Taste and the Taste of Foods - ACS Symposium Series (ACS

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

KIEU HOANG,

Downloaded by 109.163.234.5 on April 5, 2016 | http://pubs.acs.org Publication Date: December 14, 1979 | doi: 10.1021/bk-1979-0115.ch001

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

In this paper the term taste will refer to all the chemical sensory systems of the oral cavity and their sensations. These 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 Boudreau; Food Taste Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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FOOD TASTE CHEMISTRY

Figure 1.

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

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


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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.



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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 can be considered to be composed of a 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



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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.




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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 Taste Systems

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


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.



SPONTANEOUS

ACTIVITY

L - MALIC ACID


ANSERINE

GROUP I

NITRATE

SPONTANEOUS PCTIVITY

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.



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Taste of Foods

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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 l s t i m u l a t i o n (a measure of conduction v e l o c i t y and thus f i b e r s i z e ) , and types of compounds a c t i v a t i n g . 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




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


BOUDREAu ET AL.

The Taste of Foods


GROUP I

NH

HO; L-H1STIDINE

PYRIDINE

THIAZOLIDINE

L-ANSERINE

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


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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

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

Human Sensations

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

Sour

Grp. I I : Amino a c i d responsive, d i and triphosphate nucleotides, NaCl p o t e n t i a t e d

Sweety Bitterj

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

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

(Pleasant?)

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?

—


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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 Systems

The second type o f study which has c o n t r i b u t e d t o our understanding 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



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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 y high c o n c e n t r a t i o n s of i n o r g a n i c i o n s (16,17), p a r t i c u l a r l y Na, Κ and L i . The sour s e n s a t i o n i s e l i c i t e d by v a r i o u s Br^nsted a c i d s w i t h i n d i c a t i o n s 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



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




Sour

2.


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


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 sensations a s s o c i a t e d w i t h the t r i g e m i n a l 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



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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 , an a c t i v e stimulus i n the dog and c a t , e l i c i t s a s m a l l sour s e n s a t i o n (41) a t a pH of 7.4. 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



18


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 standp o i n t . S h e l l f i s h t a s t e i s p r i m a r i l y due to i n o r g a n i c i o n s , organic a c i d s , amino a c i d s and n u c l e o t i d e s (44). 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



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BOUDREAU ET AL.

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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 . The t a s t e of any food would t h e r e f o r e be a composite of d i s c r e t e t a s t e s e n s a t i o n s . A d d i t i o n a l chemical 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 ,


20



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

H i s t i d i n e Dipeptides

XXX

Nucleotides

XXX

X

XX

XX

Amines

XX

X XX

XX

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



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


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 chocolate are a l s o i n p a r t d e r i v e d from 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 , as are meat f l a v o r s . Heat produced 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.


1.

23

The Taste of Foods

BOUDREAU ET AL.

TASTE COMPOUNDS IN BEER

,CH

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NICOTINIC ACID OH

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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



24


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). In a n a t u r a l n u t r i t i o n a l ecosystem there i s great chemical complexity as a r e s u l t of the many s p e c i e s c o n t r i b u t i n g to the f l o w of compounds. Many o f these compounds 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



^

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'

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'

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'

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Figure 10.

'

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The differences in the flavor vohtiles of tree-ripened and artificially ripened fruit (95).

Journal of Food Science

CHROMATOGRAMS OF PEACH VOLATILES WITH LACTONES IDENTIFIED

10

«iNÎfill





1.

BOUDREAU ET AL.

27

The Taste of Foods


unnatural cmpds.

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


28


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 producing food s i m i l a r i n chemical composition to t h a t found i n the n a t u r a l n u t r i t i o n a l ecosystem w i t h i n which 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.

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10. 11. 12. 13. 14. 15.


16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

The Taste of Foods

29

Murray, R.G., In: "Handbook of Sensory Physiology". IV. part 2, 31-50. Shimamura, Α., Tokunaga, J., Toh, Η., Arch. Hist. Jap. (1972) 34: 52-60. Takeda, M., Hoshino, T., Arch. Hist. Jap. (1975) 37: 395-413. Miller, I . J . , J. Comp. Neurol. (1974) 158: 155-166. Boudreau, J.C., White, T., In: "Flavor Chemistry of Animal Foods" (R.W. Bullard, Ed) 102-128, Amer. Chem. Soc., Wash. D.C., 1978. Kashiwagura, T., Kamo, N., Kurihara, K., Kobatake, Y. Comp. Biochem. Physiol. (1977) 56: 105-108. Moncrief, R.W. "The Chemical Senses", L.Hill, London (1967). Skramlik, E.v., Physiologie des Geschmaksinnes, In: "Handbuch der Physiologie der Niederen Sinne", Georg Thieme Verlag, Leipzig, 1926. Beets, M.G.J., "Structure Activity Relationships in Human Chemoreception", Applied Science Pub., Ltd., Barkin, 1978. Boudreau, J.C., Nelson, T.E., Chem. Sen. Flav.(1977) 2: 353-374. Amerine, M.A., Pangborn, R.M., and Roessler, E.B., Principles of Sensory Evaluation of Food, Acad. Press, N.Y., 1965. Kirimura, J., Shimizu, Α., Kimizuka, Α., Ninomiya, T., and Katsuya, N., J. Agri. Food Chem. (1969) 17: 689695. Ney, K.H. In: "Natürliche und Synthetische Zusatzstoffe in der Nahrung des Menschen. (R. Ammon and J. Hollo, Eds) 131-143. Arctander, S., "Perfume and Flavor Chemicals" S. Arstander, Publisher, Elizabeth, N.J. (1969). Hall, M.J., Bartoshuk, L. Μ., Cain, W.S., Stevens, J.C. Nature (1975) 353: 442-443. Van der Well, H., Arvidson, K. Chem. Sen. Flav. (1978) 3: 291-297. Kuninaka, Α., In: "Chemistry and Physiology of Flavors" (H.W. Schultz, E.A. Day and L.M.Libbey, Eds.), Avi. Publishing Co., Westport, Conn, pp 515-535 (1967). Terasaki, M., Fujita, E . , Wada, S., Takemoto, T., Nakajima, T., Yokobe, T., Jrnl. Jpn. Soc. Food Nutr. (1965) 18: 172-175. Terasaki, Μ., Fujita, Ε., Wada, S., Takemoto, T., Nakajima, T., Yokobe, T., Jrnl. Jpn. Soc. Food Nutr. (1965) 18: 222-225. Yamaguchi, S. In: "Olfaction and Taste VI" (J. Le Magnen and P. Mac Leod, Eds) ρ 493, Information Retri eval, Wash. D.C., 1978. Meilgaard, M.C., MBAA Tech. Quart. (1975) 12: 151-168.



30

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RECEIVED August 9, 1979.


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