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Encyclopedia of food and color additives
 9780849394164, 0849394163, 0849394120, 0849394139, 0849394147

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E N C Y C L O P E D I A OF

F o o d andC olor A dditives G eorge A. Burdock, Ph.D. VOLUME II F -O

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

CRCPress Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 1997 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business

No claim to original U.S. Government works ISBN 13: 978-0-8493-9416-4 (hbk) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www. copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety ofusers. For organizations that have been granted a photocopy license by the CCC, a separate system of payment hasbeen arranged.

TrademarkNotice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe.

Visit the Taylor& FrancisWeb site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com Library of Congress Cataloging-in-PublicationData Burdock, George A. Encyclopedia of food and color additives / George A. Burdock. p. cm. Includes indexes. ISBN 0-8493-9416-3 (set: alk. paper). -- ISBN 0-8493-9412-0 (v. 1 : alk. paper). -- ISBN 0-8493-9413-9 (v. 2 : alk. paper). -- ISBN 0-8493-9414-7 (v. 3 : alk. paper) l. Food additives--Encyclopedias. 2. Coloring matter in food-Encyclopedias. I. Title. TX553.A3B87 1996 664'.06'03--dc20

96-42689 CIP

Library of Congress Card Number 96-42689 Cover design: Typography:

Denise Craig Roy Barnhill

F FARNESOL Synonyms

Dihydrofamesol; 2.6.10-Dodecatrien-l-ol, 3,7,11-trimethyl- (8CI)(9CI); Farnesol; Famesyl alcohol; 2.6.10-Trimethyl-2,6,10-dodecatrien-12-ol; 3.7.11-Trimethyl-2,6,10-dodecatrienol; 3,7,11 -Trimethyl-2,6,10-dodecatrien-1-ol.

Current CAS number

4602-84-0

Description

Farnesol has a characteristic flowery odor.

Empirical formula

C 15H260

Specifications

Physical/chemical characteristics1 (The physical constants vary slightly, depending on the source and the method of preparation.) Appearance: Colorless liquid. Assay: 97% min. Molecular weight: 222.36. Boiling point: 131 - 132°C at 3 mmHg (from petitgrain); 124 - 125°C at 2.2 mmHg (from Tolu balsam); 118 - 120°C at 2 mmHg (from cabreuva); 118 - 120°C at 2 mmHg (from geranyl acetone). Specific gravity: 0.887 - 0.889 at 25°C (0.8880 from petit­ grain); (0.8883 at 20°C from Tolu balsam); (0.8886 from cabreuva); (0 .8 8 8 6 from geranyl acetone). Refractive index: 1.4890 - 1.4910 at 20°C (1.4891 from petit­ grain); (1.4892 at 20°C from Tolu balsam); (1.4894 from cabreuva); (1.48906 from geranyl acetone). Solubility: 1:3 in 70% alcohol; almost soluble in water; soluble in most organic solvents.

Natural sources

Reported found in apricot, orange peel oil, bitter orange peel oil, grapefruit juice, high-bush blueberry, strawberry jam, clove bud, ginger, pork (cured, raw), beer, malt whiskey and scented rice (cooked).2 The presence of this terpene alcohol has also been reported in more than 30 essential oils; the levels are generally low (0.5 - 1.0%) with the exception of cabreuva, which contains up to 2.5% farnesol, and the distillate from flowers of Oxystigma buccholtzii Harms., which contains up to 18% farnesol; among

1 Burdock, G.A. (ed.) (1995). Fenaroli’s Handbook of Flavor Ingredients. Third edition. CRC Press. Boca Raton, FL. 2 Maarse, H., Visscher, C.A., Willemsens, L.C. and Boelens, M.H. (eds.) (1993). Volatile compounds in food. Qualitative and quantitative data. Supplement 4 and cumulative index. TNO Nutrition and Food Research. Zeist, The Netherlands.

1059

1060_______________________________________________________________________________________ F P & C

the essential oils containing farnesol are lemongrass, Ceylon citronella, cananga, ambrette seeds, ylang-ylang, Acacia famesiana, Peru balsam, palmarosa, tuberose, and others.1 Functional use in foods

Flavoring ingredient.

GRAS status

FEMA GRAS No. 2478 (Refer to FenarolVs Handbook o f Flavor Ingredients (1995) for use levels reviewed by the FEMA Expert Panel.)

Regulatory citations CITATION NUMBER/CFR PART 21 CFR 172.515 Food additives permitted for direct addition to food for human consumption. Subpart F Flavoring agents and related substances. Synthetic flavoring substances and adjuvants.

FOOD CATEGORY

PERMITTED FUNCTIONALITY

No restrictions

(12) Flavoring agents and adjuvants

USE LIMITS CGMP

FD&C Description2

Color additives may be added to food, drugs, cosmetics (i.e., FD & C) and certain medical devices for the purpose of imparting color. The three categories of color additives are: (1) “Straight colors” (color additives that have not been mixed or chemically reacted with any other substance); (2 ) lakes (color additives formed by chemically reacting a straight color with water-insoluble sub­ stances); and (3) mixtures (color additives formed by mixing a color additive with one or more other color additives or noncolored substances, without chemical reaction). Section 7 of the Food and Drugs Act of 1906 (Pub. L. 59-384) prohibited the use of poisonous or deleterious colors in confec­ tionery and the coloring or staining of food to conceal damage or inferiority. In 1907, the agency, then part of the Department of Agriculture, issued Food Inspection Decision 76 (Ref. 10), which contains a list of seven straight colors approved for use in food. Between 1907 and 1939, the agency expanded the list of straight colors approved for use in food from 7 to 15. These colors were known as “coal tar colors” because they were synthesized mainly from substances obtained from coal tar. However, prior to 1939, the agency’s list of acceptable colors did not include lakes of coal tar colors because such lakes were not used in food. Also, prior to 1938, the government program for batch analysis and certifica­ tion of colors was voluntary. The Federal Food, Drug, and Cosmetic Act of 1938 (21 U.S.C. 301 et seq. (the act)) (Pub. L. 75-717) required FDA to list coal

1 Burdock, G.A. (ed.) (1995). Fenaroli’s Handbook of Flavor Ingredients. Third edition. CRC Press. Boca Raton, FL. 2 Adapted from Federal Register 61(43):8371-8417, 1996.

FD&C

1061

tar colors “harmless and suitable” for use in foods, drugs, and cosmetics, and to certify all batches of listed colors, including lakes. The agency issued regulations under the act listing lakes for food use, as well as for drug and cosmetic use, and establishing conditions for certification of batches of lakes (4 FR 1922, May 9, 1939; 4 FR 3931, September 16, 1939; and 5 FR 1138, March 23, 1940). The agency issued the first certificate for a lake under the act on May 11, 1939 (Ref. 11). The initial listing of lakes for food use under the act restricted their use to coloring shell eggs (egg dyeing) (5 FR 1138). In 1959, at the request of industry, the agency expanded the uses of lakes prepared from FD & C straight colors to encompass general use in foods (24 FR 3818, May 13, 1959; and 24 FR 5302, June 30, 1959). The 1960 amendments amended the act by defining the term “color additive” (section 201(t) (21 U.S.C. 321(t))) for the first time and restricting the use of color additives in or on food, drugs, cosmet­ ics, or the human body to those listed in IDA regulations. (The Medical Device Amendments of 1976 (Pub. L. 94-295) extended these restrictions to the use of color additives in certain medical devices.) As amended, the act provides that a food (section 402(c) (21 U.S.C. 342(c))), drug or device (section 501(a)(4) (21 U.S.C. 351(a)(4))), or cosmetic, other than a coal tar hair dye (section 601(e) (21 U.S.C. 361(e))), is adulterated if it is, bears, or contains an unsafe color additive. Section 721 (formerly section 706) of the amended act (21 U.S.C. 379e) provides for the listing of safe and suitable color additives for use in foods, drugs, cosmetics, and medical devices; it prohibits the listing of a color additive for a proposed use unless data establish that such use will be safe. Section 721 of the act also continues the requirement for certifi­ cation of batches of color additives, with or without diluents, to determine whether each batch conforms to the purity and identity specifications in the applicable listing regulation. However, the amendments allow FDA to exempt color additives from batch certification if certification is unnecessary to protect the public health. Section 203 of the 1960 amendments also provided for the provi­ sional listing of color additives that were commercially established when the 1960 amendments were enacted, pending completion of scientific investigations necessary to determine their safety under the new standard established by the 1960 amendments. The pur­ pose of section 203 was to allow the use of such color additives on an interim basis, to the extent consistent with the public health. Section 203 directed the agency to recognize as provisionally listed the following color additives: (1) Any color additive which, on the day preceding the enactment date, was listed and certifiable for any use or uses and for which a batch or batches had been certified for such use or uses prior to the enactment date; (2 ) any color additive which was commercially used or sold prior to the enact­ ment date for any use or uses on any food, drug, or cosmetic, but

1062

FD & C BLUE NO . 1

was not required to be listed under the act; (3) synthetic beta carotene. The provisional listing was to apply only to the use or uses to which the certification applied, or for which the color additive had been commercially used or sold. Under the authority of the 1960 amendments, in the Federal Register of October 12, 1960 (25 FR 9759), the agency provisionally listed those color additives, including lakes, covered by section 203. This listing, originally codified as 21 CFR 8.501 and later recodified as Sec. 81.1 (21 CFR 81.1) (42 FR 15665, March 22, 1977), included many of the coal tar colors (including lakes) that had been previ­ ously listed. In the Federal Register of December 27, 1963 (28 FR 14311), the agency determined that batch certification was unnecessary to ensure the safety of most color additives derived from plant, ani­ mal, or mineral sources, and designated these color additives as exempt from certification. However, the agency determined that batch certification was necessary to ensure the safety of most color additives, including lakes, derived principally from coal and petro­ leum sources, and designated those colors as subject to certifica­ tion. Currently, the color additives exempt from batch certification and the permanently listed color additives subject to batch certi­ fication are listed in parts 73 and 74 (21 CFR parts 73 and 74), respectively. Since the establishment of the provisional list in 1960, the agency has gradually removed color additives from the list either by per­ manent listing or by termination of listing due to lack of interest by industry or due to safety concerns prompted by the agency’s reviews. At this time, only lakes remain provisionally listed in parts 81 and 82. After the enactment of the act in 1938, FDA established the des­ ignation “FD & C” to identify color additives listed for use in foods, drugs, and cosmetics; the designation “D&C” to identify color additives listed for general use in drugs and cosmetics, but not foods; and the designation “Ext. D&C” to identify color addi­ tives listed for use only in externally applied drugs and cosmetics (4 FR 1922 at 1923). These designations are still part of the names of certified color additives. However, the uses of some straight colors (and consequently also of their lakes) were restricted when they were permanently listed, based on the safety reviews con­ ducted by the agency under the 1960 amendments. Consequently, the designations “FD & C” or “D&C” in the name of a certified color additive can no longer be relied upon to accurately describe the approved uses of the color additive. FD & C BLUE NO. 1 Synonyms

Acid blue 9; Acid blue 9 ammonium salt; Acid sky blue A; Acilan turquoise blue AE; AF Blue No 1; A.F. Blue No. 1; Aizen brilliant blue FCF; Alphazurine; Alphazurine (indicator); Alphazurine FG; Alphazurine F G; Alphazurine FGND;

FD & C BLUE NO . 1

1063

Ammonium,ethyl(4-(p-(ethyl)(m-sulfobenzyl)amino)-a/Jp^a(o-sulfophenyl)benzy-lidene)-2,5-cyclohexadienl-ylidene)(m-sulfobenzyl)-, hydroxide, inner salt, diammonium salt; Benzenemethanaminium, n-ethyl-n(4-((4-(ethyl((3-sulfophenyl)methyl)amino)phenyl) (2-sulfophenyl)methylene)-2,5-cyclohexadien-l-ylidene)3-sulfo-, hydroxide, inner salt, diammonium salt (9CI); Blue brilliant Fcf; Blue 1; 1206 Blue; 11388 Blue; Blue dye number 1 food additive; Brilliant blue; Brilliant blue FCF,diammonium salt; Brilliant blue lake; Bucacid azure blue; Calcocid blue EG; Calcocid blue 2G; Calcocid blue 2 G; Canacert brilliant blue FCF; Cl Acid blue 9; C.I. Acid blue 9; C.I. Acid blue 9; Cl 671; C.I. 671; C.I 42090; Cl 42090 (Ammonium salt); Cl Acid blue 9, diammonium salt; C.I. Acid blue 9, diammonium salt (8CI); Cl Direct brown 78; C.I. Direct brown 78; Cl Direct brown 78, diammonium salt; C.I. Direct brown 78, diammonium salt; Cl Food blue 2; C.I. Food blue 2; D and C Blue 1; D&C Blue No 1; D and C Blue No 1; D & C Blue No. 4; D and C Blue No 4; Diammonio(ethyl)(4-((4-(ethyl(3-sulphonatobenzyl)amino) phenyl)(2- sulphonatophenyl)methylene)cyclohexa2,5-dien-l-ylidene)(3-sulpho-natobenzyl)ammonium; Disulphine lake blue EG; Dolkwal brilliant blue; E; Edicol supra blue E6 ; Erioglaucin; Erioglaucine; Erioglaucine A; Erioglaucine (biological stain); Erioglaucine E; Eriosky blue; FD & C Blue 1; FD and C Blue No. 1; FD & C Blue No.l; FD & C Blue No. 1; F D+C Blue 1; Fenazo blue XR; Food blue 1; Food blue No. 1; Hexacol brilliant blue A; Hidacid azure blue; H.K. Formula No. K. 7117; Kiton blue AR; Kiton pure blue L; Maple brilliant blue FCF; Neptune blue BRA; Neptune blue BRA concentration; Neptune blue WF; Patent blue AE; Patent blue 2Y; Peacock blue X 1756; Schultz No 770; Tortrazine c extra; Triantine light brown 3RN; Usacert blue No. 1; Xylene blue VSG. Current CAS number

2650-18-2

Other CAS number(s)

1334-07-2; 29519-65-1; 37307-55-4; 37307-56-5; 37307-78-1; 51609-24-6; 55819-29-9; 86924-52-9; 37307-56-5

Description

FD & C Blue No. 1 is principally the disodium salt of ethyl[4-[p[ethyl(m-sulfobenzyl)amino]-a-(1:6 in 90% ethanol

The resinoid once was prepared by hydrocarbon solvent extraction and subsequent evaporation of the solvent; today a high-boiling, odorless solvent is added prior to evaporation; this solvent is left in the finished commercial product. The solvent-free resinoid is a dark-amber, viscous liquid with a characteristic balsamic odor. It yields turbid solutions in alcohol. Natural sources

Botanical source Ferula galbaniflua Boiss. & Buhse and other Ferula species. Family Umbelliferae.

Functional use in foods

Flavoring ingredient.

GRAS status

FEMA GRAS No. 2501 (Refer to FenarolVs Handbook of Flavor Ingredients (1995) for use levels reviewed by the FEMA Expert Panel.)

Regulatory note

TSCA Definition 1990: Extractives and their physically modified derivatives. Ferula, Umbelliferae.

Regulatory citations CITATION NUMBER/CFR PART 21 CFR 172.510 Food additives permitted for direct addition to food for human consumption. Subpart F Flavoring agents and related substances. Natural flavoring substances and natural substances used in conjunction with flavors.

FOOD CATEGORY

PERMITTED FUNCTIONALITY

Restricted to use in alcoholic beverages

(12) Flavoring agents and adjuvants

USE LIMITS CGMP

GALBANUM, RESIN (Ferula spp.) Synonyms Galbanum gum; Galbanum, resin (Ferula spp.).

1 Burdock, G.A. (ed.) (1995). Fenaroli’s Handbook of Flavor Ingredients. Third edition. CRC Press. Boca Raton, FL.

1162

GAMBIR ( Uncaria gambir Roxb.)

Current CAS number

9000-24-2

Description

Refer to GALBANUM, OIL.

Functional use in foods

Flavoring ingredient.

GRAS status

FEMA GRAS No. 2502 (Refer to FenarolVs Handbook of Flavor Ingredients (1995) for use levels reviewed by the FEMA Expert Panel.)

Regulatory citations CITATION NUMBER/CFR PART 21 CFR 172.510 Food additives permitted for direct addition to food for human consumption. Subpart F Flavoring agents and related substances. Natural flavoring substances and natural substances used in conjunction with flavors.

FOOD CATEGORY

PERMITTED FUNCTIONALITY

No restrictions

(12) Flavoring agents and adjuvants

USE LIMITS CGMP

GAMBIR (Uncaria gambir Roxb.) Synonyms

Gambier; Gambir (Uncaria gambir Roxb.).

Current CAS number

8001-48-7

Description

Tree native to Malaysia, cultivated in tropical countries. A yellow catechu is derived from the leaves of a Malaysian woody vine (yellow or pale catechu) by boiling or infusing them in water. It is used for chewing with betel nuts and is exported for tanning and dyeing. The parts used are the leaves and young branches. The derivative is a coloring substance. Extracted from leaves and young branches, it consists of small (10 to 15 g) fragments that are externally reddish-brown and internally yellowish. The main constituents include tannic acid, quercetin, and coloring matter.

Natural sources

Botanical source Uncaria gambir (Hunter) Roxb. Family Rubiaceae (Chinchonaceae).

Functional use in foods

Flavoring ingredient.

Regulatory note

TSCA Definition 1990: Extractives and their physically modified derivatives. It is a product which may contain resin acids and their esters, terpenes, and oxidation or polymerization products of these terpenes. (Uncaria gambier or Ourouparia gambier, Rubiaceae.)

GARLIC (AND ITS DERIVATIVES)

1163

Regulatory citations CITATION NUMBER/CFR PART 21 CFR 172.510 Food additives permitted for direct addition to food for human consumption. Subpart F Flavoring agents and related substances. Natural flavoring substances and natural substances used in conjunction with flavors.

FOOD CATEGORY

PERMITTED FUNCTIONALITY

No restrictions

(12) Flavoring agents and adjuvants

USE LIMITS CGMP

GARLIC (AND ITS DERIVATIVES) Current CAS number

977001-81-2

Description

Garlic is obtained from Allium sativum, a member of the lily family. Historical records reveal garlic has been used in foods dating back to 4500 B. C. Garlic is an herbaceous, perennial plant of Mediterranean origin. Its stalk can reach lengths of 1 m (39 in.). The plant exhibits flat, keeled leaves and is terminated by an umbellated flower cluster. The bulb-like root contains several bul­ bils (cloves) enclosed in a common membrane. The bulbs are the part used. Garlic has a pungent, acrid, aromatic, garlic-like odor. Other Allium species used for food beside garlic are onion, leeks, shallots and chives. In addition to the natural form, garlic is marketed as: (a) minced dehydrated garlic, (b) garlic powder, which is ground dehydrated cloves, (c) garlic salt, which is garlic powder mixed with table salt, and if necessary, some edible starch to prevent caking, and (d) oil of garlic, which is steam distilled from crushed garlic bulbs. Major chemical constituents of whole garlic include alliin (allylsulfinyl alanine), which is rapidly converted enzymatically when garlic is crushed to allicin (responsible for the characteristic odor of the essential oil and for the odor liberated from the crushed garlic clove), (allylsulfinyl allylsulfide), volatile and fatty acids, mucilage, and albumin. The principal compounds of garlic oil obtained by steam distillation of fresh garlic are disulfides, such as allylpropyl disulfide, diallyl disulfide, and diallyl trisulfide. Dimethyl sulfide, dimethyl disulfide and dimethyl trisulfide have also been identified in the volatile fraction of the oil.1 The essential oil is obtained in 0.1 to 0.2% yields by steam dis­ tillation of the crushed bulbs or cloves; sometimes the whole plant is distilled. The essential oil obtained from bulbs is a clear, paleyellow to reddish-orange liquid bearing a very intense mercaptanlike note.2

1 Adapted from the SCOGS review of garlic, PB 223 848. 2 Burdock, G.A. (ed.) (1995). Fenaroli’s Handbook of Flavor Ingredients. Third edition. CRC Press. Boca Raton, FL.

1164

GARLIC, OIL (Allium sativum L.)

Specific gravity at 25°/25°C Refractive index at 20°C

1.040 to 0.190 1.5590 to 1.5790

Purity

Refer to Food Chemicals Codex.

Specifications

Refer to Food Chemicals Codex.

Natural sources

Botanical source Allium sativum L., family Liliaceae.

Functional use in foods

Flavor agent.

Regulatory notes

§184.1317(a) Garlic is the fresh or dehydrated bulb or cloves obtained from Allium sativum, a genus of the lily family. Its deriv­ atives include essential oils, oleoresins, and natural extractives obtained from garlic, (b) Garlic oil must meet the specifications of the Food Chemicals Codex.

Regulatory citations CITATION NUMBER/CFR PART 21 CFR 184.1317 Direct food substances affirmed GRAS.

FOOD CATEGORY

PERMITTED FUNCTIONALITY

No restrictions

(12) Flavoring agent and adjuvants.

USE LIMITS CGMP

GARLIC, OIL (Allium sativum L.) Synonyms Allium sativum; Garlic; Garlic oil; Garlic, oil (Allium sativum L.); Oil of garlic; Oils, garlic; Plant extract, garlic oil. Current CAS number

8000-78-0

Description

§184.1317(a) Garlic is the fresh or dehydrated bulb or cloves obtained from Allium sativum, a genus of the lily family. Its deriv­ atives include essential oils, oleoresins, and natural extractives obtained from garlic.

Functional use in foods

Flavoring ingredient.

GRAS status

FEMA GRAS No. 2503 (Refer to FenarolVs Handbook o f Flavor Ingredients (1995) for use levels reviewed by the FEMA Expert Panel.)

Regulatory notes

§184.1317(b) Garlic oil must meet the specifications of the Food Chemicals Codex. TSCA Definition 1990: Extractives and their physically modified derivatives. Allium sativum, Liliaceae.

GELATIN

1165

Regulatory citations CITATION NUMBER/CFR PART 21 CFR 184.1317 Direct food substances affirmed GRAS.

GELATIN Synonyms

FOOD CATEGORY

PERMITTED FUNCTIONALITY

No restrictions

(12) Flavoring agent and adjuvant

USE LIMITS CGMP

Absorbable gelatin sponge; Gelatin; Gelatin foam; Gelatins; Gelatin velvatex; Gelfoam; GT.

Current CAS number

9000-70-8

Other CAS number(s)

8052-24-2; 9013-63-2; 9013-63-2

Description

Gelatin does not occur in nature as such, but is derived by hydrol­ ysis of collagen, the chief protein component in connective tissues of the animal body. Extraction of gelatin for use as a glue by cooking hides dates back to the earliest recorded history of man and appears in the literature of the items up to the present day. During the early years of the Napoleonic era it was manufactured on a large scale in an attempt to alleviate the food shortages resulting from the English naval blockade of Europe. Gelatin was first manufactured in the U.S. in 1809. In 1845 a U.S. patent was granted for a gelatin which contained all the ingredients fitting it for table use, and required only the addition of hot water and subsequent cooling to prepare it for serving. 1 Quantitatively, collagen is concentrated in the skin, the bone of the skeletal system and the tendons attaching muscles to the skel­ eton, although it occurs throughout all of the tissues and organs to a lesser degree. Chemically, collagen and gelatin are virtually indistinguishable, but the process of collagen extraction results in converting the fibrous, water-insoluble, highly organized macro­ molecules (tropocollagens) irreversibly into gelatin which has dis­ similar physical characteristics. Variations in gelatin properties due to source and treatment make it a highly diverse, heterogeneous substance, particularly with regard to molecular weight.2 The major sources of collagen are cattle hides, pig skins and bones. The resulting gelatin is of two types commonly designated A and B, depending upon which of two processes are used to convert the collagen into gelatin. Type A gelatin is derived primarily from pig skin by acid processing; it has an isoelectric point between pH 7 and pH 9. Type B is from cattle hides and bones by alkaline or

1 Adapted from the Select Committee on GRAS Substances (SCOGS) (1975). Evaluation of the Health Aspects of Gelatin as a Food Ingredient. Life Sciences Research Office. Federation of American Societies for Experimental Biology, Bethesda, MD. Prepared for the Bureau of Foods, FDA, Washington, DC. NTIS PB-254 527. 2 Adapted from the SCOGS report on Gelatin, 1973, PB 223 857.

1166

GELATIN

lime processing and has an isoelectric point between pH 4.7 and pH 5.1. Gelatin from different sources and as prepared by the different processes exhibits small differences in amino acid com­ position as shown in the following table. The nutritionally essential amino acid, tryptophan, is absent in gelatin. Gelatin also is unusual in that it contains large proportions of glycine, proline and hydroxyproline, and a small percentage of hydroxylysine, an amino acid rare in proteins. (SCOGS, 1975) Amino Acid Composition of Different Gelatins (gram/100 gram gelatin)3

Amino Acid

Type A (percent)

Type B (skin) (percent)

Type B (bone) (percent)

Alanine Arginine Aspartic acid Cystine Glycine Glutamic acid Histidine Hydroxylysine Hydroxyproline Isoleucine Leucine Lysine Methionine Phenylalanine Proline Serine Threonine Tyrosine Valine

8.6-10.7 8.3-9.1 6.2-6.7 0.1 26.4-30.5 11.3-11.7 0.85-1.0 1.04 13.5 1.36 3.1-3.34 4.1-5.2 0.80-0.92 2.1-2.56 16.2-18.0 2.9-4.13 2.2 0.44-0.91 2.5-2.8

9.3-11.0 8.55-8.8 6.6-6.9 None-trace 26.9-27.5 11.1-11.4 0.74-0.78 0.91-1.2 14.0-14.5 1.7-1.8 3.1-3.4 4.5-4.6 0.80-0.90 2.2-2.5 14.8-16.35 3.2-4.2 2.2 0.2- 1.0 2.6-3.4

11.3 9.0 6.7 Trace 27.2 11.6 0.7 0.76 13.3 1.54 3.45 4.36 0.63 2.49 15.5 3.73 2.36 0.23 2.77

a Adapted from: Glicksman, M. (1969). Gelatin. In: M. Glicksman, Gum technology in the food industry. Academic Press, NY. Used with permission.

The major use of gelatin in the U.S. is in food products, principally in gelatin desserts, meat products, consommes, marshmallows, candies, bakery and dairy products and ice cream. A substantial portion of each year’s production (imported and domestic) is also used in the pharmaceutical, photographic and paper industries. (SCOGS, 1975) Gelatin is colorless or slightly yellow, transparent, brittle, practi­ cally odorless, tasteless, presenting as sheets, flakes or a coarse powder. On being warmed, gelatin disperses into the water result­ ing in a stable suspension. Water solutions of gelatin will form a reversible gel if cooled below the specific gel point of gelatin. The gel point is dependent on the source of the raw material. Gelatin extracted from the tissues of warm-blooded animals will have a gel point in the range of 30°C - 35°C. Gelatin extracted from the

1167

GELATIN

skin of cold-water ocean fish will have a gel point in the range of 5°C - 10°C. Gelatin is soluble in aqueous solutions of polyhydric alcohols such as glycerin and propylene glycol. It is insoluble in most organic solvents. Characteristics of Gelatin

Characteristic Moisture pH Isoelectric point (pH) Gel strength (Bloom) Viscosity Ash

Type A (pork skin, acid treated) 8- 12% 3.8-5.5 7.0-9.0 50-300 g 20-70 mps 0.3% (mainly as Na+, C1-, S 0 42-)

Type B (ossein or calfskin, lime treated) 8-12% 5.0-7.5 4.7-5.1 50-275 g 20-75 mps 0.5-2.0% (mainly Ca2+, PO4-, Cl )

Adapted from SCOGS, 1973.

Empirical formula

Unknown.

Specifications

Identification: (A) Gelatin forms a reversible gel when tested as follows: Dissolve 5 g in 100 mL of hot water in a suitable flask, and cool in a refrigerator at 2°C for 24 h. A gel forms. Transfer the flask to a water bath heated to 60°C. Within 30 min, upon stirring, the gel reverts to the original liquid state. (B) To a 1 in 100 solution of the sample add trinitrophenol TS or in a 1 in 1.5 solution of potassium dichromate previously mixed with about one-fourth its volume of 3 N hydrochloric acid. A yellow precip­ itate forms. Ash: Not more than 3.0%. Chromium: Not more than 10 mg/kg. Fluoride: Not more than 0.005%. Heavy metals (as Pb): Not more than 10 mg/kg. Lead (as Pb): Not more than 1.5 mg/kg. Loss on drying: Not more than 15.0%. Microbial Limits: Escherichia coli: Negative; Salmonella: Neg­ ative. Pentachlorophenol limit: Not more than 0.1 mg/kg. Protein: The specification conforms to the representations of the vendor. Sulfur dioxide: Not more than 0.005%.1

Functional use in foods

As a firming agent, formulation aid, processing aid, stabilizer and thickener, surface-active agent and surface-finishing agent.

1 National Academy of Sciences (1994). Draft monograph on gelatin for inclusion into the Food Chemicals Codex. Submitted to the USFDA in partial fulfillment of contract number 223-92-2250. October 10, 1994.

1168

Regulatory notes

GELATIN

§133.133(b) Optional ingredients (3) Other optional ingredients (iii) Stabilizers, in a total amount not to exceed 0.5% of the weight of the finished food, with or without the addition of dioctyl sodium sulfosuccinate in a maximum amount of 0.5% of the weight of the stabilizer(s) used. §133.134(b) Optional ingredients (2) Other optional ingredients (I) Stabilizers, in a total amount not to exceed 0.8% of the weight of the finished food, with or without the addition of dioctyl sodium sulfosuccinate in a maximum amount of 0.5% of the weight of the stabilizer(s) used. §133.162(b) Optional ingredients (3) Other optional ingredients (iii) Stabilizers, in a total amount not to exceed 0.5% of the weight of the finished food, with or without the addition of dioctyl sodium sulfosuccinate in a maximum amount of 0.5% of the weight of the stabilizer(s) used. §133.178 (b) The optional ingredients ... are: (1)(I) One or any mixture of two or more of the following: carob bean gum, gum karaya, gum tragacanth, guar gum, gelatin, sodium carboxymethylcellulose (cellulose gum), carrageenan, oat gum, algin (sodium alginate), propylene glycol alginate, or xanthan gum. The total quantity of any such substances, including that contained in the neufchatel cheese, is not more than 0 .8 % by weight of the finished food, (ii) When one or more of the optional ingredients in para­ graph (b)(l)(I) of this section are used, dioctyl sodium sulfosuc­ cinate complying with the requirements of § 172.810 of this chapter may be used in a quantity not in excess of 0.5% by weight of such ingredients. §133.179 (f) The other optional ingredients ... are: (l)(i) One or any mixture of two or more of the following: carob bean gum, gum karaya, gum tragacanth, guar gum, gelatin, sodium carboxymethylcellulose (cellulose gum), carrageenan, oat gum, algin (sodium alginate), propylene glycol alginate, or xanthan gum. The total weight of such substances is not more than 0 .8 % of the weight of the finished food, (ii) When one or more of the optional ingre­ dients in paragraph (f)(l)(i) of this section are used, dioctyl sodium sulfosuccinate complying with the requirements of §172.810 of this chapter may be used in a quantity not in excess of 0.5% by weight of such ingredients. §184.1318 Definition: The produce obtained by the hydrolysis of collagen (the chief protein components in connective tissues of the animal body). Type A gelatin is produced by acid processing of pig skins and type B is produced by alkaline or lime processing of cattle hides and bones. The bones, hides, or skins shall not have been exposed to pentachlorophenol, nor consist of tannery waste materials. Specifications (food-grade gelatin): Not less than 8 8 % protein; not more than 1 2 % moisture; not more than 2 % ash; not more than 0.4% phosphorus; not more than 1 ppm arsenic; not more than

GELLAN GUM

1169

40 ppm sulfur dioxide; and not more than 10 ppm heavy metals (as lead). TSCA Definition 1990: A complex combination of proteins obtained by hydrolysis of collagen by boiling skin, tendons, liga­ ments, bones, etc. Regulatory citations CITATION NUMBER/CFR PART

FOOD CATEGORY

PERMITTED FUNCTIONALITY

USE LIMITS

21 CFR 133.133 Food standards. Part 133 - Cheeses and related products. Subpart B - Requirements for specific standardized cheese and related products. Cream cheese.

(5) Cheeses

See above

See above

21 CFR 133.134 Food standards. Part 133 - Cheeses and related products. Subpart B - Requirements for specific standardized cheese and related products. Cream cheese with other foods.

(5) Cheeses

See above

See above

21 CFR 133.162 Food standards. Part 133 - Cheeses and related products. Subpart B - Requirements for specific standardized cheese and related products. Neufchatel cheese.

(5) Cheeses

See above

See above

21 CFR 133.178 Food standards. Part 133 - Cheeses and related products. Subpart B - Requirements for specific standardized cheese and related products. Pasteurized neufchatel cheese spread with other foods.

(5) Cheeses

See above

See above

21 CFR 133.179 Food standards. Part 133 - Cheeses and related products. Subpart B - Requirements for specific standardized cheese and related products. Pasteurized process cheese spread.

(5) Cheeses

See above

See above

21 CFR 182.70 Substances generally recognized as safe. Substances migrating from cotton and cotton fabrics used in dry food packaging.

CGMP

GELLAN GUM Synonyms

Gellan gum; Gelrite.

Current CAS number

71010-52-1

Other CAS number(s)

85087-30-5; 88402-73-7

Description

A high molecular weight polysaccharide gum produced by a pureculture fermentation of a carbohydrate with Pseudomonas elodea, and purified by recovery with isopropyl alcohol, dried, and milled. It is a heteropolysaccharide comprising a tetrasaccharide repeating unit of one rhamnose, one glucuronic acid, and two glucose units. The glucuronic acid is neutralized to mixed potassium, sodium, calcium, and magnesium salts. It may contain acyl (glyceryl and acetyl) groups as the O-glycosidically linked ester. It occurs as an off-white powder that is soluble in hot or cold deionized water.

GELLING AGENTS

1170

Empirical formula

Unknown.

Purity

It yields not less than 3.3% and not more than 6 .8 % of carbon dioxide (C02), calculated on the dried basis.

Specifications

Identification: (A) A 1% solution is made by hydrating a 1-g sample in 99 mL of deionized water. The mixture is stirred for about 2 h, using a motorized stirrer and a propeller-type stirring blade. Draw a small amount of the above solution into a widebore pipet, and transfer it into a solution of 1 0 % calcium chloride. A tough, worm-like gel will form instantly. (B) To the 1% deion­ ized water solution prepared for Identification Test A, add 0.5 g of sodium chloride, heat the solution to 80°, stirring constantly, and hold the temperature at 80° for 1 min. Stop heating and stirring the solution, and allow it to cool to room temperature. A firm gel will form. Arsenic (as As): Not more than 3 mg/kg. Heavy metals (as Pb): Not more than 0.002%. Isopropyl alcohol: Not more than 0.075%. Lead: Not more than 2 mg/kg. Loss on drying: Not more than 15.0%.1

Functional use in foods

Chewing gum base.

Regulatory citations CITATION NUMBER/CFR PART 21 CFR 172.665 Food additives permitted for direct addition to food for human consumption. Subpart G gums, chewing gum bases and related substances.

FOOD CATEGORY (6) Chewing gum

PERMITTED FUNCTIONALITY

USE LIMITS See above

GELLING AGENTS2 Description

Gels are a form of matter intermediate between a solid and a liquid. They consist of polymeric molecules cross-linked to form a tan­ gled, interconnected molecular network immersed in a liquid medium. In food systems this liquid is water. The properties of the gel are the net result of the complex interactions between these two components. The water, as a solvent, influences the nature and magnitude of the intermolecular forces which maintain the integ­ rity of the polymer network; the polymer network holds the water, preventing it from flowing away. The gel-forming agents in foods are proteins and polysaccharides (see table below).

1 National Academy of Sciences (1994). Draft monograph on gellan gum for inclusion into the Food Chemicals Codex. Submitted to the USFDA in partial fulfillment of contract number 223-92-2250. June 30, 1994. 2 Adapted from Oakenfull, D. (1987). Gelling agents. Critical Reviews in Food Science and Nutrition 26(1): 1-25.

1171

GELLING AGENTS

Gel-forming Polymers Commonly Used in Foods

Polymer

Approx. cost relative to starch

Starch

1

Gelatin

5

Pectin

9

Source

Chemical composition

Potato, wheat, Polymers of rice, maize, D-glucose: amylose cassava, (linear) and sago, amylopectin arrowroot (branched) Animal skins Protein: major and bones amino acids are glycine and proline Citrus peel and apple pomace

Alginates

10

Giant kelp, Macrocystis pyrifera or species of Laminaria

Carrageenan

12

Agar

21

Seaweed, Chondrus crispus Seaweed (many different species)

Typical uses Puddings, custard, pie fillings, confectionery

Dessert jellies, cake icings, confectionery, canned meat and fish Linear polymer Jams, preserves, of partly and esterified confectionery D-galacturonic acid Copolymer of Dessert gels, D-mannuronic puddings, pie acid and fillings, and L-glucuronic artificial fruit for cakes and acid pies (withstands heat) D-galactose and “Instant” desserts and sugar-free sulfated D-galactoses dietetic jams Agaropectin (a Pie fillings linear polymer (withstands of sulfated high D-glucuronic temperatures), acid) and confectionery, agarose (a canned meat neutral and fish polymer of agarobiose)

Molecular networks in food gels In food gels, the polymer molecules are not cross-linked by cova­ lent bonds (with the exception of disulfide bonds in some protein gels). Instead, the molecules are held together by a combination of weak intermolecular forces - hydrogen bonds, electrostatic forces, Van der Waals forces, and hydrophobic interactions. The cross-linkages are not point interactions but involve extensive seg­ ments from two or more polymer molecules, usually in welldefined structures called junction zones. The gelation process is essentially the formation of these junction zones. To understand the properties of food gels we thus need to understand how junction zones form; we need to understand the molecular structures involved and the intermolecular forces that give them their stability. There is a high degree of cooperativity; individually the intermolecular forces

1172

GELLING AGENTS

are very weak but together they form a stable cross-linkage. It is important to remember that the cross-linkages are not permanent but are free to continuously break and reform. The characteristic rheological properties of gels result directly from the presence of molecular networks. Some of the measure­ ments that can be made are load (stress) and deformation (strain) for a slab of gel under compression. The rigidity of a gel (or shear modulus, G) is given as the initial slope of stress/strain and the maximum load that the gel can sustain is its rupture strength (RS). Another quantity sometimes measured is the “cohesiveness,” which is the stress produced by a strain which is half of the RS. Also because of the viscoelastic nature of the material, the strain and stress at the breakpoint depend on the rate of testing. Conse­ quently, results are best expressed as a “failure envelope.” Interactions of the gel-forming polymers with water Food gels are always formed in an aqueous environment. Thus the interactions of protein and polysaccharide molecules with water are in themselves important factors in the gelation process. Both types of polymer are strongly hydrated in aqueous solution, so that some water molecules are so tightly bound that they fail to freeze even at temperatures as low as -60°C. Proteins Native proteins have well-defined and specific secondary and tertiary structures. Gelation occurs when the conditions are such that the molecules are induced to unfold and then refold (or partly refold) in different conformations so as to form a network. The stability of protein structures arises from a combination of weak intermolecular forces. First is the hydrophobic effect, wherein nonpolar groups or molecules are surrounded by an ordered layer of water molecules. When these nonpolar molecules approach each other, some of the ordered water molecules are “squeezed out” and the molecular rearrangements that this entails provide the thermodynamic driving force for hydrophobic inter­ action. Similarly, in electrostatic interactions the hydration shells of ordered water molecules surrounding the ions are important. Association of ions involves interpenetration of these hydration shells with consequent rearrangement of water molecules. Hydrogen bonding between solute molecules in water is also pow­ erfully influenced by the solvent. These bonds are necessarily very weak because water is itself such a powerful hydrogen bond donor and acceptor. Hydrogen bonding within and between biopolymers is therefore always a cooperative process with many individually weak interactions combining to maintain the integrity of a partic­ ular structure. Polysaccharides Like proteins, polysaccharides are strongly hydrated in aqueous solution, but they tend to have less ordered structures. The polar chemical groupings that occur in polysaccha­ rides (-OH, -NHCOCH3, -O-, -COOH, -COO”, OSO3-) obviously

1173

GELLING AGENTS

interact powerfully with the polar solvent. Some polysaccharides also contain nonpolar CH3-groups introducing the possibility of hydrophobic effects. The single most important mode of interac­ tion of carbohydrates with water is by hydrogen bonding from sugar hydroxy-groups. This appears to be highly orientation dependent. Spectroscopic and thermodynamic evidence show that sugars (and polyols) interact with water to an extent that depends upon their molecular structure. The stereoisomers glucose and fructose are hydrated differently, as are mannitol and sorbitol. It is theorized that sugar molecules induce structure in the water molecules sur­ rounding them if the orientation of OH-groups is such that some of the 0 - 0 spacings correspond with the 0 - 0 distance of 4.86 A of the water lattice. The Hofmeister Lyotropic Series The effects of electrolytes on the properties of food gels and the gelation process are of obvious technological significance. The manner in which electrolytes mod­ ify the aqueous environment of macromolecules depends on the structure and constituents of the individual ions. For example, the alkali metal halides modify the environment mainly by electro­ static ion-ion and ion-dipole interactions, whereas salts such as the tetraalkylammonium halides also interact with water through hydrophobic interaction of the hydrocarbon residues on the cation. The mass of empirical observations of effects of ions on polymers such as proteins and polysacchardes has resulted in the construc­ tion of a series in which ions are listed in order of their efficiency in promoting particular effects. This ability of ions to act independently of each other in their effect on the conformation and stability of macromolecules was first studied by Hofmeister in connection with the efficiency of various salts in precipitating “englobins” from aqueous solution. Direct and specific interactions of ions are crucial to the activity of some gelling agents. Ions such as K+, Ca2+, and HPOI' can be intimately involved in the formation of cross-linkages. Some specific gel-forming systems The carrageenans The carrageenans are alternating copolymers of 1,3-linked P-D-galactose and 1,4-linked 3,6-anhydro-a-D-galactose. They are designated iota or kappa according to the relative number of sulfate ester substituents. The choice of algae from which they are prepared and the preparative technique can favor particular structures. The linkage pattern introduces a twist into the molecule, giving rise to helical structures. Gelation involves the formation of these double helices. (Single-stranded helices analogous to the protein cx-helix are not stable.) Because of the ionic nature of the polymer, gelation is strongly influenced by the presence of electrolytes, particularly potassium ions. Under suit­ able conditions of ionic strength, temperature, and polymer con­ centration, all alkali metal ions will induce gelation of iota- and

1174

GELLING AGENTS

-carrageenan. The dependence of the elasticity modulus on cation type follows a Hofmeister series: Cs+>Rb+>K+» N a \L i+, with kappa gels stronger than iota gels. This suggests that the ions, by acting as solvent structure-makers or structure-breakers, are influencing gelation through their effects on the solvent properties of water. Such effects could promote or inhibit the formation of the hydrogen bonds that stabilize the helices. These ideas are supported by the fact that anions also affect gelation and appear to influence helix formation through an effect on the solvent prop­ erties of water. Alginates Alginates are the major structural polysaccharides of brown algae of the Phaeophycea family. They have a more com­ plex structure than the carrageenans. This is based on a linear polyuronic acid backbone with three types of block structure: poly(i-D-mannuronic acid (M), poly-a-L-glucuronic acid (G), and mixed (MG) block containing both uronic acids. Full character­ ization of the polymer would, therefore, require knowledge of the M/G ratio; the ratio of M,G, and MG blocks; the detailed compo­ sition of the MG blocks; and the length distributions and relative positions of the blocks. Only the first two of these parameters have been studied in any detail, and they have been found to vary with the source of algae, growth conditions, and the age and type of tissue from which the material was extracted. Molecular weight distributions are also influenced by the source of algae and the extraction procedure, but typical weight average molecular weights are approximately 5 x 10 5. Divalent cations are required for gelation and, unlike carrageenan, the gels are not thermoreversible. Cations have to be added in a controlled manner, otherwise precipitation occurs rather than gela­ tion. There are two methods which are used industrially: (1) cal­ cium is slowly diffused into a sodium alginate solution and (2 ) calcium is slowly released homogeneously into the alginate solu­ tion. This can be done either by dispersing a sparingly soluble calcium salt (such as citrate) or slowly lowering the pH (usually by hydrolysis of glucono-de/ta-lactone) to increase the solubility of calcium phosphate. Gels produced by diffusion have inhomogeneous structures with what appear to be locally ordered micro­ crystalline regions. Alginates with a high M/G ratio form weak, turbid gels, whereas low M/G alginates give transparent, stiff, brittle gels. The gel strength depends on the nature of the divalent cation with the order Ba2+>Sr2+>Ca2+» M g 2+. Pectins Pectins are derived from the “pectic substances” that are the major structural component of the cell walls of plants. They are a class of compounds with structures based on poly (l,4)-a-Dgalacturonic acid; the detailed structure depending on the plant source and the extraction procedure. The acid groups are partly esterified with methanol and (1—>2)-L-rhamnose may be present in the polygalacturonic acid backbone. The degree of esterification (DE) varies with the age and type of plant tissue from which the

GELLING AGENTS

1175

material was extracted. Some pectins, particularly those extracted from sugar beet, may also be partly acetylated. The gelling char­ acteristics strongly depend on the DE, and for this purpose pectins can be divided into two classes: high methyoxyl (HM) pectins with degrees of esterification typically within the range 5 5 to 80%, and low methyoxyl (LM) pectins with less than 50% esterification. It is important to remember that commercial pectin samples are very heterogeneous, and that the various physical and chemical characteristics that can be measured represent average values. HM pectins form gels at low pH (below about 3.6) when a cosolute is present (typically sucrose at a concentration of greater than 5 5 % by weight). The nature of the cosolute is not critical. Gels can be prepared by adding many different sugars, polyols, or monohydric alcohols. An important difference between HM pectin gels and those formed by most of the other gel-forming polysaccharides is the key role of hydrophobic interactions in stabilizing their junc­ tion zones. The size of the junction zones, estimated from shear modulus data, increases with the square of the DE. Gelation of LM pectins occurs only in the presence of divalent cations such as calcium or strontium. As with HM pectins, hydrophobic forces may be of some significance because sugar is also required for gelation of LM pectins with degrees of esterification at the higher end of the range. The requirement of calcium, and to some extent the gel characteristics, depends on the method used for de-esterification. Acid-catalyzed de-esterification appears to occur randomly, whereas enzymic de-esterification produces blocks of fully de-esterified polymer interspersed with blocks of the original material. If de-esterification is carried out with ammo­ nia, the resulting amide groups also have profound effects on the gelling characteristics of the polymer. Starch Starch is produced by the higher plants as an energy reserve and it is found concentrated in storage organs such as seeds, tubers and roots. In the plant it consists of spherical granules with shape, size and size distributions characteristic of the particular plant species. Starch granules can be dissolved in dimethyl sulfoxide and the solution fractionated into two structurally distinct polysaccharides: amylose and amylopectin. Amylose is usually the minor compo­ nent (-20%). It is an essentially linear polymer of a - ( l—>4) linked D-glucose. Amylopectin has a highly branched structure, again of chains of a -( l—>4)linked D-glucose but with branches linked at position 6 . Gelation occurs when starch granules suspended in water are heated above a certain critical temperature (the gelatinization temperature) and then cooled. Irreversible swelling of the granules then occurs, accompanied by solubilization of amylose. Very little amylopectin is released. Heating simply produces open, porous “amylopectin” granules suspended in an amylose solution. Starch gels consequently have a composite structure of open, porous amylopectin granules threaded by an amylose matrix. The

1176

GELLING AGENTS

physical characteristics of starch gels thus depend on the rheology of the amylose matrix, the rigidity of the amylopectin granules, the volume fraction and shape of the granules, and the filler-matrix interaction. All these factors depend on the source of the starch, on processing conditions, and product formulation. Thus, the actual gel-forming polymer in starch is amylose. The molecular weight distribution of amylose depends on the plant source, and molecular weights of several millions with broad dis­ tributions have been reported. The physical properties of amylose solutions indicate a stiff, coil-like structure for the polymer. When a hot amylose solution is cooled, gelation occurs if the concentra­ tion is above a critical value (below this concentration there is precipitation rather than gelation). These changes on cooling have been termed “retrogradation.” The rate of retrogradation increases with increasing molecular weight, with a maximum at about 80 monosaccharide units. It is also sensitive to added salt with salt effects following the Hofmeister series, and suggesting that polymer-solvent interactions are important in gelation. Several explanations have been offered as to the mechanism and make-up of starch gelation. However, it is clear that starch gelation is vastly more complex than other systems so far considered. Gelatin Gelatin is a water-soluble protein formed by partial deg­ radation of collagen from animal skins and bones. Gelatin gels from reversibly on cooling a gelatin solution. It is now well estab­ lished that the protein molecules are cross-linked to form a network by junction zones where the protein chains have partly refolded a collagen triple helix structure. There is some doubt, though, as to whether these involve three separate protein chains or only two, with the helices being partly intramolecular. Calculations from shear modulus data for very dilute gelatin gels suggest that the junction zones are wholly intermolecular, at least in the initial stages of gelatin. A study of the kinetics of gelatin supports this conclusion. The reaction approaches a third order dependence on concentration in the early stages of gelation, drop­ ping to second order as the reaction proceeds. Calculations from shear modulus data also show that the average number of amino acid residues in a junction zone is 142. This represents about 16 turns of collagen triple helix structure. Globular proteins The mechanism of gelation of the globular proteins is still poorly understood, despite their greater technolog­ ical importance in meat, fish, and dairy products. Gels of globular proteins form irreversibly and usually under the influence of heat. Gelation involves denaturation or unfolding of the native protein structure followed by intermolecular association to form a gel network. There are no readily identifiable protein characteristics crucial for gelation of globular proteins. The ability to form disulfide linkages is probably not important; neither is the ability to form an extensive

GELLING AGENTS

1177

hydrogen bonded p-sheet structure. The contribution from hydrophobic forces is more difficult to assess, but there appears to be no correlation between the ability of a protein to form gels and its proportion of hydrophobic amino acids. Gelation of meat protein occurs on heating. The first step is aggre­ gation of the myosin chains by their “head” regions in a process involving the formation of disulfide bonds. A gel network then forms with the unfolding of the tail part of the molecule. Gel strength is dependent on pH, with a maximum pH 6.0. Tarry acids enhance gel strength. This appears to be because they bind to the protein causing increased electrostatic repulsion between the pro­ tein chains leading to a more ordered pattern of aggregation on heat treatment. It has also been found that myosin gels can be formed by slowly lowering the pH, the basis of this process being acid-induced denaturation. The gels vary in strength of muscle type since gels formed from white muscle myosin are stronger than those formed under otherwise identical conditions from red muscle myosin. Myofibrils form also from gels, but weaker ones than the equivalent concentration of pure myosin. The major gel-forming protein of egg white is ovalbumin. Gelation appears to be dominated by electrostatic forces. There are maxima in gel strength against pH on either side of the isoelectric point. Between these two maxima, large compact aggregates form with little development of network structure, and at the extremes of pH there is only a low tendency for aggregation. Maximum gel strength is developed under conditions where the protein has a limited capacity for aggregation within well-defined limits. Another protein important in the gelation of egg white is ovomu­ cin. This is actually a mixture of glycoproteins, one of which is an extended state brought about by highly acidic sulfate and sialic acid groups. Cross-linking appears to occur by the formation of intermolecular disulfide bonds and weaker interactions involving the carbohydrate moiety. The heat-induced gelation of aqueous dispersions of soy protein has been extensively investigated. Gelation depends on the con­ centration of protein and on the temperature and time of heating. The effects of various salts and lipids have also been studied. The results suggest that the gel network is stabilized by a combination of hydrogen bonding, hydrophobic and electrostatic interactions, and possibly disulfide linkages. Using purified soy protein frac­ tions, it has been found that the effects of anions on the develop­ ment of viscosity on heating followed the Hofmeister series with SC>4