., Tamiya, S., Komaki, K., 2002. New sweet potato line having low gelatinization temperature and altered starch structure. Starch/Stärke 54, 51 57. Katayama, K., Tamiya, S., Ishiguro, K., 2004. Starch properties of new sweet potato lines having low pasting temperature. Starch/Stärke 56, 563 569. Katayama, K., Sakai, T., Kai, Y., Nakazawa, Y., Yoshinaga, M., 2012. “Konamizuki”: a new sweetpotato cultivar. Bull. NARO Kyushu Okinawa Agric. Res. Cent. 58, 15 36 [in Japanese with English summary]. Katayama, K., Ohara-Takada, A., Kuranouchi, T., Nakamura, Y., Kai, Y., Kumagai, T., et al., 2014. New sweetpotato cultivars bred for food recently in Japan. In: Takahata, Y. (Ed.), New Era of Sweetpotato Research in East Asia: Proceedings of 6th Japan China Korea Sweet Potato Workshop, Kagoshima, pp. 14 15. Katayama, K., Tamiya, S., Sakai, T., Kai, Y., Ohara-Takada, A., Kuranouchi, T., et al., 2015. Inheritance of low pasting temperature in sweetpotato starch and the dosage effect of wild-type alleles. Breed. Sci. 65, 352 356. Katayama, K., Kobayashi, A., Sakai, T., Kuranouchi, T., Kai, Y., 2017. Recent progress in sweetpotato breeding and cultivars for diverse applications in Japan. Breed. Sci. 67, 3 14. Kimura, T., Otani, M., Noda, T., Ideta, O., Shimada, T., Saito, A., 2001. Absence of amylose in sweet potato (Ipomoea batatas (L.) Lam.) following the introduction of granule-bound starch synthase I cDNA. Plant Cell Rep. 20, 663 666. Kimura, M., Kobori, C.N., Rodoriguez-Amaya, D.B., Nestel, P., 2007. Screening and HPLC methods for carotenoids in sweetpotato, cassava and maize for plant breeding trials. Food Chem. 100, 1734 1746.

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Kiribuchi, T., Kubota, K., 1976. Studies on cooking of sweet potato (part 1); changes in sugar content and β-amylase activity during cooking. J. Home Econ. Jpn. 27, 418 422 [in Japanese with English summary]. Kitahara, K., Mizukami, S., Suganuma, T., Nagahama, T., Yoshinaga, M., Kumagai, T., et al., 1996. A new line of sweetpotato with a low amylose content. J. Appl. Glycosci. 43, 551 554. Kitahara, K., Nakamura, Y., Otani, M., Hamada, T., Nakayachi, O., Takahata, Y., 2017. Carbohydrate components in sweetpotato storage roots: their diversities and genetic improvement. Breed. Sci. 64, 62 72. Kobayashi, M., 2010. Dispersal in Japan. In: Japan Root and Tuber Crops Development Association (Eds.), Encyclopedia of Sweetpotato. Japan Root and Tuber Crops Development Association, Tokyo, pp. 48 56 (in Japanese). Komaki, K., Yamakawa, O., 2006. R&D collaboration with industry—the Japanese sweetpotato story. In: Tan, S.L. (Ed.), Innovative Technologies for Commercialization: Proceedings of the 2nd International Symposium on Sweetpotato and cassava, Gent, pp. 23 29. Komaki, K., Yamakawa, O., Hidaka, M., Kumagai, T., 1992. Relationships between color degradation and polyphenol content in sweet potato roots. Kyushu Agric. Res. 54, 48 [in Japanese]. Kukimura, K., 1988. New sweet potato cultivars, ‘Benihayato’ and ‘Satsumahikari’, making a new turn for processing. Jpn. Agric. Res. Q. 22, 7 13 [in Japanese with English summary]. Kukimura, H., Yoshida, T., Komaki, K., Sakamoto, S., Tabuchi, S., Ide, Y., et al., 1989. ‘Satsumahikari’: a new sweet potato cultivar. Bull. Kyushu Natl Agric. Exp. Stn. 25, 225 250 [in Japanese with English summary]. Kumagai, T., Uemura, Y., Baba, T., Iwanaga, M., 1990. The inheritance of β-amylase null in storage roots of sweet potato Ipomoea batatas (L.) Lam. Theor. Appl. Genet. 79, 369 376. Kuranouchi, T., Nakamura, Y., Tamiya, S., Nakatani, M., 2006. Changes in quality traits of “hoshi-imo” steamed and dried slices of sweet potato during drying process and varietal differences. Jpn. J. Crop Sci. 75, 44 50 [in Japanese with English summary]. Kuranouchi, T., Nakamura, Y., Takada, A., Tamiya, S., Nakatani, M., Kumagai, T., 2010a. Effects of mulching with polyethylene films and weather on agronomic characters of sweetpotato varieties for steamed and cured slices processing. Jpn. J. Crop Sci. 79, 491 498 [in Japanese with English summary]. Kuranouchi, T., Nakamura, Y., Kumagai, T., Kashimura, E., Suzuki, M., Kawamata, T., et al., 2010b. Studies on sweetpotato for high quality steamed and cured slices processing with high international competitiveness. Bull. Natl Inst. Crop Sci. 11, 49 65 [in Japanese with English summary]. Kuranouchi, T., Nakamura, Y., Takada, A., Tamiya, S., Nakatani, M., Kumagai, T., 2012. Breeding of a new sweetpotato variety “Hoshikiari” suitable for hoshi-imo steamed and cured slices with excellent quality. Bull. Natl Inst. Crop Sci. 13, 1 22 [in Japanese with English summary]. Ma, S., Silva, J., Hearnsberger, J.O., Garner Jr., J.O., 1992. Prevention of enzymatic darkening in frozen sweet potatoes (Ipomoea batatas (L.) Lam.) by water blanching: relationship among darkening, phenols, and polyphenol oxidase activity. J. Agric. Food Chem. 40, 864 867. Maoka, T., Akimoto, N., Ishiguro, K., Yoshinaga, M., Yoshimoto, M., 2007. Carotenoids with a 5,6-dihydro-5,6-dihydroxy-beta-end group, from yellow sweet potato “Benimasari”, Ipomoea batatas LAM. Phytochemistry 68, 1740 1745.

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Masuda, D., Fukuoka, N., Goto, H., Kano, Y., 2007. Effect of cold treatment after harvest on sugar contents and storability in sweet potato (Ipomoea batatas L.). Hortic. Res. (Jpn.) 6, 597 601 [in Japanese with English summary]. Matsuura-Endo, C., Ohara-Takada, A., Yamauchi, H., Mori, M., Fujikawa, S., 2002a. Disintegration differences in cooked potatoes from three Japanese cultivars: comparison of starch distribution within one tuber and tissue structure. Food Sci. Technol. Res. 8, 252 256. Matsuura-Endo, C., Ohara-Takada, A., Yamauchi, H., Mukasa, Y., Mori, M., Ishibashi, K., 2002b. Disintegration differences in cooked potatoes from three Japanese cultivars: comparison of the properties of isolated starch, degree of cell separation with EDTA, and contents of calcium and galacturonic acid. Food Sci. Technol. Res. 8, 323 327. Morrison, T.A., Pressey, R., Kays, S.J., 1993. Changes in α- and β-amylase during storage of sweetpotato lines with varying starch hydrolysis potential. J. Am. Soc. Hortic. Sci. 118, 236 242. Murata, T., 1971a. Sucrose synthetase of sweet potato roots, Part 2. A kinetic study. Agric. Biol. Chem. 35, 1441 1448 [in Japanese with English summary]. Murata, T., 1971b. Enzymic mechanism of starch synthesis in sweet potato roots, part 4. Sucrose synthetase of sweet potato roots (1) changes in enzyme activity during development and some properties of sucrose synthetase. J. Jpn. Soc. Agrochem. 45, 441 448 [in Japanese with English summary]. Nakabayashi, T., 1970. Studies on tannins of fruits and vegetables. Part 5. The colour development of tannin by iron ion. Nippon Shokuhin Kogyo Gakkaishi 17, 231 236 [in Japanese with English summary]. Nakamura, Y., Kuranouchi, T., Kumagai, T., Ishida, N., Matsuda, T., Nakatani, M., 2006. “Hoshi-imo,” the local and traditional agricultural processed food of sweetpotato in Japan, and the research for its qualitative injury “Shirota”. In: Yoshinaga, M. (Ed.), Innovative Technologies for Food, Resources and Environment: Proceedings of 2nd Japan China Korea Sweetpotato Workshop, Miyakonojyo, pp. 35 36. Nakamura, Y., Kuranouchi, T., Ishida, N., Kumagai, T., Nakatani, M., 2007. The contents of starch and water in storage roots in relation to the occurrence of whiteopaque defect “Shirota” in steamed and cured slices of sweetpotato. Jpn. J. Crop Sci. 76, 576 585 [in Japanese with English summary]. Nakamura, Y., Kuranouchi, T., Ohara-Takada, A., Ishida, N., Koda, I., Iwasawa, N., et al., 2010. Cell structure, water status and starch properties in tuberous root tissue in relation to the texture of steamed sweetpotato (Ipomoea batatas (L.) Lam). Jpn. J. Crop Sci. 79, 284 295 [in Japanese with English summary]. Nakamura, Y., Ohara-Takada, A., Kuranouchi, T., Masuda, R., Katayama, K., 2014a. Mechanism for maltose generation by heating in the storage roots of sweet potato cultivar ‘Quick Sweet’ containing starch with a low pasting temperature. J. Jpn. Soc. Food Sci. Technol. 61, 62 69 [in Japanese with English summary]. Nakamura, Y., Kuranouchi, T., Ohara-Takada, A., Katayama, K., 2014b. The effects of β-amylase activity and starch pasting temperature on maltose generation in steamed storage roots of sweet potato. J. Jpn. Soc. Food Sci. Technol. 61, 577 585 [in Japanese with English summary]. Nakamura, Y., Ohara-Takada, A., Kuranouchi, T., Katayama, K., 2015. Disintegration of steamed root tissues of sweet potato and its relation to texture and the contents of starch, calcium and pectic substances. J. Jpn. Soc. Food Sci. Technol. 62, 555 562 [in Japanese with English summary]. Nakamura, Y., Masuda, R., Kuranouchi, T., Katayama, K., 2016. Effect of starch pasting temperature on remaining total ascorbic acid content in steamed sweet potatoes. J. Jpn. Soc. Food Sci. Technol. 63, 433 438 [in Japanese with English summary].

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Nakamura, Y., Masuda, R., Kuranouchi, T., Katayama, K., 2017. Texture and unsaccharified residual starch content in steamed roots of sweet potato cultivars with varying levels of beta-amylase activity. J. Jpn. Soc. Food Sci. Technol. 64, 59 65 [in Japanese with English summary]. Nakamura, Y., Kuranouchi, T., Ohara-Takada, A., Masuda, R., Kumagai, T., Katayama, K., 2018. Maltose generation by beta-amylase and its relation to eating quality of steamed storage roots of sweet potato cultivars, including recently developed varieties in Japan. Jpn. Agric. Res. Q. 52, 7 16. Nara, S., 1951. Few fundamental studies on utilization and storage of sweet potato. Bull. Facul. Educ. Nat. Sci. Mie Univ. 6, 31 37 [in Japanese]. Nara, S., 1957a. On the steamed sweet potato product (Part 3). The difference of mealiness and sogginess (1). In: Studies From the Institute of Horticulture, Kyoto University, vol. 8, pp. 99 100 (in Japanese with English summary). Nara, S., 1957b. On the steamed sweet potato product (Part 4). On the differences of mealiness and sogginess (2). Bull. Facul. Agric. Mie Univ. 14, 141 143 (in Japanese). Nara, S., 1957c. On the steamed sweet potato product (Part 5). On the differences of mealiness and sogginess (3). Bull. Facul. Agric. Mie Univ. 14, 145 147 [in Japanese]. Nara, S., 1958. On the steamed sweet potato product (Part 6). Changes of pectic substances in raw and steamed sweet potato. Bull. Facul. Agric. Mie Univ. 18, 41 46 [in Japanese]. Noda, T., Takahata, Y., Nagata, T., Shibuya, N., 1994. Chemical composition of cell wall material from sweet potato starch residue. Starch/Stärke 46, 232 236. Noda, T., Takahata, Y., Sato, T., Suda, I., Morishita, T., Ishiguro, K., et al., 1998. Relationships between chain length distribution of amylopectin and gelatinization properties within the same botanical origin for sweet potato and buckwheat. Carbohydr. Polym. 37, 153 158. Noda, T., Kobayashi, T., Suda, I., 2001. Effect of soil temperature on starch properties of sweet potatoes. Carbohydr. Polym. 44, 239 246. Ohara-Takada, A., Kumagai, T., Kuranouchi, T., Nakamura, Y., Fujita, T., Nakatani, M., et al., 2016. Aikomachi’, a new sweetpotato cultivar with good appearance and high confectionery quality. Bull. Naro Inst. Crop Sci. 16, 35 56 [in Japanese with English summary]. Oki, T., Miyoshi, A., Goto, K., Sato, M., Shiratsuchi, H., Terahara, N., et al., 2010. Determination of major anthocyanins in processed foods made from purple-fleshed sweet potato. J. Jpn. Soc. Food Sci. Technol. 57, 128 133 [in Japanese with English summary]. Ono, K., Ishimaru, K., 2006. Sucrose-phosphate-synthase: a key enzyme of sucrose synthesis in plants. Jpn. J. Crop Sci. 75, 241 248 [in Japanese with English summary]. Onoda, M., Fukuda, T., Ota, Y., Chishiki, T., Toyoda, Y., Suzuki, S., et al., 1970. On the three new sweet potato varieties ‘Kurimasari’, ‘Tamayutaka’, ‘Konasengan’. J. Cent. Agric. Exp. Stn. 14, 167 194 [in Japanese with English summary]. Otani, M., Hamada, T., Katayama, K., Kitahara, K., Kim, S.H., Takahata, Y., et al., 2007. Inhibition of the gene epression for granule-bound starch synthase I by RNA interference in sweet potato plants. Plant Cell Rep. 26, 1801 1807. Picha, D.H., 1986a. HPLC determination of sugars in raw and baked sweet potatoes. J. Food Sci. 50, 1189 1190. Picha, D.H., 1986b. Carbohydrate changes in sweet potatoes during curing and storage. J. Am. Soc. Hortic. 111, 889 892. Ross, H.A., Wright, K.M., McDougall, G.J., Roberts, A.G., Chapman, S.N., Morris, W. L., et al., 2011. Potato tuber pectin structure is influenced by pectin methyl esterase activity and impacts on cooked potato texture. J. Exp. Bot. 62, 371 381.

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Sakai, K., Marumine, S., Hirosaki, S., Kikukawa, S., Ide, Y., Shirasaki, S., 1967. On the new variety of sweet potato “Koganesengan”. Bull. Kyushu Agric. Exp. Stn. 13, 55 68 [in Japanese with English summary]. Sakai, K., Ando, T., Ishikawa, H., Takemata, T., Umehara, M., 1978. On a new sweet potato variety “Benikomachi”. J. Cent. Agric. Exp. Stn. 27, 57 68 [in Japanese with English summary]. Salvador, L.D., Suganuma, T., Kitahara, K., Tanoue, H., Ichiki, M., 2000. Monosaccharide composition of sweetpotato fiber and cell wall polysaccharides from sweetpotato, cassava, and potato analyzed by the high-performance anion exchange chromatography with pulsed amperometric detection method. J. Agric. Food Chem. 48, 3448 3454. Shen, M.C., Sterling, C., 1981. Changes in starch and other carbohydrates in baking Ipomoea batatas. Starch/Stärke 33, 261 268. Shiga, T., Sakamoto, S., Ando, T., Ishikawa, H., Kato, S., 1985. On a new sweetpotato cultivar ‘Beniazuma’. Bull. Natl Agric. Res. Cent. 3, 73 84 [in Japanese with English summary]. Shimozono, H., Tokimura, K., Ikeda, K., Baba, T., Tsushida, T., 2000. Suppression of discoloration after cooking of sweet potato by an enzymatic treatment. Nippon Shokuhin Kogyo Gakkaishi 47, 503 508 [in Japanese with English summary]. Suda, I., Oki, T., Masuda, M., Kobayashi, M., Nishiba, Y., Furuta, S., 2003. Physiological functionality of purple-fleshed sweet potatoes containing anthocyanins and their utilization in foods. Jpn. Agric. Res. Q. 37, 167 177. Suda, I., Ishikawa, F., Hatakeyama, M., Miyazaki, M., Kudo, T., Hirano, K., et al., 2008. Intake of purple sweet potato beverage affects on serum hepatic biomarker levels of healthy adult men with borderline hepatitis. Eur. J. Clin. Nutr. 62, 60 67. Takahata, Y., 2014. Sweetpotato in Japan: past and future. In: Takahata, Y. (Ed.), New Era of Sweetpotato Research in East Asia: Proceedings of 6th Japan China Korea Sweetpotato Workshop 2014, Kagoshima, pp. 6 7. Takahata, Y., Noda, T., Nagata, T., 1992. Varietal diversity of free sugar composition in storage root of sweet potato7. Jpn. J. Breed. 42, 515 521. Takahata, Y., Noda, T., Nagata, T., 1993a. Comparison of free sugar compositions of sweet potato storage roots harvested from different two locations, and the relationships of free sugar compositions and sensory evaluation. Kyushu Agric. Res. 55, 44 [in Japanese]. Takahata, Y., Noda, T., Nagata, T., 1993b. HPLC determination of β-carotene content of sweet potato cultivars and its relationship with color values. Jpn. J Breed. 43, 421 427. Takahata, Y., Noda, T., Nagata, T., 1994. Effect of β-amylase stability and starch gelatinization during heating on varietal differences in maltose content in sweetpotatoes. J. Agric. Food Chem. 42, 2564 2569. Takahata, Y., Noda, T., Sato, T., 1995. Changes in carbohydrates and enzyme activities of sweetpotato lines during storage. J. Agric. Food Chem. 43, 1923 1928. Takahata, Y., Noda, T., Sato, T., 1996. Relationship between acid invertase activity and hexose content in sweet potato storage roots. J. Agric. Food Chem. 44, 2063 2066. Takahata, Y., Kai, Y., Tanaka, M., Nakayama, H., Yoshinaga, M., 2011. Enlargement of the variances in amount and composition of anthocyanin pigments in sweetpotato storage roots and their effect on the differences in DPPH radical-scavenging activity. Sci. Hortic. 127, 469 474. Tamiya, S., Nakatani, M., Komaki, K., Katayama, K., Kuranouchi, T., 2003. New sweet potato cultivar “Purple Sweet Lord”. Bull. Natl Inst. Crop Sci. 4, 29 43. Tanaka, M., Ishiguro, K., Oki, T., Okuno, S., 2017. Functional components in sweetpotato and their genetic improvement. Breed. Sci. 67, 52 61. Tarumoto, I., Sakamoto, S., Ishikawa, H., Kato, S., Katayama, K., Tamiya, S., 1995. New sweet potato cultivar “Healthy Red”. Bull. Natl Agric. Res. Cent. 24, 75 96 [in Japanese with English summary].

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Terahara, N., Shimizu, T., Kato, Y., Nakamura, M., Maitani, T., Yamaguchi, M., et al., 1999. Six diacylated anthocyanins from the storage roots of purple sweet potato, Ipomoea batatas. Biosci. Biotechnol. Biochem. 63, 1420 1424. Tokimura, K., Shimozono, H., Ikeda, K., Tanoue, H., 2002. The retrogradation of starch gels and starch properties from various kinds of sweet potato starches. J. Appl. Glycosci. 49, 305 312. Tsukui, A., 1988. The dietary fiber contents as related to harvest years, cultivated fields, races and flesh color of tuberous roots in sweet potatoes. J. Home Econ. Jpn. 39, 89 97 [in Japanese]. Tsukui, A., Suzuki, A., Oguchi, E., Nagayama, S., 1994. Effect of cooking on dietary fiber contents in potatoes. J. Home Econ. Jpn. 45, 1029 1034 [in Japanese]. Tsukui, A., Suzuki, A., Komaki, K., Terahara, N., Yamakawa, O., Hayashi, K., 1999. Stability and composition ratio of anthocyanin pigments from Ipomoea batatas Poir. J. Jpn. Soc. Food Sci. Technol. 46, 148 154 [in Japanese with English summary]. Walter Jr., W.M., 1987. Effect of curing on sensory properties and carbohydrate composition of baked sweet potatoes. J. Food Sci. 52, 1026 1029. Walter Jr., W.M., Purcell, A., 1980. Effect of substrate levels and polyphenol oxidase activity on darkening in sweet potato cultivars. J. Agric. Food Chem. 28, 941 944. Walter Jr., W.M., Schadel, W.E., 1981. Distribution of phenols in “Jewel” sweet potato [Ipomoea batatas (L.) Lam]. J. Agric. Food Chem. 29, 904 906. Walter Jr., W.M., Purcell, A., Nelson, A.M., 1975. Effects of amylolytic enzymes on “moisture” and carbohydrate changes of baked sweet potato cultivars. J. Food Sci. 40, pp. 793-766. Woolfe, J.A., 1992. Sweet Potato: An Untapped Food Resource. Cambridge University Press, Cambridge, pp. 1 187. Yamakawa, O., Yoshimoto, M., 2001. Sweetpotato as food material with physiological functions. In: Proceedings of the 1st International Conference on Sweetpotato, pp. 179 185. Yamakawa, O., Yoshinaga, M., Hidaka, M., Kumagai, T., Komaki, K., 1997. “Ayamurasaki”: A new sweetpotato cultivar. Bull. Kyushu Natl Agric. Exp. Stn. 31, 1 22 [in Japanese with English summary]. Yamakawa, O., Yoshinaga, M., Kumagai, T., Hidaka, T., Komaki, K., Kukimura, H., et al., 1998. “J-Red”: a new sweetpotato cultivar. Bull. Kyushu Natl Agric. Exp. Stn. 33, 49 72 [in Japanese with English summary]. Yoshimoto, M., 2010. Physiological functions and utilization of sweet potato. In: Ray, R. C., Tomlins, K. (Eds.), Sweet Potato; Post Harvest Aspects in Food, Feed and Industry. Nova Science Publishers, New York, pp. 59 89. Yoshinaga, M., 2010. Low sugar content varieties. In: Japan Root and Tuber Crops Development Association (Eds.), Encyclopedia of Sweetpotato. Japan Root and Tuber Crops Development Association, Tokyo, p. 158 (in Japanese). Yoshinaga, M., 2014. Shokkan. In: Japan Root and Tuber Crops Development Association (Eds.), Encyclopedia of Yakiimo. Japan Root and Tuber Crops Development Association, Tokyo, pp. 39 40 (in Japanese). Yoshinaga, M., Yamakawa, O., Nakatani, M., 1999. Genotypic diversity of anthocyanin content and composition in purple-fleshed sweet potato (Ipomoea batatas (L.) Lam). Breed. Sci. 49, 43 47. Yoshinaga, M., Kai, Y., Katayama, K., Sakai, T., 2006. New varieties for dried sweetpotato products Hamakomachi and Kyushu No. 137. Sweetpotato Res. Front 20, 3. Yoshizumi, T., Ito, H., Kokubu, T., 1986. Amami no Keifu to Sono Kagaku, Korin, Tokyo (in Japanese).

CHAPTER 14

Global market trends, challenges, and the future of the sweet potato processing industry Dai-Fu Ma

Xuzhou Sweet Potato Research Center, Chinese Academy of Agricultural Sciences, National Sweet Potato Improvement Center, Xuzhou, People’s Republic of China

Global consumption and development status of the sweet potato and its processing technology The sweet potato is one of the most important crops in the world and has the characteristics of both food and cash crops, which has much higher yield level than cereal crops. It shows extensive adaptability and watersaving characteristics, and high yields can be obtained in arid regions of Africa and China has created a record of 15 t/ha yield in dry hilly areas (Chinese Agricultural Industry Technology Development Report, 2009 2016). The tuberous roots, stems, and leaves of the sweet potato are all edible, and its easily restored growth gives its unique disaster relief function. It is also considered good for children and adults for long-term consumption and is beneficial to cardiovascular health, longevity, the prevention of diabetes, and the reduction of cancer risk, etc. (Mohanraj and Sivasankar, 2014). It is a good raw material for processing for products, which is especially suitable for industrial development, and for providing sufficient raw materials for the development of related food processing industries and supplying abundant food products. The consumption of sweet potato around the world is closely related to the development of the social economy. In the past, sweet potato was mainly used for eating, and was given equal attention to feeding, eating and industrial processing later in Asia, while it was always consumed as satiety and nutrient sources in Africa. At present most of the developing countries mainly use sweet potato as food and feed, supplemented by a small amount of processing; the less developed countries still serve as an important source of food and nutrition; whereas developed countries use sweet potato tuberous roots as a fresh food, Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00014-4

© 2019 Elsevier Inc. All rights reserved.

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stems and leaves as high-quality vegetables, and healthcare-oriented processed leisure foods are also popular. China is the largest consumer of sweet potato in the world. In the 1950s and 1960s sweet potato was mainly used as a fresh food, accounting for more than 50% of the total sweet potato production, and the proportions of processing and feeding were about 10% and 30%, respectively. In the early 1890s the proportions of sweet potato use for fresh food, feed, and processing accounted for about one third each, respectively. Since then, the consumption of sweet potato as fresh food has gradually decreased, and the proportion of processing has increased (Fig. 14.1). In recent years, the proportion of fresh food consumption for health has increased year on year, that of feed has decreased yearly, while that of processing has been stable. According to the 2016 survey, the consumption proportion of fresh sweet potato was close to 30%; the processed products showed diversified development trends; the sales of snack foods such as sweet potato dates, French fries, preserved sweet potato, and chips increased; and the production of starch and processed products were relatively stable, but small-scale starch-processing enterprises were subject to more stringent law enforcement control due to environmental pollution. Vegetable type of sweet potato got faster development, and the reason was due to that there was a shortage and relatively high price for leafy vegetable during the early spring and hot summer days. In addition, the research into ornamental sweet potatoes has started, as well as into

Figure 14.1 Trends of sweet potato consumption in China.

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ornamental and vegetable type dual-use sweet potatoes. Furthermore, with the development of the logistics industry and the improvement of the storage technology, the sales of sweet potato have basically achieved the annual supply (Chinese Agricultural Industry Technology Development Report, 2009 2016; Ma et al., 2012). Japan is a country with a good development of sweet potato industrialization. The degree of mechanization of the sweet potato production is relatively high and production efficiency is also high. The productive labor time of planting, harvest, and management of sweet potato for starch processing has been reduced to 46 h/ha; while that of sweet potato for fresh food is much more, about 200 h/ha. The farmers usually use the plastic film covering to increase the yield and quality of sweet potato. There are many retail styles for sweet potato, for example, the sweet potato seedlings used for production are mostly provided by seedling companies, which greatly restrict the spread of diseases and provide a guarantee for the stable development of commercial sweet potato production. At present the main consumption of sweet potato is as fresh food, in winemaking, and in food processing, and the use as feed and in starch processing is reduced. Japan is the first country to successfully cultivate purple sweet potato varieties, such as Purple Okinawa, Yamagawa Murasaki, Aya Murasaki, Murasakimasari, Kankei 55, Tanegashima murasaki, Chiran Murasaki, Kyushu, Miyanou-36, Bise, Purple Bom, Kuyukei, Purple Sweet Lord, and so on (Montilla et al., 2011). Purple sweet potato powder products have been successfully developed as food colorings and ingredients, which are commonly used in the production of flowercolored snacks. In addition, the development of the variety without β-amylase has also been paid attention to, and the variety “Satsuma,” which is suitable for the production of fried potato chips, has been developed. The development of high carotene varieties has also received attention in Kyushu, and the use of sweet potatoes has become increasingly widespread (Okutsu et al., 2016). The United States has always used sweet potatoes as a fine food, especially for the varieties of orange red-fleshed sweet potatoes. In recent years the output and consumption of sweet potatoes in the United States have increased at a rate of 6% per year. As consumers pay more attention to its high vitamin A, vitamin C, and dietary fiber contents, more and more consumers like sweet potato, and the demand for sweet potato for consumption is rapid rising, increasing from per capita annual consumption of 1.9 kg in 2000 3.4 kg in 2015 (Table 14.1).

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Table 14.1 Overview of the development of sweet potato industry in the United States in the past 5 years. Parameters

2016

2015

2014

2013

2012

Planting area (hm2) Yield (t/hm2) Total output (10,000 tons) Gross output value (10, 000 US dollar)

68,030 21.03 143.09

63,500 22.16 140.69

55,560 24.15 134.19

46,820 24.01 112.42

52,810 22.75 120.12

70569.00

67558.30

70691.60

59721.70

46186.10

Note: Data are from the website of National Agricultural Statistics Service, US Department of Agriculture.

In Africa the sweet potato is considered as a crop that provides food and nutrition security as well as a feed and a raw material for processing. Due to the hot weather in most areas, the storage conditions of the sweet potato could not be met but it can grow all year-round, so many sweet potato farmers harvest according to the needs of sales or self-eating. Traditional fresh eating sweet potato is mainly white fleshed with high starch content. The method of consumption for sweet potato is still mainly steaming, although some farmers eat them after they are sliced, dried and ground. There are insufficient processing enterprises and products, while some countries have small amounts of fried products. Abong et al. (2016) reported that per capita consumption is 90 100 kg per year in Uganda and about 24 kg in Kenya. In sub-Saharan Africa, more than 40% of children under the age of 5 suffer from vitamin A deficiency. Orange-fleshed sweet potato is one of the most important crops for addressing vitamin A deficiency, being known as “children’s protectors" in some rural areas. In recent years, under the promotion of international organizations, such as the International Potato Center and the Bill Gates Foundation, carotene-rich orange red-fleshed sweet potato has received extensive attention, and has solved the night blindness caused by carotene deficiency in children to some extent (Abong et al., 2016; Low et al., 2017).

Global research focus of sweet potato processing technology The research of international sweet potato processing technology mainly includes sweet potato starch, pigments, pectin, and convenient snack food

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processing. The research on natural products of purple sweet potato has been the focus of international postharvest processing research in recent years. It mainly studies the structural identification and biological activity of phytochemicals such as polysaccharides, proteins, anthocyanins, and polyphenols in purple sweet potato, especially the introduction of the bioavailability evaluation system, the simulated gastrointestinal digestive system, the new cancer cell culture system, and other advanced research methods, and in-depth research into anticancer and antiinflammatory properties, as well as inhibition of cardiovascular and cerebrovascular diseases and other biological activities of purple sweet potato have become an international research highlight (Sun et al., 2018; Esatbeyoglu et al., 2017). Under the background of global energy supply shortage, the development of sweet potato energy utilization technology has made certain progress. As a bioenergy source, it is meaningful to study its recycling value and explore its energy release pattern, and the current research focuses on the ethanol fermentation of uncooked or unsweetened sweet potato, the mixed fermentation of sweet potato and sweet sorghum juice, and the energy consumption and environmental impact assessment of the sweet potato ethanol fermentation process (Schweinberger et al., 2016; Mussoline and Wilkie, 2017). Scientists from China, Brazil, and Indonesia have studied the parameters related to ethanol-fuel production of sweet potatoes and the stable supply of sweet potatoes as a raw material for ethanol production and food (Huang et al., 2012). In the processing of sweet potato foods, the effects of pulsed electric fields on the quality characteristics, physicochemical properties, and fermentation characteristics of sweet potato and fried products, as well as the effects of ultrasonic pretreatment on fried sweet potato, and heat and high hydrostatic pressure on sweet potato flour and bread products were carried out (Liu et al., 2016; Oladejo et al., 2017; Pérez et al., 2017). Orangefleshed sweet potato wheat composite bread was developed to meet vitamin A requirements (Nzamwita et al., 2017). Fermented milk with sweet potato pulp and purple sweet potato fermented alcoholic beverages were produced (Ramos et al., 2017; Li et al., 2017). The effects of diglycosidespecific β-primeverosidase on enhancing monoterpene alcohols in sweet potato shochu have been reported, indicating that β-primeverosidase might be useful for controlling aroma formation during shochu manufacturing and ultimately contributing to diversifying its quality (Sato et al., 2018). Bread made with orange-fleshed sweet potato puree is very popular among consumers, of which the application in bread improves the utilization value and economic benefits of sweet potato (Abong et al., 2016).

386

Sweet Potato

In China the research on the sweet potato processing technology mainly focuses on sweet potato starch, sweet potato storage and preservation, and the comprehensive utilization of sweet potato by-products. The effects of high hydrostatic pressure and heat treatment on the structure, physicochemical properties, and in vitro digestibility of sweet potato resistant starch were reported, as well as the sweet potato resistant starches produced by single or combined physical, chemical, and enzymatic modification methods (Zhao, 2015). The processing characteristics of anthocyanins, carotenoids, chlorogenic acids, sweet potato powder, composite dried sweet potato chips, sweet potato cakes (balls), purple sweet potato drinks, and purple sweet potato sake were studied, and the processing technologies were optimized. After different pretreatment, puffing, and drying conditions, the contents of polyphenols, phenolic acids, and flavonoids in sweet potatoes changed significantly, while the anthocyanin content did not change significantly (Li et al., 2017). Dietary fiber, oligosaccharide, pectin, lactic acid, and ethanol, etc. are prepared by using starch-processing waste residues, whereas protein and polysaccharides are prepared by using starch-processing wastewater. The physicochemical properties and lead scavenging ability of sweet potato dietary fiber were evaluated, as well as the health effects of purple sweet potato anthocyanins on obesity, fatty liver, and inflammation (Zhang et al., 2016). The above related research promotes the development of the sweet potato processing industry.

Global development trend of sweet potato processing industry The sweet potato is widely used, and the processing industry is the driving force behind the development of the sweet potato industry. Besides a few African countries, sweet potato is mainly used as raw material for processing (starch, pigments, and ready-eat food, etc.), vegetables, and feed. However, the sweet potato processing industry is still largely in its primary stage due to sweet potato being mainly grown only in developing countries. The development of the sweet potato processing industry should focus on the concept of green recycling. Developing countries should vigorously promote the application of primary processing technologies ,such as storage and preservation, and strive to promote the comprehensive utilization of agricultural products and processing by-products. Developed

Global market trends, challenges, and the future of the sweet potato processing industry

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countries should comprehensively improve the overall level of intensive processing and enhance industrial integration. Driven by the local conditions, the sweet potato processing industry should be integrated into the characteristic culture, national culture, leisure agriculture, rural tourism, and other industries. According to the national conditions, various professional circulation services, diversified industrial integration subjects, and multitype industrial integration methods should be developed, and multiform interest linkage mechanisms should be established to stimulate the development of industrial integration. Taking the development of the sweet potato starch-processing industry as an example, sweet potato starch has the characteristics of high viscosity and a high degree of polymerization. Sweet potato starch and its derivatives account for a large proportion of the global sweet potato products market. In developed countries, due to the impact of corn starch, the sweet potato starch-processing industry is in a stage of shrinking stagnation. At the same time, limited by labor costs, raw material costs, and insufficient production experience, developed countries should pay more attention to the development of modified starch. The production cost of sweet potato starch in the United States, Japan, South Korea, and other countries is relatively high, about twice that of the import price, and the sales price is three times that of the import price. Therefore sweet potato starch in Korea and the United States mainly depends on imports, accounting for 40% of the annual consumption. In the developing countries, due to the lack of advanced processing technology, it is difficult for sweet potato processing to reach a higher level of consumption and a wider range of consumption. In some poor countries in Africa the sweet potato starch-processing industry has not yet started. These countries can properly develop the sweet potato starch-processing industry under the framework of the “One Belt, One Road” economic model advocated by China, drawing on the different processing patterns of Chinese starch manufacturing. China is the world’s major producer and exporter of sweet potato starch and its derivatives. Although there are some problems due to the lack of special high-quality sweet potato varieties, the low degree of mechanization, starch extraction rate, and the purity of starch products—in particular, the starch-processing residues and wastewater have not been effectively treated or utilized and environmental protection pressures are high—China still has high competitiveness internationally. The prerequisite for the development of sweet potato starch-processing industry is to select varieties with high starch content, antibrowning properties,

388

Sweet Potato

low soluble sugar content and high viscosity, and appropriately increase the concentration of planting, as well as gradually moving toward largescale, mechanized, environment-friendly, and ecological starch processing. In addition, sweet potato food processing is divided into whole sweet potato processing and recombinant processing, which can be also divided into leisure and healthcare, fermentation and non-fermentation. There are so many different kinds of sweet potato products in the world. Of the developed countries, Japan is one of the most successful countries in the development of sweet potato foods, with a wide range of products, fine workmanship, and a high output value and Japan is a leader in the development of sweet potato industrialization. Some developed and developing countries (such as China, South Korea, the United States, and Indonesia) could learn from the Japanese model to develop their own sweet potato food processing industry, which might be one of the development directions of the international sweet potato food processing industry. The concept of nutrition in many developed countries has shifted from emphasizing survival, satiety, and no side effects to the use of food to maintain nutritional health and reduce morbidity. Sweet potatoes are rich in protein, dietary fiber, β-carotene, polyphenols, anthocyanins, and other nutrients and functional ingredients to meet this demand. With the increasing emphasis on the healthcare functions of sweet potato functional ingredients, the development of related health foods with sweet potato as the main raw material can greatly increase the processing output value of sweet potato. Some developing countries can learn from the model of the Chinese sweet potato food processing industry, pay more attention to the development of whole potato processed foods, and expand the processing and utilization of orange-fleshed sweet potato in areas with vitamin A deficiency. Some sweet potato varieties have special curative effects on certain diseases and have broad prospects for the development of edible and even medicinal plants (Tai et al., 1998; Wang et al., 2015). Research on the mechanisms of the medicinal healthcare of sweet potato and the development of medicinal products should also be strengthened (Mohanraj and Sivasankar, 2014). Furthermore, developing biomass energy is considered an effective way to solve the world energy crisis. Sugar cane in Brazil and corn in the United States have been used for commercialized biomass energy production. Many experts believe that high sweet potato energy production is the dominant crop for developing biomass energy. Sweet potato is considered to be a superior crop for the development of biomass energy

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(Schweinberger et al., 2016). China has also conducted in-depth research on the energy utilization of sweet potatoes, and innovatively proposed the ethanol fermentation technology of sweet potato, but it is difficult to commercialize due to various factors. Practices in recent years have proved that the possible way to use sweet potato to produce biomass energy in China is to use genetically modified technology to increase sweet potato yield and total fermentable sugar content, that is, the content of starch and soluble sugars. The development and utilization of marginal land and beaches have considerable potential. In the development of biomass energy, if the product cannot form part of a biorefinery industrial chain such as the coproduction of food, chemicals, and energy products, then it will lead to problems such as the low utilization rate of raw materials, high production cost, and difficulty in handling pollutants. There may also be a waste of raw materials. The author believes that the use of sweet potatoes as a biomass energy crop still has many technical and policy issues to be solved and may not achieve a major breakthrough in the next few years.

Global challenges and opportunities of sweet potato processing industry Nowadays there are widespread problems in the sweet potato processing industry, such as low comprehensive utilization rate, serious environmental pollution, insufficient investment on research and development of equipment, weak innovation capability on processing technology, slow updating of processed products, nonuniform food quality, the discrepancies between processing and equipment, too much fluctuation in the price of processing raw materials, nonsmooth circulation and trade channels within the domestic and international market, and especially the uneven development of the sweet potato processing industry. Economic globalization has prompted countries to pay more attention to the processing industries for agricultural products. The processing industries have become an important engine for modern agricultural development, especially in developing countries, which have a major impact on expanding value chains, creating employment opportunities, and reducing poverty and hunger (Wang et al., 2008). The modern lifestyle is convenient for the agricultural product processing industry, with high nutritional value and good palatability. The modern lifestyle provides a strong market demand for the development of functional foods by the agricultural products processing industry—foods that are convenient to

390

Sweet Potato

carry, tasty, and with high nutritional value—and it also brings development opportunities for the sweet potato processing industry (He and Qin, 2016). The development of the sweet potato processing industry should be adjusted according to the national conditions and the agricultural development models of each country in order to make overall plans, clarify the steps, measures, and main directions of development. Cooperation between relevant organizations and countries can be strengthened and promoted by building a cooperation network, jointly undertaking research on related key technologies and policy, and enhancing the mutual benefit and exchange of resources and technology (e.g. China, Japan, and South Korea have established a Sweet Potato Research Committee), thus achieving the purpose of strengthening communication, understanding and cooperation. One can learn from the Chinese “One Belt, One Road” cooperative economic development model, promote the investment of transnational corporations in the sweet potato processing industry in developing countries, and use the abundant agricultural labor resources and raw materials advantages of developing countries. One can strengthen farmers’ cooperative organizations, agricultural product processing enterprises, and supermarkets, and further develop sweet potato products processing. The promotion of farmers will increase income, and ensure the supply of raw materials. The deep processing of sweet potato products should be encouraged and the processing proportion of functional foods should be increased, thus greatly increasing the added value of agricultural products. From a global perspective, one should continue to carry out research and development and entrepreneurial exchanges in sweet potato-related fields, strengthen research on the mechanisms of the biological functions of sweet potato nutritional components, and stimulate market demand, thus promoting the development of the sweet potato processing industry. Currently, the China Sweet Potato Research System has been established, and the Post-Production Processing Group has been set up within the system, which gets strong and stable support from the government. The scientists in the processing field are cooperating with those in the breeding and cultivation areas, which is very helpful in order to promote the development of the sweet potato processing industry.

References Abong, G.O., Ndanyi, V.C.M., Kaaya, A., Shibairo, S., Okoth, M.W., Lamuka, P.O., 2016. A review of production, post-harvest handling and marketing of sweetpotatoes in Kenya and Uganda. Curr. Res. Nutr. Food Sci. 4 (3), 162.

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Department of Science, Technology and Education, Ministry of Agriculture and Rural Affairs of the People’s Republic of China, Department of Science, Education and Culture, Ministry of Finance of the People’s Republic of China, & Technology Development Center, Ministry of Agriculture and Rural Affairs of the People’s Republic of China. Chinese Agricultural Industry Technology Development Report (2009 2016). China Agricultural Science and Technology Press, Beijing (In Chinese). Esatbeyoglu, T., Rodríguez-Werner, M., Schlösser, A., Winterhalter, P., Rimbach, G., 2017. Fractionation, enzyme inhibitory and cellular antioxidant activity of bioactives from purple sweet potato (Ipomoea batatas). Food Chem. 221, 447 456. He, A.H., Qin, G.Y., 2016. The Status, problem and suggestions of the development of Chinese agricultural products processing industry. Agric. Econ. Manag. 5, 73 80. In Chinese. Huang, Y.H., Jin, Y.L., Zhao, Y., Li, Y.H., Fang, Y., Zhang, G.H., et al., 2012. Viscosity reduction during fuel ethanol production by fresh sweet potato fermentation. Chin. J. Appl. Environ. Biol. 18, 661 666. In Chinese. Li, S., An, Y., Fu, W., Sun, X., Li, W., Li, T., 2017. Changes in anthocyanins and volatile components of purple sweet potato fermented alcoholic beverage during aging. Food Res. Int. 2017 (100), 235 240. Liu, T., Dodds, E., Leong, S.Y., Eyres, G.T., Burritt, D.J., Oey, I., 2016. Effect of pulsed electric fields on the structure and frying quality of “kumara” sweet potato tubers. Innov. Food Sci. Emerg. Technol. 39, 197 208. Low, J., Ball, A., Magezi, S., Njoku, J., Mwanga, R., Andrade, M., et al., 2017. Sweet potato development and delivery in sub-Saharan Africa. Afr. J. Food Agric. Nutr. Dev. 17, 11955 11972. Ma, D.F., Li, Q., Cao, Q.H., Niu, F.X., Xie, Y.P., Tang, J., et al., 2012. Development and prospect of sweetpotato industry and its technologies in China. Jiangsu. J. Agric. Sci. 28, 969 973. In Chinese. Mohanraj, R., Sivasankar, S., 2014. Sweet Potato (Ipomoea batatas [L.] Lam)—a valuable medicinal food: a review. J. Med. Food 17, 733 741. Montilla, E.C., Hillebrand, S., Winterhalter, P., 2011. Anthocyanins in purple sweet potato (Ipomoea batatas L.) varieties. Fruit Veg. Cereal Sci. Biotechnol. 5, 19 24. Mussoline, W.A., Wilkie, A.C., 2017. Feed and fuel: the dual-purpose advantage of an industrial sweetpotato. J. Sci. Food Agric. 97, 1567 1575. Nzamwita, M., Duodu, K.G., Minnaar, A., 2017. Stability of β-carotene during baking of orange-fleshed sweet potato-wheat composite bread and estimated contribution to vitamin A requirements. Food Chem. 228, 85 90. Okutsu, K., Yoshizaki, Y., Kojima, M., Yoshitake, K., Tamaki, H., Kazunori, T., 2016. Effects of the cultivation period of sweet potato on the sensory quality of imo-shochu. J. Inst. Brew. 122, 168 174. Oladejo, A.O., Ma, H., Qu, W., Zhou, C., Wu, B., Yang, X., et al., 2017. Effects of ultrasound pretreatments on the kinetics of moisture loss and oil uptake during deep fat frying of sweet potato (Ipomea batatas). Innov. Food Sci. Emerg. Technol. 43, 7 17. Pérez, I.C., Mu, T.H., Zhang, M., Ji, L.L., 2017. Effect of heat treatment to sweet potato flour on dough properties and characteristics of sweet potato-wheat bread. Food Sci. Technol. Int. 23, 708 715. Ramos, L.R., Santos, J.S., Daguer, H., Valese, A.C., Cruz, A.G., Granato, D., 2017. Analytical optimization of a phenolic-rich herbal extract and supplementation in fermented milk containing sweet potato pulp. Food Chem. 221, 950 958. Sato, Y., Han, J., Fukuda, H., Mikami, S., 2018. Enhancing monoterpene alcohols in sweet potato shochu using the diglycoside-specific β-primeverosidase. J. Biosci. Bioeng. 125, 218 223.

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Schweinberger, C.M., Putti, T.R., Susin, G.B., Trierweiler, J.O., Trierweiler, L.F., 2016. Ethanol production from sweet potato: the effect of ripening, comparison of two heating methods, and cost analysis. Can. J. Chem. Eng. 94, 716 724. Sun, J., Zhou, B., Tang, C., Gou, Y., Chen, H., Wang, Y., et al., 2018. Characterization, antioxidant activity and hepatoprotective effect of purple sweetpotato polysaccharides. Int. J. Biol. Macromol. 115, 69 76. Tai, J.X., Hua, X., Wang, J., Niu, F., Huang, G., Fu, Q., 1998. The functional test and clinical application study on the leaf products of Tebai 1 sweet potato. Acta Agron. Sin. 24, 161 167. In Chinese. Wang, X.Z., Wang, H., Zhang, B., 2008. Trends, enlightenment and countermeasures of global agricultural industry. Agric. Prod. Process. Ind. 9, 19 20. In Chinese. Wang, H.Y., Zhang, Y., Sun, J., Niu, F.X., Xu, F., Yue, R.X., et al., 2015. Effect of water extract from special sweet potato TSP-1 on blood lipids and platelet of type II diabetic rats. Jiangsu Agric. Sci. 43, 289 291. In Chinese. Zhang, Y., Wang, H.Y., Niu, F.X., Sun, J., Xu, F., Zhu, H., et al., 2016. Identification of purple sweet potato color of cultivar Ningzi No. 1 by HPLC-QTOF/MS and its effect on preventing obesity in high-fat-diet-treated rats. Sci. Agric. Sin. 49, 1787 1802. In Chinese. Zhao, Z.K., 2015. Effect of High Pressure and Heat Treatment on the Formation of Sweet Potato Resistant Starch. Xinjiang Agricultural University, Urumqi (In Chinese).

Index Note: Page numbers followed by “f ” and “t” refer to figures and tables, respectively.

A Agricultural product processing industry, 386 389 challenges and opportunities, 389 390 Anthocyanins, 178 182, 181f effect of purple sweet potato, 190f, 191f, 192f, 193f, 194f, 195f effects of home-processed methods on, 304 305 pharmacological effects of, 189 193 physiological function, 189 193 in storage root, leaf, and cell line, 180t in sweet potato storage root, 368 371 Antioxidant hydrolysates, 69 70 Aspergillus genus, 329 330

B

β-carotene, 369 Biomass energy, 388 389 Biscuits, 315 318 dietary fiber-enriched, 317 Botanical characteristics of sweet potatoes, 5 6 nutritional needs, 5 6 Brownies, 314 315

C Caffeoylquinic acids, 182 188, 182f, 203f 5-CQA and 4-CQA, 182 185 investigations of, 186 187, 187f molecular structure and plant sources of, 183t physiological function, 193 201 antihuman immunodeficiency virus (HIV) effects of, 201 antiosteoporotic effect of, 201

antioxidant capacity and related effects, 194 196, 197t, 198t, 199f, 200f lifestyle-related disease prevention effect, 196 201 Cakes, 313 314 effect of adding different proportions of sweet potato, 314 315 Cancer, effects of lipids on, 154, 164 Carotenoids ABTS radical-scavenging activity of, 232 234, 233t antioxidant activity of, 232 234 development of sweet potato with high content of, 230 231 division of, 223 224 effects of home-processed methods on, 304 305 in Japanese yellow-fleshed cultivars/ breeding lines, 225 228 in orange- and yellow-fleshed sweet potato roots, 224 228, 226t overview, 223 224 rapid analysis methods for analysis of content, 231 232, 232f research and development, 237 238 retention and bioaccessibility/ bioavailability of, 235 237 of β-carotene, 236 during cooking, 237 in sweet potato leaves, 228 230 in sweet potato storage root, 368 371 in Taiwanese orange- and yellow-fleshed sweet potato, 228 in transgenic calli, 231 Cellulose, 117 Central America, 6, 8 9

393

394

Index

China, sweet potato production in, 7 9 consumption in, 382 383, 382f cultivation regions in, 7, 15t, 17f, 18 20 decline in cultivation area, 18 19 export, 12 future prospects, 24 25 processing technology for, 386 sweet potatoes processing industry in, 22 24 uses of sweet potatoes, 20 22, 36 37 Chips, 308 simulation, 309 310 traditional or general, 308 309 Chitin, 118 Citric acid production, 332 Colorectal cancer, 164 Consumption of sweet potatoes, 12

D Dehydrated sweet potatoes, 260 261 Dietary fiber ammonia concentration in, 126 application prospect of, 145 bacterial count in, 126 classification, 117 118 color and sensory evaluation, 123 composition of control and, 123 124, 123t definition, 117 effect on cecal fermentation products and intestinal flora in rats, 123 127, 125t pectin content, 128 propionic acid concentration in, 126 127 protective effect on mucosa, 118 119 from sweet potato, 119 127, 121t physicochemical and functional properties of, 121 123 use of, 118 119 water and oil retention capacity, 121 122, 122f water-soluble, 118 119 Docosahexaenoic acid (DHA), 149 Doughnuts, 318 319 Dried sweet potato slices, 310 313 drying methods, 312 313 process, 311 312

E Eicosapentaenoic acid (EPA), 149 Extruded sweet potato snacks, 319 320

F Fatty acids (FAs), 149 Flavonoids, 3

G Glycolipids (GLs), 149, 159 162, 164 166 effects on migration of HT-29 and Bcap-37 cells, 167 170 Guar gum, 118

H Hemicellulose, 117 118 High-velocity cross-flow hot-air dryer, 311 Hyperlipidemia, 151

I Imports/exports of sweet potatoes, 11 12, 13t, 15t value, 6f worldwide, 2013, 12 Intestinal bacteria, useful and harmful, 126 Isoelectrically precipitated sweet potato (IPSP) protein, 73 75 amino acid composition and nutritional quality, 76 80, 77t sulfur-containing amino acids, 76 79 gelation properties and network formation mechanism of, 83 86 gel dynamic rheological properties, 86 mechanical parameters, 87 88 microstructure of gels, 88 89 rheological and mechanical properties of, 80 83 trypsin inhibitory activities, 76

J Japan, sweet potatoe production in, 11 12, 326, 383, 388 production and utilization, 349 351 cooking method, 350 351

Index

public sweet potato breeding system, 371 varieties, 351 354 “Ayamurasaki” cultivar, 179, 189 191 “Yamagawamuarsaki” cultivar, 179, 189 191

K Koji, role of, 330 333 Koji manufacturing, 325 koji acidity, 332 koji fungi, 329 330 koji production, 333 334 production of enzymes, 330 332

L Lignin, 118 Linoleic acid, 149 Linolenic acid, 149 Lipids alleviation of oxidative stress, 153 154 antiatherosclerosis effect of, 152 antibacterial effect of, 152 anticarcinogenic effect of, 154 antiinflammatory effect of, 152 153 bioavailability of functional components, 150 151 definition, 149 diabetes prevention effect of, 153 154 extraction methods Bligh-Dyer method, 150 mechanical squeezing, 149 150 microwave-assisted extraction, 150 supercritical fluid extraction, 150 ultrasound-assisted extraction, 150 fatty acids composition of, 149, 157 159, 163t of glycolipids, 159 162 in neural lipids, 159 of phospholipids, 162 glycolipids (GLs), 149 hypolipidemic effect of, 151 memory improving effect of, 153 neutral lipids (NLs), 149 phospholipids (PLs), 149

395

physiological functions of, 150 154 of sweet potato, 154 162 anticancer effects of, 162 171 antiproliferative effect of, 164 166, 165f, 166f application prospect of, 171 cell adhesion inhibition effects of, 166 170, 167f composition, 155 157 glycolipids, 164 166 total lipids (TLs), 149 Lutein, 3, 228 230, 229t

M Microstructure of sweet potato, 244 246 Moromi alcohol, 336 337

O

“One Belt, One Road” cooperative economic development model, 389 390 Origin of sweet potatoes, 6 7, 13t in Mainland China, 7, 15t Oxidative browning inhibition, 70 71 in citric acid solution, 70 71 Oxygen radical absorbance capacity (ORAC), 105t

P Pectin, 118, 127 128 extraction of, 128 130, 129t extraction reagents for preparing, 129 130 extraction temperature and pH, 130 recovering pectin from extraction solution, 130 FTIR spectra of, 140 142, 143f as gelling agent, 127 128 industry, 128 properties breaking strength of pectin gel, 132 134 chemical, 131 effects of pH on breaking pressure of pectin gel, 133, 133f molecular weight, 132, 132f

396

Index

Pectin (Continued) structure, 131 132 sonication duty cycle, effect of, 136 137 sonication efficiency, 134 135, 135f sonication power, effect of, 141f on antioxidant capacity, 142 144, 143f on degree of methoxylation and galacturonic acid content, 139 140 ferric reducing antioxidant power (FRAP), 143 144 on GalA and DM, 139 140, 139t on molecular weight, 136, 137f on neutral sugar composition, 137 139, 138t oxidative radical absorbance capacity (ORAC), 142 143 on sonolysis, 135 136, 136f on structure, 140 142 uses, 128 Peruvian sweet potato, 6, 31 Phenolics, 70 71, 75 Philippines, sweet potatoe production in, 7 Plant chlorophyll, composition of, 223 224 Polydextrose, 118 Polynesia, 6 7 Polyphenols, 3 Polyphenols chemistry anthocyanins, 178 182, 180t, 181f caffeoylquinic acids, 182 188, 182f, 183t constituents of, 188t effect of cooking on, 206 210, 206f, 209t flavonoids, 178 in leaves, 185 186, 211f, 212f, 213f overview, 177 178 phenolic acids, 178 physiological function anthocyanins, 189 193 caffeoylquinic acids, 193 201 processing and utilization, 201 214 in improving bread functionality, 211 214

in palatability and preservability of food, 210 211 research and development, 214 215 Prehistoric remnants of sweet potato, 6 7 Production of sweet potatoes, 7 25 in Americas, Europe, and Oceania, 2007 16, 16f in Asia and Africa, 2007 16, 8 9, 16f comparison of yield, 17f cultural factors influencing, 11 leading countries, 13t main cultivation regions and countries, 8 9 amount (tons) of exported, 15t reasons for imbalance, 9 11 trend, 12 18 worldwide, 8 18, 14f worldwide cultivation area, 14 18 Purple sweet potato, 179 181

R Retrogradation, 33 Roast sweet potatoes, 303 308 aroma components of, 305 306 frozen, 307 308 pollution analysis and safety evaluation of, 307 sugar composition and starch morphology, 305 Roots of sweet potato, 1, 5 6 proteins, vitamins and minerals, 5 6 storage root, 5 6

S Shochu aroma, 340 343 distillation process atmospheric, 337 338 vacuum, 338 flavour contributions, 333 gas components, 339 health properties of blood glucose lowering effect of, 344 346 J-curve effect of alcohol, 343 344 thrombolytic effects, 344

Index

lipid components, 339 340 preparation method, 334 337 acidity, 336 alcohol concentration, 336 primary, 335 336 secondary, 336 337 temperature, 336 yeast concentration, 335 336 production process, 325, 326f purification process, 338 340 raw material for, 327 processing, 328 329 role of enzymes, 330 332 acidic carboxypeptidase, 331 332 acidic protease, 331 α-amylase, 331 amylolytic enzymes, 331 glucoamylase, 331 β-glucosidase, 332 lipolytic enzymes, 332 proteolytic enzymes, 331 332 sweet potato varieties and colored, 343 Joy White, 343 yeast (Saccharomyces cerevisiae), 334 335 Sonication duty cycle, 136 137 Soy lecithin, effects on hyperlipidemia, 151 SPP hydrolysates (SPPH), 2 Starch, 1 2, 328t acetylated retrograded sweet potato starch (ARS), 47 48, 52t amylopectin content of, 29 30, 33, 37 amylose content of, 29, 33, 37 ash content of, 37 digestibility of, 33 34, 43 45, 44f, 247 255, 265t factors influencing, 254 255 influence of processing on, 255 264 nutritional properties and, 265 products, 259, 260f, 264 266 type of cultivar and, 248 249, 249f from Wariyapola red (SWP1) cultivar, 248 249 gelatinization and pasting properties of, 31 32, 37 40 GI values of, 261 264, 263t, 264f glycemic index of, 247 255

397

lipid content, 30 modification, 34 36 chemical, 34, 250 252 enzymatic, 35, 250 physical, 35, 252 254 value addition, 35 36 moisture content of air-equilibrated, 30 molecular structure of, 247 248 paste properties, 32, 45 47, 46t, 50f BDV, 45 46 HPV, 45 PV, 45 47 SBV, 45 47 phosphorus content, 30 physically modified sweet potato confocal laser scanning microscopy (CLSM) profiles, 57 58 HHP treatment on, 55, 58f pasting properties, 60 polarized light microscopy (PLM) profiles, 57 58 SEM micrographs, 55 57 structural and physicochemical properties, 53 63, 56f swelling power and solubility of, 60 63 thermal properties of, 53 55 XRD profiles, 58 60 protein content of, 37 proximate composition, 37 rapidly digestible starch (RDS), 247 research and development, 63 resistant, 47 48 resistant starch (RS), 247 retrogradation tendency of, 33 rate and extent of, 33 retrograded chemically modified, 47 52 digestibility of, 49 particle size distribution parameters, 51 pasting property, 51 52 swelling capacity (SWC), 49 51 thermal properties, 48 water absorption index (WAI), 49 51 water-soluble index (WSI), 49 51 slowly digestible starch (SDS), 247 solubility of, 32 33, 45

398

Index

Starch (Continued) source of, 27 28 structural and physicochemical characteristics, 36 47, 41f amorphous layers, 30 31 color, 28 29, 40 43, 42t crystallinity, 31 of granules, 28t level of intergranular organization, 30 31 pasting property parameters, 61t proximate composition of, 29 30 semicrystalline layers, 30 31 shape, 42t size, 28 29, 40 43, 42t swelling power of, 32 33, 45 syneresis, 44 thermal properties of, 37 40, 48 utilization of, 32 viscous behavior and low gel strength, 32 wheat, 265 Suioh sweet potato cultivar, 186, 237 dishes and products from, 214 Sweet potato, 258f, 259f as alternative to French fries, 261 as an important supplement, 3 carotenoids, 3 components, 327 free sugar, 354 362, 355f, 356f dietary fiber, 2 edible parts of, 1 fatty acids (FAs) in, 2 3 food processing applications, 3 global consumption and development, 381 384, 382f glycolipids (GLs) in, 2 3 macro- and microstructural changes of cooked, 257 258, 258f, 259f pectin, 2 phenolic acids, 3 phospholipids (PLs) in, 2 3 processing industry, 386 389 challenges and opportunities, 389 390 processing technology, 384 386 production in Asia, 1 protein content, 2 roots, 1

shochu, 3 starches, 1 2 textural properties of steamed storage roots of, 362 368 total lipids (TLs) content, 2 3 weight distributions, 327f world’s production of, 1 Sweet potato dietary fiber. See Dietary fiber Sweet potato protein (SPP), 2, 69 digestibility of, 75 76 emulsifying activity, 89 100, 91t droplet size distribution, 91 93, 93f effects of HHP treatment, 89, 91 93, 92f emulsifying activity index (EAI) and (B) emulsifying stability index (ESI), 90 91, 90f emulsion microstructure, 93 94 flow index, 97 99 interfacial protein concentration and composition, 94 97 oil water emulsions, 94 rheological behavior, 97 viscosity, 99 100 extractability and recovery, 71 73 effectiveness of recovering, 71 73 extract’s pH, 71 recovered by isoelectric precipitation, 71 73 forms of sporamins A and B in, 69 gelation properties of, 80 89, 84f effect of cooling and heating, 85 viscoelastic properties, 87f hydrolysates (SPPH), 69 70, 101t antioxidant activity of peptides by ultrafiltration, 103 104 degree of hydrolysis and antioxidant activity of, 100 102 high performance liquid chromatography (HPLC) profiles of, 102 103 molecular weight (MW) distribution profiles, 102 103, 103f preparation and antioxidant activity of, 100 111 separation and identification of peptides, 104 106

Index

peptides sequences, 107t phenolic contents of, 70 71, 75 phytochemical antioxidative activities, 76 potential antioxidant peptides and conformation prediction, 106 111 active peptides, 110 111 antioxidant activities of Trp, Tyr, and Phe, 106 109 .OH scavenging activities, 102 104 from sporamin A and B, 106 structures obtained by PEPFOLD3, 110f recovery and composition of, 70 80 discoloration problem, 70 polyphenol oxidase (PPO), 70 research and development, 111 rheological and mechanical properties of, 80 83 tannin content of, 75 76 Sweet potato pulp, 119 120, 120f components of, 121t dry weight of, 120 manufacturing dietary fiber from, 120 121 uronic acid content of, 128 129 Sweet potato snacks, 303 biscuits, 315 318 dietary fiber-enriched, 317 brownies, 314 315 cakes, 313 314 effect of adding different proportions of sweet potato, 314 315 chips, 308 simulation, 309 310 traditional or general, 308 309 doughnuts, 318 319 dried sweet potato slices, 310 313 drying methods, 312 313 process, 311 312 extruded, 319 320 packaging of, 320 321 roast sweet potatoes, 303 308 trends and prospects, 321 Sweet potato staple foods breads, 274 cake, 275 definition and types of, 273 275

399

gluten-free pancakes, 275 noodles, 274 275 development of, 296 298 traditional Chinese, 297 pasta, 297 298 prospects, 298 raw ingredients for, 275 277 flour, 276 277 fresh sweet potato, 275 276 mashed sweet potato, 276 steamed breads, 273 development of, 277 278 from dietary fiber, 277 278 from flour, 277 Sweet potato storage root, 201 205 Sweet potato storage roots β-amylase in, 355 356 anthocyanins in, 368 371 carotenoid contents in, 368 371 cellulose and hemicellulose in, 367 368 darkening discoloration of, 370 fructose and glucose in, 361 maltose in, 356 357, 361 362 nonenzymatic darkening in heat-cooked, 371 pigment associated with, 368 polyphenol oxidase in, 371 processed foods from, 350 351 skin colors of, 368 starch content of, 363 365 sucrose accumulation in, 360 sweetness of, 360 textural properties of, 362 368 tissue disintegration in, 367 Sweet potato tops, 205 214 Sweet potato wheat bread, 278 296 bread-making process and quality evaluation, 285t crust and crumb color, 284 285, 293 specific volume, 286 texture analysis, 286 287 characteristics, 279 280, 279t, 281t color, 280 DSC profiles, 282 SEM profiles, 280 281 size, 280 dough properties, 294t

400

Index

Sweet potato wheat bread (Continued) DSC profile, 283t, 284 fermentation, 282 283, 284t, 292, 292t specific loaf volume, 293 texture analysis, 293 296, 295t thermal, 290 292 effect of heat treatment on, 278 287 effect of high hydrostatic pressure on dough, 287 296, 289t on color, 288 DSC profiles, 288 290, 291t on particle size, 288 SEM profiles, 288

sulfur-containing amino acids, 76 79 gelation properties and network formation mechanism of, 83 86 gel dynamic rheological properties, 86 mechanical parameters, 87 88 microstructure of gels, 88 89 rheological and mechanical properties of, 80 83 trypsin inhibitory activities, 76 Ultrasonic technology, 134 United States, sweet potatoe production in, 9, 383, 384t Uses of sweet potatoes, 20 22

U

V

Ultrafiltration/diafiltration-processed sweet potato (UDPS) protein, 73 76 amino acid composition and nutritional quality, 76 80

Vermicelli, 9 11 Vitamin A deficiency, prevention of, 5 6, 223 224 OFSP as solution to, 234 235