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Food Taste Chemistry
 9780841205260, 9780841206809, 0-8412-0526-4

Table of contents :
Title Page......Page 1
Half Title Page......Page 3
Copyright......Page 4
ACS Symposium Series......Page 5
FOREWORD......Page 6
PdftkEmptyString......Page 0
PREFACE......Page 7
1 Taste and the Taste of Foods......Page 8
Anatomy of Taste Systems......Page 11
Neurophysiology of Taste Systems......Page 13
Psychophysics of Taste Systems......Page 20
Neurophysiological Correlates of Sensation......Page 23
Taste Compounds in Foods......Page 24
Alterations in Natural Nutritional Ecosystems......Page 26
Literature Cited......Page 35
Umami Substances......Page 40
Fundamental Taste Properties of Umami......Page 42
The Synergistic Effects of Umami Substances......Page 44
Factors Affecting the Synergistic Effect......Page 47
Effects of Umami Substances on Flavors of Foods......Page 48
Literature Cited......Page 57
Pungency, an appraisal of the term......Page 59
Estimation of Pungency - Problems......Page 62
Standard methods for evaluation of pungency:......Page 63
Specific Components for Pungency......Page 66
Chillies (Capsicum Sp.)......Page 67
Pepper (Piper nigrum Linn.)......Page 73
Ginger (Zingiber officinale Roscoe)......Page 79
Other spices......Page 85
Pungency in vegetables......Page 86
Structure and Pungency......Page 88
Literature Cited......Page 94
Additional Reference......Page 98
4 Sweet and Bitter Compounds: Structure and Taste Relationship......Page 99
Literatur cited......Page 136
5 Chemistry of Sweet Peptides......Page 138
Literature Cited......Page 152
6 Bitterness of Peptides: Amino Acid Composition and Chain Length......Page 154
Summary......Page 174
Literature cited......Page 175
7 Taste Components of Potatoes......Page 179
Literature Cited......Page 187
Acknowledgements......Page 188
Taste-active Components in Fish......Page 189
Organic Acids in Shellfish......Page 191
Taste-active Components in Abalone......Page 192
Free Amino Acids in Prawns and Lobsters......Page 194
Taste-active Components in Crabs......Page 197
Conclusion......Page 205
Literature Cited......Page 206
9 Flavor of Browning Reaction Products......Page 208
Amino Acids-Sugars Model Systems......Page 217
Nitrogen-heterocyclic......Page 233
Abstract......Page 242
Literature Cited.......Page 244
10 Concluding Remarks......Page 249
A......Page 253
C......Page 254
Ε......Page 255
G......Page 256
L......Page 257
Ρ......Page 258
Q......Page 259
S......Page 260
Τ......Page 261
Ζ......Page 262

Citation preview

Food

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Food Taste Chemistry James C . Boudreau,

EDITOR

University of Texas—Houston

Based on a symposium sponsored by the Division of Agricultural and Food Chemistry at the A C S / C S J Chemical Congress, Honolulu, Hawaii, A p r i l 2 - 6 , 1979.

ACS

SYMPOSIUM

SERIES

115

AMERICAN CHEMICAL SOCIETY WASHINGTON, D. C. 1979 In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Library of CongressCIPData Food taste chemistry. (ACS symposium series; 115 ISSN 0097-6156) Includes bibliographies and index. 1. Food—Analysis—Congresses. 2. Flavor—Con­ gresses. 3. Taste—Congresses. I. Boudreau, James C., 1936- . II. American Chemi­ cal Society. Division of Agricultural and Food Chemis­ try. III. Series: American Chemical Society. ACS sym­ posium series; 115. TX511.F685 ISBN 0-8412-0526-4

612'.87 ASCMC 8

79-26461 115 1-262 1979

Copyright © 1979 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating new collective works, for resale, or for information storage and retrieval systems. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto.

PRINTED IN THE UNITED

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

ACS Symposium Series M . Joa

Comstock

Series Editor

Advisory Board Kenneth B. Bischoff

James P. Lodge

Donald G . Crosby

John L. Margrave

Robert E. Feeney

Leon Petrakis

Jeremiah P. Freeman

F. Sherwood Rowland

E. Desmond Goddard

Alan C. Sartorelli

Jack Halpern

Raymond B. Seymour

Robert A . Hofstader

Aaron W o l d

James D . Idol, Jr.

Gunter Zweig

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

FOREWORD The ACS SYMPOSIUM SERIES was founded in 1974 to provide

a medium for publishin format of the Serie IN CHEMISTRY SERIES except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable since symposia may embrace both types of presentation.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

PREFACE *"ρ1ιβ symposium on which this book is based was one of the few symposia, ever, totally devoted to the chemistry of food tastes. Most previous symposia have dealt with foodflavors,where flavor is considered to consist mostly of odorous sensations. Taste has long been considered to consist of only four sensations that contribute little to most food flavors. These four feeble sensations were linked to a simplistic taste chemistry that had little relevance to modern chemistry. Thes ignore the major role tast ingestion, but also are not followed in practice by much of the flavor industry. Thus you will often discover upon reading the literature that the "odors" of a food were best, or perhaps only, realized when food was in the mouth. Many flavor chemists have found that in order to ade­ quately define a food flavor, tastes other than the four basics must be postulated. The types of taste active compounds in foods encompass much of natural product chemistry. Many of the compounds presently identified as odors are strongly taste active. To help lay a new groundwork for the study of the tastes of foods, the speakers at the symposium presented papers on a variety of topics related to food taste chemistry. The problems in taste are of great com­ plexity, involving biological as well as chemical variables. For a taste chemist, the types of sensations elicited and their measurement are as important as the nature of the compounds eliciting them. Various aspects of these problems are treated in detail in the papers in this volume. The findings presented at this symposium have relevance far outside the narrow area of flavor chemistry. The taste measurements of the chemical properties of nutrient solutions have applications that range from physical organic chemistry to human nutrition. Special thanks are due to the Japanese cochairman M . Namiki and the members of the Agricultural and Food Chemistry Division of ACS, especially G. Charalambous, R. Teranishi, and C. Mussinan for organiz­ ing and scheduling the symposium. I thank J. Oravec, Ng. Hoang, and the ACS Books Department for assistance in preparing this volume for publication. University of Texas—Houston

JAMES C. BOUDREAU

Houston, T X 77025 September 13, 1979 ix In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1 Taste and the Taste of Foods JAMES C. BOUDREAU, JOSEPH ORAVEC, NGA and THOMAS D. WHITE

KIEU HOANG,

Sensory Sciences Center, Graduate School of Biomedical Sciences, University of Texas at Houston, Houston, TX 77030

In this paper the ter tast will refe t all th chemical sensory systems of the sensory systems are intimately involved in the selection of food items and in the regulation of food intake. As we shall see, there are a variety of different taste systems attuned to different chemical aspects of food. These taste systems perform an exact and elaborate analysis of the chemical constituents in the food we eat. The structure and function of these taste systems will be discussed in the context of a natural nutritional ecosystem, i.e. one in which man is not a disruptive element. Human taste systems are assumed to have developed to function in this natural system and to have changed little as a result of the cultural dietary changes that have occurred in the last 10- 20,000 years. The natural nutritional ecosystem of man is assumed to be one in which both plant and animal foods are eaten (Figure 1) and they are eaten raw. In a n a t u r a l n u t r i t i o n a l ecosystem, t a s t e serves a primary r o l e i n r e g u l a t i n g the flow of compounds (_1, 2 ) . C e r t a i n t h i n g s are to be eaten by us. Other things by o t h e r s . There e x i s t s wide v a r i a t i o n i n the t a s t e s o f n a t u r a l foods. Thus Cott has shown t h a t c e r t a i n b i r d s and t h e i r eggs are both conspicuous and i l l t a s t i n g (3, 4 ) . The types o f foods we consume now represent a s e l e c t i o n from the vast a r r a y of items n a t u r a l l y a v a i l a b l e during our e v o l u t i o n a r y development. The chicken egg f o r i n s t a n c e , represents a s e l e c t i o n of one o f the best t a s t i n g eggs n a t u r a l l y a v a i l a b l e (Figure 2 ) . We consume and transform p l a n t and animal substances t o promote c e r t a i n p h y s i o l o g i c a l a c t i v i t i e s (the probable r o l e o f t a s t e i n mammalian sexual behavior i s not considered h e r e ) . Primary among these p h y s i o l o g i c a l f u n c t i o n s i s the replacement of body compounds and the supply of compounds f o r metabolic energy systems. Thus t a s t e serves t o r e g u l a t e the consumption of needed compounds. Almost without e x c e p t i o n , n a t u r a l t h i n g s that t a s t e good are good f o r you and foods that are needed t a s t e good even though your stomach i s f u l l . Toxic compounds almost 0-8412-0526-4/79/47-115-001$08.00/0 © 1979 American Chemical Society In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2

FOOD TASTE CHEMISTRY

Figure 1.

Flow diagram of the natural nutritional ecosystem of the human (simplified)

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1.

BOUDREAU ET AL.

The Taste of Foods

3

Proceedings of the Zoological Society of London Figure 2.

Preferences of man ( #—·

), rat (O

O), and hedgehog

(O - · · O) for some eggs of different species of birds (A).

The species

Gallus gallus is the chicken.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

FOOD TASTE CHEMISTRY

4

i n v a r i a b l y have noxious t a s t e s . One exception ( o m i t t i n g marine substances) i s the poisonous Amanita phaloides mushroom, a fungus that t a s t e s good but w i l l k i l l you. Not o n l y do t o x i c foods have noxious t a s t e s , but the thresholds f o r many t o x i c substances are extremely low. Another p o s s i b l e f u n c t i o n of t a s t e i s the i n g e s t i o n of compounds f o r the r e g u l a t i o n of body temperature. Although there seems to e x i s t l i t t l e hard data on t h i s matter, many human c u l t u r e s c l a s s i f y foods i n t o those that warm the body and those that c o o l i t C5, 6 ) . Taste may a l s o f u n c t i o n i n the s e l e c t i o n of pharmacologically a c t i v e compounds f o r good h e a l t h or good f e e l i n g . Things that t a s t e good o f t e n make you f e e l good. In a d d i t i o n , many f l a v o r compounds have a n t i m i c r o b i a l a c t i o n s and other pharmacological p r o p e r t i e s . Anatomy of Taste Systems A t a s t e system ca receptor element f o r the t r a n s d u c t i o n of chemical s i g n a l s , a p e r i p h e r a l sensory n e u r a l system f o r the c o l l e c t i o n and t r a n s m i s s i o n of chemical n e u r a l i n f o r m a t i o n , and a complex c e n t r a l nervous system f o r the a n a l y s i s of t h i s sensory n e u r a l i n f o r mation (7). The chemoreceptors that have been described i n the o r a l c a v i t y are of two b a s i c m o r p h o l o g i c a l types: f r e e nerve endings and t a s t e buds. The s o - c a l l e d " f r e e nerve endings" are d i s t i n g u i s h e d on the b a s i s of l i g h t microscopy as possessing no r e c o g n i z a b l e r e c e p t o r or encapsulated ending. These f r e e nerve endings are found throughout the o r a l c a v i t y and are responsive to a v a r i e t y of chemical compounds. A t a s t e bud, on the other hand, i s a receptor n e u r a l complex c o n s i s t i n g of nerve f i b e r s and 20-50 s p e c i a l i z e d c e l l s organized i n a f a i r l y elaborate manner (Figure 3 ) . The elongated t a s t e bud c e l l s are grouped together w i t h one end forming the f l o o r of the t a s t e p i t which opens up, through the t a s t e pore, to the o r a l f l u i d s . The t a s t e bud c e l l s p r o j e c t i n t o the t a s t e pore w i t h e i t h e r m i c r o v i l l i or an elongated bulb. The t a s t e bud c e l l s have been c l a s s i f i e d m o r p h o l o g i c a l l y i n t o three or more d i s t i n c t types (8-12). Taste buds, u n l i k e f r e e nerve endings, are not d i s t r i b u t e d throughout the o r a l c a v i t y but r a t h e r are on the dorsum of the tongue, the s o f t p a l a t e , pharynx, e p i g l o t t i s , l a r y n x and upper t h i r d of the esophagus (Figure 3 ) . On the tongue, t a s t e buds are l o c a l i z e d on protuberances known as p a p i l l a e . The t a s t e buds on the f r o n t two t h i r d s of the tongue are l o c a t e d on the d o r s a l surface of the small fungiform p a p i l l a e . At the rear of the tongue the t a s t e buds are l o c a t e d i n the f o l i a t e p a p i l l a e and the v a l l a t e p a p i l l a e . The p o s t e r i o r l y l o c a t e d chemosensory complexes c o n t a i n l a r g e numbers of t a s t e buds together w i t h s p e c i a l i z e d s e c r e t o r y glands. The p e r i p h e r a l sensory neurons that supply the chemo* receptors i n the o r a l c a v i t y r e s i d e i n four d i s t i n c t c r a n i a l g a n g l i a (Figure 4 ) . The t r i g e m i n a l ganglion contains the sensory

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1. BOUDREAU ET AL.

The Taste of Foods

5

Figure 3. Location of some oral chemosensory receptor systems. Taste buds (schematic upper right) are found on specialized papillae on the tongue and scattered on the palate and posterior oral structures. Free nerve endings are found on all oral surfaces (94).

Figure 4. Peripheral sensory ganglia that supply nerve endings to taste buds in the mammalian oral cavity. Trigeminal ganglion, which supplies free nerve endings to all oral surfaces, not shown.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

FOOD TASTE CHEMISTRY

6

neurons p r o v i d i n g f r e e nerve endings to a l l p a r t s of the o r a l c a v i t y . The other three sensory g a n g l i a i n n e r v a t e the t a s t e buds, w i t h each g a n g l i o n i n n e r v a t i n g buds on d i s t i n c t l o c a t i o n s . The t a s t e buds on the fungiform p a p i l l a e and the a n t e r i o r s o f t p a l a t e are innervated by sensory neurons i n the g e n i c u l a t e ganglion of the f a c i a l nerve. The t a s t e buds on the f o l i a t e p a p i l l a e , the c i r c u m v a l l a t e p a p i l l a e , the p o s t e r i o r p a l a t e , the t o n s i l s and the fauces are innervated by c e l l s i n the petrous g a n g l i o n of the glossopharyngeal nerve. Taste buds on the e p i g l o t t i s , the l a r y n x and the upper t h i r d of the esophagus are innervated by neurons i n the nodose g a n g l i o n of the vagus nerve. P h y s i o l o g i c a l and psychophysical s t u d i e s on the f u n c t i o n a l p r o p e r t i e s of these d i f f e r e n t nerves and g a n g l i a i n d i c a t e t h a t the chemosensory systems i n the d i f f e r e n t g a n g l i a are s e l e c t i v e l y responsive to d i f f e r e n t chemical aspects of foods. Neurophysiology

of Tast

In examining the f u n c t i o n of t a s t e systems,various p h y s i o l o g i c a l measures are a v a i l a b l e to the i n v e s t i g a t o r . Although, t h e o r e t i c a l l y the n e u r o p h y s i o l o g i c a l responses of e i t h e r the r e c e p t o r s or from any of the neurons i n the s e n s o r i n e u r a l c h a i n may be u t i l i z e d , i n p r a c t i c e , the most exact procedure i s to measure the p u l s e t r a i n s being t r a n s m i t t e d from the p e r i p h e r y to the c e n t r a l nervous system (Figure 5 ) . Receptor p o t e n t i a l s are s u b j e c t to s e v e r a l sources of e r r o r , at l e a s t as regards q u a n t i t a t i v e measures of n e u r a l responses. For the p r e c i s e study of n e u r a l responses to chemical s t i m u l a t i o n the n e u r a l pulse i s the measure of choice i n sensory neurophysiology. These pulses are p r e f e r a b l y measured from p e r i p h e r a l sensory neurons s i n c e n e u r a l i n t e r a c t i o n i s minimized. Pulses may be measured from e i t h e r the p e r i p h e r a l f i b e r s of the sensory ganglion c e l l s or from the somas. One advantage i n r e c o r d i n g pulses from the c e l l s themselves i s that s m a l l f i b e r systems are sampled, w h i l e there i s a s t r o n g b i a s toward l a r g e f i b e r p o t e n t i a l s i n f i b e r r e c o r d i n g s . The o n l y ganglion c e l l system that has been examined i n any d e t a i l i s the g e n i c u l a t e ganglion system which innervates r e c e p t o r s on the fungiform p a p i l l a e . The p r o p e r t i e s of these neurons w i l l be reviewed f o r the c a t , dog, and goat. T y p i c a l l y , a g e n i c u l a t e ganglion c e l l i n n e r v a t e s receptors on more than one fungiform p a p i l l a (Figure 5 ) . The number of fungiform p a p i l l a e innervated by a -single neuron ranges from one to as many as twelve. W i t h i n a t a s t e bud, a nerve f i b e r w i l l contact many receptor c e l l s . Almost a l l g e n i c u l a t e ganglion neurons e x h i b i t p u l s e a c t i v i t y i n the absence of experimenter designed s t i m u l a t i o n (Figure 6 ) . This "spontaneous a c t i v i t y " i s u s u a l l y of a complex i r r e g u l a r type that i s c h a r a c t e r i s t i c of chemical sensory systems. Pulses are o f t e n emitted i n b u r s t s w i t h f i x e d i n t e r s p i k e i n t e r v a l s , w i t h the b u r s t i n t e r v a l

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

BOUDREAU ET AL.

The Taste of Foods

JJ1JJ

L SIGNAL

TASTE BUDS

GENICULATE GANGLION VIA

LINGUAL A N D C H O R D A TYMPANI NERVES

\JD>-

[ED

EM

GANGLION CELL

STIMULUS INPUT A N D RECEPTORS

C.N.S.

Figure 5. (lower) Diagram of the peripheral and central connections of a sensory ganglion cell innervating the taste buds of the tongue, (upper) Illustration of the connections of sensory ganglion cells and the pulse signals used to encode sensory information.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

FOOD TASTE CHEMISTRY

SPONTANEOUS

ACTIVITY

L - MALIC ACID

ANSERINE

NITRATE

GROUP I

L - CYSTEINE

L -

iaOLEUCINE

SPONTANEOUS

ACTIVITY

INOSINE - 5' - TRIPHOSPHATE

BUTYRYL

CHOLINE

CHLORIDE

200.0 MSEC Figure 6. Spontaneous and evoked spike activity recorded from taste neurons of the geniculate ganglion of the cat. The chssification of the three different sensory neurons is indicated by Groups I, II, and III.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1.

BOUDREAU ET AL.

The

Taste of Foods

9

decreasing as f u n c t i o n of i n t e r v a l order. T h i s spontaneous a c t i v i t y may be i n h i b i t e d by the a p p l i c a t i o n of a chemical s o l u t i o n to the tongue, or the neuron may be e x c i t e d by a d i f f e r e n t s o l u t i o n . M i x i n g an i n h i b i t o r y compound i n t o an e x c i t a t o r y s o l u t i o n may r e s u l t i n an i n h i b i t o r y s o l u t i o n . The neuron may be e x c i t e d or i n h i b i t e d by chemical s t i m u l a t i o n of a s i n g l e p a p i l l a i n i t s p a p i l l a e system (13), and e x c i t a t i o n of two p a p i l l a e simultaneously may r e s u l t i n an increase i n d i s c h a r g e , w i t h the i n c r e a s e being u s u a l l y l e s s than a l g e b r a i c . The discharge r e s u l t i n g from the e x c i t a t i o n of the neuron by s t i m u l a t i o n of one p a p i l l a may be i n h i b i t e d by the simultaneous s t i m u l a t i o n of another p a p i l l a w i t h an i n h i b i t o r y s o l u t i o n . Neurons of the g e n i c u l a t e ganglion have been found to be of more than one type when examined i n terms of v a r i o u s p h y s i o l o g i c a l measures such as spontaneous a c t i v i t y r a t e and type, l a t e n c y of s p i k e discharge to e l e c t r i c a v e l o c i t y and thus f i b e Neurons i n both the cat and the dog can be c l a s s i f i e d i n t o at l e a s t three d i f f e r e n t groups (10). Neurons i n the goat have a l s o been t e n a t i v e l y d i v i d e d i n t o three d i f f e r e n t groups, although only one of these groups i s comparable to those i n the two c a r n i v o r e s . The n e u r a l groups i n the c a t , goat, and dog tend to p r e f e r e n t i a l l y innervate fungiform p a p i l l a e on somewhat d i f f e r e n t p a r t s of the tongue (Figure 7 ) , although there i s extensive o v e r l a p , e s p e c i a l l y i n the dog. The determination of the types of compounds s t i m u l a t i n g g e n i c u l a t e ganglion neurons c o n s t i t u t e s an extensive f i e l d of c o n t i n u i n g i n v e s t i g a t i o n . Not s u r p r i s i n g l y , i t has been found that many of the neurons are s e n s i t i v e to s o l u t i o n s of foods commonly present i n the animal's environment. Thus a goat neuron may respond to a c a r r o t or herb .solution and a cat to chicken or l i v e r . Cats and dogs have been found to be h i g h l y responsive to many of the compounds found i n meats, such as amino a c i d s , the d i p e p t i d e s , anserine and carnosine, and n u c l e o t i d e s . The goat has been l e s s w e l l i n v e s t i g a t e d , but seems h i g h l y responsive to s a l t s and a l k a l o i d s . E s p e c i a l l y prominent i n the s t i m u l a t i o n of the c a r n i v o r e are n i t r o g e n and s u l f u r compounds, e s p e c i a l l y f i v e and s i x member r i n g h e t e r o c y c l e s . The d i f f e r e n t n e u r a l groups tend to be d i f f e r e n t i a l l y responsive to chemical s t i m u l i , i l l u s t r a t i n g t h e i r s e l e c t i v i t y i n the measurement of food compounds (Figure 8 ) . Some of the s i m i l a r i t i e s and d i f f e r e n c e s among the g e n i c u l a t e ganglion n e u r a l groups of the c a t , dog, and goat are summarized i n Table I . As evident i n t h i s t a b l e , the dog g e n i c u l a t e ganglion systems are q u i t e s i m i l a r to the c a t . One major d i s t i n c t i o n i s t h a t the dog amino a c i d s e n s i t i v e neurons ( c l a s s A u n i t s ) , although h i g h l y s i m i l a r to cat group I I u n i t s , are a l s o responsive to sugar as w e l l as the most s t i m u l a t i n g amino a c i d s and d i - and t r i - phosphate n u c l e o t i d e s a l t s . The goat n e u r a l groups on the o t h e r hand seem q u i t e d i s t i n c t from c a r n i v o r a l t a s t e systems w i t h only the a c i d responsive group

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

FOOD TASTE CHEMISTRY

Figure 7. Peripheral innervation of fungiform chemoreceptors by neurons of the eniculate ganglion of three different speoies. In each species the neurons have een separated into three distinct neural groups (see Table I for comparison of chemical stimuli for the different neural groups).

f

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

BOUDREAu ET AL.

The Taste of Foods

GROUP I

NH

HO; L-H1STIDINE GROUP M H

2

2

0

π

NH HSCH CHO0 H

Û

2

^Νγ(:θ Η 2

H

MORPHOLINE

PYRROLIDINE

L-CYSTEINE L-PROLINE GROUP III OH I 0=P—OH

CCH,CH,CH(CH , CH, 2 3

y-OCTAL AC TONE

HO

OH

HOCH (CH ) CHO

MP

5-HYDROXYPENTANAL

2

2

3

p r =

-N.

o H

CH

HO

NH

2

OH ITP

Figure 8. Chemical formulas for some of the most active stimuli for the cat geniculate ganglion neural groups

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

12

FOOD TASTE CHEMISTRY

Table I : Summary o f N e u r o p h y s i o l o g i c a l I n v e s t i g a t i o n s on Mammalian G e n i c u l a t e Ganglion Taste Systems.

Species CAT

Human Sensations

C e l l Groups and S t i m u l i

Sour

Grp. I : Organic A c i d s and H i s t i d i n e Compounds. A l s o t o A l k a l o i d s Grp. I I : Amin a c i d responsive triphosphat NaCl p o t e n t i a t e

di

d

Grp. I l l : Α-Nucleotide Responsive B-Carbonyl Responsive DOG

(Pleasant?)

C l a s s A: L i k e Cat grp. I I , except respond t o sugars

Sweety Bitter

Class B: L i k e Cat Group I

Sour

Class C: GOAT

Sweety

P a r t i a l l y l i k e Cat Grp. I l l

x

-

Grp.

: L i k e c a t grp. I & dog grp. B, o n l y a l s o respond t o S a l t s

Sour

Grp.

: Respond to NaCl and L i C l

(Salty?)

Grp.

: A l k a l o i d s plus?



In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1. BOUDREAU ET AL.

The Taste of Foods

13

common w i t h the other two s p e c i e s . No t a s t e system h i g h l y responsive t o amino a c i d s has been found i n the goat; and, u n l i k e the carnovore, l a r g e numbers o f neurons, i n c l u d i n g the a c i d responsive system, a r e discharged by NaCl. Although the other major tongue t a s t e system - t h a t represented by the petrous ganglion o f the glossopharyngeal nervehas been examined i n mammals by s e v e r a l i n v e s t i g a t o r s , they have not u t i l i z e d a s u f f i c i e n t number o f chemical compounds f o r us t o determine t h e i r p o s s i b l e r o l e i n the measurement o f food compounds. One study (15) i n the f r o g glossopharyngeal nerve (not d i r e c t l y comparable t o mammalian) found that many compounds assumed t o be f u n c t i o n i n g as odors were strong s t i m u l i . Thus a c t i v e s t i m u l i were found t o be^l-ionone, I - o c t a n o l , s k a t o l , isoamyl a c e t a t e , e t h y l b u t y r a t e , coumarin, phenol and s i m i l a r compounds. Psychophysics

o f Taste System

The second type o f study which has c o n t r i b u t e d t o our under standing o f the f u n c t i o n a l p r o p e r t i e s o f o r a l chemoreceptor systems i s human psychophysics, where v e r b a l r e p o r t s a r e taken on the t a s t e p r o p e r t i e s o f food and beverages and t h e i r chemical c o n s t i t u e n t s . I t i s o f t e n p o s s i b l e f o r an i n d i v i d u a l t o break a f l a v o r complex down i n t o a v a r i e t y o f d i s t i n g u i s a b l e s e n s a t i o n s . These sensations a r e end products o f n e u r a l p r o c e s s i n g t h a t a r e a v a i l a b l e t o consciousness. Any n a t u r a l food i s o f complex chemical composition and thus a c t i v a t e s a wide v a r i e t y of o r a l and n a s a l chemoreceptors. These f l a v o r sensations may a r i s e e n t i r e l y from t h e o r a l c a v i t y or r e q u i r e both o r a l and n a s a l stimulation. Although i t i s common t o a s s e r t t h a t there a r e o n l y f o u r d i s t i n c t t a s t e s e n s a t i o n s , even a c a s u a l i n t r o s p e c t i o n r e v e a l s that other o r a l sensations can be d i s t i n g u i s h e d . As one may expect, f l a v o r chemists have discovered t h a t many separate o r a l sensations a r e r e q u i r e d t o r e c o n s t r u c t the f l a v o r s o f foods and beverages. Some o f these sensations have d i s t i n c t o r a l l o c i from which they a r e e l i c i t e d by s p e c i f i e d types o f chemical compounds, thus i n d i c a t i n g t h a t d i f f e r e n t n e u r a l systems a r e i n v o l v e d . Many of these sensations a r e d i f f i c u l t t o t y p i f y v e r b a l l y and a l s o o f t e n have a f f e c t i v e overtones. These sensations a r e the r e s u l t of c o n s i d e r a b l e p e r i p h e r a l and c e n t r a l n e u r a l processing and a r e only i n d i r e c t l y r e l a t e d t o the p e r i p h e r a l n e u r a l p u l s e s i g n a l s as discussed above. The type o f s e n s a t i o n e l i c i t e d and the l o c u s o f e l i c i t a t i o n provide us w i t h f u r t h e r measures o f the f u n c t i o n a l p r o p e r t i e s o f o r a l chemoreceptor systems. Studies on human t a s t e sensations confirm and extend our understanding o f t h e types o f chemical s i g n a l s measured by these o r a l chemoreceptor systems. There a r e , f o r i n s t a n c e , s e v e r a l d i s t i n c t sensations e l i c i t e d by chemical s t i m u l a t i o n o f fungiform p a p i l l a e innervated by t h e g e n i c u l a t e g a n g l i o n , i n d i c a t i n g t h a t a n e u r a l f u n c t i o n a l complexity s i m i l a r t o t h a t d e s c r i b e d above f o r

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

14

FOOD TASTE CHEMISTRY

the c a t , dog and goat u n d e r l i e s these human t a s t e systems. Table I I summarizes some of the d i f f e r e n t types of t a s t e sensations t h a t can be e l i c i t e d by chemical s t i m u l a t i o n of the human o r a l c a v i t y and the types of chemical compounds found to e l i c i t them. I n some cases i t i s p o s s i b l e to a s s i g n a s e n s a t i o n to a p a r t i c u l a r g a n g l i o n because of the l o c u s of e l i c i t a t i o n . The sensations i n t h i s t a b l e c o n s i s t of o n l y some of the more commonly e l i c i t e d sensations and those w i t h some degree of experimental s p e c i f i c a t i o n . The amount of i n f o r m a t i o n a v a i l a b l e v a r i e s w i d e l y f o r the d i f f e r e n t s e n s a t i o n s , and some of the sensations i n t h i s t a b l e may not be c l e a r l y d i f f e r e n t i a t e d from one another. Four of the sensations commonly d i s t i n g u i s h e d are the s a l t y , sour, sweet, and b i t t e r s e n s a t i o n s . A l l of these sensations can be e l i c i t e d from the fungiform p a p i l l a e systems. The s a l t y s e n s a t i o n i s a s s o c i a t e d w i t h r e l a t i v e l high c o n c e n t r a t i o n f i n o r g a n i c i o n s (16,17) s e n s a t i o n i s e l i c i t e d by t h a t proton donating n i t r o g e n groups may be a c t i v e a t n e u t r a l pH (18-20). Sweety and b i t t e r ^ have been given s u b s c r i p t s to d i s ­ t i n g u i s h them from s i m i l a r sensations e l i c i t a b l e from the back of the mouth. Sweety i s evoked by s o l u t i o n s of low c o n c e n t r a t i o n s of i n o r g a n i c s a l t s , s u g a r s , and v a r i o u s n i t r o g e n compounds, e s p e c i a l l y amino a c i d s (21) such as L-hydroxyproline and L - a l a n i n e . B i t t e r ^ can be a s s o c i a t e d w i t h hydrophobic amino a c i d s (22) and a l k a l o i d s . The s e n s a t i o n of pleasant i s p o s t u l a t e d on the b a s i s of cat neurophysiology and human psychophysics. The pleasant s e n s a t i o n i s assumed to a r i s e from the s t i m u l a t i o n of a. s m a l l f i b e r g e n i c u l a t e g a n g l i o n system. The s t i m u l i e l i c i t i n g the pleasant s e n s a t i o n are l a c t o n e s and other carbon-oxygen compounds (23). The general i n d i s t i n c t n e s s of the pleasant s e n s a t i o n i s assumed to be a s s o c i a t e d w i t h the a c t i v a t i o n of extremely s m a l l f i b e r systems. The sensations sweet2 and b i t t e ^ can be d i s t i n g u i s h e d because they are e l i c i t e d from p o s t e r i o r o r a l l o c i by s t i m u l i d i s t i n c t from those a c t i n g on the f r o n t (17,24). Dihydrochalcones are a c t i v e s t i m u l i f o r sweet2;and bitter£ s e n s a t i o n i s e l i c i t e d by c e r t a i n s a l t s l i k e MgS04, and probably v a r i o u s polyphenols. Addi­ t i o n a l "sweet" and " b i t t e r " sensations could probably be d i s ­ t i n g u i s h e d . The sweet t a s t i n g p r o t e i n s thaumatin and m o n e l l i n have been found to maximally s t i m u l a t e fungiform p a p i l l a e on the l a t e r a l edge of the tongue as opposed to sucrose which s t i m u l a t e s the t i p (25). C e r t a i n foods seem to e l i c i t a b i t t e r s e n s a t i o n l o c a l i z e d to the f o l i a t e p a p i l l a e . "Umami" i s the Japanese word used to d e s c r i b e the s e n s a t i o n e l i c i t e d by compounds such as monosodium glutamate, sodium i n o s i n a t e , sodium guanylate, and i b o t e n i c a c i d (26-29). The umami s e n s a t i o n i s sometimes t r a n s l a t e d as the s e n s a t i o n of " d e l i c i o u s ness". The p o s s i b i l i t y of more than one umami s e n s a t i o n e x i s t s , s i n c e the monophosphate n u c l e o t i d e s s t i m u l a t e f a r back i n the o r a l

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Pleasant

Sweet

Bitter

Astringent

Pungent

Umami}

Umami2

Metallic

5.

6.

7.

8.

9.

10.

11.

12.

2

Bitte^

4.

2

Ant. Tongue, P a l a t e

Sweet ι

3.

Tongue

Post. Mouth

Tongue

Oral Cavity

Oral Cavity

Post. Tongue

Post. Tongue

Ant. Tongue, P a l a t e

Ant. Tongue, P a l a t e

Ant. Tongue, P a l a t e

Sour

2.

Ant. Tongue, P a l a t e

Salty

Locus

1.

Sensation

Silver

Nitrate

IMP, GMP

Monosodium Glutamate

Capsaicin

Theaflavin

MgS04, P h e n o l i c s

Dihydrochalcone

Lactones

L-Tryptophan

L - A l a n i n e , Fruc to s e

Malic Acid

NaCl, KC1

Stimuli

Taste buds (?)

?

?

Free Nerve

Free Nerve

Taste buds

Taste buds

Taste buds

Taste buds

Taste buds

Taste buds

Taste buds

Receptor

Table I I : P a r t i a l Summary o f some Human Taste Sensations

Petrous

?

Trigeminal

Trigeminal

Petrous

Petrous

Geniculate

Geniculate

Geniculate

Geniculate

Geniculate

Ganglion

16

FOOD TASTE CHEMISTRY

c a v i t y . These umami sensations may be i n d i s t i n c t and d i f f i c u l t to c h a r a c t e r i z e . I t i s probable t h a t they a r i s e from s m a l l f i b e r sensory systems. Compounds a c t i n g s i m i l a r l y to MSG a r e L - c y s t e i n e - S - s u l f o n i c a c i d , L-homocysteic a c i d , and muscimol (22). The m e t a l l i c s e n s a t i o n a r i s e s from s t i m u l a t i o n w i t h c e r t a i n m e t a l l i c s a l t s , such as s i l v e r n i t r a t e , and i s a l s o a s s o c i a t e d w i t h l-octen-3-one (30). M e t a l l i c t a s t e sensations may a l s o a r i s e w i t h p a t h o l o g i e s o f the glossopharyngeal nerve. Two o f the sensations a s s o c i a b l e w i t h t r i g e m i n a l ganglion systems are the pungent s e n s a t i o n and the a s t r i g e n t s e n s a t i o n . These sensations a r e e l i c i t a b l e from much o f the o r a l c a v i t y . The compounds e l i c i t i n g the pungent s e n s a t i o n (31) a r e found i n c h i l l i e s , pepper, g i n g e r , mustard and h o r s e r a d i s h . Pungent compounds i n c l u d e p i p e r i n e , c a p s a i s i n , g i n g e r o l , and s i n i g r i n . Common s t i m u l i f o r the a s t r i g e n t s e n s a t i o n (32-34), i n c l u d e many polyphenols such as those found i n f r u i t s c i d e r t e a s wines and beer. Other chemical sensation trigemina ganglion i n c l u d e temperature sensations such as the coolness (35) a s s o c i a t e d w i t h menthol and the heat a s s o c i a t e d w i t h c a p s a i s i n . O r a l sensations e l i c i t e d by chemical s o l u t i o n s a l s o i n c l u d e t a c t i l e sensations such as smooth, dry, o r powdery, and such d i s a g r e e a b l e sensations as p a i n . Some o f these sensations may represent d i f f e r e n t degrees o f a c t i v a t i o n o f a s i n g l e system o r the a c t i v a t i o n of s e v e r a l separate systems. Many other o r a l chemical sensations may be d i s t i n g u i s h e d , although i n general there has been l i t t l e study of them, t h e i r l o c u s of e l i c i t a t i o n o r t h e i r chemical s t i m u l u s determinants. Sensations such as yeasty, n u t t y , soapy, f r u i t y , papery, a c r i d , a c i d (as d i s t i n g u i s h e d from sour) a r e o f t e n d i s t i n g u i s h e d (36-38). Taste sensations w i t h d i s t i n c t hedonic tones (23,39) such as sweetish, creamy, coconut, peachy, and so f o r t h a r e o f t e n e l i c i t e d by p h e n o l i c compounds such as v a n i l l i n and v a r i o u s oxygen h e t e r o c y c l e s (e.g. e t h y l m a l t o l ) , e s p e c i a l l y l a c t o n e s . Disagreeable o r a l sensations such as b u r n t , stale,, t a i n t e d and noxious a r e o f t e n reported. Many o f these sensations doubtless i n c l u d e a n a s a l component. N e u r o p h y s i o l o g i c a l C o r r e l a t e s of Sensation D i f f e r e n t sensations may a r i s e because of the a c t i v a t i o n o f two d i s t i n c t n e u r a l t a s t e systems (e.g. c a t group I and group I I ) , or the d i f f e r e n t i a l a c t i v a t i o n o f a s i n g l e system. D i f f e r e n t i a l a c t i v a t i o n o f the same system could occur when d i f f e r e n t segments of an ordered p o p u l a t i o n a r e a c t i v a t e d by d i f f e r e n t chemicals o r when one chemical compound e x c i t e s the n e u r a l group and another i n h i b i t s . On the b a s i s o f neurophysiology on the c a t , dog and goat g e n i c u l a t e g a n g l i a , the n e u r o p h y s i o l o g i c a l c o r r e l a t e s o f sour, sweety, and b i t t e r ^ can be p o s t u l a t e d w i t h some degree o f

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1.

BOUDREAU ET AL.

The

Taste of Foods

17

c e r t a i n t y . The sour s e n s a t i o n a r i s e s when a l a r g e f i b e r subgroup i s a c t i v a t e d . T h i s l a r g e f i b e r system i s a general mammalian system found i n c a t s , dogs, humans, and goats. The s t i m u l i a c t i v a t i n g t h i s system are s i m i l a r i n a l l species s t u d i e d . The system i s h i g h l y responsive to a c i d i c compounds, e s p e c i a l l y c a r b o x y l i c and phosphoric a c i d s . The s t i m u l i f o r the a c t i v a t i o n of t h i s system can be t y p i f i e d as Brtfnsted a c i d s (14). At n e u t r a l pH, c a r b o x y l i c and phosphoric a c i d groups are d i s s o c i a t e d and hence n o n s t i m u l i . Most of the food eaten by animals i s near n e u t r a l i t y and the most a c t i v e compounds w i l l be n i t r o g e n Br^nsted a c i d s . Thus the cat and dog sour system w i l l be s t i m u l a t e d by n a t u r a l s t i m u l i such as carnosine and h i s t i d i n e , and the goat by v a r i o u s p l a n t compounds i n c l u d i n g a l k a l o i d s (Table 1). The sourness of n i t r o g e n compounds has been l i t t l e s t u d i e d , although meat (40) has been reported to e l i c i t a sour s e n s a t i o n and L - h i s t i d i n e a c t i v stimulu i th do d cat e l i c i t s a s m a l l sour s e n s a t i o The sweet s e n s a t i o n i s a s s o c i a t e d w i t h the a c t i v a t i o n of a g e n i c u l a t e ganglion n e u r a l group (cat group I I , dog group A) and the b i t t e r s e n s a t i o n w i t h the i n h i b i t i o n of t h i s same n e u r a l group (42). The equating of t h i s c a r n i v o r e system w i t h the human s w e e t - b i t t e r sensations i s made on the b a s i s of s i m i l a r i t y of s t i m u l i , e s p e c i a l l y amino a c i d s (42). Amino a c i d s that s t i m u l a t e the cat and dog system tend to t a s t e sweet, those t h a t i n h i b i t t a s t e b i t t e r . I n both the cat and the dog t h i s system i s a c t i v a t e d by NaCl and KC1 compounds which t a s t e sweet i n low concentration. In the dog and the human, t h i s system i s a c t i v a t e d by sugars. The p r o p e r t i e s of sweet and b i t t e r n i t r o g e n compounds have been described by Wieser et a l . (43). I t i s evident that t h i s system i s s p e c i a l l y modified f o r the d i f f e r e n t s p e c i e s , e.g., w i t h d i s t i n c t i o n s among the types of amino a c i d s s t i m u l a t i n g the system. No amino a c i d s e n s i t i v e system seems present i n the goat, thus making the human more l i k e the cat and dog than the goat. As i n d i c a t e d elsewhere, there i s evidence f o r a p o s s i b l e c o r r e l a t e of a pleasant s e n s a t i o n i n a cat u n i t group, but t h i s system has been l i t t l e i n v e s t i g a t e d i n e i t h e r the cat or i n other s p e c i e s . Although the goat has a system that responds maximally to Na and L i s a l t s , t h i s system has not been seen i n the c a r n i v o r e . The chemical s t i m u l i e l i c i t i n g the human s e n s a t i o n of s a l t y are s a l t s i n r e l a t i v e l y high c o n c e n t r a t i o n , concentrations that i n other species may s t i m u l a t e more than one group. Taste Compounds i n Foods In d i s c u s s i n g n a t u r a l t a s t e compounds one faces a dilemma. On the one hand almost every compound o c c u r r i n g i n nature i s a p o s s i b l e t a s t e compound, e s p e c i a l l y i f i t i s a t a l l water s o l u b l e . A v a s t number of p o s s i b l e t a s t e compounds i s thus arrayed before us. On the other hand, r e l a t i v e l y few food compounds have been

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

18

FOOD TASTE CHEMISTRY

studied for t h e i r taste properties. Invariably, v o l a t i l e compounds are assigned the r o l e of o l f a c t o r y s t i m u l i , even though o f t e n the compounds must be put i n t o the mouth to produce the f l a v o r . In o n l y a few cases has there been any r e c o g n i t i o n of t a s t e p r o p e r t i e s of v o l a t i l e f l a v o r products. Therefore, a t the present time there e x i s t s l i t t l e knowledge of the t a s t e a c t i v i t i e s of many n a t u r a l f l a v o r compounds; and i n assembling any l i s t of t a s t e a c t i v e compounds one o f t e n must operate p a r t l y on c o n j e c t u r e . Man i s capable of l i v i n g on an a l l p l a n t or a l l animal d i e t , although omnivory i s most common i n human s o c i e t i e s . Animal foods may be d i v i d e d i n t o two major c a t e g o r i e s as f a r as we are concerned: v e r t e b r a t e and i n v e r t e b r a t e . Although i n v e r t e b r a t e animals may p l a y a s i g n i f i c a n t p a r t i n the d i e t of many human s o c i e t i e s , there has been l i t t l e work on i n v e r t e b r a t e t a s t e chemistry (except f o r s h e l l f i s h ) from a human consumption standpoint. S h e l l f i s h tast organic a c i d s , amino a c i d Hunting and c a r n i v o r y are found i n humans, baboons and chimpanzees (45). For perhaps 2 m i l l i o n y e a r s , man has k i l l e d and eaten a l l v a r i e t i e s of v e r t e b r a t e s . As the u l t i m a t e carnivorous ape, man has exterminated most of the l a r g e animals of the e a r t h and cut down most of the t r e e s to cook them. D i f f e r e n t animals, b i r d s and eggs have w i d e l y d i f f e r e n t t a s t e s ; and, i n a d d i t i o n , d i f f e r e n t p a r t s of the body w i l l have d i f f e r e n t t a s t e s . Through s t u d i e s on the f l a v o r chemistry of raw f i s h and meat, much i s known about v e r t e b r a t e muscle f l a v o r compounds (4649). Prominent i n meat and f i s h t a s t e are i n o r g a n i c s a l t s , n u c l e o t i d e s , amino a c i d s ( e s p e c i a l l y s u l f u r amino a c i d s ) , the d i p e p t i d e s anserine and carnosine (which o f t e n occur i n extremely h i g h c o n c e n t r a t i o n ) , and v a r i o u s other compounds found i n f l e s h such as t a u r i n e , t h i a m i n , and organic a c i d s . Egg f l a v o r compounds are i n l a r g e p a r t s i m i l a r to those found i n meats. M i l k f l a v o r , however, l a r g e l y d e r i v e s from organic a c i d s , simple p h e n o l i c s , sugars and l a c t o n e s . P l a n t foods present another order of chemical complexity as compared to animal foods. The types of compounds present i n p l a n t s are much more v a r i e d than those present i n animal t i s s u e s (48, 50-56). The chemical composition of the seeds or f r u i t of a p l a n t w i l l be d i f f e r e n t from t h a t of the r o o t s , bark, l e a v e s , or stems. The f l a v o r of f r u i t s i s u s u a l l y determined by compounds d i s t i n c t from those f u n c t i o n i n g i n the f l a v o r of vegetables. Prominent i n f r u i t f l a v o r (39,51,57) f o r i n s t a n c e are sugars, a l c o h o l s , aldehydes, e s t e r s , organic a c i d s and l a c t o n e s (58). Vegetable f l a v o r s (48, 59-62), on the other hand, are u s u a l l y a t t r i b u t e d to v a r i o u s n i t r o g e n and s u l f u r compounds, e s p e c i a l l y amino a c i d s , n u c l e o t i d e s , and v a r i o u s n i t r o g e n and s u l f u r h e t e r o c y c l e s (63-66). Lactones (58), however, are prominent i n c e l e r y and tomato f l a v o r ; and sugars are major f a c t o r s i n many r o o t foods such as c a r r o t s and beets. P h e n o l i c compounds (67,68) occur i n a l l c l a s s e s of vegetables and f r u i t s . Mushroom f l a v o r

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1.

BOUDREAU ET AL.

The

Taste of Foods

19

(69) comes p r i m a r i l y from n i t r o g e n and s u l f u r compounds, and some h i g h l y unuaual compounds such as l - o c t e n - 3 - o l and muscimole may be a c t i v e . S u l f u r compounds are h i g h l y prominent i n the f l a v o r of g a r l i c and onions (70,76), and are a l s o important i n the f l a v o r of asparagus, tomatoes, and cabbage. The t a s t e of any food item would c o n s i s t of the d i f f e r e n t o r a l chemical sensations e l i c i t e d when t h i s food i s consumed. The types of sensations e l i c i t e d would be a f u n c t i o n of the c l a s s e s of compounds present i n the food (Table I I I ) , s i n c e d i f f e r e n t sensations w i l l be evoked by d i f f e r e n t compounds. I n Table IV are t a b u l a t e d some of the t a s t e sensations l i k e l y to be a s s o c i a t e d w i t h d i f f e r e n t foods. Raw monkey meat would e l i c i t a v a r i e t y of s e n s a t i o n s , i n c l u d i n g a s a l t y , sour, s t r o n g sweety, umamii, and umami2. An apple from a n a t u r a l n u t r i t i o n a l ecosystem would l i k e l y e l i c i t sensations of sweety, b i t t e ^ , sour, sweet2, a s t r i n g e n t , and p l e a s a n t Various other foods would e l i c i t other s e n s a t i o n s be a composite of d i s c r e t sensations would d e r i v e from the o l f a c t o r y and t r i g e m i n a l systems. Some of these sensations may r e q u i r e both o r a l and n a s a l i n p u t , s i n c e o r a l and n a s a l chemoreceptor systems have been demonstrated to have converging input on b r a i n stem neurons (72,73). A l t e r a t i o n s i n N a t u r a l N u t r i t i o n a l Ecosystems Man has made b a s i c a l t e r a t i o n s i n h i s n u t r i t i o n a l ecosystem. Two of these changes have c l e a r l y been to i n t e n s i f y the f l a v o r of the foods he e a t s . The f i r s t of these changes, f e r m e n t a t i o n , i s seminatural i n t h a t the f l a v o r compounds are q u i t e l i k e l y to occur n a t u r a l l y i n foods. Many of man's foods are subjected to fermentation before being consumed. Examples of such foods are a l c o h o l i c beverages such as wine and beer, many breads, pifckles and condiments (e.g., kim c h i and sour k r a u t ) , cheeses, many f l a v o r sauces such as soy sauce and f i s h sauce ( i n c l u d i n g anchovies), and beverages such as c o f f e e , tea and cocoa. The f l a v o r products developed d u r i n g m i c r o b i a l fermentation depend i n l a r g e p a r t on the s u b s t r a t e and the microbe. Some of the common fermentation f l a v o r products are a l c o h o l s , e s t e r s , f a t t y a c i d s , v a r i o u s mono and d i c a r b o n y l s , p h e n o l i c compounds, and many l a c t o n e s (74-77). In g e n e r a l , f l a v o r s from fermented foods are strong and complex. The major man-induced change i n the chemistry of h i s n u t r i t i o n a l ecosystem i s the p r o d u c t i o n of f l a v o r compounds by cooking foods; whereas m i c r o b i a l production of f l a v o r compounds i s s e m i n a t u r a l , many heat produced compounds are not found i n nature i n any q u a n t i t y . Although i t has been speculated t h a t food i s cooked f o r h y g i e n i c purposes or to i n c r e a s e the n u t r i t i o n a l value of foods eaten ( c e r t a i n starches are rendered e d i b l e by h e a t i n g ) , most foods man p r e s e n t l y eats can be eaten raw w i t h l i t t l e or no l o s s i n n u t r i t i o n a l v a l u e . With h e a t i n g , i n f a c t ,

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

20

FOOD TASTE CHEMISTRY

Table I I I :

S i m p l i f i e d Summary o f some o f the Major Taste A c t i v e Compounds found i n d i f f e r e n t foods.

Compound

Meat

Vegetables

Fruit

Roots

Seeds

Inorganic Ions

XX

X

X

X

X

Amino A c i d s

XXX

XX

X

XX

XX

Peptides and P r o t e i n s

XXX

XX

X

XX

Nucleotides

XXX

X

XX

XX

Amines

XX

X XX

XX

H i s t i d i n e Dipeptides

XXX

Sugars

X

Phenols - Simple

XX

XX

Hydroxy Compounds

X

XX

P o l y p h e n o l i c Compounds

XX

XX

X

Carbonyl Compounds

XX

XXX

X

XX

Esters S u l f u r Compounds

XX

X

XX

Acids

XX

XXX

Furans

XX

XX

Lactones

XX

XXX

XXX

X

N, S H e t e r o c y c l e s

X

X

X

X

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

X

Lettuce

XX

xxxx

XX

Onion

Honey

Ginger

XX

XX

X

X

XXX

XXX

XXX

XXX

XX

XXX

Mushroom

Wheat

XXX

X

Tomato

X

XXX

Orange

X

Carrot

X

XX

XX

Oyster

X

Apple

XX

X

X

XX

X

XX

X

X

X

XX

X

X

XX

XX

XX

X

XX

XXX

XX

XX

X

X

XX

X

XXX

XX

X

X

XX

X

XX

XX

S a l t y Sour Sweety B i t t e ^ P l e a s a n t Sweet 2 B i t t e r

Monkey

Food 2

X

X

XX

XX

X

XX

XXX

X

X

XXX

XX

XX

XX

XX

XX

XX

XX

XXX

XXX

XX

X

A s t r i n g e n t Pungent Umami^ Umami 2 M e t a l l i c

TASTE SENSATION

Table IV: S i m p l i f i e d Summary o f the Taste o f some Foods, uncooked.

22

FOOD TASTE CHEMISTRY

the opposite may occur s i n c e many v i t a m i n s and amino a c i d s are destroyed. Furthermore, many of the compounds produced by cooking have been reported to be d e t r i m e n t a l to h e a l t h (78,79). To any f l a v o r chemist the reason f o r cooking foods i s obvious: i t g i v e s them f l a v o r . The v a r i e t y of f l a v o r compounds produced by h e a t i n g foods i s a s t r o n o m i c a l , e s p e c i a l l y when, as i s common i n p r e p a r i n g many d i s h e s , foods of d i f f e r e n t chemical composition are heated together. The f l a v o r compounds produced by h e a t i n g (63-66,79-82) i n c l u d e aldehydes, ketones, phenols, t h i o l s , and a wide v a r i e t y of s u l f u r n i t r o g e n and oxygen h e t e r o c y c l e s . F i v e and s i x member r i n g h e t e r o c y c l e s are among the most f l a v o r a c t i v e compounds a r i s i n g from cooking. Those w i t h oxygen i n the r i n g are most c h a r a c t e r i s t i c of p l a n t foods, e s p e c i a l l y g r a i n products, or of the l i p i d d e r i v e d f l a v o r s of m i l k and f a t . Popcorn and potato c h i p s are f l a v o r e d i n p a r t by these compounds The complex f l a v o r s of c o f f e e and chocolat s u l f u r and n i t r o g e n h e t e r o c y c l e s pro duced h e t e r o c y c l i c s u l f u r and n i t r o g e n compounds i n c l u d e p y r i d i n e s , p y r a z i n e s , t h i a z o l e s , p y r r o l e s , and thiophenes. For many of these compounds only o l f a c t o r y sensations have been r e ported. They a r e , however, s i m i l a r i n s t r u c t u r e to many h e t e r o c y c l i c compounds a c t i v e neurophysiologicàlly i n the cat and dog and are t h e r e f o r e l i k e l y t a s t e a c t i v e i n humans. R e l a t i v e l y l i t t l e i s known of the n u t r i t i o n a l p r o p e r t i e s of many of these heat produced compounds. Many of them are of n a t u r a l occurrence, although u s u a l l y found at much lower l e v e l s . Others are u n l i k e l y to be encountered i n a n a t u r a l n u t r i t i o n a l ecosystem, e.g. oxazoles and o x a z o l i n e s (83). In many cases fermentation and h e a t i n g are i n v o l v e d together i n food p r e p a r a t i o n . Thus both fermentation and h e a t i n g are used i n the p r o d u c t i o n of t e a , c o f f e e , chocolate and a l c o h o l i c beverages. Although some n a t u r a l foods can be t y p i f i e d by t h e i r simple m i l d t a s t e s , these processed foods g i v e r i s e to s t r o n g complex s e n s a t i o n s . These complex sensations a r i s e from the many t a s t e a c t i v e substances present'. The t a s t e of beer f o r i n s t a n c e , would r e s u l t from compounds n a t u r a l l y present i n grain, and hops, such as amino a c i d s , n u c l e o t i d e s , and v a r i o u s p h e n o l i c compounds and those produced by fermentation and h e a t i n g such as a l c o h o l s , l a c t o n e s and s u l f u r and n i t r o g e n h e t e r o c y c l e s (Figure 9 ) . Other major changes t h a t man has i n s t i t u t e d i n the chemical composition of h i s n a t u r a l n u t r i t i o n a l ecosystem d e r i v e from a g r i c u l t u r e and i n d u s t r y ; and, u n l i k e the changes i n c u r r e d by fermentation and h e a t i n g , many of these chemical changes have been d e t r i m e n t a l f o r t a s t e a c t i v i t y . I n the a g r i c u l t u r a l r e v o l u t i o n of the n e o l i t h i c p e r i o d , man s u b s t i t u t e d a n u t r i t i o n a l ecosystem over which he had some c o n t r o l f o r one over which he had no c o n t r o l . T r a d i t i o n a l a g r i c u l t u r e approximated a n a t u r a l n u t r i t i o n a l ecosystem i n t h a t man was normally p a r t of an i n t e r grated system and food was produced by s m a l l d i v e r s i f i e d farms.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1.

23

The Taste of Foods

BOUDREAU ET AL.

TASTE COMPOUNDS IN BEER

,CH

hn: HN'

2

-CH 2,3-DIMETHYLPYRAZINE 3

NICOTINIC ACID OH

H O - P - 0 - H , C JO, OH

H OH OH GUANOSINE-5 MONOPHOSPHATE

CH CH N(CH ) HORDENINE 2

1

2

y-VALEROLACTONE

3 :

CH OH

CH OH

OH

OH

2

MQ Ο Y^CH CH(CH ) 2

3

H C:OC0 H PYRUVIC ACID

2

C

3

2

2

MALTOSE

(CH ) CHCH CH OH 3 2

2

j^N^C0 H 2

2

ISOAMYL ALCOHOL

L-TRYPTOPHAN

CH (CH ) CH C.OCH:CH

CH CH CHOHCH

1-OCTEN-3-ONE

2-BUTANOL

3

PHENYLACETIC ACID

Η

L-PROLINE

2

3

2

3

2

3

Figure 9. Some of the taste active compounds found in beer

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

24

FOOD TASTE CHEMISTRY

Food was grown and s e l e c t e d i n l a r g e p a r t on the b a s i s of t a s t e . In the l a s t 100 years however, changes have occurred i n the methods of food p r o d u c t i o n and d i s t r i b u t i o n t h a t are d e t r i m e n t a l to good t a s t e . Taste i s no longer a f a c t o r i n food p r o d u c t i o n . Rather, a g r i c u l t u r e i s geared toward q u a n t i t y p r o d u c t i o n and v a r i o u s d i s t r i b u t i o n needs. The chemical composition of the prod u c t i o n system has been s i m p l i f i e d by the e l i m i n a t i o n of most n a t u r a l l y o c c u r r i n g s p e c i e s and the farming of one or two crops. F r u i t s and vegetables are grown f o r y i e l d and ease i n t r a n s p o r t a t i o n and are ripened by a r t i f i c i a l means. These changes have taken p l a c e l a r g e l y without regard to t a s t e (84). As a r e s u l t many foods have l o s t t a s t e , as exemplif i e d by turkey and tomatoes (85). S e v e r a l p r a c t i c e s c o n t r i b u t e to the g e n e r a l i n e d i b i l i t y of American a g r i c u l t u r a l produce. A r t i f i c i a l l y r i p e n i n g f r u i t s r e s u l t s i n a decrease i n f l a v o r compounds ( F i g u r e 10). I n a t u r a l n u t r i t i o n a l ecosyste ther i s great chemical complexit c o n t r i b u t i n g to the f l o compounds y compound such as amino a c i d s and n u c l e o t i d e s are u t i l i z e d by p l a n t s and hence c y c l e d i n the system. The r e l i a n c e on a few f e r t i l i z e r s w i t h h i g h n i t r o g e n content and a few simple compounds r e s u l t s i n an imbalance i n the ecosystem d i s r u p t i n g n a t u r a l systems and changing chemical composition of food. The l o s s of f l a v o r i n onion and g a r l i c w i l l r e s u l t w i t h s u l f u r d e f i c i e n c y (86); a d e f i c i e n c y becoming more and more common i n farms. I n England there has occurred a s h i f t from t r a d i t i o n a l growing of c i d e r apples to i n t e n s i v e c l o s e packed orchards u t i l i z i n g h i g h n i t r o g e n f e r t i l i z e r s . As a r e s u l t f l a v o r has d e c l i n e d markedly (87). In f a c t i t seems t h a t a g r i c u l t u r a l chemistry as now p r a c t i c e d i s i n i m i c a b l e to good t a s t e . Besides the e f f e c t of i n d u s t r i a l f e r t i l i z e r s upon food composition, the u l t i l i z a t i o n of chemical compounds f o r v a r i o u s a g r i c u l t u r a l purposes has been found to a l t e r the chemical composition of our foods. The chemicals used to l o o s e n oranges f o r mechanical p i c k i n g , f o r i n s t a n c e , have been found to i n t r o d u c e n o v e l chemicals w i t h o f f t a s t e s i n t o the orange (88,89). Nematicides have a l s o been r e p o r t e d to produce l a r g e changes i n the chemical composition of tomatoes (90). The u t i l i z a t i o n of v a r i o u s chemical compounds f o r h e r b i c i d e s , f u n g i c i d e s , i n s e c t i c i d e s , and medication has introduced v a r i o u s new compounds i n t o our foods (91,92). Many of these compounds are i n c o r p o r a t e d i n t o the food we eat o f t e n i n h i g h c o n c e n t r a t i o n . Some of the t o x i c a n t s present i n foods (93) are e n d r i n , DDT, toxaphine, a l d r i n and d i e l d r i n , h e p t a c h l o r , d i a z i n o n , p a r a t h i o n , c h l o r o b e n z i l a t e , d i t h i o c a r b i m a t e , dalapon, dimethoate and many other compounds employed f o r v a r i o u s purposes. Besides n o v e l food compounds d i r e c t l y added by a g r i c u l t u r e , many i n d u s t r i a l compounds such as p o l y c h l o r i n a t e d b i p h e n o l s have found t h e i r way i n t o our food supply. Some of the compounds o f common occurrence i n today's food are i l l u s t r a t e d i n F i g u r e 11. These compounds and s i m i l a r d e r i v e d products are assumed to d e t r a c t from the

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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CHROMATOGRAMS OF PEACH VOLATILES WITH LACTONES IDENTIFIED

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FOOD TASTE CHEMISTRY

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1.

BOUDREAU ET AL.

27

The Taste of Foods

unnatural cmpds.

Figure 12. Schematic of present human nutritional ecosystem with diminished components

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

28

FOOD TASTE CHEMISTRY

p a l a t a b i l i t y o f foods. Many o f the foods s e l e c t e d over m i l l e n i a f o r f l a v o r have, i n the l a s t 20 t o 30 years because o f changes i n t h e i r chemical composition, become bland o r even o b j e c t i o n a b l e i n f l a v o r . Some o f the changes i n s t i t u t e d i n man's n a t u r a l n u t r i t i o n a l ecosystem are i l l u s t r a t e d i n F i g u r e 12. Concluding Statement Taste i n present day food p r o d u c t i o n i s o f t e n not a r e l e v a n t v a r i a b l e , being secondary t o such f a c t o r s as y i e l d , s h i p p i n g , storage and appearance. Often the food i n d u s t r y g i v e s the impression t h a t food i s something t o which f l a v o r can be added. The food f u t u r e o f t e n p r o j e c t e d i s one i n which we w i l l be e a t i n g v a r i o u s processed foods such as f l a v o r e d soy w i t h i n f i n i t e storage l i f e . F l a v o r , however, i s i n h e r e n t i n the n a t u r a l chemical composition o f the food and the o n l y way t o improve food f l a v o r i s by producin to t h a t found i n the n a t u r a the t a s t e systems were designed t o f u n c t i o n .

Acknowledgements We thank J . Lucke f o r computer programming and C. H o l l e y f o r s e c r e t a r i a l a s s i s t a n c e . T h i s r e s e a r c h was financed i n p a r t by N a t i o n a l Science Foundation Research Grants.

Literature Cited 1. Freeland, W.J., Janzen, D.H., Amer. Nat. (1974) 108: 269-289. 2. Swain, T. Ann. Rev. Plant Physiol (1977) 28: 479-501. 3. Cott, H.B., Proc. Zool. Soc., London (1946) 116: 371524. 4. Cott, H.B., Proc. Zool. Soc., London (1953) 123: 123141. 5. Ahem, E.M., In: "Medicine in Chinese Cultures" (A. Kleinman et a l . , Ed's), 91-113, U.S. Dept. Health Ed. Welfare, NIH, DHEW. Pub. No. 75-653, Wash. D.C., 1975. 6. Anderson, E.N., Anderson, M.L., In: "Medicine in Chinese Culture" (A. Kleinman et a l . , Ed's), 143-175, U.S. Dept. Health Ed. Welfare, NIH, DHEW. Pub. No. 75-653, Wash. D.C., 1975. 7. Boudreau, J.C. and Tsuchitani, C., "Sensory Neurophysio­ logy", Van Nostrand Reinhold Co., N.Y., 1973. 8. Andres, K.H., Arch. Oto. -Rhino.-Laryng. (1975) 210: 141. 9. Graziadei, P.P.C., In: "Olfaction and Taste III" (C. Pfaffmann Ed.), 315-330, Rockefeller University, N.Y., 1969.

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53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79.

The Taste of Foods

31

220-233. Lee, C.Y., Shallenberger, R.S., Vittum, M.T. Food Sciences (Geneva, N.Y.) No. 1, 1970. Linner, Κ., Qual. Plant. (1973) 23: 251-262. Schmidtlein, H., Herrmann, K.Z. Lebensm. Unters.-Forsch. (1975) 159: 139-148. Smith, T.Α., Phytochem. (1975) 14: 865-890. Nursten, H.E. In: "Sensory Properties of Foods" (G.G. Birch, J.C. Brennan, and K.J. Parker, Eds), 151-166, Appl. Sci. Pub., Barkin, 1977. Maga, J.A., Crit. Rev. Fd. Sci. Nutr., (1976) 8: 1-56. Salunkhe, D.K., Do, J.Y., Crit. Rev. Fd. Sci. Nutr., (1976) 8: 161-190. Schutte, L . , Crit. Rev. Fd. Tech. (1974) 4: 457-505. Virtanen, A.I., Phytochem. (1965) 4: 207-228. Maga, J.A., CRC Crit Rev Fd Sci Nutr. (1978) 10 373-403. Maga, J.A., CRC Crit. Rev. Fd. Sci. Nutr., (1976) 7: 147-192. Maga, J.A., CRC Crit. Rev. Fd. Sci. Nutr., (1975) 6: 241-270. Maga, J.A., CRC Crit. Rev. Fd. Sci. Nutr., (1975) 6: 153-176. Maga, J.A. and Sizer, C.E., J. Agr. Food Chem., 21 (1973) 22-30. Murray, K.E., Whitfield, F.B., J. Sci. Fd. Agric. (1975) 26: 973-986. Maga, J.A., CRC Crit. Rev. Fd. Sci. Nutr., (1978) 10: 323-372. Dijkstra, F.Y., Wiken, T.O., Z. Lebensm. Unters.-Forsch. (1976) 160: 255-262. Freeman, G.G., Whenham, R.J., J. Sci. Fd. Agric. (1975) 26: 1869-1886. Whitaker, J.R., Adv. Fd. Res. (1976) 22: 73-133. Van Buskirk, R.L.v., Erickson, R.P. Neurosc. Let. (1977) 5: 321-326. Van Buskirk, R.L.v., Erickson, R.P., In: "Olfaction and Taste VI", p 206, Information Retrieval, Wash.D.C., 1977. Haymon, L.W., In: "Lipids as a Source of Flavor" (M.K. Supran, Ed.), 94-115, Am. Chem. Soc., Wash.D.C., 1978. Litman, I., Numrych, S., In: "Lipids as a Source of Flavor" (M.K. Supran, Ed.), 1-17, Am. Chem. Soc., Wash. D.C., 1978. Tressl, R., Apetz, M., Arrieta, R., Grunewald, K.G., In: "Flavor of Foods and Beverages Chemistry and Technology" (G. Charalambos and G.E. Inglett, Eds.) 145-168. Artman, N.R., Adv. Lip. Res. (1969) 7: 245-330. Commoner, B., Vithayathil, A . J . , Dolara, P., Nair, S., Madyastha, P., Cuca, G.c., Science (1978) 201: 913-916. Chang, S.S., Peterson, R.J., Ho, C.T., In: "Lipids as a

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

FOOD TASTE CHEMISTRY

80. 81.

82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95.

Source of Flavor" (M.K. Supran, Ed.) 18-41, Am. Chem. Soc., Wash.D.C., 1978. Schutte, L . , In: "Phenolic, Sulfur, and Nitrogen Compounds in Food Flavors" (G. Charalambous and I. Katz, Eds) 96-113, Am. Chem. Soc., Wash.D.C., 1976. Mussinan, C.J., Wilson, R.A., Katz, I., Hruza, Α., Vock, M.H., In: "Phenolic, Sulfur and Nitrogen Compounds in Food Flavors", (G. Charalambous and I. Katz, Eds), 133145, Am. Chem. Soc., Wash.D.C., 1976. Wilson, R.A., Agr. Fd. Chem. (1975) 23: 1032-1037. Maga, J.A., J. Agr. Fd. Chem. (1978) 26: 1049-1050. Dirinck, P., Schreyen, L . , Schamp, Ν., Agr. Fd. Chem. (1977) 25: 759-763. Whiteside, T., "The New Yorker", 1977, Jan 14, 36-61. Freeman, G.G., Whenham, R. J., Int. Flav. (1976) 7 Lea, A.G.H., Beech F.W. J. Sci Fd Agic (1978) 29: 493-496. Moshonas, M.G., , , Agric (1977) 25: 1151-1153. Moshonas, M.G., Shaw, P.E., J. Agric. Fd. Chem. (1978) 26: 1288-1290. Bajaj, K.L. Mahajan, R., Qual. Plant. (1977) 27: 335338. Oehme, F.W., Toxicology (1973) 1: 205-215. Menn, J.J., Still, G.G., CRC Crit. Rev. Toxicol. (1977) 5: 1-21. Salunkhe, D.K., Wu, M.J., CRC Crit. Rev. Fd. Sci. Nutr., (1977) 9: 265-324. Miller, I . J . , In: "Food Intake and Chemical Senses" (Y. Katsuki et al, Eds) 173-185, University Park Press, Baltimore, 1978. Do, J.Y., Salunkhe, D.K., Olson, L.E., J. Food Sci. (1969) 34: 618- 621.

RECEIVED August 9, 1979.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2 The Umami Taste SHIZUKO YAMAGUCHI Central Research Laboratories, Ajinomoto Co., Inc., Kawasaki, Japan

The characteristic tast f monosodiu glutamat d 5'-ribo nucleotides is called "umami role in the flavor of foods, such as meats, poultry, fish and other sea foods, dairy products, or vegetables. The taste was first discovered by Ikeda (1908) (1), and has been studied by a large number of researchers from different points of view (refer to, e.g. 2-9). We have systematically investigated the umami taste using psychometric procedures. In this paper, a part of our studies will be outlined. Umami Substances Most typical umami substances are divided into two series of compounds. One is a group of L-α-amino acids represented by mono­ sodium glutamate (MSG) (Table I) (10-15), and another is that including 5'-ribonucleotides and their derivatives, represented by disodium 5'-inosinate (IMP) or disodium 5'-guanylate (GMP) (Table II) (16, 17, 18, 19). The latter group of substances have only very Table I. Umami Substances Related to MSG R e l a t i v e umami i n t e n s i t y Monosodium L-glutamate H 0 Monosodium DL-iforeo-P-hydroxy glutamate H 0 Monosodium DL-homocystate H 0 Monosodium L - a s p a r t a t e H 0 Monosodium L-a-amino a d i p a t e H 0 L-Tricholomic a c i d (erythro form) L-Ibotenic a c i d

1 0.86

2

2

2

2

2

a

a

0.77 0.077 0.098 5-30 5-30

Table from Yamaguchi et al. (38). From T e r a s a k i et al. (14, 15). a

0-8412-0526-4/79/47-115-033$05.00/0 © 1979 American Chemical Society In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

34

FOOD TASTE CHEMISTRY

Table I I . Umami Substances R e l a t e d t o IMP , ^ Substance (Disodium s a l t ) /TV

1f

R e l a t i v e potency ^ i

N

o

1

5 -Inosinate.7.5H20 5'-Guanylate.7H 0 5'-Xanthylate.3H 0 5'-Adenylate Deoxy 5'-guanylate.3H2O 2-Methyl-5 -inosinate.6H2O 2-Ethyl-5 -inosinate.1.5H 0 2-Phenyl-5'-inosinate.3H20 2-Methylthio-5'-inosinate.6H2O 2-Ethylthio-5 -inosinate.2H2O 2-Ethoxyethylthio-5'-inosinat 2-Ethoxycarbonylethylthio 2-Furfurylthio-5 -inosinate.H2O 2-Tetrahydrofurfurylthio-5 -inosinate.H2O 2 - I s o p e n t e n y l t h i o - 5 - i n o s i n a t e (Ca) 2-(3-Methallyl)thio-5 -inosinate 2-(Y-Methallyl)thio-5 -inosinate 2-Methoxy-5'-inosinate.H2O 2-Ethoxy-5'-inosinate 2-i-Propoxy-5 -inosinate 2-n-Propoxy-5'-inosinate 2 - A l l y l o x y - 5 - i n o s i n a t e (Ca) O.5H2O 2-Chloro-5 -inosinate.1.5H2O N -Methyl-5 -guanylate.5.5H 0 N , N -Dimethyl-5'-guanylate.2.5H2O N ^ M e t h y l - S ' - i n o s i n a t e . H2O Ν -Methy1-5'-guanylate.H2O Ν -Methy1-2-methy11hio-5'-ino s i n a t e 6-Chloropurine r i b o s i d e 5'-phosphate.H2O 6-Mercaptopurine r i b o s i d e 5'-phosphate.6H2O 2-Methyl-6-mercaptopurine r i b o s i d e 5'-phosphate. H2O 2-Methylthio-6-mercaptopurine r i b o s i d e 5'phosphate.2.5H2O 2',3'-0-Isopropylidene 5 ' - i n o s i n a t e 2',3'-0-Isopropylidene 5'-guanylate 2

2

1

1

2

a

1

f

a

l

a

l

a

f

a

f

a

a

1

a

a

t

a

1

2

1

2

2

2

1

1

u m a m

1 2.3 0.61 0.18 0.62 2.3 2.3 3.6 8.0 7.5

17 8 11 10 11 4.2 (3.7) 4.9 4.5 2 6.5 3.1 2.3 2.4 0.74 1.3 8.4 2 3.4 8 7.9 0.21 0,35

From Yamaguchi et al. (38). From Imami et al. (19). a

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2. YAMAGucHi

The Umami Taste

35

weak tastes. It is notable, however, that they synergistically increase the umami of the former substances (17, 20). In addition to these two groups, some peptides have been reported to have tastes similar to MSG (21, 22). Some researchers regard the taste of succinic acid (23, 24) or theanine (25) as umami, although their taste qualities are considerably different from that of MSG. Fundamental Taste P r o p e r t i e s o f Umami Threshold Value. The t h r e s h o l d values of umami and the other four t a s t e substances have been reported by many researchers (e.g. 1 26, 27, 28, 29). However, because o f the measurement c o n d i t i o n s are d i f f e r e n t , p r e c i s e comparisons w i t h one another a r e d i f f i c u l t . We have measured the d e t e c t i o n thresholds o f MSG and the four t a s t e substances simultaneously as c a r e f u l l y as p o s s i b l e u s i n g a s i n g l e panel under i d e n t i c a panel was composed o f 3 and 40. T r i a n g l e t e s t s were used, where each t r i a n g l e c o n s i s t e d o f two samples of pure water and one sample of a t e s t s o l u t i o n . The p a n e l i s t s were asked t o s e l e c t the odd sample. A s e r i e s o f t r i a d s were presented i n descending order. The lowest c o n c e n t r a t i o n which could be s i g n i f i c a n t l y d i s t i n g u i s h e d from pure water was obtained f o r each t e s t substance (Table I I I ) . Table I I I . MSG

Detection Sucrose

0.012

0.086

Concentrations

Thresholds f o r F i v e Taste Subsatnces (n = 30) Quinine Tartaric Sodium sulfate acid chloride 0.0037

given as g/100ml.

0.00094

0.000049

From Yamaguchi and Kimizuka(9).

The d e t e c t i o n t h r e s h o l d f o r MSG was as low as 0.012 g/100ml or 6.25 χ 1 0 " % . I t was higher than that o f quinine s u l f a t e o r t a r t a r i c a c i d , lower than t h a t of sucrose and almost the same as that of sodium c h l o r i d e i n the molar c o n c e n t r a t i o n . Some umami substances have lower t h r e s h o l d s than that of MSG. S u b j e c t i v e I n t e n s i t y Scale f o r Umami. Thresholds do not always express the r e l a t i v e potency o f d i f f e r e n t t a s t e s t i m u l i , because the i n t e n s i t y o f t a s t e does not increase w i t h concentra­ t i o n i n the same manner f o r each substance. Several kinds o f t a s t e i n t e n s i t y s c a l e s have been e s t a b l i s h e d f o r the four t a s t e s . T y p i c a l examples a r e the gust s c a l e by Beebe-Center (31) and the τ s c a l e by Indow (32). I n order t o deal w i t h the umami on the same b a s i s w i t h the four t a s t e s , we newly e s t a b l i s h e d a new s u b j e c t i v e t a s t e i n t e n s i t y s c a l e f o r umami as w e l l as f o r the four t a s t e s (9, 30). S i x s o l u t i o n s f o r each o f the f i v e t a s t e substances were prepared. The t a s t e i n t e n s i t y o f

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36

FOOD TASTE CHEMISTRY

each s o l u t i o n was r a t e d by the 30 panel members. The p a n e l i s t s kept 10 ml of the sample i n t h e i r mouths f o r 10 seconds. Then they were asked to assess the i n t e n s i t y of the t a s t e on a 100 p o i n t s c a l e w i t h 0 being no d i s c e r n i b l e t a s t e and 100 the h i g h e s t i n t e n s i t y . The p a n e l i s t s r a t e d a l l the samples twice. The r e s u l t s are shown i n Figure 1 using mean v a l u e s of a t o t a l of 60 ratings. The r e l a t i o n s h i p between the c o n c e n t r a t i o n and the perceived t a s t e i n t e n s i t y of MSG was l o g a r i t h m i c a l l y l i n e a r l i k e those of the four common t a s t e s , although the slope f o r MSG was somewhat l e s s steep than the others. I t means that Weber-Fechner s law holds f o r a l l of the f i v e t a s t e substances. The r e l a t i o n of the t a s t e i n t e n s i t y (S) to the c o n c e n t r a t i o n (x) can be expressed by 1

S =

α log

2

(x/3),

where α represents the increas c e n t r a t i o n and, 3, the c o n c e n t r a t i o of the e x t r a p o l a t e d l i n e w i t h the c o n c e n t r a t i o n a x i s , seen i n Figure 1. This equation was a p p l i e d to the f i v e t a s t e substances and the r e s u l t s were: MSG

S

M

=

9.69

log

2

(x

M

/0.0195),

Sucrose

S

s

= 14.98

log

2

(x

s

/0.873),

Sodium c h l o r i d e

S

s c

= 15.50

log

2

(x /0.0943),

Tartaric acid

S

T

= 14.45

log

2



Quinine s u l f a t e

SQ

= 14.16

log

2

(XQ /0.000169),

sc

τ

/0.00296),

where χ i s given i n terms of g/100 ml. In t h i s experiment, only the i n t e n s i t y of t a s t e was r a t e d and the q u a l i t y of t a s t e was d i s r e g a r d e d . Consequently, the same v a l u e of S i n the above mentioned equations represents the same i n t e s n i t y of t a s t e . Beebe-Center defined the u n i t of t a s t e i n t e n ­ s i t y as gust. One gust means the t a s t e i n t e n s i t y of 1% sucrose s o l u t i o n . However, the gust s c a l e i s not always convenient be­ cause i t does not d e f i n e the upper l i m i t of the s c a l e . In our s c a l e , the u n i t of S was adjusted so that the v a l u e of S i s zero at the c o n c e n t r a t i o n β and 100 at the saturated sucrose c o n c e n t r a t i o n at 20°C (89.27g/100ml). I n t e r a c t i o n s between Umami and the Four Tastes. Interaction of t a s t e s i s another important problem i n the study of phenomena of t a s t e s . In order to examine the e f f e c t of umami on the four common t a s t e s , the i n f l u e n c e of MSG on the thresholds of the four t a s t e s has been examined by s e v e r a l researchers (27, 28, 33), but the r e s u l t s are c o n f l i c t i n g . In order to c l a r i f y the i s s u e , the thresholds of the four t a s t e substances were measured again i n 5mM

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2.

YAMAGucHi

37

The Umami Taste

s o l u t i o n of MSG o r IMP (9, 30). The panel and experimental cond i t i o n s were e x a c t l y the same as that i n the aforementioned experiment. The d e t e c t i o n t h r e s h o l d of q u i n i n e s u l f a t e was s l i g h t l y r a i s e d by the presence o f 5mM of IMP. The t h r e s h o l d o f t a r t a r i c a c i d was c o n s i d e r a b l y r a i s e d by both umami substances, no doubt because o f the change i n pH. No e f f e c t was observed on the t h r e sholds o f sucrose and sodium c h l o r i d e (Table I I I and I V ) . Table IV.

D e t e c t i o n Thresholds f o r the Four Taste Substances i n S o l u t i o n of Umami Substance (n = 30) Quinine Tartaric Sodium Sucrose Base s o l u t i o n sulfate acid chloride

0.094g/100 ml MSG

a

0.26g/100 ml

a

IMP

0.086

0.0037

0.0019

0.000049

Concentrations g i v e n as g/100ml. 5mM. From Yamaguchi and Kimizuka ( 9 ) .

a

The S y n e r g i s t i c E f f e c t s o f Umami Substances Q u a n t i t a t i v e a n a l y s i s o f the s y n e r g i s t i c e f f e c t . When f r u c tose and sucrose are mixed together, the sweetness of the mixture becomes s l i g h t l y g r e a t e r than the sum of the sweetness o f the separate substances (34). Such phenomenon i s c a l l e d the s y n e r g i s t i c e f f e c t . A c l e a r and p r e c i s e d e f i n i t i o n o f the s y n e r g i s t i c e f f e c t along w i t h s e v e r a l n u m e r i c a l l y t r e a t e d examples has been presented elsewhere (34). The magnitude o f the s y n e r g i s t i c e f f e c t between the two groups o f umami substances i s u n p a r a l l e l e d . F i g u r e 2 shows the r e l a t i o n s h i p between the i n t e n s i t y of umami and the p r o p o r t i o n of IMP i n the mixture of MSG and IMP (35). The t o t a l c o n c e n t r a t i o n was kept constant a t 0.05 g/100ml and the p r o p o r t i o n of IMP was v a r i e d from 0 t o 100%. Since the umami i n t e n s i t i e s of the samples on both extremes are very weak and almost the same, the curve would have proven to be h o r i z o n t a l l y l i n e a r i f the s y n e r g i s t i c e f f e c t had been absent. The symmeLric curve i l l u s t r a t e s the remarkable s y n e r g i s t i c e f f e c t . I n t h i s curve, t h e i n t e n s i t y of umami at i t s maximum i s e q u i v a l e n t t o that o f 0.78g/ 100 ml of MSG alone. The mixture i s 16 times as s t r o n g as that o f MSG. This a m p l i f i c a t i o n f a c t o r i s concentration-dependent, and becomes higher w i t h i n c r e a s i n g c o n c e n t r a t i o n . The s y n e r g i s t i c e f f e c t between MSG and IMP can be expressed by means of the f o l l o w i n g simple equation: y = u + 1200 uv

(1)

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

FOOD TASTE CHEMISTRY

876

M S G + I M P : 0.05g/ml

5 4

3 2 1 1

10

,

20

,

30

,

40

,

50

,

60

,

70

,

80

,

90

5

100

{%)

Proportion of IMP Journal of Food Science

Figure 2.

Refotionship between umami intensity and mixing ratio of MSG and IMP (35)

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2.

YAMAGucHi

The

Umami Taste

39

where u and ν are the r e s p e c t i v e c o n c e n t r a t i o n s of MSG and IMP i n the mixture and y i s the equi-umami c o n c e n t r a t i o n of MSG a l o n e ( 3 5 ) . The s y n e r g i s t i c e f f e c t can be demonstrated between any conb i n a t i o n of substances i n Table I and Table I I ; and the i n t e n s i t y of umami can a l s o be expressed by an equation e s s e n t i a l l y equal to e q u a t i o n (1) (36, 37, 3*0 . The i n t e n s i t i e s of a l l substances i n Table I are always p r o p o r t i o n a l to that of MSG. Therefore, u'g/ 100ml of any substance i n Table I i s r e p l a c e a b l e w i t h cru g/100ml of MSG. The constant α f o r each substance i s l i s t e d i n Table I . On the other hand, the t a s t i n g a c t i v i t i e s of a l l n u c l e o t i d e s i n Table I I are c o n s i s t e n t l y p r o p o r t i o n a l to that of IMP. Hence, v'g/lOOml of any n u c l e o t i d e i s r e p l a c e a b l e w i t h 3v g/100ml of IMP. The constant 3 f o r each n u c l e o t i d e i s l i s t e d i n Table I I . Therefore, the umami i n t e n s i t y of the mixture of any combination of substances i n Table I and Table I I can be c a l c u l a t e d by s u b s t i ­ t u t i n g 35 - 45 8 - 10 1.5 - 2 0.5 - 0.7 +

n.b.: not bitter Table XXI Taste of piperidines and pyridines (11) R

Q-

c (mmol/1) tbi

H 1-Methyl 2-Methyl 3-Methyl 4-Methyl 4-Phenyl

8 10 5 1 1 0.3-

2-C00H

25 - 30

2- C00H 3- C00H 3-C0NH

5 - 7 20 - 25 6 - 8

2

10 15 8 4 2 0.5

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FOOD TASTE CHEMISTRY

Figure 8. Fixation of sweet and bitter compounds in rectangular coordinates— amino acids

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

4. BELiTz ET AL.

Sweet and Bitter Compounds

log c

t b i

121

=a · η+ b

As with the 2-alkanones in some cases this seems to be true only up to a limited value of n. All together the examples show that monopolar (electrophilic) hydrophobic compounds can cause bitter taste and that the intensi­ ty of bitter taste depends on the size and shape of the hydropho­ bic part of the molecule. Expanding SHALLENBERGER and ACREE's concept for sweet com­ pounds further, we regard not only AH/B-systems capable of forming hydrogen bonds as a potential polar contact grouping but all elec­ trophil ic/nucleophil ic systems. As a further development of KIER's concept we assume the existence of an expanded hydrophobic contact, not a restricted one. Size and shape of the hydrophobic part are important for the quality and for the intensity of taste. The individual results can be classified formally i are - for sweet compounds 2 polar groups, an electrophilic one ρ (+), a nucleophilic one ρ (-). An apolar group "a" is not essential but important for the intensity of sweet taste. - for bitter compounds one polar group, an electrophilic one ρ (+) and an apolar group "a". The coordinate system was so chosen that in the case of 2-aminocarboxylic acids the C-atom 2 is at the origin and the polar groups ρ (+) and ρ (-) occupy the positions resulting from Fig.8. In this system, by superimposing probable conformers of 2amino-carboxylic acids - as in Fig. 5 - one can indicate positions allowed and forbidden for sweet compounds (Fig.9). Even if there is only information regarding one forbidden position on the +z side and the dotted line is therefore hypothetical, it is s t i l l clearly recognizable that the forbidden positions as a whole are not arranged symmetrically to the x-axis. A D-amino acid, e.g. D-njrleucine (sweet), could occupy the positions ρ (+) ρ (-) a (+x, -y, +z) (Fig.10), whereby the expres­ sion in brackets after "a" indicates the expansion directions of the apolar group. A short-chain L-amino acid, e.g. L-alanine (sweet) can occupy the positions ρ (+) ρ (-) a (+x, -y, -z), whereas L-norleucine (bitter) in stretched conformation cannot do so because of forbid­ den positions (Fig.10), altough it can occupy ρ (+) a (+x, -y, +z) after 60° rotation about the y-axis (Fig.10). This rotation about the y-axis is only one possibility, for with monopolar occupation, where fixation of the origin is no longer meaningful, rotation about other axes is also possible, so that this situation must be generally denoted as D (+) a (+x, -y, ± z ) . 1-Aminocyclohexane-1-carboxylic acid (sweet/bitter) would occupy the position ρ (+) ρ (-) a (+x, -y, ± z ) (Fig.11). These examples show, that compounds can be arranged with re-

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FOOD TASTE CHEMISTRY

Figure 9. Fixation of sweet and bitter compounds in rectangular coordinates— amino acids: (O) allowed and (Φ) forbidden positions for sweet taste

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

4.

BELITZ ET AL.

Sweet and Bitter Compounds

123

D-norleucine sweet p(+) p(-) a(+x, + y, + z) allowed Θ

ρ(-) 00Ο

>

a ( + x , y,+z) y,+ +



a(+x,^y, - z )

θ

ρ(-) 00Ο L- norleucine

bitter

p(+) p(-) a(+x,iy,-z) forbidden

L-norleucine

bitter

p(+) a(+x, + y, + z) allowed

Figure 10. Fixation of sweet and bitter compounds in rectangular coordinates— Ό-norleucine and L-norleucine in allowed and forbidden positions

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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FOOD TASTE CHEMISTRY

Figure 11. Fixation of sweet and bitter compounds in rectangular coordinates— 1-amino-cyclohexane-l-carboxylic acid (sweet/bitter) p(-{-)p(—)a(-\-x, ±y, ztz)

Figure 12. Fixation of sweet and bitter compounds in rectangular coordinates— pohr (p(+), p(—)) and apolar (a(±x, ±y, ±z)) occupation possibilities for compounds with the taste qualities sweet: p(-\-)p(—)a(-\-x, ±y, + z), p(-\-)p(—)a(-\-x, ±y, —z); sweet-bitter: p(-\-)p(—)a(-\-x, ±y, ±z); bitter: p(-\-)a(±x, ±y,

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

4. BELiTz ET AL.

Sweet and Bitter Compounds

125

gard to their sweet and/or bitter taste qualities according to their varying occupation possibilities(-it is generally assumed that a compound occupies all positions to which it has access -) in the following (Fig.12): sweet:

p(+) p(-) a(+x, ± y , +z) or p(+) p(-) a(+x, ± y , -z) (In the latter case the C-atom at the coordinate origin may only carry substituents R = Et.) sweet-bitter:p(+) p(-) a(+x, -y, *z) bitter: p(+) a ( ± x , ± y , ± z ) The aromatic compounds investigated also fit in well with this formal model. The benzene ring is located in the x/y plane, but it extends towards the sides +z and -z with its^T-orbitals, so that in the presence of a suitable bipolar system sweet-bitter taste can be expected. The deviations observed, i.e. the occurrence of either sweet or bitter tast must be traced back to substituents, which either disturb the bipolar system or are located in sterically forbidden positions. The benzoic acids can occupy the positions p(+) p(-) a(+x, -y) (Fig.13), likewise the nitrobenzenes (Fig.14). For the phenols the corresponding positions would be p(+) p(-) a(-x, *y) (Fig.15). The other possible arrangement (dotted line in Fig.15) would bring the phenyl residue into a position which in the case of many other compounds (carboxylic acids, nitro compounds) is not occupied by a hydrophobic group but by a polar one. Accordingly a hydrophobic contact in the -x direction too, is not out of the question. This possibility is also advantageous for the placing of bitter peptides and 2.5-dioxopiperazines, which could bring several apolar groups into contact by occupying p(+) a(*x, -y, ± z ) . The general model developed for sweet and bitter compounds leads to a sweet-bitter receptor, which can be given formal representation as a hydrophobic pocket with a bipolar system (Fig. 16). If we designate the contact of a stimulus with one, resp. both polar groups, as monopolar, resp. bipolar, and the hydrophobic contact with one, resp. both sides (+z or -z, resp. *z) of the pocket, as monohydrophobic, resp. bihydrophobic, then the taste qualities sweet and bitter can be formally traced back to the following contacts: sweet sweet-bitter bitter

— » bipolar-monohydrophobic — * bipolar-bihydrophobic — » monopolar-monohydrophobic or bihydrophobic.

This formal system cannot claim to make any statement regarding a real receptor, but it has the advantage of making it possible for the first time, to treat the taste qualities sweet and bitter uniformly and to classify them in one model.

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FOOD TASTE CHEMISTRY

Figure 14. Fixation of sweet and bitter compounds in rectangular coordinates— nitrobenzene

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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Sweet and Bitter Compounds

127

Figure 16. Representation of the hydrophobic pocket and the polar contact groups of a formal sweet-bitter receptor

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

FOOD TASTE CHEMISTRY

128

Figure 17.

Atomic charge distribution of selected compounds (e · 10 ) (26, 27, 28)

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

3

4.

BELITZ ET AL.

Sweet and Bitter Compounds

Figure 17. Continued

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

129

130

FOOD TASTE CHEMISTRY

Acknowledgments. We are indebted to Prof. Dr. Unterhalt, Marburg for supplying us with cycloalkane sulfamates and to Prof. Dr. Tressl, Berlin for supplying us with lactones.

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In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

5 Chemistry of Sweet Peptides YASUO ARIYOSHI Central Research Laboratories, Ajinomoto Co., Inc. 1-1, Suzuki-cho, Kawasaki-ku, Kawasaki, 210, Japan

It is known that sweet-tastin and their chemical structure a structure-taste relationship, a large number of compounds have been tested, and several molecular theories of sweet taste have been proposed by different groups. At present, the phenomenon of sweet taste seems best explained by the tripartite functioning of the postulated AH, Β (proton donor-acceptor) system and hydro­ phobic site X (1, 2, 3, 4, 5). Sweet-tasting compounds possess the AH-B-X system in the molecules, and the receptor site seems to be also a trifunctional unit similar to the AH-B-X system of the sweet compounds. Sweet taste results from interaction between the receptor site and the sweet unit of the compounds. Space-filling properties are also important as well as the charge and hydro­ phobic properties. The hydrophile-hydrophobe balance in a molecule seems to be another important factor. After the finding of a sweet taste in L-Asp-L-Phe-OMe (aspar­ tame) by Mazur et al. (6), a number of a s p a r t y l d i p e p t i d e e s t e r s were synthesized by s e v e r a l groups i n order to deduce s t r u c t u r e t a s t e r e l a t i o n s h i p s , and to o b t a i n potent sweet peptides. I n the case of the peptides, the c o n f i g u r a t i o n and the conformation o f the molecule are important i n connection w i t h the s p a c e - f i l l i n g p r o p e r t i e s . The p r e f e r r e d conformations o f amino a c i d s can be shown by a p p l i c a t i o n of the extended Hiickel theory c a l c u l a t i o n . However, p r o j e c t i o n of reasonable conformations f o r d i - and t r i peptide molecules i s not e a s i l y accomplished. In the course of i n v e s t i g a t i o n s of a s p a r t y l d i p e p t i d e e s t e r s , we had to draw t h e i r chemical s t r u c t u r e s i n a u n i f i e d formula. I n an attempt to f i n d a convenient method f o r p r e d i c t i n g the sweett a s t i n g property of new peptides and, i n p a r t i c u l a r , to e l u c i d a t e more d e f i n i t e s t r u c t u r e - t a s t e r e l a t i o n s h i p s f o r a s p a r t y l d i p e p t i d e e s t e r s , we p r e v i o u s l y a p p l i e d the F i s c h e r p r o j e c t i o n technique i n drawing sweet molecules i n a u n i f i e d formula ( 4_). The sweet-tasting property of a s p a r t y l d i p e p t i d e e s t e r s has been s u c c e s s f u l l y explained on the b a s i s o f the g e n e r a l s t r u c t u r e s shown i n Figure 1 (4). A peptide w i l l t a s t e sweet when i t takes 0-8412-0526-4/79/47-115-133$05.00/0 © 1979 American Chemical Society In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

FOOD TASTE CHEMISTRY

134

COOH

COOH

I

H^C-*NH H^C—NH CO

2

CO

I

I

NH

NH Figure 1. General structure for sweet peptides: Rj = small hydrophobic side chain (1 ~ 4 atoms); R = larger hydrophobic side chain (3 ~6 atoms) (4) 2

R^C-^H

H^Ç—Ri R Ri £ R (A) Sweet

R 2

(B) Not sweet

COOH Figure 2. General structure for sweet amino acids: R, is not restricted; R = H,CH ,orC H (12) 2

3

2

5

2

2

R ^C^NH r Ri 2

2

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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the formula (A), but not when i t takes the formula (Β), where R i s l a r g e r than R R i s a small hydrophobic s i d e chain with a chain l e n g t h of 1^4 atoms and R i s a l a r g e r hydrophobic s i d e chain with a chain l e n g t h of 3^6 atoms. Ri i n formula (A) serves as the hydrophobic binding s i t e (X). In the formula (A), when Ri and R are s u f f i c i e n t l y d i s s i m i l a r i n s i z e , the sweetness potency w i l l be intense, whereas when Ri and R are of s i m i l a r s i z e , the potency w i l l be weak (Table 1). The s t r u c t u r e - t a s t e r e l a t i o n s h i p s w i l l be discussed i n d e t a i l . Dipeptide e s t e r s are c l o s e l y r e l a t e d to amino a c i d s i n chemical s t r u c t u r e and p r o p e r t i e s . Hence, we s e l e c t e d amino a c i d s as the standard to which sweet peptides were r e l a t e d . The s t r u c ­ t u r a l features of sweet-tasting amino a c i d s have been best explained by Kaneko (12) as shown i n Figure 2, i n which an amino a c i d w i l l t a s t e sweet when R i s H, CH or C H , whereas the s i z e of R i s not r e s t r i c t e In the case of a s p a r t y a f r e e α-amino group, and proton acceptor Β i s a f r e e β-carboxyl group. Therefore, the a s p a r t y l part could be r e a d i l y arranged to meet the s t r u c t u r a l requirements f o r sweet t a s t e defined by Kaneko through the F i s c h e r p r o j e c t i o n formula. The distance between the f r e e α-amino and β-carboxyl groups was considered to be w i t h i n the range defined f o r sweet molecules. In the case of the second amino a c i d such as Phe-OMe of aspartame, however, somewhat greater f l e x i b i l i t y i n drawing c o n f i g u r a t i o n s was afforded by the interchange of atoms or groups attached to the asymmetric carbon atom of the second amino a c i d . This part of amino a c i d could be replaced by a great v a r i e t y of L- or D-amino a c i d e s t e r s without l o s i n g the sweetness. This suggests that the sweet t a s t e receptor s i t e sees only the s i z e and shape of t h i s p a r t , apart from the AH-B system of L - a s p a r t i c a c i d . I t seems that the t a s t e receptor sees the second amino a c i d s as an a l k y l side chain i n the case of sweet amino a c i d s . In order to avoid confusion and to u n i f y the system, the molecular s t r u c t u r e was p r o j e c t e d so that the l a r g e s t s i d e chain attached to the asymmetric carbon atom would be at the bottom of the formula and the amino group of the peptide bond i n the upper p o s i t i o n as shown i n Figure 1. The remaining two groups such as a hydrogen atom and a smaller side chain, then l a i d i n f r o n t of the p r o j e c t i o n p l a i n . The o r i e n t a t i o n of the hydrogen atom and the smaller s i d e chain depends on the c o n f i g u r a t i o n and the s i z e of the two s i d e chains of the amino a c i d e s t e r . I t was considered that the t a s t e of the d i p e p t i d e e s t e r s changed according to the s i z e and shape of the second amino a c i d . For instance, a sweet peptide, L-Asp-L-Phe-OMe (1), corresponds to the formula (A), where R i s a methyl e s t e r group and R i s a benzyl group, whereas a nonsweet peptide, L-Asp-D-Phe-OMe (2), corresponds to the formula (B), where R i s a methyl e s t e r group and R i s a benzyl group. This evidence suggests that a peptide w i l l t a s t e sweet when i t takes the formula (A), but not when i t takes the formula 2

lt

x

2

2

2

2

3

2

5

x

x

2

x

2

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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

6

71

n

L-Asp-L-Phe-OMe L-Asp-D-Phe-OMe ε-Ac-D-Lys L-Asp-Gly-OMe L-Asp-Gly-OEt L-Asp-Gly-0C Hn L-Asp-D-Ala-OMe L-Asp-L-Ala-OMe L-Asp-D-Abu-OMe L-Asp-L-Abu-OMe L-Asp-Gly-OPr^ L-Asp-D-Ala-OPr L-Asp-D-Abu-OPr^ L-Asp-L-Nva-OMe L-Asp-L-Nva-OMe L-Asp-L-Nva-OEt L-Asp-D-Nva-OPr L-Asp-L-Nle-OMe L-Asp-L-Nle-OEt L-Asp-L-Nle-OEt L-Asp-L-Cap-OMe L-Asp-L-Cap-OEt L-Asp-L-Ser (Ac)-OMe L-Asp-L-Ser(Bt^)- OMe L - A s p - L - S e r ( B t i ) - OEt L-Asp-L-Thr(Bt^)- OMe L-Asp-L-aThr(Bt^) -OMe

Compounds

A,Β A,Β A,Β A Β A Β A,Β A A A Β Β A A A Β A A A A A A A

A Β

Projection formula*

Table 1.

3

3

3

3

CH2CH3 COOCH3 CH 3 CH 2 CH 2 CH 3 CH 2 CH 2 CH 2 CH 3 COOCH3 COOCH2CH3 C H 3 CH 2 CH 2 CH 2 COOCH3 COOCH2CH3 COOCH3 COOCH3 COOCH2CH3 C00CH COOCH3

Η CH

CH3CH2

CH2CH3

3

Η Η Η CH HC

CH3OOC

C00CH

Ri R 2

2

3

3

2

3

3

2

2

2

3

3

2

CH 0C0CH(CH ) CH 0C0CH(CH ) CH(CH )0C0CH(CH )2 CH(CH )OCOCH(CH )

COOCH2CH2CH3 COOCH2CH2CH3 CH 2 CH 2 CH 3 COOCH3 COOCH2CH3 COOCH2CH2CH3 CH 2 C H 2 C H 2 CH 3 CH 2 C H 2 CH 2 C H 3 COOCH2CH3 C H 2 C H 2 C H 2 CH 2 CH 2 C H 3 C H 2 CH 2 CH 2 CH 2 CH CH 3 CH2OCOCH3

3

COOCH3 COOCH COOCH2CH2CH3

COOCH3 COOCH2CH3 COOCeHxx COOCH3 COOCH3

C H 2 C eHg CH2C6H3

Taste of a s p a r t y l peptides

-

50 2-3

-10

45 45 5 0 47

-

0 14 170,125 95 4 0

-16

5-10 8 13 13 25

-

180

Taste**

6 6 4 4 7 4 8 8 9 9 4 8,9 9 9 9 9 4 9 7 7 7 7 9 9 9 9 9

Lit.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

n

A A A A A A A A A A A A A,Β A A A A Β

*See F i g u r e 1 f o r p r o j e c t i o n f o r m u l a .

n

L-Asp-L-Ile-OMe L-Asp-L-Lys-OMe L-Asp-L-Lys(Ac)-OMe L-Asp-L-t-HyNle-OMe L-Asp-L-e-HyNle-OMe L-Asp-L-MPA L-Asp-L-HMPA L-Asp-D-Ser-OPr L-Asp-D-Ala-OBu« L-Asp-D-Ser-OBu L-Asp-D-Thr-OPr^ L-Asp-D-aThr-OPrn L-Asp-Gly-Gly-OMe L-Asp-D-Ala-Gly-OMe L-Asp-D-Abu-Gly-OMe L-Asp-D-Val-Gly-OMe L-Asp-L-Ama (OFn) -OMe L-Asp-D-Ama(OFn)-OMe

Projection formula* 3

3

3

COOCH3 CH3OOC

3

2

3

2

2

3

3

2

CH CH OH CH OH CH CH OH CH(CH )0H CH(CH )0H Η CH CH 2 CH 3 CH(CH )

COOCH3 COOCH3 COOCH3 COOCH3

C00CH

Ri 2

2

2

2

3

3

2

3

3

2

3

CONHCH 2 COOCH 3 CONHCH 2 COOCH CONHCH 2 COOCH CONHCH COOCH3 COO-fenchyl COO-fenchyl

COOCH2CH2CH3

2

C00CH CH CH

COOCH 2 CH 2 CH 2 CH 3

CH2C5H5 CH2C6H5 COOCH2CH2CH3 COOCH2CH2CH2CH3

2

2

CH(0H)CH CH CH CH(OH)CH CH CH

CH2CH2CH2CH2NHCOCH3

CH(CH3)CH CH3 CH 2 CH 2 CH 2 CH 2 NH 2

R

**Numbers represent the sweetness potency of the compound as a m u l t i p l e of s u c r o s e . In a d d i t i o n , 0 = t a s t e l e s s , - = b i t t e r .

26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43.

Compounds

6 9 9 9 9 10 10 9 7 9 9 9 7 9 7 7 11 11

_

1.2 7 18 50 1 320 50 70 150 40 0 3 1 0 22000 0

-

Lit.

Taste**

138

FOOD TASTE CHEMISTRY

(B). T h i s a l s o suggests that the AH-B concept r e p r e s e n t s o n l y a f i r s t approximation i n the case of p e p t i d e s . C e r t a i n l y , the AH-B system i s r e q u i r e d i n the molecule. However, the s t r u c t u r a l c h a r a c t e r i s t i c s of the second amino a c i d sometimes may completely mask any AH-B e f f e c t . To t e s t the above h y p o t h e s i s , we have s y n t h e s i z e d a number of p e p t i d e s w i t h o r w i t h o u t a sweet t a s t e . The C-C bonding i n R has been replaced by e t h e r , t h i o e t h e r , amide or e s t e r bond without l o s i n g sweetness. Ri i s a s m a l l s i d e c h a i n such as a methyl, e t h y l , i s o p r o p y l , or hydroxymethyl group, or an e s t e r having a s m a l l s u b s t i t u e n t . The exact chemical nature of these groups i s not c r u c i a l . The s t u d i e s on p e p t i d e s began w i t h a c o r r e l a t i o n between sweet amino a c i d s and p e p t i d e s . Since the p r o j e c t i o n formula of L-Asp-Gly-OMe (4) i s s i m i l a r i n s i z e and shape to that of ε-Ac-DLys (3) which i s sweet, we p r e d i c t e d t h a t L-Asp-Gly-OMe would t a s t e sweet i n s p i t e o Therefore, we synthesize i t was sweet and i t s sweetness potency was almost equal to that of ε-Ac-D-Lys. Thus, the d i p e p t i d e could be c o r r e l a t e d to the amino a c i d . Lengthening (5) or enlargement (6) of the a l k y l group of the e s t e r d i d not a f f e c t i t s sweetness potency (Table 1 ) . However, when a methyl group was introduced so as to protrude on the r i g h t of the p r o j e c t i o n formula of L-Asp-Gly-OMe, the r e s u l t a n t L-Asp-D-Ala-OMe (7) (8) was sweeter than L-Asp-Gly-OMe. This r e s u l t suggests that the methyl group i s i n v o l v e d i n a hydro­ phobic i n t e r a c t i o n a t the r e c e p t o r s i t e and causes the increased sweetness potency. On the other hand, when a methyl group was introduced so as to protrude on the l e f t of the p r o j e c t i o n formula of L-Asp-Gly-OMe, the r e s u l t a n t L-Asp-L-Ala-OMe (8) (8) was not sweet but b i t t e r . Loss of sweetness suggests that i n t e r a c t i o n w i t h the receptor s i t e may be blocked by the methyl group. This a l s o supports the idea t h a t a d i p e p t i d e w i l l not t a s t e sweet when i t takes the formula (Β), i n which R protrudes on the l e f t . I n t r o d u c t i o n of an e t h y l group i n s t e a d of the methyl group so as to protrude on the r i g h t of the p r o j e c t i o n formula of L-Asp-GlyOMe gave L-Asp-D-Abu-OMe ( 9 ) , which was 16 times sweeter than sucrose. I n t r o d u c t i o n of an e t h y l group on the o p p o s i t e s i d e gave L-Asp-L-Abu-OMe (10), which was devoid of sweetness. The low l e v e l of sweetness of ( 9 ) , as compared w i t h ( 7 ) , may show that the p o p u l a t i o n of the sweet formula (A) may s i g n i f i c a n t l y decrease because the two groups do not d i f f e r g r e a t l y i n s i z e . This idea gained f u r t h e r support when a methyl group was introduced on the r i g h t of the p r o j e c t i o n formula of L-Asp-GlyOPrtt (11) to g i v e L-Asp-D-Ala-OPr^ (12), which was 125 times sweeter than sucrose. L-Asp-D-Ala-OPr^ was about 9 times sweeter than L-Asp-Gly-OPrtt. I n the molecule of L-Asp-D-Ala-OPt^, the s i z e s of CH (Ri) and C00CH CH CH (R ) are s u f f i c i e n t l y d i s s i m i l a r . An e t h y l group was introduced i n s t e a d of the methyl group to give L-Asp-D-Abu-OPr (13), which was 95 times sweeter than sucrose and was l e s s sweeter than L-Asp-D-Ala-OPr^. Lengthening the a l k y l 2

x

3

2

2

3

2

n

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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group of the e s t e r of L-Asp-D-Abu-OMe increased the sweetness potency; L-Asp-D-Abu-OPr was 6 times sweeter than L-Asp-D-AbuOMe. This f a c t may show that the sweeter compound (13) takes predominantly the sweet formula (A), s i n c e the s i z e s of the two groups are s u f f i c i e n t l y d i s s i m i l a r . More i n t e r e s t i n g i s the case of L-Asp-L-Nva-OMe (14). Since the s i z e s of COOCH and CH CH CH are almost equal, both the sweet formula(A) and the nonsweet formula (B) could be drawn f o r the d i p e p t i d e , so that i t could be p r e d i c t e d that the peptide would be s l i g h t l y sweet. In f a c t , i t was only 4 times sweeter than sucrose. Of course, replacement of the methyl group by an e t h y l group r e s u l t e d i n a compound (L-Asp-L-Nva-OEt (15)) l a c k i n g i n sweetness as expected from i t s p r o j e c t i o n formula. Some d i peptides c o n t a i n i n g D-norvaline were a l s o sweet, when R and R matched the sweet formula (A), e.g., L-Asp-D-Nva-OPr (16) was 45 times sweeter than sucrose Thus, i t i s p l a u s i b l depend on the c o n f i g u r a t i o y depends on the s i z e and shape of t h i s amino a c i d e s t e r . L-Asp-L-Nle-OMe (17) was s t r o n g l y sweet, but L-Asp-L-Nle-OEt (18) was only s l i g h t l y sweet. These d i f f e r e n c e s could be e a s i l y p r e d i c t e d from examination of t h e i r p r o j e c t i o n formulas. Both the sweet formula (A) and the nonsweet formula (B) could be drawn f o r the l a t t e r peptide. L-Asp-L-Cap-OMe (19) was sweet, whereas the e t h y l e s t e r (20) was not sweet but b i t t e r , though we could draw the sweet formula (A) to i t . This may show that the increased hydrophobicity i n the molecule changed the property of the sweet peptide to a b i t t e r property, because i t has been known that b i t t e r - t a s t i n g compounds are composed of charge and hydrophobic p r o p e r t i e s . This a l s o suggests that the hydrophile-hydrophobe balance i n a sweet molecule i s a very important f a c t o r . An a l k y l s i d e c h a i n of the second amino a c i d could be replaced by an e s t e r group without l o s i n g the sweetness, e.g., L-Asp-L-Ser(Ac)-OMe (21), L-Asp-L-Ser(Bt^)-OMe (22) and L-Asp-LS e r ( B t i ) - O E t (23) were sweet. The replacement of the L - s e r i n e by L-threonine or by L-aZZothreonine r e s u l t e d i n b i t t e r compounds (24, 25). These r e s u l t s matched that the i n t r o d u c t i o n of a methyl group i n t o a sweet peptide, L-Asp-L-Nva-OMe, r e s u l t e d i n a b i t t e r substance (L-Asp-L-Ile-OMe (26)). The methyl group may b l o c k the i n t e r a c t i o n between the peptides and the sweet r e c e p t o r . From the above d i s c u s s i o n , we have concluded that a hydrophobic b i n d i n g s i t e i s necessary f o r a s e r i e s of potent sweet peptides. Next, we examined how a h y d r o p h i l i c group would a f f e c t the sweetness potency. The i n t r o d u c t i o n of an amino group to L-Asp-L-Nle-OMe (17) r e s u l t e d i n a b i t t e r compound (L-Asp-L-Lys-OMe (27)) and b l o c k i n g the amino group recovered the sweetness by some extent (28). The i n t r o d u c t i o n of a hydroxyl group i n t o a peptide with the L-L c o n f i g u r a t i o n (17, 31) r e s u l t e d i n a diminution i n the potency n

3

2

2

3

1

n

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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140

FOOD TASTE CHEMISTRY

(29, 30, 32). Contrary to the peptides with the L-L c o n f i g u r a t i o n , the i n t r o d u c t i o n of a hydroxyl group i n t o the L-D peptides d i d not always r e s u l t i n a diminution of t h e i r potencies, but sometimes increased t h e i r potencies. L-Asp-D-Ser-OR (R=Me, E t , P r , Pr^, Butt, Bu*- or c-hexyl) was sweeter than the corresponding peptides without a hydroxyl group, L-Asp-D-Ala-OR (9), e.g., compounds (33) and (35) were sweeter than compounds (12) and (34), r e s p e c t i v e l y . In the case of threonine-containing peptides, L-Asp-D-Thr-OR (R=Me or Pr^) was sweeter than L-Asp-D-Abu-OR which l a c k s a hydroxyl group of the D-threonine. On the contrary, when the Dthreonine was replaced by D-α Ζ Z.0 threonine, the potency diminished significantly. The IR s p e c t r a of these peptides showed that the hydroxyl a b s o r p t i o n had disappeared due to strong hydrogen bonding. I t i s considered that the apparent exceptions may be due to r i g i d i n t r a m o l e c u l a p h i l i c i t y and allowing Returning to stereoisomerism, the r e l a t i o n s h i p s between stereoisomerism and t a s t e w i l l be discussed by using stereoisomers of aspartame (L-Asp-L-Phe-OMe) as model compounds. The l a c k of sweet t a s t e i n a-L-Asp-D-Phe-OMe (6) i s r e a d i l y explained a f t e r c o n s i d e r i n g the p r o j e c t i o n formula ((2) i n Figure 3), i n which a small s i d e chain on the l e f t may cause e l i m i n a t i o n of sweetness. According to Mazur et al β-D-Asp-L-Phe-OMe (6) i s b i t t e r though i t s p r o j e c t i o n formula ((44) i n Figure 3) would suggest that i t has a sweet t a s t e . This r e s u l t can not be explained f u l l y . However, d i p e p t i d e e s t e r s c a r r y i n g a small hydrophobic group on the 5th carbon from the carbon bearing the AH(NH ) o f t e n t a s t e d b i t t e r ; e.g., L-Asp-L-Ile-OMe (26) and L-Asp-L-aThr(Bt^)-OMe (25) were b i t t e r . The l o c a t i o n of C00CH from the AH(NH ) i n the peptide (44) corresponds to that of CH i n these peptides. The l a c k of sweet t a s t e i n β-L-aspartyl d i p e p t i d e e s t e r s such as β-LAsp-Gly-OMe (7) and β-L-Asp-L-Phe-OMe (6) i s r e a d i l y explained a f t e r c o n s i d e r i n g t h e i r p r o j e c t i o n formulas ((45) i n Figure 3), i n which the second amino a c i d l i e s on the l e f t . This formula i s incompatible with that d e f i n e d f o r sweet amino a c i d s , i n which the second amino a c i d corresponds to R of Figure 2. And a l s o the peptide does not f i t the s p a t i a l b a r r i e r model f o r the receptor s i t e proposed by Shallenberger et al. (13). The l a c k of sweet t a s t e i n α-D-aspartyl d i p e p t i d e e s t e r s such as a-D-Asp-L-Phe-OMe (6) i s i n t e r p r e t e d analogously a f t e r c o n s i d e r i n g t h e i r p r o j e c t i o n formulas ((46) i n F i g u r e 3). Therefore, we have concluded that sweet-tasting a s p a r t y l d i p e p t i d e e s t e r s can be drawn as the u n i f i e d formula (A), whereas nonsweet peptides as (B) as shown i n Figure 1. There i s no asymmetric carbon atom i n aminomalonic a c i d molecule. When both of the c a r b o x y l i c a c i d s are s u b s t i t u t e d by e s t e r i f i c a t i o n with d i f f e r e n t a l c o h o l s , o p t i c a l isomers are generated. I t i s known that aminomalonic a c i d d e r i v a t i v e s r e a d i l y racemize i n s o l u t i o n under o r d i n a r y c o n d i t i o n s . L-Asp-Ama (OFn) n

9

2

3

2

3

2

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

ARiYOSHi

Sweet Peptides

COOH

COOH

CH 2

CH 2

I

I

H*-C-*NH

H^C—NH

CO

CO

2

I f o

2

I

o

H*. C-*C-0-CH

CHs-O-C^C-^

3

180 α-L-L

(1)

a-L-D (2)

β-D-L (44)

COOH

I

0

COOH

R-C-CH ^ Ç - * N H 2

2

H

0

ÇH

2

R-C*-C—NH

2

H

3-L-L(or D) (45)

a-D-L(or D) (46)

R=Phe-OMe Figure 3. Projection formulas of isomers of aspartame (L-Asp-L-Phe-OMe)

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

142

FOOD TASTE CHEMISTRY

OMe was found by Fujino et al.(11) to be 22000^33000 times sweeter than sucrose. I t i s not e x a c t l y known whether the sweett a s t i n g isomer has the L-L(or S-R) or the L-D(or S-S) c o n f i g u r a t i o n because of ready racemization. From the examination of i t s p r o j e c t i o n formula, i t could be p r e d i c t e d that the L-L(or S-R) isomer (42), i n which aminomalonic a c i d d i e s t e r takes an L ( o r R)c o n f i g u r a t i o n , would be sweet. This p r e d i c t i o n agreed with that reported i n the l i t e r a t u r e (14). In Ama-L-Phe-OMe (47) (14, 15), i t i s a l s o not known whether the sweet-tasting isomer has the L-L(or S-S) or the D-L(or R-S) c o n f i g u r a t i o n . In the case of a s p a r t y l d i p e p t i d e e s t e r s , the L-L isomer was sweet. By analogy, other researchers deduced that the L-L(or S-S) isomer ((47b) i n Figure 4) would be sweet. However, i t seemed to us that the D(or R)-configuration would be p r e f e r r e d for the aminomalonic a c i d because the D-L(or R-S) isomer ((47a) i n Figure 4) was compatible with the sweet formula and could a l s o f i t the s p a t i a l b a r r i e r mode could n e i t h e r f i t the recepto Further examinations of the molecular f e a t u r e s and of the model of receptor have suggested that s e v e r a l a s p a r t y l t r i p e p t i d e e s t e r s may a l s o t a s t e sweet. In confirmation of the idea, s e v e r a l t r i p e p t i d e e s t e r s have been synthesized. In the f i r s t p l a c e , LAsp-Gly-Gly-OMe (38) was synthesized as an a r b i t r a r i l y - s e l e c t e d standard of t r i p e p t i d e s , because i t was considered that t h i s peptide e s t e r had the simplest s t r u c t u r e , and c o r r e l a t i o n of other peptides to (38) was easy. The t r i p e p t i d e e s t e r was p r e d i c t e d that i t would be s l i g h t l y sweet or t a s t e l e s s because i t s p r o j e c t i o n formula was s i m i l a r i n s i z e and shape to that of L-Asp-Gly0Bu which i s 13 times sweeter than sucrose (16) and because i t i s more h y d r o p h i l i c than the d i p e p t i d e . The t r i p e p t i d e (38) was devoid of sweetness and almost t a s t e l e s s . Next, L-Asp-D-Ala-Gly-OMe (39) was synthesized i n order to evaluate the c o n t r i b u t i o n of a small s i d e c h a i n , which i s properly o r i e n t e d to e l i c i t sweetness i n the p r o j e c t i o n formula. The peptide was speculated to be sweet. As expected, i t was sweet. L-Asp-D-Abu-Gly-OMe (40) was s e l e c t e d as a next candidate i n order to determine i t s sweetness i n t e n s i t y r e l a t i v e to L-Asp-DAla-Gly-OMe (39). The sweetness i n t e n s i t y of t h i s peptide was p r e d i c t e d to be lower than that of L-Asp-D-Ala-Gly-OMe a f t e r examining t h e i r formulas. As expected, the synthesized L-Asp-DAbu-Gly-OMe was sweet, and i t s sweetness i n t e n s i t y was lower than that of L-Asp-D-Ala-Gly-OMe. F i n a l l y , L-Asp-D-Val-Gly-OMe (41) was synthesized i n order to see whether i t remained sweet. The peptide was devoid of sweetness and almost t a s t e l e s s , though D - v a l i n e - c o n t a i n i n g a s p a r t y l d i p e p t i d e e s t e r s such as L-Asp-D-Val-OPr^ (17) and L-Asp-D-ValOPr^ (8, 17), which are s i m i l a r to the t r i p e p t i d e e s t e r i n s i z e and shape and have potent sweet t a s t e . As mentioned above, the second amino a c i d of the sweet a s p a r t y l d i p e p t i d e e s t e r s could be replaced by d i p e p t i d e e s t e r s n

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

5.

ARiYOSHi

Sweet Peptides

143

such as D-Ala-Gly-OMe and D-Abu-Gly-OMe without l o s i n g the sweetness. However, t h e i r sweetness potencies were c o n s i d e r a b l y lower than those of a s p a r t y l d i p e p t i d e e s t e r s w i t h the s i m i l a r s i z e and shape. Replacement of the second amino a c i d of a sweet a s p a r t y l d i p e p t i d e e s t e r such as L-Asp-Gly-OBu by Gly-Gly-OMe r e s u l t e d i n l o s i n g the sweetness ((38) i n Figure 5 ) , i n s p i t e of i t s s i m i l a r i t y i n the p r o j e c t i o n formula to that of the sweet d i p e p t i d e e s t e r . These f a c t s suggest t h a t the t r i p e p t i d e e s t e r s are more h y d r o p h i l i c than the d i p e p t i d e e s t e r s and the h y d r o p h i l i c property caused the sweetness i n t e n s i t y to decrease. The conformation of the t r i p e p t i d e e s t e r s has, of course, i n f l u e n c e on the e l i c i t a t i o n of sweetness i n connection w i t h the s p a c e - f i l l i n g p r o p e r t i e s of sweet compounds. However, the conformational problem can not be discussed here because i t has not been i n v e s t i g a t e d . In the case of s m a l l - s i z e d sweeteners such as g l y c i n e (48) and a l a n i n e (49), the sweetness s e n s a t i o n occurs only by the AH-B system and the sweetnes l y . In the case of medium-size asparty d i p e p t i d e e s t e r s , two types of i n t e r a c t i o n have been considered. Among the a s p a r t y l d i p e p t i d e e s t e r s without a hydrophobic b i n d i n g s i t e such as L~Asp-Gly-OPr (11), the sweetness s e n s a t i o n has occurred only by the AH-B system, l i k e g l y c i n e , and the sweetness i n t e n s i t y i s comparatively low. On the other hand, i n t r o d u c t i o n of a small hydrophobic group i n t o the sweet molecule so as to i n t e r a c t w i t h a hydrophobic s i t e of the receptor r e s u l t s i n a sweeter compound such as L-Asp-D-Ala-OPr (12). The small hydrophobic group introduced p l a y s a r o l e i n enhancing the sweetness i n t e n s i t y by forming a hydrophobic bond w i t h the r e c e p t o r s i t e . This f a c t has been s u c c e s s f u l l y explained by the theory of the AH-B-X system, i n which X i s the " d i s p e r s i o n " s i t e proposed by K i e r and has been proved e x p e r i m e n t a l l y by us to be a hydrophobic b i n d i n g s i t e ( 4 ) . Therefore, i n the case of medium-sized molecules, we have been able to conclude that formation of a hydrophobic bond causes the sweetness potency to i n c r e a s e . On the other hand, i n the case of a s p a r t y l t r i p e p t i d e e s t e r s (39, 40), i t appears that a s m a l l hydrophobic s i t e f o r hydrophobic i n t e r a c t i o n i s necessary to f i t the receptor s i t e . One problem that remains i s the mode of i n t e r a c t i o n between the sweet peptides and the receptor s i t e . Despite a great number of s t u d i e s , the mechanism of a c t i o n of sweet s t i m u l i on the receptor i s not w e l l known. Stereoisomerism can be r e s p o n s i b l e f o r d i f f e r e n c e s i n t a s t e responses, and s p a c e - f i l l i n g p r o p e r t i e s are a l s o very important. These f a c t s suggest that the receptor s i t e e x i s t s i n a three-dimensional s t r u c t u r e . In t h i s connection, the sense of sweet t a s t e i s subject to the " l o c k and key" of biological activity. The above d i s c u s s i o n s , i n c o n j u n c t i o n w i t h previous r e s u l t s , support our previous idea that the receptor s i t e f o r sweet t a s t e i s composed of the AH-B-X system and i t s most l i k e l y shape i s a "pocket" as shown i n F i g u r e 6 ( 5 ) . In t h i s model, the s p a t i a l w

n

n

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

144

FOOD TASTE CHEMISTRY

COOH H^C^NH

COOH 2

R-\

@

-Ala-Met-Ala-Pro-Lys-His-Lys-Glu-Met-Pro-Phe-Pro-Lys-Tyr-Pro-Val-Gin-Pro-Phe-Thr® -Gln-Ser-Glu-Ser-Leu-Thr-Leu-Thr-Asp-Val-Glu-Asn-Leu-His-Leu-Pro-Pro-Leu-Leu-Leu-

Θ

- Gin -Ser - Tyr-Met-His

- Gin - Pro-His - Gin-Pro-Leu-Pro-Pro

®

@

- Thr - Val-Met-Phe-Pro-Pro-Glu

® -Ser-Val-Leu-Ser-Leu-Ser-Gln-Ser-Lys-Val-Leu-Pro-Val-Pro-Glu-Lys-Ala-Val-Pro-Tyr-

®

®

\Q--2497,Pelissier">\

-

-

@

Pro-Gin-Arg-Asp-Met-Pro-lle-Glu-Ala-Phe-Leu-Leu-Tyr-Gln-Gln-Pro-Val-Leu-Gly-ProQ = 1700, Gordon * 64

Q=2085,Pelissier > 62

-Val-Arg-Gly-Pro-Phe-Pro-lle-lle-Val I Q-- 1700,

@

Gordon * 64

Figure 3.

Bitter peptides from β-casein

Table XV B i t t e r peptides from peptic Zein hydrolysates Peptide Ala-Ile-Ala Ala-Ala-Leu Leu-Gin-Leu Leu-Glu-Leu Leu-Val-Leu Leu-Pro-Phe-Asn-Glu-Leu Leu-Pro-Phe-Ser-Glu-Leu

Q 1477

1293 1613 1797 2177 1682 1688

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

|

162

FOOD TASTE CHEMISTRY

course of a h y d r o l y s i s of a protein, b i t t e r peptides would be formed (72). Generally pure proteins are considered to be without any taste* Secondary, t e r t i a r y and quaternary structures generally prevent a taste impression being ob­ tained* The following Table XVI gives the Q-values of some proteins* Table XVI Q-values of proteins and b i t t e r hydrolysates derived Protein Collagen Gelatin Bovine muscular tissue Wheat Gluten Zein Soybean p r o t e i n Potato p r o t e i n Casein

Q 1280 1280 1300

B i t t e r hydrolysates known

1480

no no no yes yes

1600

yes

1420

It i s i n t e r e s t i n g to see that proteins with high Qvalues above 1400 as e.g. soybean protein, c a s e i n wheat gluten, potato protein, Zein are the "parents" of b i t t e r peptides, whereas no b i t t e r peptides have been i s o l a t e d from hydrolysates prepared from c o l l a g e n or g e l a t i n , proteins with Q-values below 13ΟΟ. Petrischek (74) confirmed that the p r o t e i n and not the protease i s responsible f o r the occurence of b i t t e r pep­ t i d e s * However, when the "parent" proteins are not b i t t e r but the peptides derived from them are b i t t e r , the questions a r i s e as to why t h i s i s so and as to where we must place the molecular weight l i m i t s of peptides with Q > 1400 that are also not b i t t e r * An i n d i c a t i o n of the values to be expected can be obtained from the r e s u l t s of our synthesis of b i t t e r peptides with Q 1400 and molecular weights up to 2000 Dalton* Fujimaki (75) i s o l a t e d from the peptic hydrolysate of soybean p r o t e i n a non-dialysable b i t t e r peptide of a molecular weight of about 2800 Dalton* P i l n i k (76) found by the p r o t e o l y s i s of soybean p r o t e i n i n a membrane-filtration apparatus that no b i t t e r peptides e x i s t e d with molecular weights above 6000 Dalton* Clegg (49) obtained from digests of Casein with Papain a b i t t e r peptide having a molecular weight of about 3OOO Dalton* Fujimaki (77*78) condensed b i t t e r soybean p r o t e i n hydro­ l y s a t e s i n a Plastein-Reaction (79) and obtained non-bitter p r o t e i n - l i k e products, unfortunately without determination of molecular weights* We studied the influence of chain length on the b i t t e r ­ ness of peptides by gel permeation chromatography of

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

6.

NEY

163

Bitterness of Peptides

enzymatic protein hydrolysates (80). Table XVII sums up the r e s u l t s of these experiments* We can conclude, that a l i m i t of about 6000 Dalton can be placed on the molecular weight* Table XVII Molecular weights and tastes of enzymatic Parent Protein

Soybean p r o t e i n Soybean protein Casein Casein Wheat Gluten Potato protein Potato protein Gelatine

Q

1540 1540

16Ο5 1605 142 1567 1567 1280

Molecular weight of hydrolysate i n Dalton

hydrolysates Taste b i t t e r nonbitter

4000

X

125ΟΟ 4000 8000

X

X

8000 3000

X

X X

Above t h i s molecular weight, also peptides with a Qvalue above 1400 w i l l no longer e x h i b i t b i t t e r taste* I t i s c l e a r therefore, that 2 ways e x i s t to come to non-bitter protein hydrolysates* As demonstrated i n Figure 4 a) choice of the s t a r t i n g material, t h i s means proteins with Q-values below 13ΟΟ b) choice of the working conditions, t h i s means, i f the Qvalue of the s t a r t i n g protein i s above 1400, c a r e f u l h y d r o l y s i s to obtain peptides with main molecular weights of above 6000 Dalton* It should be pointed out, that we were concerned with presence or absence of bitterness* B i t t e r n e s s i n terms of sensory threshold values or b i t t e r n e s s r a t i n g s was not assessed* What i s now the current state of a f f a i r s of the Q-ruleî As mentioned i t has been accepted and applied by the s c i e n t i s t s working i n t h i s f i e l d . The most comprehensive and c a r e f u l assessment of the Q-rule has been c a r r i e d out by Guigoz and Solms (54). They found that the r u l e can be applied to the majority of the b i t t e r peptides known and observed, that only peptides containing glycine sometime do not comply f u l l y with the rule* They therefore propose, that glycine should be l e f t out of the c a l c u l a t i o n s , which then gives Q-values higher than 1400 f o r a l l b i t t e r peptides* Guigoz and Solms conclude that the Q-values should be a usef u l assessment of the r e l a t i o n s h i p between amino acid composition and the b i t t e r taste of peptides* Wieser and B e l i t z (Ql) have suggested a very i n t e r e s t i n g extension of the r u l e * They obtained the b i t t e r n e s s threshold values of

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

164

FOOD TASTE CHEMISTRY

a) Q < 1300 non bitter Da I ton to*

ι to* Amino acid residues Protein

Amino acid

b) Q >1400 6000 Mol. wt. Dalton bitter

;

Amino acid

non bitter

W

4

to* Amino acid residues Protein

Figure 4. Molecular weight, average hydrophobicity Q, and bitter taste of peptides

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

6.

NEY

Bitterness of Peptides

165

d i - and t r i p e p t i d e s by c a l c u l a t i n g the sum of the hydro­ phobicity of the "backbone" peptide c o n s i s t i n g only of glycine residues adding to i t the hydrophobicities of the side chains* In t h i s way an estimation of the threshold values of d i - and t r i p e p t i d e s was obtained* We now i n v e s t i g a t e d the hypothesis i f the b i t t e r n e s s of l i p i d s - and carbohydrates - could also be linked to hydro­ phobic i n t e r a c t i o n s (82,241

40

2,400

40

1,400

40 40

480 480

25

300

50 >275

b- Non-Auqeous F l a v o r Precursors 1- N e u t r a l l i p i d s 25 50 2- P o l a r L i p i d s 25 50 3- Isoprenoids 150 5 4- Carotenoids 10 25 5- U n s a p o n i f i a b l e compounds Subtotal >210 22

Grand t o t a l

>451

2,500 >8,960 1,250 1,250 750 250

>3,500 >12,460

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

9.

MABROUK

Browning Reaction Products

209

peptides i n p l a n t s c o n s i s t of γ-glutamyl d i p e p t i d e s along w i t h some t r i p e p t i d e s , one of which i s homoglutathione ( y - g l u t a m y l c y s t e i n y l - g - a l a n i n e ) which r e p l a c e s g l u t a t h i o n e i n some enzymic r e a c t i o n s Q ) . About t h i r t y - f i v e amino a c i d s and amines occur n a t u r a l l y i n foods. D-ribose i s found i n nucleo­ t i d e s , D - r i b u l o s e , D-lyxose, D - x y l u l o s e , glucose, f r u c t o s e , sedoheptulose, and glyceraldehyde are present as phosphoric a c i d e s t e r s i n the products of carbohydrate metabolism. Fucose i s a c o n s t i t u t e n t of blood group substances which c o n t a i n g a l a c t o s e and mannose. The presence of carbohydrate a l c o h o l s i n vegetables, f r u i t s and animals may be accounted f o r by a r e d u c t i o n of the reducing sugars, or through d e c a r b o x y l a t i o n of the corresponding higher a l d o n i c a c i d s . Sorbose, glucose, sucrose, i n o s i t o l , m a n n i t o l , p e n t i t o l and two u n i d e n t i f i e d sugars were reported i n cocoa beans (4). Sugar a c i d s , mono- d i d t r i - c a r b o x y l i a c i d s ketohydroxy-acids as w e l l a i n foods. While u r o n i c a c i d s p a r t i c i p a t e the browning r e a c t i o n , others might a c c e l e r a t e or r e t a r d the r e a c t i o n . Non-aqueous f l a v o r p r e c u r s o r s c o n t r i b u t e d i r e c t l y and i n d i r e c t l y to food f l a v o r s . Boar meat l i p i d s c o n t a i n 5-androst16-ene-3-one which i s r e s p o n s i b l e f o r n o t i c e a b l e u n d e s i r a b l e notes (5). The c h a r a c t e r i s t i c f l a v o r s of mutton and goat meat are a t t r i b u t e d to the presence of odd-numbered η-fatty a c i d s , and abnormal p r o p o r t i o n of branched chain f a t t y a c i d s (6). During p r o c e s s i n g of foods, l i p i d s undergo a u t o x i d a t i o n , h y d r o l y s i s , dehydration, d e c a r b o x y l t i o n , and degradation. Thermal degradation of n e u t r a l l i p i d s , p o l a r l i p i d s and f r e e f a t t y a c i d s produce the f o l l o w i n g c l a s s e s o f compounds (7-13) : - ( l ) n - and i s o - a l k a n a l s , (2) a l k a d i a n a l s , (3) o x o - a l k a n a l s , (4) a l k e n a l s , (5) a l k a d i e n a l s , (6) aromatic aldehydes, (7) methyl ketones, (8) s a t u r a t e d and unsaturated a l c o h o l s , (9) γ- and 6- l a c t o n e s , (10) hydrocarbons, (11) keto- and hydroxy-acids, (12) d i c a r b o x y l i c a c i d s , and (13) s a t u r a t e d and unsaturated f a t t y a c i d s , c o n t a i n i n g l e s s carbon atoms than the parent ones. L i p i d s i n foods vary from t r a c e s as i n c e r e a l s to 30-50% as i n nuts. The p h y s i c a l s t a t e and d i s t r i b u t i o n of l i p i d s vary c o n s i d e r a b l y among food items. In each item l i p i d d i s t r i b u t i o n s a f f e c t i t s f l a v o r as i t undergoes chemical r e a c t i o n s and a c t as a f l a v o r components v e h i c l e or p a r t i t i o n i n g medium. Furthermore, l i p i d s have a pronounced e f f e c t upon the s t r u c t u r e of food items. F a t t y a c i d s of n e u t r a l ( t r i g l y c e r i d e s ) and p o l a r l i p i d s of beef and pork are t a b u l a t e d i n Table I I I . Pork f a t contains more unsaturated f a t t y a c i d s than beef and i t s l i n o l e i c a c i d content i s double t h a t i n beef. Heat produced v o l a t i l e s i n red meat f a t s are l i s t e d i n Table IV. The presence of p y r a z i n e s i n the v o l a t i l e s of beef f a t i s due to the presence of nitrogenous compounds i n the f a t amounting to 0.1-0.2%N, ( K j e l d h a l method). These compounds might be p r o t e i n s , p e p t i d e s , amino a c i d s , and amine moitiés i n p o l a r l i p i d s . Such compounds

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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210

Table I I I F a t t y Acids Composition of Beef and Pork L i p i d s

F a t t y Acids

Triglcerides Polar L i p i d s Beef Pork Beef Pork % of T o t a l F a t t y A c i d Content

a.- Saturated Capric Laurie Myristic Palmitic Stearic

0.1 0.1 2.2 27.5

0.1 0.2 1.2 23.9

2.6 13.2

2.0 20.0

Total

46.8

37.0

31.4

33.0

1.0 4.7 41.3

7.4 45.3

0.9 2.2 21.2

0.2 2.3 16.2

47.0

52.6

24.3

18.8

0.6 4.4 -

-

1.3 20.2

-

0.6 27.9 0.9

5.0

8.7

21.5

29.3

b. - Monounsaturated Tetradecenoic Palmitoleic Oleic Total - Dienoic Tetradecenoic Linoleic Docasadienoic Total d. - T r i e n o i c Linolenic Eicosatrienoic Total Tetraenoic Arachidonic Total

8.7

-

-

-

1.1

1.6

-

-

1.8 1.9

1.0 1.6

1.1

1.6

3.7

2.6

0.1

0.1

19.1

16.3

0.1

0.1

19.1

16.3

Reference (14)

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

9.

MABROUK

Browning Reaction Products

Table IV Heat Produced Carbonyls I n Red Meat Fats V o l a t i l e Compounds ALKANALS Ethanal(Acetaldehyde) n-Propanal n-Pentanal n-Hexanal η-Hep t a n a l n-Octanal n-Nonanal n-Decanal n-Undecanal n-Dodecanal METHYL-ALKANALS Methylpropanal Me t h y l b u t a n a l

Pork

Beef

10 10 7 7,10 7 7,10 7,10 7 7 7

9a, 10 9a, 10 8 8,9,10 8,9 8,9 8,9 8,9

7

9a

OXOALKANAL 2-0xopropanal(Pyruvaldehyde)

9a

2-ALKENAL 2t-Butenal(Crotonal) 2t-Heptenal 2t-0ctenal 2t-Nonenal 2t-Decenal 2t-Undecenal

7 7,10 7,10 7,10 10

9a 8,9 8,9 8,9 8,9 8

2,4-Alkadienal 2,4-Heptadienal 2,4-Nonadienal 2,4-Decadienal

10 10 10

8,10

7

10

10

9a 8,9a

ALKANEDIAL Ethanedial(Glyoxal)

2-ALKAN0NE Acetone 2-Butanone 2-Decanone 2-Undecanone 2-Tridecanone 2-Pentadecanone 2-Hep tadecanone

Lamb

10 10 10

10 9a 7 7 8 8 8

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

212

FOOD TASTE CHEMISTRY

Table IV (cont'd) Heat Produced Carbonyls V o l a t i l e Compounds

i n Red Meat Fats Pork

Beef

HYDROXY-2-ALKANONE 3-Hydroxy-2-butanone(acetoin)

9a

PYRAZINE 2,5-Dimethyl2-Ethyl2-Ethyl-3,6-dimethyl2-Ethyl-5-methyl2,3,5-Trimethyl-

8 8 8 8 8

Reference (7,8,9,10) "a designates h e a t i n g 11

nitroge

Lamb

atmosphere,

degrade d u r i n g h e a t i n g and produce ammmonia and b a s i c compounds. When ammonia, amino a c i d s and degraded nitrogenous compounds r e a c t w i t h α-dicarbonyls produced from l i p i d o x i d a t i o n , p y r a z i n e s a r e formed. Cooked chicken f l a v o r concentrate (13) contained the f o l l o w i n g aldehydes; 3c-nonenal; 4c-decenal; 2 t , 4 c - d e c a t r i e n a l ; 2t,5c-undecadienal; 2t-dodecenal; 2 t , 4 c - d o c a d i e n a l ; 2t,6c- and 2t,6t-dodecadienal 2 t - t r i d e c e n a l ; 2 t , 4 c - t r i d e c a n d i e n a l ; 2 t , 4 c , 7 c - t r i d e c a t r i e n a l ; and 2 t , 4 c - t e t r a d e c a d i e n a l . Three of these aldehydes: 4c-decenal; 2t,6c-dodecadienal; and 2 t , 4 c , 7 c - t r i d e c a t r i e n a l a r e t y p i c a l breakdown o f a r a c h i d o n i c a c i d , and to a major extent a l s o 2t,5c-undecadienal. These aldehydes p l a y an important r o l e i n cooked chicken f l a v o r v i a the browning r e a c t i o n . 2,4-Decadienal which i s considered t o be a key compound of chicken aroma, was found i n the v o l a t i l e s of cooked chicken (15,16). 2,4-Heptadienal; 2,4-nonadienal and 2,4-decadienal were i d e n t i f i e d i n heated pork f a t (10) and heated beef f a t (8,10). D i c a r b o n y l s , d i e n a l s , t r i e n a l s undergo amino-carbonyl r e a c t i o n s , r e s u l t i n g probably i n the formation of meaty f l a v o r notes i n s p e c i f i c p r o p o r t i o n s c h a r a c t e r i s t i c of each s p e c i e s . Although t h e i r y i e l d i s s m a l l i n comparison w i t h monocarbonyls, t h e i r h i g h r e a c t i v i t i e s a r e r e s p o n s i b l e f o r t h i e r important r o l e i n f l a v o r p r o d u c t i o n . The complexity of food f l a v o r p r e c u r s o r s i s manifested by the number of compounds estimated i n each c l a s s of compounds and by the p o s s i b l e t o t a l number i n a s i n g l e raw food. The p o s s i b l e number o f f l a v o r components produced p e r one compound of f l a v o r p r e c u r s o r i s estimated i n the range 10-150, but i s reduced t o 5-40 t o e l i m i n a t e compounds that might be produced

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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213

by more than one precursor and to account f o r v a r i a t i o n s i n p r o c e s s i n g methods. One hundred compounds were i d e n t i f i e d i n thermal degradation products o f glucose (17,18,19): aldehydes ketones, aromatics, f u r a n s , oxygenated f u r a n s , n o n - v o l a t i l e s . Sugars r e a c t w i t h ammonia at temperatures ranging from 20 t o 260°C., to produce h e t e r o c y c l i c compounds : s u b s t i t u t e d i m i d a z o l e s , s u b s t i t u t e d p y r a z i n e s , s u b s t i t u t e d p i p e r a z i n e s , p y r i d i n e and s u b s t i t u t e d p y r i d i n e s ( 2 0 ) . A t 0-120°C., a-dicarbonyls (2,3-butadione, e t h a n e d i a l " g l y o x a l " , 2-oxopropanal "pyruvaldehyde , and α-hydroxycarbonyl " g l y c o l a l d e h y d e " r e a c t w i t h ammonia i n presence o f formaldehyde to g i v e s u b s t i t u t e d i m i d a z o l e s . Dipeptides r e a c t w i t h sugars e i t h e r as an e n t i t y or as amino a c i d s produced by h y d r o l y s i s during p r o c e s s i n g . I n the f i r s t case, one compound i s formed, pyrazinone (21), i n the second case numerous f l a v o r compounds are produced from the r e a c t i o n of the two amino a c i d s w i t h sugar. The r e a c t i v i t y of d i p e p t i d e s towards r e a c t i o higher than t h a t of amin i s very b i t t e r , the product o f i t s r e a c t i o n w i t h g l y o x a l has an a s t r i n g e n t , a l i t t l e sour and l a t e r a m i l d t a s t e . Some products from the browning r e a c t i o n and p y r o l y s i s o f f l a v o r precursors r e a c t w i t h ammonia and hydrogen s u l f i d e , thus i n c r e a s i n g the number o f f l a v o r components produced from food p r e c u r s o r s . The molecular s t r u c t u r e o f the amino a c i d i n f l u e n c e s the f l a v o r notes i n the food. For example, thiophenes are reported i n the v o l a ­ t i l e s produced from the p y r o l y s i s of c y s t e i n e o r from i t s r e a c t i o n w i t h glucose or pyruvaldehyde (22,24) but were absent i n the case of c y s t i n e (23,24). Hundreds o f compounds have been i d e n t i f i e d i n the v o l a t i l e f l a v o r components of processed foods. Hydrocarbons, a l c o h o l s , e t h e r s , aldehydes, ketones, a c i d s , a c i d anhydrides, e s t e r s , aromatic, l a c t o n e s , pyrones, f u r a n s , p y r i d i n e s , p y r r o l e s , n-alkylpyrrole-2-aldehydes, pyrazines, s u l f i d e s , d i s u l f i d e s , t h i o l s , thiophenes, t h i a z o l e s , t r i t h i o l a n e s , t h i a l d i n e . . . e t c . Each compound has a c h a r a c t e r i s t i c note and a s p e c i f i c t h r e s h o l d which makes i t s c o n t r i b u t i o n to food f l a v o r unique. But none o f the i n d i v i d u a l compounds were reported to completely produce the c h a r a c t e r i s t i c f l a v o r of the processed food. This i n d i c a t e s that the food f l a v o r s are of mixed f l a v o r notes o f numerous compounds, w h i l e others are necessary f o r i t s s y n e r g i s t i c e f f e c t , others exert a background of modifying e f f e c t . Browning r e a c t i o n has become a major t o p i c o f food research because o f i t s relevance t o the d e s i r a b l e and u n d e s i r a b l e changes ( f l a v o r , c o l o r , t e x t u r e , and n u t r i t i v e value) o c c u r r i n g i n foods during p r o c e s s i n g and storage. During the Second World War and i t s post e r a , research on food d e t e r i o r a t i o n due to nonenzymic browning r e c e i v e d c o n s i d e r a b l e a t t e n t i o n due t o the problems r e s u l t i n g from the n e c e s s i t y to prepare dehydrated foods and food concentrates which had to be s t o r e d under t r o p i c a l c o n d i t i o n s without s e r i o u s d e t e r i o r a t i o n . 11

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

214

FOOD TASTE CHEMISTRY

The purpose o f t h i s paper i s t o review the numerous papers p u b l i s h e d on f l a v o r s , t a s t e s and odors r e s u l t i n g from the browning r e a c t i o n . I n v e s t i g a t i o n s of model systems which have been observed under l a b o r a t o r y c o n d i t i o n s are considered and t h e i r p o s s i b l e s i g n i f i c a n c e i n b a s i c and i n d u s t r i a l processes w i l l be discussed. S p e c u l a t i o n on the p o s s i b l e c o r r e l a t i o n between model system r e s u l t s and s p e c i f i c processed food items w i l l be presented. R e s u l t s o f recent work i n our l a b o r a t o r y on f l a v o r notes developed upon heating r i b o s e w i t h v a r i o u s amino a c i d s w i l l be discussed. The numerous p u r e l y chemical papers are considered t o be o u t s i d e the scope of t h i s paper. The reader i s r e f e r r e d t o s e v e r a l reviews on the s u b j e c t (25-30). Comprehensive reviews on nonenzymic browning i n r e l a t i o n t o s p e c i f i c food problems had been published (31,32,33). As f l a v o r productio complicated r e a c t i o n s du chemists concentrated t h e i r research e f f o r t s on simpler systems to understand the r e a c t i o n s i n v o l v e d and t h e i r products. Amino Acids-Sugars Model Systems Model system s t u d i e s o f the r e a c t i o n between s i n g l e amino a c i d s and n a t u r a l l y o c c u r r i n g substances capable of r e a c t i n g w i t h them has f u r n i s h e d v a l u a b l e i n f o r m a t i o n l e a d i n g toward an under­ standing of food f l a v o r s . The w e l l known S t r e c k e r degradation of α-amino a c i d s , Schonberg and Moubacher (25), has been used as the c e n t r a l r e a c t i o n around which other amino compound degradation systems may be o r i e n t e d . I n t h i s r e a c t i o n α-amino a c i d s a r e deaminated and decarboxylated by s p e c i f i c carbonyl compounds or others t o y i e l d aldehydes and ketones c o n t a i n i n g one carbon atom less. 3-amino a c i d s a l s o undergo o x i d a t i v e deamination and d e c a r b o x y l a t i o n t o a ketone w i t h one l e s s carbon atom; e.g., 3-amino-n-butyric a c i d produces 2-propanone, which a l s o r e s u l t s rrom the S t r e c k e r degradation of α-amino-isobutyric a c i d . Organic d i - and t r i c a r b o n y l s are not the only oxidants e f f e c t i v e i n S t r e c k e r degradations. Hydrogen peroxide i n presence o f f e r r o u s s u l f a t e degrade α-amino a c i d s a t room temperature ; f o r example g l y c i n e y i e l d s formaldehyde. The c o - o x i d a t i o n of s u l f u r c o n t a i n i n g amino a c i d s i n an a u t o - o x i d i z i n g l i p i d system i n d i c a t e s that l i p i d peroxides a c t as oxidants (34). The r e s u l t i n g carbonyl compounds from amino a c i d s through S t r e c k e r degradation p a r t i c i p a t e i n the formation o f d e s i r a b l e c h a r a c t e r i s t i c f l a v o r s during p r o c e s s i n g o r u n d e r s i r a b l e ones r e s p o n s i b l e f o r f l a v o r d e t e r i o r a ­ t i o n d u r i n g storage. F l a v o r s and odors given by amino a c i d s and sugars i n d i l u t e aqueous s o l u t i o n s a t d i f f e r e n t temperatures has been the s u b j e c t of i n t e n s i v e s t u d i e s by v a r i o u s researchers. Tables V and VI summarize the d e s c r i p t i v e aroma evolved from r e a c t i n g carbonyl compounds w i t h amino a c i d s a t 100, and 120/180°C., r e s p e c t i v e l y

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

weak caramelÇ35) strong,yeasty protein

hydrolyzate(35)

α-Alanine Valine

α-Aminobutyr i c Leucine

Phenylglycine Methionine

Threonine

Serine

Isoleucine

baked potato(35)

Glycine

baked potato(35)

moderate c r u s t (35) vaguely breadl i k e (35) v e r y weak(35)

strong,cheesy,

Dihydroxyacetone

Amino Compounds

chocolate(37) maple (38) *" b i t t e r almond(38) overcooked sweet potato(36) potato(37)

objectionable chopped cabbage(36)

overcooked cabbage(36)

obj e c t i o n a b l e weak NH (36)

weak(36)

caramelized sugar unpleasant (36,39), f a i n t carmel beer(40). smell(36). beer aroma(40) rye bread(37) fruity,aromatic (39) maple(38) sweet c h o c o l a t e ( 3 7 ) toast(39),bread(40) rye bread(38) musty(37),fruity, aromatic(39) maple syrup(38)

unpleasant burned wood(36)

3

Sucrose

Maltose

Glucose

Fructose

Aromas Developed by the Reactions of Carbonyl Compounds and V a r i o u s Amino Compounds at 100°C

Table V

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Phenylalanine

Glutamine Me thy lamine Ammonia

Histidine

Arginine

Hydroxyproline

Proline

Cystine

Cysteine

Amino Compounds

9

very s t r o n g , hyacinth(35)

v e r y weak(35)

very s t r o n g cracker,crust, toast(35) weak, vaguely l i k e proline(35) very weak(35)

(35)

mer cap tan

Dihydroxyacetone

Fructose

Maltose

popcorn(37) butterscotch(39) buttery note(39), none(37) chocolate(37) empyreumatic t a s t e ( 4 0 ) t a r r y odor, b i t t e r taste(40) r a n c i d caramel, s t i n g i n g smell, pleasant unpleasant(36) very objecsweet v i o l e t s ( 3 7 ) rose tionable(36) caramel(36) perfume(39)

meat(39) sulfide(37) meat, burnt turkey s k i n ( 3 9 ) corn-like(39) burnt p r o t e i n (37) potato(39)

Glucose

Aromas Developed by the Reactions o f Carbonyl Compounds and V a r i o u s Amino Compounds a t 100°C

Table V (cont'd)

unpleasant sweet caramel(36)

Sucrose

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

References

(35,40)

a-Methylaminobutyric a-Aminoisobutyric

s t r o n g dark corn syrup(35)

very weak(35) c h i c k e n broth(35)

Tyrosine Aspartic Glutamic

Arginine Lysine

Dihydroxyacetone

Amino Compounds

maple(38)

maple(38)

caramel(37) rock candy(37) oldwood pleasant(36) b u t t e r y note(39) baked sweet potato(36)

Glucose

objectionable fried butter (36)

too weak(36)

Fructose

Sucrose

unpleasant wet wood (36)

r o t t e n wet potato(36

too weak(36) p l e a s a n t caramel(36)

Maltose

Aromas Developed by the Reactions of Carbonyl Compounds ans V a r i o u s Amino Compounds a t 100°C

Table V (cont'd)

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

References

Glutamic Arginine

(37,38,41)

brunt brunt

sugar(37) sugar(37)

violets,lilac (41) caramel(37)

strong flower (41) s t r o n g breadc r u s t (41) moderate(41) no s i g n i f i c a n t aroma(41)

Phenylalanine

Aspartic

burnt(37)

objectionable burnt cheese(37)

penetrating chocolate(37) burnt cheese(37)

weak(41)

moderate breadcrust(41) moderate breadcrust(41) moderate breadcrust(41)

Aroma D e s c r i p t i o n 120°C 180°C

Threonine

Isoleucine

Leucine

Valine

Amino A c i d

Tryptophan

Glutamine

Histidine

strong(41)

s t r o n g baked potato(41) s t r o n g breadcrust cracker (41) no s i g n i f i c a n t aroma(41)

Methionine Proline

weak(41)

cornbread(37) b u t t e r y note(38) butterscotch(37)

p l e a s a n t bakery aroma(37)

potato(37)

bread-like(37)

Aroma D e s c r i p t i o n 120°C 180°C

Lysine

Amino A c i d

Aromas Developed by Heating Glucose w i t h V a r i o u s Amino A c i d s at 120°C and 180°C

Table VI

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(35-41). The aroma produced by the i n t e r a c t i o n o f sugars and amino a c i d s ranged from p l e a s a n t t o o b j e c t i o n a b l e . The molecular s t r u c t u r e o f sugar i n f l u e n c e s the r e s u l t i n g aroma o f the products of i t s r e a c t i o n w i t h the amino a c i d . While a p l e a s a n t caramel sweet aroma developed upon r e a c t i n g p h e n y l a l a n i n e w i t h maltose, an unpleasant caramel aroma developed w i t h f r u c t o s e , and a h y a c i n t h aroma i n case o f dihydroxyacetone. While the aroma o f methionine-sugar browning product was r e m i n i s c e n t o f a p l e a s a n t baked potato i n presence o f dihydroxyacetone, aroma notes o f overcooked potatoes developed w i t h glucose. I n presence o f a reducing d i s a c c h a r i d e (maltose), overcooked cabbage aroma was n o t i c e d and an unpleasant burnt wood aroma developed i n presence of non-reducing d i s a c c h a r i d e (sucrose). P r o l i n e , v a l i n e and i s o l e u c i n e i n presence o f glucose gave a p l e a s a n t bakery aroma (41). Keeney and Day (42) s t u d i e d the c h a r a c t e r odor o f the products of S t r e c k e r Degradation o f amino a c i d s u s i n g i s a t i n as the oxidant (Table V I I ) Table V I I Aroma o f S t r e c k e r Degradation Products o f α-Amino A c i d s with I s a t i n (Diketodihydroindole) Sensory D e s c r i p t i o n Amino A c i d s Aldehyde Keeney & Day(42) Others(43.44) α-Alanine acetaldehyde reminiscent of wine, malty coffee(43) Valine 2-methy1apple f r u i t y , b a n a n a - l i k e (4 3) propanal green,pungent sweet(44) Leucine 3- methylmalty fruity,peach-like(43) butanal burnt,green,sickly(44) Isoleucine malty-apple 2-methylburnt,sickly(44) butanal Norleucine pentanal flower slightly fruity,herb­ aceous , n u t - l i k e (43) burnt-green(44) Phenyl­ h y a c i n t h odor,resenfole phenylacetviolets alanine benzaldehyde i n aldehyde taste(43) Glutamic b a c t e r i a l agar Cystine methional cheesy-brothy References (42,43,44) There i s some agreement and disagreement between the sensory notes s t a t e d i n Tables V and V I I , which may be a t t r i b u t e d t o v a r i a t i o n s i n the c o n c e n t r a t i o n o f the r e a c t a n t s and l a c k o f agreement on d e s c r i p t i o n o f sensory notes. The d i s c r e p a n c i e s can be r e s o l v e d , by a c a r e f u l c o n s i d e r a t i o n of the s p e c i f i c c o n d i t i o n s of the r e a c t i o n . I t i s p o s s i b l e t h a t both the nature and extent o f the

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r e a c t i o n vary widely with the p r i n c i p a l v a r i a b l e s ( c o n c e n t r a t i o n , temperature, h e a t i n g p e r i o d , s p e c i f i c r e a c t a n t s ) . U n f o r t u n a t e l y , many of the papers i n s u f f i c i e n t l y d e f i n e the experimental c o n d i t i o n s f o r an adequate e v a l u a t i o n of the r e s u l t s . The d e s c r i p t i o n of odor and t a s t e notes of a product or a compound v a r i e s according to i t s c o n c e n t r a t i o n , the media used f o r e v a l u a t i o n ( water, p a r a f f i n o i l , skim m i l k . . .etc ) > and the sensory e v a l u a t o r . The data i n Table VI i n d i c a t e that the aromas developed when sugars and amino a c i d s were h e a t i n g a t 120 and 180°C, were q u i t e d i f f e r e n t from those produced a t 100°C. N o t i c e a b l e d i f f e r e n c e s e x i s t between aroma notes developed a t 120°C and 180°C, t h i s may be a t t r i b u t e d t o sugar degradation notes which become n o t i c e a b l e i n the system heated a t 135*C and above. A l l a l i p h a t i c compounds c o n t a i n i n g primary o r secondary amine group r e a c t i n brownin depending upon the molecula r e a c t a n t s (amino a c i d s , carbonyl compounds) i n the browning r e a c t i o n had been reported i n the l i t e r a t u r e as : a. i n t e n s i t y o f c o l o r developed, b. amount of carbonyl compounds determined by GC o r as dinitrophenylhydrazones. c. percent l o s s of r e a c t a n t s (one r e a c t a n t or both). I t i s very hard t o compare r e s u l t s reported i n the l i t e r a t u r e , as there are v a r i o u s v a r i a b l e s i n f l u e n c i n g the data. Concentrat i o n of r e a c t a n t s , r a t i o of amine compound to sugar o r c a r b o n y l , r e a c t i o n i n b u f f e r o r water, m o l a r i t y and type of b u f f e r used, pH o f b u f f e r , d u r a t i o n o f r e a c t i o n . Rate of carbonyl compounds formation w i l l be the c r i t e r i o n used f o r r e a c t i v i t y except i n few occasions where the percent l o s s of one o f the r e a c t a n t s or both w i l l be r e f e r r e d t o i n the manuscript. Rooney e t a l (£5) reported that the r a t e of carbonyl formation v a r i e d w i t h the molecular s t r u c t u r e of sugar. Xylose was most r e a c t i v e as i t produced the g r e a t e s t q u a n t i t y of c a r b o n y l s , f o l l o w e d by glucose, then maltose. I n the presence of these sugars i s o l e u c i n e was more r e a c t i v e than phenylalanine. In a study on the S t r e c k e r degradation of v a l i n e - c a r b o n y l , d i a c e t y l showed the g r e a t e s t r e a c t i v i t y f o l l o w e d by sorbose> arabinose>xylose>fructose>glucose>sucrose>rhamnose, S e l f ( 4 6 ) . The number of v o l a t i l e carbonyls produced by the r e a c t i o n of glucose and glutamic a t pH 5.0,6.5 and 8.0 a t 100°C was almost equal. When these systems were heated a t 180°C, the number of v o l a t i l e s was g r e a t e r and increased more d r a s t i c a l l y as pH increased to a l k a l i n i t y (36). The amount of aldehydes produced from mixtures of 0.01M amino a c i d and 0.1M glucose i n water was f a r below those produced i n 0.01M phosphate b u f f e r a t pH6.5, which i n t u r n was f a r below the amount obtained a t pH 7.5. The a d d i t i o n of phosphate b u f f e r increased the amount of aldehydes produced s i g n i g i c a n t l y . The amount of aldehyde produced i s a l s o i n f l u e n c e d by the chain l e n g t h of the a c i d . I n g e n e r a l ,

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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s t r a i n g t chain amino acids produced more aldehydes than branched chain amino acids o f the same number o f carbon atoms (k6). Alanine, v a l i n e , s e r i n e , glutamic, glutamine, methionine, t a u r i n e , h i s t i d i n e , c r e a t i n e , c i t r u l l i n e , carnosine, c y s t i n e , s y s t e i n e , a s p a r t i c , asparagine, l e u c i n e , i s o l e u c i n e , t y r o s i n e , p h e n y l a l i n i n e , tryptophan, α-aminobutyric, p r o l i n e , o r n i t h i n e , 2 - p y r r o l i d o n e - 5 - c a r b o x y l i c , threonine, g l y c i n e , l y s i n e , a r g i n i n e , cysteine were i d e n t i f i e d i n aqueous r e d meat f l a v o r precursors ( V f ) . Ribose was a l s o reported as the major sugar. F l a v o r evalu­ a t i o n o f the browning r e a c t i o n products o f these amino acids and r i b o s e were undertaken. 0.1M s o l u t i o n o f each amino a c i d was pre­ pared by d i s s o l v i n g the amino a c i d i n 0.05M S^rensen phosphate b u f f e r and r e a d j u s t i n g the pH by the a d d i t i o n o f monopotassium o r dipotassium phosphate. 0.1M r i b o s e s o l u t i o n was prepared i n the same b u f f e r . Ten ml o f amino a c i d and 10ml o f ribose s o l u t i o n s were p i p e t t e d i n 50ml glas with 2k/kO glass j o i n t weighed d i r e c t l y i n r e a c t i o n v e s s e l s , and 10ml b u f f e r added. To study the e f f e c t o f heating on f l a v o r o f amino acids i n absence of r i b o s e , 10ml b u f f e r were p i p e t t e d i n glass ampoules and f l a s k s along with the amino a c i d . The ampoules were sealed a f t e r evacu­ a t i o n , and heated f o r one hour at l80°C. The f l a s k s were heated i n g l y c e r o l bath at 100°C, a f t e r f i t t i n g with condensors. A f t e r the heating p e r i o d , the f l a s k s and the ampoules were removed, cooled i n wet i c e , and the contents t r a n s f e r r e d t o v i a l s and kept at -10°C. A l l experiments were run i n t r i p l i c a t e s . In absence of r i b o s e , while s u l f u r containing amino acids s o l u t i o n s produced s u l f u r y notes upon heating, none o f the others developed any f l a v o r notes. Table VIII l i s t s the s t r i k i n g f l a v o r notes produced by heating various amino a c i d s - r i b o s e s o l u t i o n s at 100° and l80°C. Table VIII F l a v o r Notes Produced by Heating Amino Acid-Ribose AT 100°C and l80°C Amino A c i d

Alanine Valine Serine Glutamic

Methionine Cysteine Taurine

Flavor 100°C

Description

very m i l d caramel s i c k l y sweet sweet b o u i l l i o n brothy, s l i g h t l y sweet, l i n g e r i n g i n mouth s u l f u r y , savory s u l f u r y , r o t t e n egg pleasant t o f f e e

180°C sweet burnt caramelized sugar penetrating burnt chocolate burnt sugar roasted meat

c r u s t o f roast meat s u l f u r y , s p i c y meat s i c k l y , sweet, burnt caramel

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Table V I I I (cont'd) F l a v o r Notes Produced by Heating Amino Acid-Ribose At 100°C and 180°C Amino A c i d

Tryptophan Phenylalanine Histidine

Creatine Tyrosine Leucine Aspartic Cystine Glutamine Asparagine a-Aminob u t y r i c γ-Aminobuturic 3-Aminobutyric Arginine Ornithine Proline Cysteic

Flavor Description 100°C 180°C

o i l y aromatic, sugar sweet sharp f l o w e r salty, slightly bitter toffee slightly salty s l i g h t caramel b i t t e r almond bread crumb hard b o i l e d egg yolk caramel w i t h burnt sugar note d e s i r a b l e burnt sugar undersirable burnt sugar burnt sugar custard burnt sugar bread crumb bread crumb undersirable, sulfury brothy

2-Pyrrolidone -5-carboxylic Isoleucine aromatic, undesirable Glycine caramel, f a i n t Lysine custard Homocystine canned m i l k Threonine c u s t a r d weak Citrulline toffee-like Carnosine buttery-toffee

o i l y aromatic, naphththalene flowery w i t h aromatic and caramel notes, u n d e r s i r a b l e pleasant s l i g h t l y burnt

s l i g h t l y sweet caramel custard s l i g h t l y burnt sugar toasted bread caramelized bread c r u s t meaty w i t h H2S note butterscotch creamy b u t t e r s c o t c h sugar maple maple maple b u t t e r y , burnt sugar bread-like cracker, toast meaty, s u l f u r y meaty, p l e a s a n t burnt cheese burnt sugar bread scorched b o i l e d m i l k burnt c u s t a r d meaty meaty

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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The previous d i s c u s s i o n covered S t r e c k e r degradation o f amino a c i d s sugar aqueous s o l u t i o n s . A l i m i t e d number o f i n v e s t i ­ gations were c a r r i e d out on the nature of the browning r e a c t i o n a t h i g h temperature, i . e . low temperature p y r o l y s i s ana at low moisture content. Rohan and coworkers had provided an e x t e n s i v e research on model systems s i m u l a t i n g cocoa n i b r o a s t i n g i n absence of l i p i d . Amino a c i d s and sugars were r e s p o n s i b l e f o r chocolate f l a v o r development (48,49,50,51). While the degradation o f amino a c i d s i n cocoa beans d u r i n g r o a s t i n g was incomplete, the degrading agent, reducing sugar was completely destroyed. In a study on model systems prepared from s i n g l e amino a c i d and glucose i n molar r a t i o 2:1, approximating the composition of s h e l l ~ f r e e cocoa beans, Rohan and Stewart (52) reported t h a t amino a c i d d e s t r u c t i o n from h e a t i n g was temperature dependent and p r a c t i c a l l y ceased a f t e r one hour. Sugar d e s t r u c t i o n c o n t i n t u e d at a r a t e dependen the end o f the experimen Table IX D e s t r u c t i o n of Amino A c i d s and Ρeducing Sugars on Heating Chocolate Aroma P r e c u r s o r E x t r a c t Heating D e s t r u c t i o n Percent °C Min. Amino A c i d s Red. Sugars T o t a l Sugars Aroma 50 50 50 100 100 100 120 120 120 150 150 150 150

6.2 8.9 8.9 18.8 29.6 30.9 43.0 54.4 54.4 58.8 81.7 85.4 85.1

30 60 120 30 60 120 30 60 120 10 30 60 120

Reference

(52)

27.0 41.1 27.5 56.6 81.4 84.1 100 93.5

35.8 79.6 22.8 38.9 79.2 91.2

Cocoa Cocoa Cocoa Cocoa Cocoa Cocoa Cocoa Cocoa

journal of Food Science

The f a i l u r e of amino a c i d degradation to go to completion and the r e l a t i o n between maximal d e s t r u c t i o n and temperature might be a t t r i b u t e d to the f a c t that i n d i v i d u a l amino a c i d s reacted a t d i f f e r e n t r a t e s at d i f f e r e n t temperatures. For example c e r t a i n amino a c i d s , l e u c i n e , a r g i n i n e , methionine, and l y s i n e showed l i t t l e or no d e t e c t a b l e r e a c t i o n a t 100°C a f t e r one hour, w h i l e others were r e a c t i v e a t t h i s temperature(Table X). At higher temperatures, r a t e o f amino a c i d degradation was measurable and p r o p o r t i o n a l t o the temperature o f the r e a c t i o n .

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The e f f e c t of i n c r e a s i n g temperature on the amount of amino a c i d destroyed i n one hour was n o t i c e a b l y greater a t temperatures above 140°C than below 120°C. Between 120° and 140 C,,increase i n r e a c t i o n temperature had l e s s marked e f f e c t on the r e a c t i o n due to changes i n the p h y s i c a l c o n d i t i o n of the r e a c t i o n mixture. At about 150°C. glucose melts and , as i t might be expected, i t s mixture w i t h amino a c i d s would begin to melt a t lower temperature. e

Table X D e s t r u c t i o n of Amino A c i d s A f t e r Heating With Glucose a t D i f f e r e n t Temperatures f o r One Hour Temp. °C 80 90 100 110 120 130 135 140 150

D e s t r u c t i o n Percent Threo G l u Meth Leu V a l A r g L y s

A l PhAl

11.3 24.3 33.4 44.8 76.9

13.9 21.0 22.2 23.2 32.0

2.0 22.9 41.2 42.2 59.4 90.0

59.8

23.1 28.8 28.5 26.2 40.5 39.0 44.3

10.5 19.5 27.2 39.9

23.1 26.2 28.2 30.2 42.5

50.9 51.9 62.1

1.0 14.5 29.0 33.8 40.0

4.5 19.5 28.0 30.2 44.5

51.5

Reference (52)

Journal of Food Science

Table XI i n d i c a t e s that the degree of degradation was i n f l u e n c e d by the molecular s t r u c t u r e of the amino a c i d . A t 120°C the r a t e of d e s t r u c t i o n of amino a c i d s by c l a s s was i n the f o l l o w i n g order: s u l f u r - c o n t a i n i n g , hydroxy>acidic>neutral, basic,aromatic. Table XI Degradation of Amino Compounds A f t e r Heating f o r One Hour i n Presence of Glucose Class of Amino Compounds Percent Degradation a t ^ 120 135 150 Neutral a- n - A l k y l b*- i s o - a l k y l Hydroxy Aromatic Acidic Sulfur-containing Basic

20-28 42 23 36 41 20-29

40 42.5 90 — — —

40-45

45 — —

60 51 52 —

Reference (,52)

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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Moisture content o f the r e a c t i o n mixture i n f l u e n c e d the degradation o f amino a c i d s , Rohan and Stewart (52) reported that chocolate precursor aroma e x t r a c t s when heated i n the dry s t a t e f o r one hour a t 100°C, l o s t 30% o f the amino a c i d s by degradation. When very l i g h t l y moistened the amino a c i d s degradation dropped t o 9% under the same r e a c t i o n c o n d i t i o n s . When the r e a c t i o n mixture was wetted w i t h an equal weight o f water, no amino a c i d degradation was n o t i c e d . Cocoa beans moisture content averaged 5 t o 6 % which might be s u f f i c i e n t to i n h i b i t browning r e a c t i o n a t lower temperatures, thus i n the e a r l y stages o f r o a s t i n g the removal of moisture i s important. Roasting cocoa beans r e s u l t s i n the p r o d u c t i o n of v o l a t i l e and n o n - v o l a t i l e compounds which c o n t r i b u t e t o the t o t a l f l a v o r complex. 5-Methyl-2-phenyl-2-hexenal, which e x h i b i t e d a deep b i t t e r p e r s i s t a n t coco f r a c t i o n (53). I t was condensation of phenylacetaldehyde and i s o v a l e r a l d e h y d e w i t h the subsequent l o s s of water. The two aldehydes were the p r i n c i p a l products o f S t r e c k e r degradation products of phenylal a n i n e and l e u c i n e , r e s p e c t i v e l y . N o n - v o l a t i l e s contained d i k e t o p i p e r a z i n e s ( d i p e p t i d e anhydride) which i n t e r a c t w i t h theobromine and develop the t y p i c a l b i t t e r n e s s o f cocoa (54). Theobromine has a r e l a t i v e l y s t a b l e m e t a l l i c b i t t e r n e s s , b u t cocoa b i t t e r n e s s i s r a p i d l y n o t i c e d and disappears q u i c k l y . While cocoa b i t t e r n e s s i s f e l t i n the whole mouth, theobromine i s recognized by the h i n d p a r t o f the tongue. Furthermore, cocoa b i t t e r n e s s i s more intense than t h a t o f concentrated aqueous s o l u t i o n of theobromine. The f o l l o w i n g d i k e t o p i p e r a z i n e s had been i d e n t i f i e d as the components r e s p o n s i b l e f o r cocoa b i t t e r n e s s : - cyclo(-Pro-Leu-), cyclo(-Val-Phe-), cyclo(-Pro-Phe-), cyclo(-Pro-Gly-) , c y c l o ( - A l a - V a l - ) , c y c l o ( - A l a - G l y - ) , c y c l o (Ala-Phe-), c y c l o ( - P h e - G l y - ) , c y c l o ( - P r o - A s n - ) , and c y c l o (-Asn-Phe-). Those c o n t a i n i n g phenylalanine e x h i b i t e d b i t t e r n e s s resembling that of theobromine. D i k e t o p i p e r a z i n e s have stronger b i t t e r n e s s than the corresponding d i p e p t i d e s o r i t s two i n d i v i d u a l amino a c i d s . They develop upon h e a t i n g p r o t e i n s t o temperature above 100°C Kato e t a l (55) reported that r o a s t i n g s e r i n e a t 280°C f o r 30 min under slow n i t r o g e n stream, r e s u l t e d i n the p r o d u c t i o n o f 2,5-diketo-3,6-dimethylpiperazine, which upon a l k a l i n e h y d r o l y s i s produced the d i p e p t i d e a l a n y l a l a n i n e . A b i t t e r p e p t i d e , cyclo(-L-Leu-L-Trp-), was i s o l a t e d from c a s e i n enzymic d i g e s t (J>6). The formation o f c y c l o (-Pro-Leu-) i n aged sake i s r e s p o n s i b l e f o r i t s b i t t e r n e s s (57)· Recently, f i v e b i t t e r L - p r o l i n e - c o n t a i n i n g d i k e t o p i p e r a z i n e s were reported i n roasted malt (210°C) used i n dark beer brewing. These compounds and t h e i r approximate " b i t t e r t h r e s h o l d % v a l u e s " i n aqueous s o l u t i o n s were: cyclo(-L-Phe-L-Pro-),0.1; cyclo((-L-Leu-L-Pro) , 0.05; cyclo(-L-Pro-L-Pro-),0.1; c y c l o ( - L - V a l - L - P r o - ) , 0 . 1 ; and c y c l o ( - L - I l e - L - P r o - ) , 0 . 0 5 . The authors concluded that these f

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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d i k e t o p i p e r a z i n e s d i d not c o n t r i b u t e d i r e c t l y to beer b i t t e r n e s s and questioned t h e i r r o l e i n i n f l u e n c i n g the t a s t e of the product (58). More than 300 compounds had been i d e n t i f i e d i n cocoa v o l a t i l e s , 10% of which were c a r b o n y l compounds (59,60). Acetaldehyde, 2-methylpropanal, 3-methylbutanal, 2-methylbutanal, phenylacetaldhyde and propanal were products of S t r e c k e r degradation of a l a n i n e , v a l i n e , l e u c i n e , i s o l e u c i n e , phenylacetaldehyde, and α-aminobutyric a c i d , r e s p e c t i v e l y . Eckey (61) reported that raw cocoa beans c o n t a i n about 50-55% f a t s , which c o n s i s t e d of p a l m i t i c (26.2%), s t e a r i c (34.4%), o l e i c (37.3%), and l i n o l e i c (2.1%) a c i d s . During r o a s t i n g cocoa beans these a c i d s were o x i d i z e d and the f o l l o w i n g c a r b o n y l compounds might be produced:- o l e i c : 2-propenal, b u t a n a l , v a l e r a l d e h y d e , hexanal, h e p t a n a l , o c t a n a l , nonanal, decanal, and 2-alkenals of C3 to C - Q . L i n o l e i c : ethanal, propanal C3 to C , 2 , 4 - a l k a d i e n a l and h e x e n - l , 6 - d i a l . Carbonyl compounds p l a y a major r o l e i n the formation of cocoa f l a v o r components. Another example of food i n which the browning r e a c t i o n occurs at near anhydrous c o n d i t i o n i s c o f f e e . Coffee beans are u s u a l l y r o a s t e d a t temperatures ranging from 180° to 260°C. f o r s p e c i f i c p e r i o d to o b t a i n the d e s i r e d degree of r o a s t i n g ( l i g h t , medium, and d a r k ) . P r o t e i n , sucrose, and c h l o r o g e n i c a c i d were the compounds d r a s t i c a l l y destroyed i n c o f f e e beans upon r o a s t i n g , Table X I I "the data i n t h i s t a b l e were not c o r r e c t e d f o r dry weight l o s s which v a r i e d from 2 to 5%"(62). 1Q

Table X I I

Composition of Green and Roasted Coffee Percent, Dry B a s i s Roasted Green

Constituent Hemicelluloses Cellulose Lignin Fat Caffeine Sucrose Chlorogenic a c i d Protein(Based on n o n a l k a l o i d nitrogen) Trigonelline Reducing sugars Unknown Total

23.0 12.7 5.6 11.4 1.2 7.3 7.6

24.0 13.2 5.8 11.9 1.3 0.3 3.5

11.6 1.1 0.7 14.0

3.1 0.7 0.5 31.7

100.0 Reference

100.00

(62) Journal of Agricultural and Food Chemistry

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

9.

MABROUK

227

Browning Reaction Products

The degree of r o a s t i n g i n f l u e n c e d the magnitude of degradation of sucrose, the major sugar present i n c o f f e e , Table X I I I . Table X I I I E f f e c t of Roasting On Sucrose Content I n Coffee (Percent Dry B a s i s ) Colombian Sucrose % Loss Content Green 4.59 L i g h t Roast 0.45 Medium Roast 0.17 Dark Roast 0.06 Reference (62)

90.20 96.30 98.69

Santos Sucrose % Loss Content 5.47 0.68 0.27 0.10

87.57 95.06 98.17

Feldman et a l (62) found no n o t i c e a b l e changes i n the contents of the f o l l o w i n g amino a c i d s i n roasted c o f f e e beans proteins: alanine, glutamic, g l y c i n e , i s o l e u c i n e , l e u c i n e , p h e n y l a l a n i n e , p r o l i n e , t y r o s i n e , and v a l i n e . These f i n d i n g s are i n agreement w i t h those reported by Underwood and Deatherage (63). Table XIV showed the degree of d e s t r u c t i o n of amino a c i d s i n c o f f e e bean a f t e r r o a s t i n g . I t i s q u i t e obvious that the magnitude of nonenzymic browning of amino a c i d s was i n f l u e n c e d by i t s molecular s t r u c t u r e and degree of r o a s t i n g . Coffee o i l contains about 47% l i n o l e i c , 8% o l e i c , 1% hexadecenoic, 32% p a l m i t i c , 8% s t e a r i c , and 5% behenic and longer c h a i n f a t t y a c i d s (64). As l i n o l e i c a c i d i s the major unsaturated f a t t y a c i d i n c o f f e e o i l , i t s major o x i d a t i o n products 2,4a l k a d i e n a l s and hexen-1,6-dial would p l a y a major r o l e i n v o l a t i l e p r o d u c t i o n . Green c o f f e e beans c o n t a i n 50 to 60% carbohydrates: 18% nonhydrolyzable c e l l u l o s e , 13% h y d r o l y z a b l e c e l l u l o s e , 13% starches and p e c t i n s e a s i l y s o l u b l i z e d , and 9-12% s o l u b l e carbohydrates of which sucrose i s the major component. R a f f i n o s e and stachyose are the t r i - and t e t r a saccharides reported i n robusta c o f f e e beans. Arabinogalactan and galactomannon are the w a t e r - s o l u b l e p o l y s a c c h a r i d e s reported i n c o f f e e beans (62). The above mentioned carbohydrates, amino a c i d s , l i p i d s along w i t h other f l a v o r p r e c u r s o r s produce s e v e r a l hundreds of v o l a t i l e and n o n v o l a t i l e compounds through d i f f e r e n t r e a c t i o n s during r o a s t i n g . Molecular s t r u c t u r e s , q u a n t i t i e s , and r a t i o s of these compounds i n f l u e n c e c o f f e e f l a v o r . The v a r i e t y of c o f f e e as w e l l as the degree of r o a s t i n g e x e r t c h a r a c t e r i s t i c f l a v o r s . The molecular s t r u c t u r e of carbonyls i n f l u e n c e the type of p r y a z i n e formed. While r e f l u x i n g rhamnose (100g) and ammonia (28%; 40ml) i n water (100g) produced methyl- and e t h y l - s u b s t i t u t e d p y r a z i n e s , glucose and ammonia r e a c t i o n r e s u l t e d i n formation of m e t h y l - s u b s t i t u t e d

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

5.60 3.73

Serine Threonine

Reference (62)

Loss %

39.09

2.85 6.19 2.06

Histidine Lysine Methionine

Total

4.72 10.50 3.44

Green

Arginine Asparagine Cysteine

Amino A c i d

Table XIV

47.56

20.50

1.77 2.43

1.99 2.54 2.32

0.00 9.07 0.38

Roast I

Haita

51.80

18.84

1.26 1.83

2.17 2.74 1.48

0.00 9.02 0.34

Roast I I

37.85

5.88 3.82

2.79 6.81 1.44

3.61 10.61 2.89

Green

40.79

22.41

2.60 2.71

2.27 3.46 1.08

0.00 9.53 0.76

Roast I

Columbia

(After Acid Hydrolysis),%

60.02

15.63

0.80 1.38

1.61 2.76 1.26

0.00 7.13 0.69

Roast I I

17.73

32.48

55.26

14.53

0.00 1.08

0.85 2.56 1.71

0.00 8.19 0.14

Roast I I

Journal of Agricultural and Food Chemistry

45.41

0.14 2.37

2.23 2.23 1.68

0.00 8.94 0.14

4.97 3.48

1.79 5.36 1.29

2.28 9.44 3.87

Roast I

Angola Robusta Green

Composition of Amino A c i d s i n Green and Roasted Coffee

9.

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229

p y r a z i n e s o n l y (53). The f l a v o r s o f the products o f c y s t e i n e glucose and cysteine-pyruvaldehyde i n anhydrous c o n d i t i o n at d i f f e r e n t temperatures 80-190°C. are l i s t e d i n Table XV (24). Five-mM cysteine+5-mM glucose or pyruvaldehyde r e a c t e d a t d i f f e r e n t temperatures f o r 5 minutes and then evaluated by f l a v o r panel. Table XV F l a v o r s Produced by Cysteine-glucose And Cysteine-pyruvaldehyde a t D i f f e r e n t Temperatures F l a v o r D e s c r i p t i o n , a t °C. Reactants 80° 100° 130° 160° Cysteine & Pyruvaldehyde

Japanese rice cracker weak.

Cysteine & Glucose

no odor

Reference (24)

190°

cracker with sesamelike. no odor

Japanese Japanese rice rice cracker cracker with with seasame- sesamelike. like, sweet.

Sweet sesame, burnt,

Agricultural and Biological Chemistry

Pyruvaldehyde i s a l i q u i d a t room temperature and b o i l s a t 72°C, thus when cysteine-pyruvaldehyde mixture was heated a t 80°C, the components are i n s o l u t i o n and f l a v o r notes r e m i n i s c e n t o f Japanese r i c e c r a c k e r developed. As r e a c t i o n temperatures i n c r e a s e d g r a d u a l l y other f l a v o r notes developed. I n the case o f c y s t e i n e - g l u c o s e system, no r e a c t i o n took p l a c e u n t i l the r e a c t i o n temperature reached 130°C. The f l a v o r o f c y s t e i n e - g l u c o s e was comparable t o that o f cysteine-pyruvaldehde a t 160°C, w i t h one e x c e p t i o n , the glucose system had a sweet note. As temperature increased the f l a v o r impression of both systems increased i n s i m i l a r i t y . The v o l a t i l e compounds produced a t 160°C i n the presence of pyruvaldehyde were d i f f e r e n t from those i n presence of glucose. While t h i a z o l e and t h a z o l i n e s were absent i n the v o l a t i l e s o f c y s t e i n e - g l u c o s e , cysteine-pyruvaldehyde v o l a t i l e s were devoid o f p y r i d i n e s , p i c o l i n e s and furans (24).

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

230

FOOD TASTE CHEMISTRY

Food f l a v o r s c o n s i s t of numerous compounds, none of which alone i s c h a r a c t e r i s t i c o f s p e c i f i c food. Classes of compounds which emcompass food f l a v o r s are:- hydrocarbons ( a l i p h a t i c , a l i c y c l i c , aromatic); carbonyls (aldehydes, ketones); c a r b o x y l i c a c i d s , e s t e r s , imides, anhydrides; a l c o h o l s , phenols, e t h e r s ; alkylamines, a l k y l i m i n e s ; a l i p h a t i c s u l f u r compounds ( t h i o l s , mono-, d i - and t r i - s u l f i d e s ) ; n i t r o g e n h e t e r o c y c l i c s ( p y r r o l e s , p y r a z i n e s , p y r i d i n e s ) ; s u l f u r h e t e r o c y l i c s (thiophenes, t h i a z o l e s , t r i t h i o l a n e , t h i a l i d i n e ) ; and oxygen-heterocyclics (lactone, pyrone, furan). D i s c u s s i o n w i l l be l i m i t e d to s t r i k i n g developments i n h e t e r o c y c l i c s . H e t e r o c y c l i c Compounds During browning r e a c t i o n i n foods, h e t e r o c y c l i c compounds of known f i v e - and six-membered r i n g systems c o n t a i n i n g one or more atoms than carbon as r i n encompass s e v e r a l c l a s s e c h a r a c t e r i s t i c o r g a n o l e p t i c notes, i . e . t o a s t e d , b r e a d - l i k e , roasted, brothy, mushroom-like, nutty, ... e t c . , or undesirable notes, e.g. peppery ammoniacal, obnoxious, ... e t c . The nomenc l a t u r e s and the molecular formulas of h e t e r o c y c l i c compounds that most f r e q u e n t l y encountered i n food v o l a t i l e s are given i n Figures 1 and 2. Nitrogen-heterocyclic P y r r o l e and p y r r o l e d e r i v a t i v e s . The chemical and b i o l o g i c a l value o f p y r r o l e and i t s d e r i v a t i v e s cannot be overemphasized, n a t u r a l pigments, heme, c h l o r o p h y l l , b i l e pigments and enzymes l i k e cytochromes, contain p y r r o l e nucleus. A l s o , many a l k a l o i d s , p r o l i n e and hydroxyproline contain the reduced p y r r o l e r i n g (pyrrolidine). P y r r o l e and i t s d e r i v a t i v e s are found among the products of the browning r e a c t i o n products i n processed foods. A l k y l p y r r o l e s have intense petroleum-like odor, but they give sweet, s l i g h t l y burnt aroma on extreme d i l u t i o n . On the other hand a c y l p y r r o l e s have c h a r a c t e r i s t i c sweet smoky, and a l i t t l e m e d i c i n e - l i k e odor (65). Although a l k y l - and a c y l - p y r r o l e s do not e x h i b i t favorable aroma l i k e tne d e s i r a b l e roasty aroma o f p y r a z i n e s , they may p l a y an important r o l e i n the c h a r a c t e r i s t i c r o a s t y f l a v o r o f processed foods. In anhydrous c o n d i t i o n , Na c e t o n y l p y r r o l e , which had a b r e a d - l i k e aroma was i s o l a t e d from the r o a s t i n g products of p r o l i n e - g l u c o s e , hydroxyproline-glucose, and pyrrolidine-pyruvaldehyde (66). 1 - P y r r o l i n e which e x h i b i t e d an amine l i k e or c o r n - l i k e odor was the product of Strecker degradation of p r o l i n e - , and ornithione-sugar or- polycarbonyl reagents i n aqueous s o l u t i o n s ( 6 j ) . 2-Pyrrolealdehyde, 2 - a c e t y l p y r r o l e , 2 - p r o p i o n y l p y r r o l e , N-methyl-2-pyrrolealdehyde, N-methyl-2-acetyl p y r r o l e , 5-methyl-2-pyrrolealdehyde, N-methyl-5-methyl-2-pyrrole-

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

9.

Browning Reaction Products

MABROUK

ο ο

Η pyrrole (azole)

o

Η pyrrolidine (azolidine)

Η 2,3-dihydropyrrole (2,3-dihydroazole)

ο Ο

thiophene (thiole)

/r-N

ΟΗ

imidazole (1,3-diazole)

tetrahydrothiophene (thiolane)

trithiolane

tetrahydrofuran (oxolone)

Figure 1.

2,5-dihydrofuran 2,5-dihydroxole

o

0

t-pyran

S

S

piperidine perhydroazine

pyridazine (1,2-diazine)

1,3,5oxadithiane

S

1,3,5dithiazine

1,3,5trithione

0

pyrimidine (1,3-diazine)

ΗΝ

0

pyrazine (1,4-diazine)

NH

1,3,5thiadiazine

0 0

y-pyran

9

isoxazole

oxazole (1,3-oxazole)

Fwe-membered ring heterocyclics

S 1,2,6trithiane

Η 3H-pyrazole (3H-I,2-diazole)

thiazole (1,3-thiazole)

O O G O

pyridine (azine)

f

0

S-S

Q Q

furan (oxole)

231

a-pyrone

y-pyrone

9

1,3,5dioxathiane Figure 2.

Six-membered ring heterocyclics

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

232

FOOD TASTE CHEMISTRY

aldehyde were i d e n t i f i e d i n the p y r r o l i c f r a c t i o n o f coffee v o l a t i l e c o n s t i t u e n t s (68, 69). N- ethylpyrrole-2-aldehyde and 5-methylpyrrole-2-aldehyde are two o f the eighteen compounds r e ­ ported i n s t o r e d dehydrated orange j u i c e c r y s t a l s due to nonenzymic browning (JO). The type o f p y r r o l e d e r i v a t i v e s produced de­ pended on r e a c t i o n temperature, i t s d u r a t i o n , i t s c o n d i t i o n , i . e . , anhydrous, aqueous, or a l c o h o l i c ; pH o f the media, and molecular s t r u c t u r e o f r e a c t a n t s . While, N - a l k y l p y r r o l e - 2 - a l d e h y d e s were produced upon h e a t i n g D-xylose and alkylamine or amino a c i d i n n e u t r a l aqueous or methanolic s o l u t i o n s at 55 100°C; the c o r r e ­ sponding 5-methyl d e r i v a t i v e s o f N - s u b s t i t u t e d pyrrole-2-aldehydes were formed i n the presence o f L-rhamnose (71, 72). Odor o f the products were: N-methylpyrrole-2-aldehyde (formed from D-xylose and methylamine) cinnamonaldehyde-like, N-n b u t y l p y r r o l e - 2 - a l d e hyde (a product o f butylamine-xylose); and l - n - b u t y l - 5 - m e t h y l pyrrole-2-aldehyde (rhamnos d butylamin interaction product) xylene-like. 2-Formylpyrrol-l-yl-alky the products o f the browning xylos alkylamine or amino acids i n aqueous s o l u t i o n s (73). N-substituted-5-(hydroxymethyl)-pyrrole-2-aldenydes were formed from the r e a c t i o n o f the aldohexose "glucose" and alkylamines or amino a c i d s c o n t a i n i n g primary amino group at 70 t o 100°C, i n n e u t r a l i z e d aqueous, methanolic or e t h a n o l i c s o l u t i o n s . The r e s u l t i n g compounds were considerably unstable and had no odor i n the pure s t a t e but d e v e l ­ oped r o a s t e d aroma with browning (jh). Upon r o a s t i n g glucose and s e v e r a l alkyl-n-amino a c i d ( g l y c i n e , α-alanine, α-amino-n-butyric, v a l i n e , l e u c i n e , α-amino-n-caproic) at 200-250°C i n two components systems; 2 - 5 - h y d r o x y - m e t h y l - 2 ' - f o r m y l p y r r o l - l ^ y l ) a l k a n o i c a c i d lactones were formed as the main v o l a t i l e products (75)· The aroma d e s c r i p t i o n o f the prepared l a c t o n e d e r i v a t i v e s were: pro­ p i o n i c a c i d l a c t o n e (from α-alanine and glucose) caramel and a l i t t l e s c o r c h i n g ; i s o b u t y r i c a c i d l a c t o n e (α-amino-n-butyric and glucose) maple and strong sweet; i s o v a l e r i c a c i d lactone ( v a l i n e and glucose) and i s o c a r p r o i c a c i d l a c t o n e (leucine and glucose) miso, soy sauce and a c h o c o l a t e - l i k e . The y i e l d o f p r o p i o n i c a c i d l a c t o n e a f t e r h e a t i n g an equimolar mixture o f 0.01 mole o f glucose and α-alanine at 150, 200, and 250°C were: 1

Temperature, °C

Heating p e r i o d , min.

Y i e l d , unmoles h2

1 5 3 5 5

250 250 200 200 150

3 - h 50 6 16*

I t i s q u i t e obvious from the above data that the p r o p i o n i c a c i d lactone disappeared during the r e a c t i o n f o r longer periods and at higher temperatures. 2-(5 -hydroxymmethyl-2 -formylpyrrol-1 -yl) -3-methylbutanoate, and 2 -(5 -hydroxymethyl-2 -formyl1

,

f

f

1

f

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

9.

Browning Reaction Products

MABROUK

233

p y r r o l - l - y l ) - 3 - m e t h y l b u t a n o i c a c i d lactone were i d e n t i f i e d i n the products o f the browning r e a c t i o n o f glucose and v a l i n e i n aqueous s o l u t i o n at 65°C f o r three weeks (76). Reducing d i s a c c h a r i d e s ( l a c t o s e , maltose and melibiose) r e a c t e d with alkylamine i n aqueous s o l u t i o n s o f pH 6 . 5 , to form l-alkyl-5-hydroxymethylpyrrole-2-aldehyde (77)· N - A l k y l - 2 - a c y l p y r r o l e s and a l i p h a t i c a l d i mines were the products o f the r e a c t i o n between f u r f u r a l and i t s homologs with α-amino acids (78). Reactions o f f u r f u r a l and 2a c e t y l f u r a n with g l y c i n e and v a l i n e produced a small amount of the corresponding a c y l a l k y l p y r r o l e s which had pleasant aromas r e m i n i ­ scent of benzaldehyde. The r e s u l t i n g considerable amount o f a l i ­ phatic aldimines from the r e a c t i o n o f f u r f u r a l with v a l i n e and l e u c i n e possessed a strong odor from b i t i n g and unpleasant t o m i l d and f o o d - l i k e . Nine a l k y l p y r r o l e s , three a c y l p y r r o l e s , three a l k y l p y r r o l e - 2 - a l d e h y d e s , three f u r f u r y l p y r r o l e , eight a l k y l p y r a z i n e s , and one oxazolin i d e n t i f i e d i th v o l a t i l flavorou products o f r o a s t i n g DL-alanin i n n i t r o g e n atmosphere (79) y compound the r e a c t i o n products of model systems had been i s o l a t e d from roasted and cooked foods. For example, p y r r o l e , 1-methylpyrrole, 2-methylpyrrole, 1-formylpyrrole, 2 - f o r i L y l p y r r o l e , l - f o r m y l - 2 methylpyrrole, 2-fοrmyl-1-methylpyrrole, 2-fοrmyl-1-methylpyrrole, 2-formyl-5-methylpyrrole, 2 - f o r m y l - l - e t h y l p y r r o l e , 1-acetylpyrr o l e , 2 - a c e t y l p y r r o l e , 1 - f u r f u r y l p y r r o l e , 2 - p r o p i o n y l p y r r o l e , 2f o r m y l - 1 - f u r f u r y l p y r r o l e were reported i n roasted peanut (8θ). Roasted f i l b e r t s v o l a t i l e s contained 1-methylpyrrole, 2-pentylpyrr o l e , 2 - i s o b u t y l p y r r o l e , and 2 - p e n r y l p y r r i d i n e ( 8 l ) . f

P y r i d i n e and i t s d e r i v a t i v e s . The most unique p y r i d i n e d e r i ­ v a t i v e i s o l a t e d from processed food i s l , ^ , 5 6 - t e t r a h y d r o - 2 - a c e t o p y r i d i n e . This compound was prepared by r o a s t i n g p r o l i n e and dihydroxyacetone at 92°C i n presence o f sodium b i s u l f a t e , and ex­ h i b i t e d a strong odor reminiscent o f f r e s h l y backed soda crackers (J32). 2 - E t h y l p y r i d i n e and 2 - p e n t y l p y r i d i n e were reported i n v o l a t i l e f l a v o r components of shallow f r i e d (83). P y r i d i n e , 2methylpyridine, 3-methylpyridine, 2 - e t h y l p y r i d i n e , 3 - e t h y l p y r i dine, 5-ethyl-2-methylpyridine, 2 - b u t y l p y r i d i n e , 2 - a c e t y l p y r i d i n e , 2- p e n t y l p y r i d i n e , 2 - h e x y l p y r i d i n e , 3 - p e n t y l p y r i d i n e , 5-methyl-2p e n t y l p y r i d i n e , and 5 - e t h y l - 2 - p e n t y l p y r i d i n e were i d e n t i f i e d i n the v o l a t i l e s of roasted lamb f a t (Qh). 2,5-Dimethylpyridine and 3,5-dimethylpyridine were t e n t a t i v e l y i d e n t i f i e d i n roasted lamb f a t v o l a t i l e s . The odor t h r e s h o l d o f 2 - p e n t y l p y r i d i n e was 0 . 5 - 0 . 7 parts per 1 0 p a r t s o f water. The d i l u t e s o l u t i o n o f 2-pentylpy­ r i d i n e has a f a t t y or t a l l o w - l i k e odor. The authors a t t r i b u t e the unacceptance of lamb by some consumers to the high content o f a l k y l p y r i d i n e s i n roasted lamb. While p y r i d i n e d e r i v a t i v e s have burnt, heavy f r u i t y odors (JB5 ) ; p y r i d i n e has a disagreeable char­ a c t e r i s t i c odor and sharp t a s t e ; and p i p e r i d i n e has a peppery ammoniacal odor (06). P y r i d i n e , α-picoline (2-methylpyridine), 3- p i c o l i n e (^-methylpyridine), and U-ethylpyridine were i d e n t i f i e d 5

9

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

234

FOOD TASTE CHEMISTRY

i n c o f f e e aroma

(87).

P y r a z i n e s . In the t h i r t i e s , the a t t e n t i o n on p y r a z i n e s was focused on i t s i n d u s t r i a l r o l e i n dyes, photographic emulsions and chemotherapy. I t s importance i n l i f e processes was i n d i c a t e d i n i t s derivative, vitamin B ( r i b o f l a v i n , 6,7-dimethyl-9-(l -Dr i b i t y l i s o a l l o x a z i n e ) . L a t e r , i n the m i d s i x t i e s , i t was i d e n t i f i e d i n foods and i t s c o n t r i b u t i o n s t o the unique f l a v o r and aroma o f raw and processed foods a t t r a c t e d the a t t e n t i o n o f f l a v o r chemista Pyrazine d e r i v a t i v e s c o n t r i b u t e to the r o a s t i n g , t o a s t i n g , n u t t y , chocolaty, c o f f e e , earthy, caramel, m a p l e - l i k e , b r e a d - l i k e , and b e l l pepper notes i n foods. The reader i s r e f e r r e d t o the reviews on Krems and S p o e r r i (88) on the chemistry o f p y r a z i n e s , and the review o f pyrazines i n foods by Maga and S i z e r (89, 90.)· Table XVI summaries sensory d e s c r i p t i o n and t h r e s h o l d o f s e l e c t e d pyrazines. f

2

Oxygen H e t e r o c y c l i c s . During heat p r o c e s s i n g , sugars degrade to aldehydes and ketones which might r e a c t with amino compounds forming caramelized sugar f l a v o r s . C y c l i c diketones, pyrones, and furan d e r i v a t i v e s are examples o f the products o f t h i s r e a c t i o n . Table XVII gives the o r g a n o l e p t i c d e s c r i p t i o n o f s e l e c t e d compounds. U-Hydroxy-2,5-dimethyl(2H)-furanone has intense f r a g r a n t caramel note described as burnt sweet t a s t e (TO ), burnt pineapple ( l O l ) , beef b r o t h (lOO), strawberry preserve (103), nutty sweet aroma o f almonds (lOU), and major c o n t r i b u t o r to sponge cake f l a v o r (105). Seven terms were used to describe the o r g a n o l e p t i c note o f one compound. The question a r i s e s , what was the concent r a t i o n and media used f o r e v a l u a t i o n ? Change i n c o n c e n t r a t i o n alone can be quite s u f f i c i e n t t o a l t e r the c h a r a c t e r o f the f l a v o r or odor note. For example, t r i m e t h y l a m i n e - a i r mixtures only smell f i s h y over a narrow range o f d i l u t i o n s (1:1,500-1:8,000), with a maximum at about ( 1 : 6 , 0 0 0 ) , a l s o ammonia at a d i l u t i o n o f about (1:2,000) smells f i s h y (106). Hodge and Moser (107) r e p o r t e d that i n s p i t e o f the f a c t that the aromas o f pure m a l t o l , e t h y l m a l t o l , i s o m a l t o l , and h-hydroxy-2,5-dimethyl-3(2H)-furanone vary s i g n i f i c a n t l y from each o t h e r , p a n e l i s t s d e s c r i p t i o n was caramel or burnt sugar. The f r u i t y caramel aroma o f i s o m a l t o l i s weaker than that o f 4-hydroxy-2,5-dimethyl-3(2H)-furanone (108). Our sensory vocabulary should be adequate t o express the impact o f f l a v o r , odor, t a s t e notes components. This could be achieved by the p a r t i c i p a t i o n o f food chemist, organic chemist, sensory a n a l y s t and f l a v o r ist. Soy sauce (Shoyu) contains tautomers U-hydroxy-2-ethyl-5ethyl-3(2H)-furanone and U-hydroxy-5-ethyl-2-methyl-3(2H)-furanone (about 3:2 r a t i o ) which has sweet odor s i m i l a r t o that o f short cake (109). The same tautomers were s y n t h e s i z e d and described as possessing the f l a v o r o f cooked f r u i t s (103). M a l t o l , i s o m a l t o l and 2-methyl-5-hydroxy-6-ethyl-a-pyrone are c o n t r i b u t o r s to the c h a r a c t e r i s t i c aroma of molasses (110).

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

sweet f r i e d odor

2 . 6 - Dimethyl

2-Ethyl-3methyl2 - E t h y l -5methyl 2-Ethyl-3,6dimethyl-

2,3,5-Trimethyl 2,3,5,6-Tetramethyl 2-Ethyl

earthy raw potato (2)

(3)

deep b i t t e r p e r s i s t e n t notes (5)

Sensory D e s c r i p t i o n

2,3-Dimethyl2 . 5 - Dimethyl-

2-Methyl-

Pyrazine

cocoa

17 (91)

60 (92) 22 (91) 0.13 (92)

O.OOOU (92)

0.10 (92)

38 (91)

10 (91)

8 (91)

7 (91)

27 (91)

27 (91)

(92)

(91)

0.02U (92)

0.32

2.6

Odor Threshold ppm Mineral O i l Vegetable O i l

(91) (92) (92) (91) (92) (91) (92) (91)

105 60 2.5 3.5 1.8 5^ 1.5 9

Water

P y r a z i n e s , Sensory D e s c r i p t i o n and Odor Threshold

Table XVI

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In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2-methylamino-3methyl 2-dimethylamino-3methyl 2-dimethylamino-6methyl

2,6-dimethoxy-3isopropyl-5-methyl 2,5-dimethoxy-3i sopropyl-6-methyl 5-methyl 5H-6,7dihydrocyclopenta 5-methyl-2H-3,U,5,7retrahydrocyclopenta 2-acetyl2-methoxy-3-ac e t y l -

Pyrazine

(53)

breadcrust, nutty (96) weak breadcrust, nutty notes (96) r o a s t e d peanuts, green cocoa notes (96) P e a n u t - l i k e more green and l e s s cocoa notes (96) p e a n u t - l i k e , no cocoa notes reminiscent o f burnt c o f f e e (96)

r o a s t e d nuts, burnt

nutty with g r e e n - l i k e b e l l pepper, woody notes (96) nutty notes, green ( b e l l p e p p e r - l i k e ) (96) peanuts (53)

Sensory D e s c r i p t i o n

Water

(Cont.)

Odor Threshold ppm Mineral O i l Vegetable O i l

P y r a z i n e s , Sensory D e s c r i p t i o n and Odor Threshold

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In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

238

FOOD TASTE CHEMISTRY

Table XVII Oxygen H e t e r o c y c l i c Compounds

Compound

Sensory D e s c r i p t i o n

Spicy, smoky, s l i g h t l y cinnamonl i k e odor (98).

Fur an

Sweet b r e a d - l i k e , caramel-like t a s t e (98_). Furfury1 a l c o h o l 5-methyl-2-furfural

Sharp grape (98) ·

M a l t o l (3-hydroxy-2methyl-pyrone)

P l e a s i n g strong f r u i t s y f r e s h bread (99)»

Isomaltol

F r u i t y caramel, f r e s h bread odor, overtone m e d i c i n a l grassy (99)»

N-Furfuryl pyrrole

Green h a y - l i k e aroma

^-hydroxy-5-methy13(2H)-Furanone

Beef broth (lOO) burning sweet t a s t e (JO), burnt pineapple (101).

2,5-Dimethyl-U-hydroxy3(2H)-Furanone

Odor of roasted c h i c o r y roots (102).

References (98

-

(98).

102).

S u l f u r H e t e r o c y c l i c s . S u l f u r c o n t a i n i n g compounds ( t h i o l s , thiophenes, t h i a z o l e s , ... etc.) p l a y a major r o l e i n the f l a v o r o f raw and processed foods. These compounds have c h a r a c t e r i s t i c f l a v o r notes and the f l a v o r thresholds are mostly low. Several reviews ( i l l , 112, 113) demonstrate the important r o l e o f s u l f u r compounds i n food f l a v o r s . Organoleptic p r o p e r t i e s of these compounds may be pleasant, strong n u t - l i k e odor of i+-methyl-5v i n y l t h i a z o l e which i s present i n cocoa (ilk); objectionable p y r i d i n e - l i k e odor o f t h i a z o l e (115.); q u i n o l i n e - l i k e odor of benzothiazole ( l l 6 ) ; strong tomato l e a f - l i k e odor of i s o b u t y l t h i a z o l e (ill); and bread c r u s t f l a v o r o f a c e t y l - 2 - t h i a z o l i n e ( l l 8 ) . A mixture o f oxazoles, t h i a z o l e s , t h i a z o l i n e s , imidazoles, t r i t h i o l a n e s and

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

9.

MABROUK

Browning Reaction Products

239

d i t h i a n e s which had a meaty f l a v o r was obtained from a model sys­ tem c o n s i s t i n g o f α-dicarbonyls, e t h a n a l , hydrogen s u l f i d e and ammonia (119)« Unsaturated aldehydes r e a c t with hydrogen s u l f i d e and t h i o l s t o give mainly a d d i t i o n products t o the carbon-carbon double bond (120). The nomenclature o f the r e s u l t i n g compounds and t h e i r o r g a n o l e p t i c d e s c r i p t i o n s are given i n Table XVIII. T h i o l s , thiophenes, t h i a z o l e s , s u l f i d e s and furans were i d e n t i f i e d i n the v o l a t i l e s o f heating glucose, hydrogen s u l f i d e , and ammonia at 100°C f o r two hours (121). These v o l a t i l e s gave a roast beef­ l i k e aroma. Complex mixtures o f mercapto-substituted furans and thiophene d e r i v a t i v e s , which were reminiscent o f r o a s t meat were produced upon h e a t i n g t-hydroxy-5 methyl-3(2H)-furanone and i t s t h i o analog with hydrogen s u l f i d e (122). L i p i d Browning. L i p i d browning r e a c t i o n groups (provided by sugars o suga degradatio products and those r e s u l t i n g from unsaturated f a t t y a c i d s o x i d a t i o n ) and the f r e e amino groups present i n phospholipids have been recognized as potent causes o f undesirable f l a v o r , c o l o r and t e x t u r e changes i n dehydrated foods (123). Phosphatidyl ethanolamine, p h o s h a t i d y l s e r i n e , ethanolamine and s e r i n e plasmalogens c o n t a i n f r e e amino groups which can undergo l i p i d browning r e a c t i o n s . Phospolipids are u s u a l l y r i c h i n h i g h l y unsaturated f a t t y acids i n comparison with n e u t r a l l i p i d s , thus they are good sources o f carbonyls. A l s o , the primary amine moitiés o f p o l a r l i p i d s c a t a l y z e the a l d o l condensation o f C i ^ - C i e aldehydes r e s u l t i n g from plasmalogen h y d r o l y s i s , thus forming a,3-unsaturated aldehydes (12k). Phosp h a t i d y l ethanolamine r e a c t e d with propanal and n-hexanal forming phosphatidyl l - ( 2 - h y d r o x y e t h y l ) - 2 - e t h y l - 3 , 5 - d i m e t h y l p y r i d i n i u m , and p h o s p h a t i d y l - l - ( 2 - h y d r o x y e t h y l ) - 2 - h e x y l - 3 , 5 - d i p e n t y l p y r i d i nium, r e s p e c t i v e l y (125). The p e r i d i n i u m r i n g i s formed by the r e a c t i o n between one mole o f amino-N o f p h o s p h a t i d y l ethanolamine and three moles o f n-alkanals. The same r e a c t i o n took p l a c e i n the s y n t h e s i s o f s u b s t i t u t e d p y r i d i n e s by condensation o f carbonyl compounds with ammonia (126, 1 2 7 ) ·

Abstract In processed foods, non-enzymic browning reaction is the major source of its desirable flavors. Flavors of the products of this reaction depend upon: the molecular structure of nitrogenous com­ pounds (amines, amino acids, peptides, glycopeptides, proteins, . . . etc.); aldoses, ketoses, non-reducing, deoxy sugars, sugar acids, ... etc.); heating temperature; duration of the reaction; initial hydrogen ion concentration, moisture content, and the media of the reaction (alcoholic, aqueous, or anhydrous). The ratio of the nitrogenous compound to sugar or carbonyl compound has great effect on the flavor notes. Comparison betwen browning

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Reference

(120)

1-Methyl thiooctan-3-one

l-Octene-3-one

thiobutanal thiohexanal thioheptanal thiononal

3-methyl 3-Methyl 3-Methyl 3-Methyl

A d d i t i o n Product

2-Butenal 2-Hexenal 2-Hebtenal 2-Nonenal

Carbonyl Added Impression

Cheese-like ( B r i t e ) Cabbage (Rubbery) Unripe tomato Bast-like, slightly floral Radish-like

Flavor

0.5 5 5 3

Water

0.5 50 80 20

Paraffin O i l

F l a v o r o f Products o f the A d d i t i o n Between Unsaturated Carbonyls and Methanethiol

Table XVIII

9.

MABROUK

Browning Reaction Products

241

reaction products and rate of degradation of reactants under anhy­ drous condition, aqueous and alcoholic media was discussed. Flavor notes of products of amino compounds-sugars reaction were reviewed with emphasis on amino-ribose system. Meat-like flavor was imparted by browning reaction products of carnosine-, citrul­ line-, histidine-, glutamic-, 2-pyrrolidone-5-carboxylic-, methionine-, cysteine-, cysteic-, and taurine-ribose. Recent advancements in nitrogen-, oxygen- and sulfur hetercyclics and lipid browning were presented. Literature Cited. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Economics, Statistics, and Cooperative Service, USDA, National Review, 1978, April; p. 53. Yamasaki, Y.; Kaekawa, Κ., Agr. Biol. Chem., 1978, 42, 1761. Waley, S.G., Advan Protei Chem. 1966 21 1. Reineccius, G.A.; P.G., J. Agr. Food Chem., 1972, 20, 1. Patterson, R.L.S., J. Sc. Food Agr., 1968, 19, 31. Wong, E . ; Nixon, L.N.; Johnson, C.B., J. Agr. Food Chem., 1975, 23, 495. Watanabe, K.; Sato, Υ., Agr. Biol. Chem., 1970, 34, 88. Watanabe, K.; Sato, Υ., Agr. Biol. Chem., 1971, 35, 756. Yamato, T.; Tadao, K.; Kato, H.; Fujimaki, Μ., Agr. Biol. Chem., 1970, 34, 88. Hornstein, J., Chemistry and Physiology of Flavors, Schultz, H.W.; Day, E.A.; Libbey, L.M., Eds., Avi Publishing Co., Westport, CT, 1967; pp. 228-250. Watanabe, K.; Sato, Υ., Agr. Biol. Chem., 1970, 34, 467. Watanabe, K.; Sato, Υ., Agr. Biol. Chem., 1971, 35, 278. Harkes, P.D.; Begemann, W.J., J. Am. Oil Chemists' Soc., 1974, 51, 356. Hornstein, I.; Crowe, P.F.; Heimberg, M.J., J. Food Sci., 1961, 26, 581. Pippen, E.L.; Nonaka, M.; Jones, F.T.; Stitt, F., Food Res., 1958, 23, 103. Pippen, E.L.; Nonaka, Μ., Food Res., 1960, 25, 764. Heyns, K.; Stute, R.; Paulsen, Η., Carbohydrate Res., 1966, 2, 132. Heyns, K.; Klier, Μ., Carbohydrate Res., 1968, 6, 436. Walter, R.; Fagerson, I.S., J. Food Sci., 1968, 33, 294. Kort, M.J., Advan. Carbohydrate Chem. Biochem., 1970, 25, 311. Chuyen, N.V.; Kurata, T.; Fujimaki, Μ., Agr. Biol. Chem., 1973, 37, 327. Kato, S.; Kurata, T.; Fujimaki, Μ., Agr. Biol. Chem., 1973, 37, 1759. Fujimaki, M.; Kato, S.; Kurata, T., Agr. Biol. Chem., 1969, 33, 1144. Kato, S.; Kurata, T.; Fujimaki, Μ., Agr. Biol. Chem., 1973, 37, 539.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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25. 26. 27. 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. 55. 56. 57. 58. 59.

FOOD TASTE CHEMISTRY

Schonberg, Α.; Moubacher, R., Chem. Rev., 1952, 50, 261. Hodge, J.E., J. Agr. Food Chem., 1953, 1, 928. Hodge, J.E., Advan. Carbohydrate Chem., 1955, 10, 169. Ellis, G.P., Advan. Carbohydrate Chem., 1959, 14, 163. Reynolds, T.M., Advan. Food Res., 1963, 12, 1. Reynolds, T.M., Advan. Food Res., 1965, 14, 167. Lightbody, H.D.; Feuold, H.R., Advan. Food Res., 1948, 1, 149. Ross, A.F., Advan. Food Res., 1948, 1, 325. Stadman, E.R., Advan. Food Res., 1948, 1, pp. 325-372. Wodemeyer, G.A.; Dollar, A.M., J. Food Sci., 1963, 28, 537. Wiseblatt, L.; Zoumut, H.F., Cereal Chem., 1963, 40, 162. El'Ode, K.E.; Dornseifer, T.P.; Keith, E.S.; Powers, J.J., J. Food Sci., 1966, 31, 351. Herz, W.J.; Shallenberger, R.S., Food Res., 1960, 25, 491. Barnes, H.M.; Kaufman, C.W., Ind. Eng. Chem., 1947, 39, 1167. Kiley, P.J.; Nowlin A.C.; Mariarty J.H., Cereal Sci Today 1960, 5, 273. Buckdeschel, W.Z., , 1914, , , 1915, 9, 118. Morimoto, T . ; Johnson, J.A., Cereal Chem., 1966, 43, 627. Keeney, M.; Day, E.A., J. Dairy Sci., 1957, 40, 847. Actander, S., Perfume and Flavor Chemicals (Aroma Chemicals); Published by the Author, Montclair, NJ, 1969; two volumes. Persson, T . ; von Sydow, E . , J. Food Sci., 1973, 38, 377. Rooney, L.W.; Salem, Α.; Johnson, J.A., Cereal Chem., 1967, 44, 539. Self, R., Chemistry and Physiology of Flavors, Schultz, H.W.; Day, E.A.; Libbey, L.M., Eds., Avi Publishing Co., Westport, CT, 1967; pp. 362-389. Mabrouk, A.F.; Holms, L.G., Abstracts of Papers, 159, Division of Agricultural and Food Chemistry, 172nd ACS National Meet­ ing, San Francisco, CA, September 1976. Rohan, T.A.; Stewart, T., J. Food Sci., 1966, 31, 202. Rohan, T.A.; Stewart, T., J. Food Sci., 1966, 31, 206. Rohan, T.A.; Stewart, T., J. Food Sci., 1967, 32, 395. Rohan, T.A.; Stewart, T., J. Food Sci., 1967, 32, 399. Rohan, T.A.; Stewart, T., J. Food Sci., 1967, 32, 625. van Praag, M.; Stein, S.H.; Tibbets, M.S., J. Agr. Food Chem., 1968, 16, 1005. Pickenhagen, W.; Dietrich, P.; Keil, B.; Polansky, J.; Nouaille, F.; Lederer, Ε., Helv. Chem. Acta., 1975, 58, 1078. Kato, S.; Kurata, T . ; Ishitsuka, R.; Fujimaki, Μ., Agr. Biol. Chem., 1970, 34, 1826. Shiba, T . ; Nunami, K.I., Tetrahedron Letters, 1974, 6, 509. Takanishi, K.; Tadenuma, M.; Kitamoto, K.; Sato, S., Agr. Biol. Chem., 1974, 38, 927. Sakamura, S.; Furukawa, K.; Kasai, T., Agri. Biol. Chem., 1978, 42, 607. Boyd, E.N.; Keeney, P.G.; Patton, S., J. Food Sci., 1965, 30, 854.

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60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88.

MABROUK

Browning Reaction Products

van Straten, S.; de Vrijer, F., List of Volatile Compounds in Food, Central Institute for Nutrition and Food Research, Zeist, Holland, 1973. Eckey, E.W., Vegetable Fats and Oils, Reinhold Publishing Corp., NY, 1954; p. 676, 760. Feldman, J.R.; Ryder, W.S.; Kung, J.T., J. Agr. Food Chem., 1969, 17, 733. Underwood, G.E.; Deatherage, F.E., Food Res., 1952, 17, 425. Khan, N.A.; Brown, J.B., J. Am. Oil Chemists' Soc., 1953, 30 606. Shigematsu, H.; Kurata, T.; Kato, H.; Fujimaki, Μ., Agr. Biol. Chem., 1972, 36, 1631. Kobayasi, N.; Fujimaki, Μ., Agr. Biol. Chem., 1965, 29, 1059. Yoshikawa, K.; Libbey, L.M.; Cobb, W.Y.; Day, E.A., J. Food Sci., 1965, 30, 991 Gianturco, M.A.; Giammarino Tetrahedron, 1964, 20, 2951. Gianturco, M.A.; Giammarino, A.S.; Friedel, P., Nature, 1966, 210, 1358. Tatum, J.H.; Shaw, P.E.; Berry, R.E., J. Agr. Food Chem., 1967, 15, 773. Kato, Η., Agr. Biol. Chem., 1966, 30, 822. Kato, H., Agr. Biol. Chem., 1967, 31, 1086. Kato, H.; Fujimaki, M., J. Food Sci., 1968, 33, 445. Kato, H.; Fujimaki, Μ., Agr. Biol. Chem., 1970, 34, 1071. Shigematsu, H.; Kurata, T.; Kato, H.; Fujimaki, Μ., Agr. Biol. Chem., 1971, 35, 2097. Kato, H.; Sonobe, H.; Fujimaki, Μ., Agr. Biol. Chem., 1977, 41, 711. Sonobe, H.; Kato, H.; Fujimaki, Μ., Agr. Biol. Chem., 1977, 41, 609. Rizzi, G.P., J. Agr. Food Chem., 1974, 22, 279. Shigematsu, K.; Kurata, T.; Kato, H.; Fujimaki, Μ., Agr. Biol. Chem., 1972, 36, 1631. Walradt, J.P.; Pittet, A.O.; Kinlin, T.E.; Muralidhara, R.; Sanderson, Α., J. Agr. Food Chem., 1971, 19, 972. Kinlin, T.E.; Muralidhara, R.; Pittet, A.O.; Sanderson, Α.; Walradt, T.P., J. Agr. Food Chem., 1972, 20, 1021. Hunter, I.R.; Walden, M.K.; Scherer, J.R.; Lundin, R.E., Cereal Chem., 1969, 46, 189. Watanabe, K.; Sato, Y., J. Agr. Food Chem., 1971, 19, 1017. Buttery, R.G.; Ling, L.C.; Teranishi, R.; Mont, T.R., J. Agr. Food Chem., 1977, 25, 1277. MacLeod, G.; Coppock, B.M., J. Agr. Food Chem., 1977, 25, 113. Webster's Third New International Dictionary, G&C Merriam Co., Publishers, Springfield, MA, 1967. Goldman, I.M.; Seibl, J.; Flament, I.; Gautschi, F.; Winter, M.; Willham, B.; Stoll, Μ., Helv. Chem. Acta., 1967, 50, 694. Krems, I.J.; Spoerri, P.E., Chem. Rev., 1947, 40, 279.

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Maga, J.A.; Sizer, C.E., Crit. Rev. Food Technol., 1973, 4, 39. Maga, J.A.; Sizer. C.E., J. Agr. Food Chem., 1973, 21, 22. Seifert, R.M.; Buttery, R.G.; Guadagni, D.G.; Black, D.R.; Harris, J.G., J. Agr. Food Chem., 1970, 18, 246. Seifert, R.M.; Buttery, R.G.; Guadagni, D.G.; Black, D.R.; Harris, J.G., J. Agr. Food Chem., 1972, 20, 135. Deck, R.E.; Chang, S.S., Chem. Ind., 1965, 1343. Buttery, R.G.; Seifert, R.M.; Guadagni, D.G.; Ling, L.C., J. Agr. Food Chem., 1969, 17, 1322. Polak's Fruital Works, Improvements relating to flavorings, Br. Patent 1,248,380, September 29, 1971. Takken, H.J.; van der Linde, L.M.; Boelens, M.; van Dort, J. Μ., J. Agr. Food Chem., 1975, 23, 638. Parliment, T.H.; Epstein, Μ., J. Agr. Food Chem., 1973, 21, 714. Guadagni, D.G.; Buttery 1435. Shaw, P.E.; Tatum, J.H.; Berry, R.E., J. Agr. Food Chem., 1969, 17, 907. Tonsbeek, C.H.T.; Planchen, A.J.; van de Weerdhof, T., J. Agr. Food Chem., 1968, 16, 1016. Rodin, J.O.; Himel, C.M.; Silverstein, R.M.; Leeper, R.W.; Gortner, W.A., J. Food Sci., 1965, 30, 280. Tonsbeek, C.H.T.; Koenders, Ε.Β.; van der Zijden, Α.S.M.; Losekoot, J.A., J. Agr. Food Chem., 1969, 17, 397. Re, V.L.; Maurer, B.; Ohloff, G., Helv. Chem. Acta., 1973, 56, 1882. Takei, Y.; Yamanishi, T., Agr. Biol. Chem., 1974, 38, 2329. Takei, Y., Agr. Biol. Chem., 1977, 41, 2361. Stansby, M.E., Food Technol., 1962, 16, 28. Hodge, J.E.; Moser, H.A., Cereal Chem., 1961, 38, 221. Hodge, J.E.; Mills, F.D.; Fisher, B.E., Cereal Sci. Today, 1972, 17, 34. Numomura, N.; Saaki, M.; Asao, Y.; Yokotsuka, T., Agr. Biol. Chem., 1976, 40, 491. Ito, H., Agr. Biol. Chem., 1976, 40, 827. Schutte, L . , Crit. Rev. Food Technol., 1974, 4, 457. Maga, J.A., Crit. Rev. Food Sci. Nutr., 1975, 6, 153. Maga, J.A., Crit. Rev. Food Sci. Nutr., 1975, 6, 241. Maga, J.A., Crit. Rev. Food Sci. Nutr., 1976, 7, 147. Stoll, M.; Dietrich, P.; Sunte, E.; Winter, M., Helv. Chem. Acta., 1967, 50, 2065. Merck Index 9th ed., 1976; p. 1313. Viani, R.; Bricout, J.; Marion, J.P.; Muggler-Chavan, F.; Reymond, D.; Egli, R.H., Helv. Chem. Acta., 1969, 52, 887. Tonsbeek, C.H.T.; Copier, H.; Plancken, A . J . , J. Agr. Food Chem., 1971, 19, 1014.

90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118.

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9.

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119.

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Boelens, M.; van der Linde, L.M.; de Valois, P.J.; van Dort, H.M.; Takken, H.J., Proc. Int. Symp. Aroma Research, Zeist, Netherland, 1975; pp. 95-100. 120. Badings, H.T.; Maarse, H.; Kleipool, R.J.C.; Tas, A.C.; Neeter, R.; ten Noever de Brauw, M.C., Proc. Int. Symp. Aroma Research, Zeist, Netherland, 1975; pp. 63-73. 121. Shibamoto, T.; Russell, G.F., J. Agr. Food Chem., 1976, 24, 843. 122. van den Ouweland, G.A.M.; Peer, H.G., J. Agr. Food Chem., 1975, 23, 501. 123. Lea, C.H., J. Sci. Food Agr, 1957, 8, 1. 124. Nakanishi, T.; Suyama, Κ., Nippon Chikusan Gakkai-Ho, 1970, 41, 29. 125. Nakanishi, T.; Suyama, Κ., Agr. Biol. Chem., 1974, 38, 1141. 126. Frank, R.L.; Seven R.P. J Amer Chem Soc. 1949 71 2629. 127. Farley, C.P.; Eliel, E.L., J. Amer. Chem. Soc., 1956, 78, 3477. RECEIVED August 2, 1979.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

10 Concluding Remarks MITSUO NAMIKI Department of Food Science and Technology, Nagoya University, Chikusa-ku, Nagoya, Japan

Dear friends, we ar unique Symposium with suc this symposium, I would like to say a few words to close this session on taste. Since this symposium is held in the Joint Chemical Congress of ACS and CSJ, I hope you will allow me to talk a little while about my rather mixed up ideas on taste by using Japanese language. Figure I shows how we write "Symposium on the Taste of Foods" in Japanese. The first example is simply the phonetic translitera­ tion of the word Symposium written in "Katakana", a kind of Japanese traditional script. The next two Chinese characters are pronounced "shokuhin" and mean "food", the last character is "aji" which means "taste" or "flavor", our common interest. The character between them is "no" in "Hiragana", another kind of Japanese traditional script, and means "of". As you see, the Japanese language uses three kinds of s c r i p t s w i t h the words completely reversed from the arrangement found i n European languages. So I hope you might have some sympathetic f e e l i n g s f o r our Japanese s c i e n t i s t s here, i n c l u d i n g myself. Anyway, l e t me t e l l you how I d i d my l i t t l e a n a l y t i c a l work on these words. Since Chinese i s p i c t u r e w r i t i n g , the c h a r a c t e r f o r " a j i " can be separated i n t o two components. The l e f t one i s simply a square, meaning mouth. The other h a l f i s considered t o be p h o n e t i c a l l y e q u i v a l e n t t o "mi" o r " b i " which means "beauty" or "goodness". Therefore the composition of the c h a r a c t e r f o r " a j i " i n d i c a t e s t h a t t a s t e i s p r i m a r i l y "good to mouth", namely, p a l a t a b l e , d e l i c i o u s , and t a s t y . I n t h i s sense, I h e a r t i l y agree w i t h Dr. Boudreau who w i s e l y p o i n t e d out t h a t "umami" should be counted as a b a s i c t a s t e . The Chinese c h a r a c t e r f o r "umami", as shown i n t h i s f i g u r e , has i t s o r i g i n i n "a spoon and mouth", namely a good t a s t e e l i c i t e d by e a t i n g a d e l i c i o u s soup. As Dr. Yamaguchi s t a t e d , t h i s t a s t e "umami" i s known to be a t t r i b u t e d to mainly two f a c t o r s , MSG and n u c l e o t i d e s . I t happens t h a t the use of both of these f a c t o r s as f l a v o r i n g agents was o r i g i n a t e d by Japanese s c i e n t i s t s ;

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FOOD TASTE CHEMISTRY

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Symposium

The Taste of.Foods

v y *° *J V A

Taste

tr

S

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Bitter

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Salty

Umami

Pungent

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JJ: ^ ^ Tj- ^

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^ or spoon *~ X7 mouth

Figure 1.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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ΝΑΜίκι

Concluding Remarks

249

MSG by Dr. Kikunae Ikeda>who was once the president of the Chemical S o c i e t y of Japan, and n u c l e o t i d e s by Dr. Kodama, Dr. Kuninaka and o t h e r s . Dr. Kuninaka s f i n d i n g that a s m a l l amount of n u c l e o t i d e enhanced remarkably the "umami" of MSG has been of i n t e r e s t to the w o r l d, e s p e c i a l l y to the food i n d u s t r y . Here i t i s i n t e r e s t i n g to note t h a t i n the Japanese language the word "umami" sometimes means "unexpected p r o f i t " which j u s t f i t s t h i s case. I might add t h a t manufacture of these f l a v o r i n g agents has developed i n t o a b i g fermentation i n d u s t r y , which was developed mainly by the e f f o r t s of the a g r i c u l t u r a l chemists of Japan. The umami t a s t e , however, i s not always caused by j u s t these two agents, r a t h e r i t i s e l i c i t e d by a w e l l harmonized mixture of v a r i o u s food c o n s t i t u e n t s , such as amino a c i d s , p e p t i d e s , nucleo­ t i d e s , organic a c i d s , and i n o r g a n i c i o n s . This f a c t i s c l e a r l y demonstrated by the s t u d i e s of Dr. Konosu on the t y p i c a l umami of seafoods, and by Dr. M o r i on the t a s t e of soy sauce. Speaking of the mixtur Japanese people have a goo have seen i n the Japanese l e t t e r s , the Japanese people are them­ s e l v e s a w e l l homogenized mixture both i n c u l t u r a l and e t h n i c aspects. Their food h a b i t s are t y p i c a l l y omnivorous. Moreover, owing to the v a r y i n g c l i m a t e and a marine i s l a n d country, the food i s abundant i n k i n d , v a r y i n g from raw f i s h to v a r i o u s d e l i c i o u s fermentation products as shoyu, miso and, e s p e c i a l l y , sake. Here, I must c a l l a t t e n t i o n to the r e p o r t s by Dr. Solms and Dr. Mabrouk who informed us that the umami substances were developed and i n c r e a s e d by cooking and pr ocessin g of foods. It is these types of s t u d i e s combined w i t h a s o p h i s t i c a t e d psycophysics which w i l l help us b e t t e r understand food f l a v o r . Now, I f e e l I have dwelt too long on the s u b j e c t of "umami", but the word "umami" i n Japanese language sometimes means "sweet­ ness". As to the sweeteners, i t i s no wonder that such a great d e a l of work has been done on new sweeteners of n a t u r a l and a r t i f i c i a l o r i g i n . U n t i l now, such work has been a k i n d of h i t and miss business. Therefore, the l a s t h a l f of the day was devoted to understanding some of the s t r u c t u r a l f e a t u r e s of molecules that determine t h e i r t a s t e p r o p e r t i e s . Based on the advanced stereo-chemical s t u d i e s on a l a r g e number of sweet and b i t t e r compounds by Dr. A r i y o s h i , Dr. B e l i t z and Dr. Ney, our understanding of the molecular p r o p e r t i e s of c e r t a i n t a s t e compounds has advanced markedly. N o t i c e a b l e a l s o i s an i n c r e a s e i n our understanding of the chemical p r o p e r t i e s of amino a c i d s , peptides and s i m i l a r n i t r o g e n compounds, s i n c e as we saw i n the f i r s t h a l f of the symposium, they are primary f l a v o r components i n many foods. At t h i s p o i n t , I would l i k e to i n d i c a t e t h a t t a s t e chemistry i s e s s e n t i a l l y s o l u t i o n chemistry. I t i s therefore e s p e c i a l l y s i g n i f i c a n t t h a t the b i t t e r n e s s of many compounds can be d i r e c t l y r e l a t e d to the hydrophobic p r o p e r t i e s of the molecules. The sour s e n s a t i o n has a l s o been shown to be r e l a t e d to the Br^nsted a c i d 1

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p r o p e r t i e s o f the molecules. Thus, both sour and b i t t e r t a s t e s can be shown t o be r e l a t e d t o the s o l u t i o n p r o p e r t i e s o f the molecules. I n the f i n a l a n a l y s i s , t h a t which s t i m u l a t e s i s a molecular complex of the a c t i v e molecule and water. S o l u t i o n chemistry i s i n as p r i m i t i v e a s t a t e as i s t a s t e chemistry and they must develop together. On another important t a s t e , pungency, Dr. Govindarajan s a i d i n h i s paper t h a t t h i s s e n s a t i o n must be measured t o a c c u r a t e l y d e s c r i b e the f l a v o r o f c e r t a i n foods. He has a l s o suggested a r e l a t i o n s h i p between t a s t e and chemical s t r u c t u r e as has a l r e a d y been done i n the cases o f other t a s t e s . L a r g e l y untouched by t h i s symposium, but p o s s i b l y present i n the minds of many, i s the n u t r i t i o n a l s i g n i f i c a n c e o f t a s t e . What i s the exact r e l a t i o n s h i p between t a s t e and n u t r i t i o n ? I f we cook foods to produce f l a v o r f u l compounds, what i s the n u t r i t i o n a l s i g n i f i c a n c e o f these compounds? These and many other questions await answers. In ending these f i n a , happy t o l e a r n d i r e c t l y t h a t the e f f o r t s o f s c i e n t i s t s a l l over the world a r e r e s u l t i n g i n a steady progress i n t h i s d i f f i c u l t f i e l d , and a r e c o n t r i b u t i n g to the w e l f a r e o f men by appealing t o t h e i r most fundamental and p e a c e f u l d e s i r e s o f e a t i n g good t h i n g s . I b e l i e v e t h i s o c c a s i o n w i l l be an important m i l e s t o n e i n the development o f t h i s r e s e a r c h . F i n a l l y , I would l i k e t o extend my hearty thanks t o the speakers f o r performing so w e l l , the audience f o r being so a t t e n t i v e and to Dr. Boudreau f o r h i s great e f f o r t s i n o r g a n i z i n g t h i s symposium. Thank you. RECEIVED August 16,

1979.

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

INDEX

A Acids, Br0nsted 17 Acylpyrroles 230 Agricultural revolution 22 Alanine 143,190,201 L-Alanine 121 Abalone (Haliotis gigantea discus) taste-active components in 188-19 Aldehydes 212,220 Aliphatic compounds 220 2-Alkanones 117 Alkyl-n-amino acid 232 Alkylpyridines 233 Alkylpyrroles 230 Alkyl-ureas, open chain 117 Allium 80 Allyl isothiocyanates 80 Amanita phaloides mushroom 4 American Spice Trade Association (ASTA) 57,73 Amides, open chain 117 Amides, taste of 118* Amines, taste of 119t Amino acids 94,175-184 aromas from heating glucose with .. 218f bitter taste of 99f composition 149-169 degradation 223 effect of heating onflavorof 221 free 190 in crab extracts 194,195/ and nucleotides of boiled potatoes 177f in prawns 190,193 general structure of sweet 134/ in green and roasted coffee, composition of 228f on heating, destruction of 223f after heating with glucose, destruction of 224£ hydrophobicity of 105f, 178 interaction of sugars and 219 in lobsters, free 190,193 quality and intensity of taste of ΙΟΟί residue, hydrophobocity of 149-173 residues of a peptide 149-173 -ribose solutions,flavorproduced by heating 221f-222f in scombroid fish, free

Amino acids (continued) side chains, hydrophobic action of 149-169 structures of 110/ -sugars model systems 214 sweet taste of 95f, 96f tast f lOlf 105f 106f a-Amino acids, aroma of Strecker degradation products of 219t a-Amino acids, Strecker degradation of 214 L-a-Amino acids 33 sweet dipeptide esters of 97 1-Amino acids 153 taste of 154f 1- Amino cycloalkane-l-carboxylic acids 109 taste of 107* 2- Aminobenzoic acid 115 2-Aminocarboxylic acids 121 2-Aminonorbornan-carboxylic acid, bicyclic 109 Aminoalonic acid 140,142 Aminobenzenes, taste of 113i Amino compounds, aromas from reactions of carbonyl and 215f-217i Ammonia 54 AMP 189,193,194,201 5'-AMP 175-184 Animal proetins 93-131 Aniline 115 Animal cells, peptides in 205-245 Arginine 189,201 Armoracia lapathiofolia (horseradish) 80 Aroma 53-92, 219 from heating glucose with amino acids 218f of lactones 232 from reactions of carbonyl and amino compounds 2l5#-217i of Strecker degradation products of α-amino acids 219£ Asparagine 155 Aspartame conformational analyses of 145 projection formulas of isomers of .... 141/ stereoisomers of 140

253

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Aspartic acid 175-184 L-Aspartic acid, sweet dipeptide esters of 97 Aspartyl dipeptide esters 133 sweet-tasting 140,142 D-valine-containing 142 peptides, taste of 136*-137* tripeptide esters 142 α-D-Aspartyl dipeptide esters 140 ^L-Aspartyl dipeptide esters 140 ASTA ( American Spice Trade Association ) 57 Astringenecy 16, 53-92,178 Azacycloalkanes 117 bitter Ill

Β Bavarian cream, effect of suga flavor of 48 Bavarian cream, sucrose contents of .. 47 Beef broth onflavorof beef consomme, effects of MSG and 45/ consommé 43 effects of MSG and beef broth on flavor of 45/ effect of NaCl onflavorof 46/ lipids 209 fatty acids composition of 210* Beer bitterness 225-226 Benzenes, diamine 115 Benzoic acid(s) 115,125 taste of 114* Bicyclic 2-aminonorbornan-carboxylic acid 109 Binding site, hydrophobic 139,143,145 Bipolar hydrophobic compounds 117 Bitter azacycloalaknes Ill compounds 139 in rectangular coordinates 120/ structure and taste relationship 93-128 cycloalkanones Ill dipeptides 153* lactams Ill lactones Ill peptides from