Phenolic, Sulfur, and Nitrogen Compounds in Food Flavors

6 F l a v o r Precursors i n F o o d Stuffs LEONARD SCHUTTE

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0026.ch006

UniMillsΒ.V.,Lindtsedijk 8, Zwijndrecht, The Netherlands

The Role of Food. Flavor i s one of the p r o p e r t i e s of a food by which it i s recognized and enjoyed. The p r o p e r t i e s may be subdivided i n t o primary and secondary ones. The primary function of food i s nutrition or f u e l for the maintenance of life. Food should there fore be n o u r i s h i n g , have a proper nutritive value and be devoid of t o x i c or harmful e f f e c t s . Few m a t e r i a l s i n nature fulfill these requirements entirely: some are poisonous, others may c o n t a i n enzymes that i n t e r f e r e with the function of the body (biochemical p o i s o n s ) , and still others are simply not d i g e s t i b l e . Animals are g e n e r a l l y thought to know by i n s t i n c t what i s good for them and what i s not. In human beings this i n s t i n c t has been suppressed and replaced by an other process - that of l e a r n i n g from experience. The n a t u r a l m a t e r i a l s t h a t , through the ages, have been found to be nutritive and b e n e f i c i a l are fed to chil dren, who in turn become accustomed to them and con tinue to f l o u r i s h on them. The same mechanism may a l s o apply to animals; what we call "instinct" may be no more than a process whereby customs are passed on from parents to o f f s p r i n g . Years of experience have determined not only the range of n u t r i t i o u s foods a v a i l a b l e , but a l s o the meth od of t r e a t i n g them. For instance, n a t u r a l m a t e r i a l s sometimes have to be heated to kill enzymes which have adverse e f f e c t s . The nutritive effect and harmlessness of food have been e s t a b l i s h e d by experience, however, t h i s choice i s no longer determined on t h i s b a s i s . One does not eat a n a t u r a l m a t e r i a l at random and wait to see whether i t does one good or makes one ill. Instead, the food i s recognized by its secondary p r o p e r t i e s . The process by which memory and tradition link the secondary prop erties of the food i s represented i n Figure 1. 96

Charalambous and Katz; Phenolic, Sulfur, and Nitrogen Compounds in Food Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

6. SCHUTTE

Flavor Precursors in Food Stuffs

97


B e n e f i c i a l or adverse p h y s i o l o g i c a l e f f e c t s of food a r e e s t a b l i s h e d by e x p e r i e n c e and can be c o r r e l a t e d o n l y i n d i r e c t l y w i t h the p r i m a r y p r o p e r t i e s . On the o t h e r hand, r e c o g n i t i o n o f a f o o d by i t s secondary o r s e n s o r y p r o p e r t i e s , which may be d i v i d e d i n t o a p p e a r a n c e , t e x t u r e and f l a v o r , i s d i r e c t . Today, i n the c a s e o f packaged f o o d s , " a p p e a r a n c e " may be e x tended t o the package and to the image c r e a t e d by a d v e r t i s e m e n t s , i n which c o n s u m e r ' s a t t e n t i o n may be c a l l e d to the primary p r o p e r t i e s . Such f a c t o r s tend to c o m p l i c a t e the p i c t u r e . F l a v o r i n Food. The p r e f e r e n c e f o r p a r t i c u l a r c o m b i n a t i o n s o f secondary p r o p e r t i e s i s known as p e r s o n a l l i k i n g , or " t a s t e " i n the broad sense o f the word. The p l e a s u r e of e a t i n g i s d e r i v e d from t h e s e secondary p r o p e r t i e s . Taste i s a c q u i r e d , o f t e n over generations. T h i s e x p l a i n s why e a t i n g i s a s o c i a l c u l t u r a l phenomenon, not o n l y the method o f s e l e c t i n g and p r e p a r i n g the f o o d , but even the way o f consuming i t , the s o - c a l l e d t a b l e manners. This also explains why the i n t r o d u c t i o n o f n o v e l food p r o d u c t s i s so difficult. These p r o d u c t s w i l l o n l y s t a n d a chance o f s u c c e s s i f they bear some resemblance t o f a m i l i a r foods which a p p e a l t o the t a s t e o f the consumer. The secondary o r s e n s o r y p r o p e r t i e s by which a f o o d p r o d u c t i s judged a r e appearance, c o l o r , t a s t e , t e x t u r e , temperature and i n the c a s e o f popcorn and c h i p s f o r i n s t a n c e , even s o u n d . As shown i n F i g u r e 2, t h e s e f a c t o r s are c l o s e l y connected and i t i s o f t e n i m p o s s i b l e t o s e p a r a t e one from another (.1). They determine whether, and to what e x t e n t , a p a r t i c u l a r d i s h i s a c c e p t a b l e to the consumer. The secondary p r o p e r t i e s a r e very important as p e o p l e do not eat n u t r i e n t s , they eat f o o d ; the n u t r i t i o n a l impact o f a b a l a n c e d m i x t u r e o f n u t r i e n t s t h a t i s not a c c e p t e d as a food i s n i l . T a s t e i n the broad sense i s not v e r y f l e x i b l e , but t h i s does not mean t h a t some t a s t e s have not g r a d u a l l y changed. For i n s t a n c e , i n the r e c e n t p a s t p e o p l e i n the U n i t e d S t a t e s and i n Europe have grown a c customed to canned tomato soup and tomato j u i c e , and many p r e f e r the " s t e r i l i z e d f l a v o r " o f t h e s e p r o d u c t s to f r e s h tomato f l a v o r . F r e s h a p p l e sauce i s no l o n g e r p r e f e r r e d to canned a p p l e s a u c e . Many o f the younger g e n e r a t i o n brought up on margarine do not l i k e the t a s t e o f b u t t e r .


98

PHENOLIC, SULFUR, A N D NITROGEN COMPOUNDS I N FOOD FLAVORS

Flavor

i n Food Systems


T a s t e Compounds. A l t h o u g h f l a v o r i s m a i n l y a s s o c i a t e d w i t h t a s t e and o d o r , the o t h e r e n t i t i e s i n F i g u r e 2 p l a y an i n s e p a r a b l e r o l e . For instance, i t i s v e r y hard t o i d e n t i f y banana f l a v o r i n a p i n k i c e cream. F l a v o r c h e m i s t s , however, t r y t o i s o l a t e those f a c t o r s i m p a r t i n g t a s t e and odor from the o t h e r entities. Consequently, f l a v o r chemistry deals with t h o s e compounds r e s p o n s i b l e f o r t h e t a s t e and odor o f food. These f l a v o r compounds can be s u b d i v i d e d i n t o two c l a s s e s : t a s t e compounds and odor compounds. T h e i r c h a r a c t e r i s t i c s a r e summarized i n T a b l e I . TABLE I CHARACTERISTICS

OF TASTE AND ODOR COMPOUNDS

T a s t e Compounds P e r c e i v e d by tongue Non-Volatile Polar, water-soluble Present i n r e l a t i v e l y high concentrations Salt,

sweet,

sour,

bitter

Odor Compounds P e r c e i v e d by nose More o r l e s s v o l a t i l e More o r l e s s n o n - p o l a r Present i n low c o n c e n t r a t i o n s (mg/kg o r A i g / k g range) Many d i f f e r e n t f l a v o r sensat ions

One can p e r c e i v e t a s t e compounds on t h e i r own by p i n c h i n g the nose d u r i n g e a t i n g o r when one s u f f e r s from a heavy c o l d . In t h i s way t h e odor compounds a r e p r e v e n t e d from r e a c h i n g t h e n a s a l c a v i t y , e i t h e r d i r e c t l y or i n d i r e c t l y . T a s t e compounds a r e i n g e n e r a l n o n - v o l a t i l e , water s o l u b l e and u s u a l l y p r e s e n t i n the food i n r e l a t i v e l y h i g h c o n c e n t r a t i o n s . They g i v e a c o m b i n a t i o n o f s a l t , sweet, s o u r and b i t t e r i m pressions. Examples a r e common s a l t , s u c r o s e , citric a c i d , c a f f e i n e and monosodium g l u t a m a t e . Odor Compounds. Odor compounds have a f i n i t e vapor p r e s s u r e . They a r e never c o m p l e t e l y p o l a r o r n o n - p o l a r , a l t h o u g h some a r e more s o l u b l e i n water and others i n organic s o l v e n t s . They a r e u s u a l l y p r e s e n t i n low c o n c e n t r a t i o n s , i n the mg p e r kg o r / u g p e r kg r a n g e , and they g i v e a l a r g e v a r i e t y o f f l a v o r s e n sations. In c o n t r a s t t o t h e n o n - v o l a t i l e s , few v o l a t i l e s are e s s e n t i a l intermediates i n the biochemistry o f the l i v i n g o r g a n i s m . They may be c o n s i d e r e d as b e i n g d e r i v e d from the b a s i c b u l k i n g r e d i e n t s o f t h e



6. SCHUTTE


99

l i v i n g o r g a n i s m , i . e . c a r b o h y d r a t e s , p r o t e i n s and f a t s (triglycerides). The l a t t e r can thus be c o n s i d e r e d as the p r e c u r s o r s o f the v o l a t i l e f l a v o r compounds. As t h e s e b u l k i n g r e d i e n t s a r e s i m i l a r i n most l i v i n g s p e c i e s , i t i s not s u r p r i s i n g t h a t some odor compounds a r e common t o q u i t e d i f f e r e n t groups o f f o o d s . Examples are furanones i n f r u i t s and meat, s u l f i d e s i n meat and v e g e t a b l e s , and p y r a z i n e s i n v e g e t a b l e s and baked o r r o a s t e d p r o d u c t s l i k e bread and c o f f e e . The p r e c u r s o r s , however, a r e p r e s e n t i n d i f f e r e n t p r o p o r t i o n s i n the v a r i o u s l i v i n g s p e c i e s and f u r t h e r more the mode o f f o r m a t i o n o f the v o l a t i l e s may v a r y from one food t o a n o t h e r . P o s s i b l e modes o f f o r m a t i o n a r e b i o f o r m a t i o n i n the l i v i n g s p e c i e s , enzymatic f o r m a t i o n by f l a v o r enzymes or i n f e r m e n t a t i o n p r o c e s s e s , or non-enzymic M a i l l a r d - t y p e r e a c t i o n s . The l e v e l at which the v a r i o u s odor compounds are p r e s e n t , their p r o p o r t i o n s and t h e i r mode o f r e l e a s e determine the o v e r a l l f l a v o r o f the f i n a l food p r o d u c t . In t h e o r y , a l l v o l a t i l e s p r e s e n t i n a f o o d p r o d u c t c o n t r i b u t e t o the o v e r a l l o d o r , which makes the f l a v o r o f e v e r y food p r o d u c t u n i q u e . Some compounds a r e , however, more important than o t h e r s and t h e s e a r e o f t e n r e f e r r e d to as " k e y compounds". It would though be an o v e r - s i m p l i f i c a t i o n to t h i n k t h a t the f l a v o r o f a food p r o d u c t can be r e c o n s t i t u t e d by p r o p e r c o m b i n a t i o n o f t h e s e key compounds. T h i s has been p o s s i b l e o n l y i n some e x c e p t i o n a l c a s e s , l i k e v a n i l l a or cucumber. The importance o f f l a v o r i n d e t e r m i n i n g the a c c e p t a b i l i t y o f food c e r t a i n l y w a r r a n t s the s t u d y o f n a t u r a l f l a v o r compounds. S t r u c t u r a l e l u c i d a t i o n of t h e s e compounds not o n l y adds t o our knowledge o f the t y p e s o f m o l e c u l e s which produce f l a v o r , but i t a l s o s u p p l i e s us w i t h s u i t a b l e m a t e r i a l s f o r making h i g h l y n u t r i t i o u s but l e s s t a s t y foods more p a l a t a b l e . Moreo v e r , f l a v o r i n g can h e l p to make a c c e p t a b l y p r i c e d , a t t r a c t i v e p r o d u c t s more w i d e l y a v a i l a b l e . Knowledge o f the mechanism o f f l a v o r f o r m a t i o n from p r e c u r s o r s i s t h e r e f o r e not o n l y o f s c i e n t i f i c i n t e r e s t . This knowledge may be a p p l i e d i n s e l e c t i n g good f l a v o r i n g methods and i n c h o o s i n g p r o c e s s i n g c o n d i t i o n s , so t h a t o f f - f l a v o r formation is prevented. It i s t h e r e f o r e the t a s k o f the f l a v o r chemist not o n l y to i s o l a t e and i d e n t i f y o d o r i f e r o u s compounds i n f o o d p r o d u c t s , but a l s o to s t u d y t h e i r f o r m a t i o n from p r e c u r s o r s i n o r d e r t o be a b l e t o a p p l y the r e s u l t s t o the f l a v o r i n g o f p r o c e s s e d and n o v e l food p r o d u c t s . T h i s w i l l be i l l u s t r a t e d u s i n g meat f l a v o r as an example.


100

PHENOLIC, SULFUR, A N D NITROGEN COMPOUNDS I N FOOD FLAVORS

Meat F l a v o r Compounds


T a s t e Compounds. There a r e , of c o u r s e , many t y p e s of meat f l a v o r s , e . g . b o i l e d , b r o t h y , r o a s t , e t c . and b e e f , p o r k , lamb, c h i c k e n , e t c . A l l these f l a v o r s have many c h a r a c t e r i s t i c s i n common and I w i l l o n l y d i s t i n g u i s h between the v a r i o u s types when a p propriate. The t a s t e compounds i n meat a r e r a t h e r universal. The most important t a s t e compounds i n meat are given i n Table I I . The n a t u r a l pH o f meat i s about 5 . 5 , which s h o u l d be borne i n mind when a s s e s s i n g t h e i r c o n t r i b u t i o n t o the t a s t e o f meat. TABLE

II

SOME IMPORTANT TASTE COMPOUNDS IN MEAT FLAVOR (pH VALUE 5.5) glutamic a c i d inosinic acid succinic acid lactic acid phosphoric a c i d pyrrolidonecarboxylie

acid

Odor Compounds and t h e i r P r e c u r s o r s . The v o l a t i l e p a r t o f meat f l a v o r i s more c o m p l i c a t e d . For the sake o f argument l e t us assume t h a t an i d e a l gas c h r o matogram o f meat f l a v o r v o l a t i l e s i s as shown i n F i g u r e 3. In t h i s gas chromatogram a l l v o l a t i l e s a r e r e p r e s e n t e d as s e p a r a t e peaks and the h e i g h t s a r e p r o p o r t i o n a l to the r e l a t i v e amounts p r e s e n t i n the h e a d space o f the meat p r o d u c t c o n c e r n e d . By d i v i d i n g the c o n c e n t r a t i o n o f each compound by i t s f l a v o r t h r e s h o l d v a l u e , an "aromagram" which shows the r e l a t i v e f l a v o r i m p r e s s i o n of each compound i s o b t a i n e d . This is shown i n F i g u r e 4 which r e p r e s e n t s the h y p o t h e t i c a l aromagram of meat f l a v o r , more or l e s s comparable w i t h the s k y l i n e o f a c i t y . The f l a v o r compounds i n the aromagram o f a cooked meat p r o d u c t a r e formed from p r e c u r s o r s p r e s e n t i n the raw meat. P r e c u r s o r s a r e those i n g r e d i e n t s i n the food t h a t are the most r e a c t i v e d u r i n g i t s p r e p a r a tion. Some s o r t o f heat treatment i s u s u a l l y r e s p o n s i b l e f o r f l a v o r f o r m a t i o n i n meat. The p r e c u r s o r s of meat f l a v o r a r e c e r t a i n amino a c i d s and s u g a r s which react together i n s o - c a l l e d M a i l l a r d r e a c t i o n s . Some o f t h e s e r e a c t i o n s w i l l be b r i e f l y d i s c u s s e d . The f i r s t r e a c t i o n i s the s o - c a l l e d Amadori rearrangement ( F i g u r e 5) i n which monosaccharides l i k e g l u c o s e ,


6. SCHUTTE

Flavor Precursors in Food Stuffs PRIMARY PROPERTIES

FOOD

SECONDARY PROPERTIES

101

PHYSIOLOCICAL EFFECTS

> EATER

EXPERIENCE


Figure I. Classical relations between primary and secondary properties of food and its effect on the eater

Figure 2. Interconnection between sensory properties of food

Recorder response

Figure 3. Hypothetical gas chromatogram of meat flavor


102

PHENOLIC, SULFUR, AND NITROGEN COMPOUNDS I N FOOD FLAVORS

Flavor impression


Figure 4. Hypothetical aromagram of

CH

hydroxyfuranone

3

C=0

1 C-OH Ν C-OH

isomaltol

1 H-C-OH I

1

maltol

R 1

1

H-C-OH 1 H-C-OH

R2NH

1

H-C-OH H-C-OH R

^

O=0 CH

furfural

R 0 CHO A

ÇH H-C-CH

A

f

f\ V

N

rm

° y{~

CHO

pyrrole

1

R

Ο

Η ΛΗΟ

ÇH3 C=0

V*

0 0 \

H-C-OH R

*

Η 3

°^ C H Ç

3

AMADOR I INTERMEDIATE

d i 3 C e t y l

hydroxy,, acetone H3C ^ CH2OH

O-OH HEXOSE PENTOSE

akM

3

1

C=0 H-C-OH

pyruv-

c

clotene

y

II

Ο

ODOR COMPOUND

Figure 5. Amadou rearrangements of sugars



6.

SCHUTTE

Flavor

Precursors

in Food

Stuffs

103

f r u c t o s e or r i b o s e r e a c t w i t h amines by which c a r b o n y l groups a r e moved w i t h i n the sugar m o l e c u l e ( 2 ) . In subsequent c y c l i s a t i o n r e a c t i o n s h e t e r o c y c l i c com pounds l i k e f u r a n o n e s , pyranones o r even p y r r o l e s a r e formed. The r e a r r a n g e d sugar may a l s o break down i n t o s m a l l e r m o l e c u l e s l i k e d i k e t o n e s o r hydroxy a c e t o n e , which i n t u r n may form a c y c l o t e n e i n a c o n d e n s a t i o n reaction. Another type o f r e a c t i o n i s the S t r e c k e r d e g r a d a t i o n ( F i g u r e 6 ) , i n which a l d e h y d e s are formed from amino a c i d s which have been o x i d i z e d by d i k e t o n e s . Examples a r e the f o r m a t i o n o f a c e t a l d e h y d e from a l a n i n e , m e t h i o n a l from m e t h i o n i n e , p h e n y l a c e t a l d e h y d e from p h e n y l a l a n i n e and benzaldehyde from p h e n y l g l y cine. Strecker degradation of cysteine y i e l d s hydro gen s u l f i d e , which i t s e l f i s a meat f l a v o r compound as w e l l as b e i n g an important p r e c u r s o r o f o t h e r s u l f u r c o n t a i n i n g meat f l a v o r compounds ( 3 ) . For i n s t a n c e , r e a c t i o n o f hydrogen s u l f i d e w i t h h e t e r o c y c l i c com pounds l i k e f u r a n o n e s , l e a d s t o the f o r m a t i o n o f s u l f u r c o n t a i n i n g h e t e r o c y c l e s which have a r o a s t e d meat f l a v o r (Figure 7). M e t h a n e t h i o l , which i s developed from m e t h i o n i n e , i s a s i m i l a r compound, y i e l d i n g mono-, d i - and t r i s u l f i d e s . Some o t h e r compounds formed from these s m a l l precursor molecules are 1 - m e t h y l t h i o e t h a n e t h i o l and d i m e t h y l t r i t h i o l a n e . P y r a z i n e s are a l s o formed as a r e s u l t o f S t r e c k e r degradations. Here the d i k e t o n e moiety i s h e l d r e s p o n s i b l e f o r the d i r e c t f o r m a t i o n o f the p y r a z i n e s (4). As shown i n F i g u r e 8 some r e a c t i o n s i n v o l v e the f o r m a t i o n o f a c y l t h i a z o l e s from c y s t e i n e and d i k e t o n e s , such as p y r u v a l d e h y d e ( 3 ) , and the format i o n o f u n s a t u r a t e d l a c t o n e s from α - k e t o b u t y r i c a c i d , an o x i dat i o n p r o d u c t o f t h r e o n i n e (5). F i g u r e 9 shows the f l a v o r compounds formed from thiamine or vitamin B l . As the b i o s y n t h e s i s of the t h i a z o l e moiety of t h i a m i n e i n v o l v e s r e a c t i o n o f m e t h i o n i n e w i t h a l a n i n e , t h e r e i s some c o n n e c t i o n b e tween the f o r m a t i o n of t h e s e f l a v o r compounds and t h a t o f t h o s e formed d i r e c t l y from amino a c i d s . It a g a i n i n d i c a t e s t h a t the f o r m a t i o n o f meat f l a v o r compounds may f o l l o w c o m p l i c a t e d pathways i n v o l v i n g v a r i o u s i n termediates . The r e a c t i o n s d i s c u s s e d l e a d to w e l l - i d e n t i f i e d f l a v o r v o l a t i l e s , which c o r r e s p o n d t o the prominent t a l l b u i l d i n g s i n the s k y l i n e shown i n F i g u r e 4 . They may be r e g a r d e d as the " k e y compounds" o r t o p - n o t e s . In meat f l a v o r such compounds are : c a r b o n y l compounds, h e t e r o c y c l e s , s u l f u r and n i t r o g e n compounds and f i n a l l y , some p h e n o l s . Some o f them c o n t r i b u t e more to



P R O D U C T

R

CH3CH3-S-CH2-CH2-

acetaldehyde /

JO \


(R-CT

&

\

methional

) Η/

phenylacetaldehyde benzaldehyde

C H - C t J • H S*CH3-SH-* 3

2

CH3-CH-S-CH3 1-methylthio^ ethanethiol CH3- " HS-CH2- • CH3-S-S-CH3 di-Me-disulfide di-Me-sulfide CH3-S-CH2-CH2 · CH3-S-CH3 CH3-S-S-S-CH3 di-Me-trisulfide H

di - Me - trithiolane

R

V

9

2 -C-C

NH2

[0]

L

->J^

y N y Π pyrazines N

Figure 6. Strecker degradation of amino acids

HS

Figure 7.

Flavor compounds from the reaction of hydrogen sulfide with heterocyclic furanones


SCHUTTE


H CT 2


HC—NH2 COOH cysteine

\

tC

pyruvaldehyde

ι

*

2-acetylthiazoline

I

\

H3C

HC-CH NH threonine

*

W

}={ H3C OH 2-oxobutyric acid 3-methyl-2-hydroxy-2-hexen -4-olide Figure 8. Flavor compounds derived from cysteine and threonine

0

HC-C

2-acetyl thiazole

2

0

CH3-S-CH2-CH2-CH-COOH

NH2 .S ^CH2-CH20H NS

NH2

OTiethionine)

φ

+ CH3-CH-COOH NH2

(alanine)

thiazoles

V

.CH2

Jj

^

2

φ

Χ

®

0 />« (thiamine)

0H0

J ^jT

thiophenes

2-thiomethyl-5-methylfuran

,SH

3-mercapto propanol

hydrogen sulfide

Figure 9. Flavor compounds from thiamine



106

PHENOLIC, SULFUR, AND NITROGEN COMPOUNDS IN FOOD FLAVORS

meat f l a v o r s than o t h e r s and some may g i v e a t y p i c a l c h a r a c t e r to the meat f l a v o r , f o r i n s t a n c e , thiophenes g i v e a r o a s t e d c h a r a c t e r and d e c a d i e n a l a more c h i c k e n like flavor. A l t h o u g h q u i t e a v a r i e t y o f meat f l a v o r compounds has been mentioned, t h e r e a r e s t i l l many o t h e r s t h a t c o n t r i b u t e to a s m a l l e r d e g r e e , many o f which have not yet been i d e n t i f i e d . These compounds may be e n v i s a g e d as the s m a l l b u i l d i n g s i n the s k y l i n e , g i v i n g a more o r l e s s b a s i c meat f l a v o r . Without them the s k y l i n e would seem u n n a t u r a l and the f l a v o r would have a s y n thetic character. In o t h e r words, i n a b a l a n c e d meat f l a v o r many f l a v o r compounds p l a y a r o l e , which themselves a r e not at a l l r e m i n i s c e n t o f meat f l a v o r . An a p p r o p r i a t e name f o r such an e f f e c t , i n which two or more f l a v o r compounds t o g e t h e r g i v e a c o m p l e t e l y d i f f e r e n t f l a v o r s e n s a t i o n , i s a "synosmic e f f e c t " . It may be compared w i t h the m i x i n g of c o l o r s , e . g . b l u e and y e l l o w t o gether give a completely d i f f e r e n t c o l o r : green; or w i t h the sound o f an o r c h e s t r a , which i s d i f f e r e n t from t h a t o f any o f the i n d i v i d u a l i n s t r u m e n t s . A " s y n o s m i c e f f e c t " s h o u l d be d i s t i n g u i s h e d from an "additive effect", f l a v o r s w i t h the same f l a v o r c h a r a c t e r a d d i n g t h e i r i n t e n s i t i e s , a n d from a " s y n e r g i s t i c e f f e c t " , w e l l - k n o w n i n t a s t e m i x t u r e s , where one compound enhances the f l a v o r s e n s a t i o n o f a n o t h e r , t o a l a r g e r degree t h a n i n an a d d i t i v e e f f e c t . A p p l i c a t i o n o f Meat

Flavors

General. How can t h i s knowledge be a p p l i e d i n the f l a v o r i n g o f food p r o d u c t s ? F i r s t we must r e a l i z e what t o o l s we have and s e c o n d l y where and how t h e s e t o o l s c a n be used ( 6 ) . I f we s t a r t by l o o k i n g at the meat f l a v o r i n g s d e v e l o p e d so f a r , we can f i r s t d i s t i n g u i s h t a s t e m i x t u r e s and t a s t e enhancers i n which monosodium g l u t a m a t e and r i b o n u c l e o t i d e s are the components most f r e q u e n t l y a p p l i e d . Then we have t o p note m i x t u r e s c o n s i s t i n g o f v o l a t i l e odor compounds. R e l e a s e of t h e s e compounds may be c o n t r o l l e d by m i c r o e n c a p s u l a t i o n (7) . In some c a s e s , use o f d e r i v a t i v e s o f f l a v o r v o l a t i l e s w i t h a l a b i l e " b l o c k i n g " group may e n a b l e the g r a d u a l f o r m a t i o n , and hence g r a d u a l r e l e a s e , of the volatiles in question. Examples o f such a r t i f i c i a l f l a v o r p r e c u r s o r s are the O - t - a l k y l t h i o c a r b o n a t e s o f t h i o l s , which upon h e a t i n g form the c o r r e s p o n d i n g t h i o l f l a v o r compounds ( 3 ) . This is i l l u s t r a t e d in F i g u r e 10.



6.

SCHUTTE


107

F i n a l l y , t h e r e are the b a s i c f l a v o r m i x t u r e s which b a l a n c e the o v e r a l l meat o d o r . They are u s u a l l y formed by l e t t i n g s u i t a b l e p r e c u r s o r s r e a c t t o g e t h e r . P r o t e i n h y d r o l y s a t e s a r e f r e q u e n t l y employed f o r t h i s purpose. The f l a v o r o f p r o t e i n h y d r o l y s a t e s i s formed by r e a c t i o n s between the amino a c i d s and c a r b o h y d r a t e s l i b e r a t e d d u r i n g the h y d r o l y s i s o f p r o t e i n a c e o u s mater i a l s such as soy o r g l u t e n . More s o p h i s t i c a t e d p r e c u r s o r m i x t u r e s y i e l d i n g meat f l a v o r s on h e a t i n g have been w i d e l y p a t e n t e d (3) and ( 8 ) . As shown i n F i g u r e 11, the p r e c u r s o r systems g e n e r a l l y c o n s i s t of s u l f u r - c o n t a i n i n g compounds s u c h as c y s t e i n e , t h i a m i n e o r hydrogen s u l f i d e on one hand, and c a r b o n y l compounds l i k e m o n o s a c c h a r i d e s , aldehydes or furanones on the o t h e r . When such p r e c u r s o r s y s tems a r e heated i n w a t e r , b o i l e d meat f l a v o r s a r e formed, whereas h e a t i n g i n f a t g i v e s r i s e t o a meat f l a v o r with a roasted c h a r a c t e r . The f a t i s a l s o h e l d r e s p o n s i b l e f o r the development o f the t y p i c a l f l a v o r c h a r a c t e r o f the v a r i o u s types o f meat, such as c h i c k en, pork o r lamb. For instance, poly-unsaturated a l d e hydes and gamma l a c t o n e s i n c h i c k e n f l a v o r a r e d e r i v e d from u n s a t u r a t e d f a t t y a c i d s ( 9 ) . The p r e c u r s o r s may a l s o be added as such to the food p r o d u c t b e f o r e p r o c e s s i n g , i n which case the meat f l a v o r i s formed d u r ing c o o k i n g . The b a s i c meat f l a v o r may a l s o be d e r i v e d from n a t u r a l p r o d u c t s such as meat e x t r a c t , c h i c k e n powder o r a smoke s o l u t i o n . In t h e s e c a s e s , of c o u r s e , the compounds p r e s e n t i n the n a t u r a l m a t e r i a l s have p e r formed as the f l a v o r p r e c u r s o r s . It goes w i t h o u t s a y ing t h a t the use o f more t r a d i t i o n a l f l a v o r i n g s l i k e s a l t and s p i c e s remains i n d i s p e n s a b l e ( 6 ) . The c a s e s i n which the t o o l s i . e . the meat f l a v o r m i x t u r e s are a p p l i e d may be s u b d i v i d e d i n t o t h r e e c a t e g o r i e s . P r o c e s s e d Meat P r o d u c t s . F i r s t t h e r e i s the p r o c e s s e d meat p r o d u c t which d u r i n g p r o c e s s i n g , e . g . heat s t e r i l i z a t i o n or d r y i n g , has l o s t some of the f l a v o r c h a r a c t e r i s t i c s t h a t a r e u s u a l l y p r e s e n t i n the e q u i v a l e n t f r e s h l y cooked f o o d . In some c a s e s a r t i f a c t s i . e . f l a v o r compounds not predominant i n the f r e s h food may be f o r m e d . These are u s u a l l y p e r c e i v e d as o f f flavors. The aromagram or " s k y l i n e " w i l l l o o k l i k e F i g u r e 12. G e n e r a l l y the most v o l a t i l e o r the most l a b i l e f l a v o r compounds have d i s a p p e a r e d . T h i s i s most r e a d i l y n o t i c e d by the l a c k o f " t a l l b u i l d i n g s " . Changes i n the s m a l l e r b u i l d i n g s o f the s k y l i n e are l e s s d r a matic. The b a s i c meat f l a v o r i s t h e r e f o r e s t i l l p r e -



0II CH 3 I R-S-C-0-C-CH3

CH3

OH

ÇH3


R-S-C-O-C-CH3 • έ Η 3

A

R-S-C

+ ©C-CH3

CH3

1 I ÇH3

l§ i%- - F".* ?:ΐ'

F

U

10

alkyl thiocarbonates of tht~ oh for flavor release

SULFUR SOURCE

CARBONYL COMPOUND hexose pentose aldehyde Juranoney

R

4

-S PH Q ^

9

HO-Ç-CH3 I

0

CH3

MEDIUM

TYPE MEAT FLAVOR

water

boiled

fat

roasted

food

boiled

Figure 11. Preparation meat flavorsfrom of precursors


6.

SCHUTTE


109


sent. In a d d i t i o n , b u i l d i n g s t h a t do not b e l o n g t o the s k y l i n e a p p e a r . These a r e " a r t i f a c t s " . Examples o f a r t i f a c t s due t o p r o c e s s i n g a r e e x c e s s e s o f a l d e hydes o r o f c e r t a i n s u l f u r compounds l i k e hydrogen s u l f i d e , m e t h a n e t h i o l and d i m e t h y l s u l f i d e i n canned meat p r o d u c t s . T h i s e f f e c t may be p r e v e n t e d o r a t l e a s t reduced by the c h o i c e o f p r o p e r s t e r i l i z a t i o n c o n d i t i o n s and o f pH, o r the a d d i t i o n o f i n h i b i t o r s l i k e s a l t s of fumaric, maleic or s o r b i c a c i d (10). Such p r o c e s s e d f o o d s , i n which the b a s i c meat f l a v o r i s s t i l l present can u s u a l l y b e s t be f l a v o r e d by the a d d i t i o n of a mixture of top-notes, e i t h e r i n excess or i n a s t a b i l i z e d form. P r o d u c t s w i t h L i t t l e Meat. The second type o f a p p l i c a t i o n i s i n p r o d u c t s t h a t f o r economic r e a s o n s p a r t o f the meat has been l e f t out o f the f o r m u l a t i o n . T h i s r e s u l t s i n a d i l u t i o n of the meat f l a v o r . An e x ample i s a f a b r i c a t e d soup, which i s not u s u a l l y p r e p a r e d i n the same way as a home-made s o u p . The meat f l a v o r i n such a p r o d u c t s h o u l d be enhanced by t a s t e m i x t u r e s and m i x t u r e s based on p r e c u r s o r s e . g . h y d r o l ysates. As t h e s e p r o d u c t s a l s o f a l l i n the f i r s t c a t e g o r y , t o p - n o t e m i x t u r e s a r e recommended as w e l l . P r o d u c t s w i t h o u t Meat - V e g e t a b l e P r o t e i n P r o d u c t s . F i n a l l y , t h e r e i s the extreme c a s e o f a m e a t - l i k e p r o d u c t which c o n t a i n s no meat at a l l . Examples a r e some o f the p r o d u c t s based on t e x t u r e d v e g e t a b l e p r o t e i n m a t e r i a l s , e . g . the s o - c a l l e d f u l l analogues (8). The development o f such p r o d u c t s , which i s c u r r e n t l y of g r e a t i n t e r e s t , p r o v i d e s a major c h a l l e n g e t o the f l a v o r c h e m i s t , and i t t h e r e f o r e i s w o r t h w h i l e t o d w e l l upon i t f o r a few moments. T e x t u r e d v e g e t a b l e p r o t e i n m a t e r i a l s a r e a new type o f food i n g r e d i e n t s , which may a p p e a l t o the c o n sumer p r o v i d e d the food i t s e l f f i t s i n t o the g e n e r a l consumption p a t t e r n . S i n c e the n u t r i t i o n a l p r o p e r t i e s o f t h e s e v e g e t a b l e p r o t e i n m a t e r i a l s , p a r t i c u l a r l y soy p r o t e i n m a t e r i a l s , resemble those o f meat, i t i s l o g i c a l t o a l l o w meat to s e r v e as a model f o r t h e s e new ingredients. It s h o u l d , however, be s t r e s s e d t h a t v e g e t a b l e p r o t e i n m a t e r i a l s s h o u l d not be seen j u s t as meat i m i t a t i o n s , but r a t h e r as n o v e l , p r o t e i n - r i c h f o o d i n g r e d i e n t s , no more than s p a g h e t t i s h o u l d be seen as an i m i t a t i o n o f p o t a t o e s . There a r e some problems a s s o c i a t e d w i t h the f l a v o r i n g o f t e x t u r e d soy p r o t e i n m a t e r i a l s . Appearance and t e x t u r e o f the p r o d u c t s based on them a r e d i f f e r e n t from those o f e x i s t i n g p r o d u c t s ; t h i s d i f f e r e n c e i n



110


t e x t u r e r e s u l t s i n a d i f f e r e n t mode o f f l a v o r r e l e a s e during chewing. Then t h e r e i s the t y p i c a l soy f l a v o r i n the u n r e f i n e d soy m a t e r i a l s t o which p e o p l e do not grow a d j u s t e d . Masking o f t h i s o f f - f l a v o r , f o r i n s t a n c e w i t h s t r o n g s p i c y and s a v o r y n o t e s i s not s a t i s f a c t o r y , as the o f f - f l a v o r p e n e t r a t e s the masking f l a v o r s on r e p e a t e d c o n s u m p t i o n . The e f f e c t i s not u n l i k e t h a t of p a i n t i n g a c a r w i t h o u t removing the o l d paint. A f t e r some weeks the new p a i n t w i l l p e e l o f f and the o l d l a y e r w i l l g r a d u a l l y become v i s i b l e . The o n l y way i n which the i n c o r p o r a t i o n o f soy p r o t e i n m a t e r i a l s at h i g h l e v e l s i n food p r o d u c t s can be made a c c e p t a b l e i s by removing the o f f - f l a v o r compounds and t h e i r p r e c u r s o r s , f o r i n s t a n c e by s o l v e n t extraction. P e o p l e i n the O r i e n t have always been aware o f t h i s , s i n c e the t r a d i t i o n a l soy p r o d u c t s consumed i n those c o u n t r i e s have been p r e t r e a t e d i n some way, e . g . by e x t r a c t i o n o r f e r m e n t a t i o n , which e f f e c t s the removal o f the o f f - f l a v o r s . I f we assume t h a t the o f f - f l a v o r i n h e r e n t to the soy p r o t e i n m a t e r i a l s has been removed, i t s aromagram w i l l be s i m i l a r to t h a t shown i n F i g u r e 13. T h u s , the meat f l a v o r o f the p r o d u c t to be developed has t o be b u i l t up from s c r a t c h . T h i s would mean t h e r e f o r e a c o m b i n a t i o n o f n o n - v o l a t i l e t a s t e compounds, b a s i c meat f l a v o r (the s m a l l b u i l d i n g s ) and the t o p - n o t e s ( t a l l b u i l d i n g s ) . The most d i f f i c u l t p a r t i n t h i s seems t o be the b a s i c meat f l a v o r ; the c o m b i n a t i o n o f t a s t e compounds and t o p - n o t e s a l o n e l e a d s to a s y n t h e t i c , u n r e a l f l a v o r i m p r e s s i o n . Meat f l a v o r s based on p r e c u r s o r systems t h e r e f o r e p l a y a v e r y important r o l e i n t h i s type o f a p p l i c a t i o n . Of c o u r s e , i t h e l p s tremendously i f some n a t u r a l meat f l a v o r i s p r e s e n t i n the form o f meat i t s e l f o r as meat e x t r a c t , c h i c k e n powder, e t c . The food p r o d u c t then moves i n t o the second c a t e g o r y mentioned b e f o r e , i n which o n l y p a r t o f the n a t u r a l meat i n g r e d i e n t has been r e p l a c e d , and the n a t u r a l f l a v o r i s o n l y d i l u t e d . Therefore, textured vegetable p r o t e i n materials w i l l f r e q u e n t l y be i n t r o d u c e d as meat e x t e n d e r s . In t h i s c o n t e x t I would l i k e to comment on the recommendations o f some l e g i s l a t i v e a u t h o r i t i e s c o n c e r n i n g the s u p p l e m e n t a t i o n o f soy p r o t e i n m a t e r i a l s w i t h f r e e m e t h i o n i n e w i t h the aim of enhancing t h e i r protein value. A p a r t from the q u e s t i o n whether t h i s i s n e c e s s a r y i n view o f the r e a s o n a b l y h i g h q u a l i t y o f soy p r o t e i n and the more than adequate p r o t e i n i n t a k e i n Western c o u n t r i e s , t h e r e a r e a number o f d i s a d v a n tages a s s o c i a t e d w i t h such s u p p l e n t a t i o n s , which are r e l e v a n t to the subject of t h i s paper. Free methion i n e i s l a b i l e and a c t s as a p r e c u r s o r of v o l a t i l e s .


6.

SCHUTTE


111


Flavor impression

t Figure 12. Hypothetical aromagram of processed meat flavor

Flavor impression

t Figure 13. Aromagram offlavorlessmeat analog


112

PHENOLIC, SULFUR, AND NITROGEN COMPOUNDS IN FOOD FLAVORS


It has been found that 30 - 80% of the added methionine i s degraded, depending on the type of processing (11). Thus, only part of the added methionine i s e f f e c t i v e as a n u t r i t i o n a l supplement ; the r e s t gives r i s e to a r t i f a c t s , some of which, e . g . methional, cause objectionable o f f - f l a v o r s . And the l e v e l s of methionine recommended for n u t r i t i o n a l f o r t i f i c a t i o n are an order of magnitude higher than the l e v e l s of amino acids normally used for f l a v o r i n g purposes. Conclusion. In t h i s survey I have attempted to i n d i c a t e the importance of a c q u i r i n g knowledge on the r o l e of precursors i n the formation of f l a v o r . At present i t i s s t i l l very d i f f i c u l t to compound c o m p l i cated f l a v o r s l i k e meat f l a v o r without using some of the n a t u r a l product i t s e l f , or an e x t r a c t thereof as the foundation onto which the added f l a v o r i s b u i l t . Appropriate use of precursors may lead to a wide range of processed and novel food products, which p a r t l y on account of t h e i r good f l a v o r c h a r a c t e r i s t i c s w i l l be acceptable and even a t t r a c t i v e to the consumer i n our s o p h i s t i c a t e d Western s o c i e t y . This may be a goal i n i t s e l f , but i t i s a l s o the only way by which increased amounts of food can be made a v a i l a b l e to the growing world p o p u l a t i o n . ABSTRACT Three types of f l a v o r s may be d i s t i n g u i s h e d : t a s t e mixtures composed of n o n - v o l a t i l e s ; mixtures of volatile compounds ("top-notes"); like furanones, and s o - c a l l e d " r e a c t i o n f l a v o r s " . The latter comprise precursors which react upon heating to yield mixtures with a basic f l a v o r balancing the r a t h e r harsh top-notes. The precursors of meat f l a v o r g e n e r a l l y c o n s i s t of s u l f u r compounds like c y s t e i n e , thiamine or hydrogen s u l f i d e r e l e a s e r s and carbonyl compounds like monosaccharides, aldehydes or furanones. They may r e act i n water, fat or the food product itself. Each of the three f l a v o r types makes its s p e c i f i c c o n t r i b u t i o n to the formation of meat f l a v o r . A w e l l balanced comb i n a t i o n of the three leads to a total meat f l a v o r , although the presence of some n a t u r a l meat f l a v o r is still required for best r e s u l t s . 1. 2.

Literature Cited Kramer, A., Food T e c h n o l . , (1972) 26, 34. Hodge, J.E., Mills, F.D. and F i s h e r , B . E . , Cereal Sci. Today (1972) 17, 34.


6.

SCHUTTE 3. 4. 5. 6. 7. 8.


9. 10. 11.


Schutte, L., C.R.C. Crit. Rev. Food T e c h n o l . , 4 (1974) 4, 457. R i z z i , G.P., J. A g r i c . Food Chem., (1972) 10, 1081. S u l s e r , Η . , D e P i z z o l , J. and Büchi, W., J. Food Sci., (1967) 32, 611. Blake, A., Food Manuf., (1975) 50, 35. Balassa, L.L. and Fauger, G . O . , C.R.C. Crit. Rev. Food Technol., (1971) 1, 245. Wilson, R . A . and Katz, I., The Flavor Industry, (1974) 5, 30. Harkes, P . D . and Begeman, W.J., J. Am. Oil Chem. Soc., (1974) 51, 356. Persson, T. and Von Sydow, E., J. Food Sci., (1974) 39, 406. Shemer, M. and P e r k i n s , E.G., J. A g r i c . Food Chem., (1975) 23, 201.


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Phenolic, Sulfur, and Nitrogen Compounds in Food Flavors

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