Chapter 8
Formation and Aroma Characteristics of Heterocyclic Compounds in Foods 1
Chi-Tang Ho and James T. Carlin
Department of Food Science, New Jersey Agricultural Experiment Station, Cook College, Rutgers University, New Brunswick, NJ 08903 Heterocyclic compounds have been identified as important volatile components of many foods. The odor strength and complexity of these compounds makes them desirable as flavoring ingredients. Heterocyclic compounds are primarily formed through nonenzymatic browning reactions. Recent studies of deep-fat fried food flavors led to the identification of pyrazines, pyridines, thiazole, oxazoles and cyclic polysulfides which had long-chain alkyl substitutions on the heterocyclic ring. The involvement of lipid or lipid decomposition products in the formation of these compounds could account for the long-chain alkyl substitutions. Our knowledge o f the c h e m i c a l c o m p o s i t i o n o f food f l a v o r s has made c o n s i d e r a b l e p r o g r e s s d u r i n g the l a s t twenty y e a r s . This i s mainly due t o advances i n a n a l y t i c a l t e c h n i q u e s , such as the c o u p l i n g o f GC w i t h MS and the development o f f u s e d - s i l i c a c a p i l l a r y columns. H e t e r o c y c l i c compounds occupy a prominent p o s i t i o n among the more than 10,000 compounds o c c u r r i n g i n the v o l a t i l e s o f f o o d s . This r e s u l t s from t h e i r e x c e p t i o n a l s e n s o r y p r o p e r t i e s ( 1 ) . Heterocyclic compounds c o n t a i n one o r more heteroatoms (0, S a n d / o r N) i n r i n g s or fused r i n g systems. The m a j o r i t y o f h e t e r o c y c l i c compounds a r e formed t h r o u g h t h e r m a l i n t e r a c t i o n s o f r e d u c i n g sugars and amino a c i d s , known as the M a i l l a r d r e a c t i o n . Other t h e r m a l r e a c t i o n s such as h y d r o l y t i c and p y r o l y t i c d e g r a d a t i o n o f food components ( e . g . s u g a r s , amino a c i d s , v i t a m i n s ) and the o x i d a t i o n o f l i p i d s a l s o c o n t r i b u t e t o the f o r m a t i o n o f h e t e r o c y c l i c compounds r e s p o n s i b l e f o r the complex f l a v o r o f many f o o d s t u f f s . H e t e r o c y c l i c compounds may a l s o be formed e n z y m a t i c a l l y i n v e g e t a b l e s ( t o m a t o e s , b e l l p e p p e r s , a s p a r a 1
Current address: Joseph E. Seagrams & Sons, White Plains, NY 10604 0097-6156/89A)388-0092$06.00/0 • 1989 American Chemical Society
8.
H O AND CARLIN
Aroma Characteristics ofHeterocyclic Compounds 93
g u s ) , f r u i t s ( p i n e a p p l e , p a s s i o n f r u i t ) and d u r i n g t h e r i p e n i n g o f cheese. Recent s t u d i e s i n o u r l a b o r a t o r y showed t h a t l i p i d s may be d i r e c t l y a s s o c i a t e d w i t h t h e M a i l l a r d r e a c t i o n i n t h e f o r m a t i o n o f some h e t e r o c y c l i c compounds. The e f f e c t o f l i p i d s on t h e f o r m a t i o n o f h e t e r o c y c l i c compounds i n a model M a i l l a r d r e a c t i o n has a l s o been r e p o r t e d by Mottram and W h i t f i e l d ( 2 ) . T h i s paper d i s c u s s e s t h e f o r m a t i o n and aroma c h a r a c t e r i s t i c s o f s e l e c t e d c l a s s e s o f h e t e r o c y c l i c compounds i m p o r t a n t t o t h e f l a v o r of foods, e s p e c i a l l y deep-fat f r i e d foods. Pyrazines A l k y l p y r a z i n e s have been r e c o g n i z e d as important t r a c e f l a v o r components o f a l a r g e number o f cooked, r o a s t e d , t o a s t e d and d e e p - f a t f r i e d foods ( 3 ) . As a r u l e , a l k y l p y r a z i n e s have a r o a s t e d n u t - l i k e odor and f l a v o r . F o r m a t i o n pathways f o r a l k y l p y r a z i n e s have been proposed by numerous r e s e a r c h e r s ( 4 , 5, 6 ) . Model s t u d i e s suggest t h a t they a r e minor p r o d u c t s o f t h e M a i l l a r d r e a c t i o n . The r e c e n t i d e n t i f i c a t i o n o f 2 - h e p t y l p y r a z i n e i n f r e n c h f r i e d p o t a t o f l a v o r , and 2-methyl-3(or 6 ) - p e n t y l p y r a z i n e , 2-methyl-3(or 6) - h e x y l p y r a z i n e and 2 , 5 - d i m e t h y l - 3 - p e n t y l p y r a z i n e i n a h e a t e d and e x t r u d e d corn-based model system, d e s e r v e s p e c i a l a t t e n t i o n ( 7 , 8, 9). These a l k y l p y r a z i n e s have a l o n g - c h a i n s u b s t i t u t i o n on t h e pyrazine r i n g . Only t h e involvement o f l i p i d s o r l i p i d - d e c o m p o s i t i o n p r o d u c t s i n t h e f o r m a t i o n o f t h e s e compounds c o u l d account f o r the l o n g - c h a i n a l k y l s u b s t i t u t i o n on t h e p y r a z i n e r i n g . A mechanism f o r t h e f o r m a t i o n o f 2 , 5 - d i m e t h y l - 3 - p e n t y l p y r a z i n e was proposed and i s shown i n F i g u r e 1. 3 , 6 - D i h y d r o p y r a z i n e , formed by t h e condensat i o n o f aminoketones, r e a c t s w i t h p e n t a n a l , a l i p i d o x i d a t i o n p r o d u c t , and r e s u l t s i n t h e f o r m a t i o n o f 2 , 5 - d i m e t h y l - 3 - p e n t y l p y r a z i n e . The p o s s i b l e r e a c t i v i t y o f 3 , 6 - d i h y d r o p y r a z i n e w i t h c a r b o n y l compounds has been d i s c u s s e d by Flament ( 1 0 ) . R i z z i (11) r e p o r t e d t h e f o r m a t i o n o f 2 , 5 - d i m e t h y l p y r a z i n e and 2 , 6 - d i m e t h y l p y r a z i n e i n t h e r e a c t i o n o f 1-hydroxy-2-propanone ( a c e t o l ) w i t h ammonium a c e t a t e under m i l d c o n d i t i o n s and a c i d i c pH. The proposed mechanism was a l s o s u p p o r t e d by o u r i d e n t i f i c a t i o n o f 2 , 5 - d i m e t h y l - 3 - p e n t y l p y r a z i n e and 2 , 6 - d i m e t h y l - 3 - p e n t y l p y r a z i n e as t h e major p r o d u c t s when p e n t a n a l was added t o a m i x t u r e o f a c e t o l and ammonium a c e t a t e and r e a c t e d a t an e l e v a t e d temperature (100°C). 2 - H e p t y l p y r a z i n e has g r e e n , waxy and e a r t h y n o t e s and c o u l d be an important c o n t r i b u t o r t o t h e f l a v o r o f f r e n c h f r i e d p o t a t o e s o r o t h e r f r i e d food systems. V a r i o u s i s o p e n t y l - s u b s t i t u t e d p y r a z i n e s , such as 2 - i s o p e n t y l 3-methylpyrazine, 2-isopentyl-5-methylpyrazine, 2 - i s o p e n t y l - 6 methylpyrazine, 2-isopentyl-5,6-dimethylpyrazine, 2-isopentyl-3,5d i m e t h y l p y r a z i n e and 2 - i s o p e n t y l - 3 , 6 - d i m e t h y l p y r a z i n e were i d e n t i f i e d from t h e t h e r m a l r e a c t i o n o f g l u c o s e and l e u c i n e ( 1 2 ) . The f o r m a t i o n mechanisms f o r t h e s e compounds may a l s o i n v o l v e t h e r e a c t i o n o f 3,6-dihydropyrazine with isovaleraldehyde, the Strecker a l dehyde o f l e u c i n e . Kitamura and Shibamoto (13) d e s c r i b e d 2 - i s o p e n t y l - 5 , 6 - d i m e t h y l p y r a z i n e as h a v i n g a c a r a m e l - l i k e , c o f f e e and sweet aroma. A l t h o u g h i s o p e n t y l - s u b s t i t u t e d p y r a z i n e s have n o t y e t been r e p o r t e d i n c o c o a , they c o u l d , i f p r e s e n t , be v e r y important cont r i b u t o r s t o t h a t c h a r a c t e r i s t i c aroma.
FLAVOR CHEMISTRY: TRENDS AND
PROTEINS
CH
+
DEVELOPMENTS
CARBOHYDRATES
/OH CHf
^NH, 2
+
I
'
XH
I^ Η,ο'
H N
N
+ (NH ) OAc 4
2
0
2
CH CH CH CH CH0 * 3
2
/ N _ C H
2
2
LIPIDS
3
H C^N^CHCH CH CH CH, 3
2
2
2
OH -H 0 2
^CH
XX
3
H C^ ^CH CH CH CH CH, 3
N
2
2
2
2
F i g u r e 1. Mechanism f o r t h e f o r m a t i o n o f 2,5-dimethyl-3pentylpyrazine.
8.
H O AND CARLIN
Aroma Characteristics ofHeterocyclic Compounds 95
A l t h o u g h most o f t h e a l k y l p y r a z i n e s a r e formed through t h e r m a l i n t e r a c t i o n s o f components i n food, m e t h o x y - s u b s t i t u t e d pyrazines are m a i n l y d e r i v e d from b i o s y n t h e t i c pathways. 2-Isobutyl-3-methoxyp y r a z i n e i s o l a t e d from b e l l pepper by B u t t e r y e t a l . (14) i s one o f the most s i g n i f i c a n t f l a v o r compounds d i s c o v e r e d . This c h a r a c t e r i s t i c b e l l pepper aroma compound has an e x t r e m e l y low odor t h r e s h o l d o f 0.002 ppb i n water ( 1 5 ) . Pyridines The o c c u r r e n c e o f p y r i d i n e s i n food has been reviewed ( 1 6 ) . 2 - A l k y l p y r i d i n e s were proposed t o form from t h e c o r r e s p o n d i n g unsaturated η-aldehydes w i t h ammonia upon heat t r e a t m e n t (17, 1 8 ) . T a b l e I l i s t s p y r i d i n e s i d e n t i f i e d i n t h e v o l a t i l e s o f f r i e d c h i c k e n (19) and french f r i e d potatoes ( 7 ) . Table
I.
P y r i d i n e s I d e n t i f i e d i n F r i e d C h i c k e n and French F r i e d Potato F l a v o r
Compound Pyridine 2-Me t h y l p y r i d i n e 2-Ethylpyridine 3-Ethylpyridine 4-Ethylpyridine 2-Acetylpyridine 2-Methyl-5-ethylpyridine 2-Ethyl-3-methylpyridine 2-Butylpyridine 2-Pentylpyridine 2-Heptylpyridine 2-Pentyl-3,5-dibutylpyridine 2-Isobutyl-3,5-dipropylpyridine
Fried Chicken
French-Fried Potato
+ + + + + + + + + +
+ + + +
+
2 - P e n t y l p y r i d i n e was i d e n t i f i e d i n b o t h f r i e d c h i c k e n and f r e n c h f r i e d potato f l a v o r s . T h i s compound has a s t r o n g f a t t y and t a l l o w l i k e odor and was t h e major p r o d u c t i n t h e v o l a t i l e s g e n e r a t e d from the t h e r m a l i n t e r a c t i o n o f v a l i n e and l i n o l e a t e ( 2 0 ) . I t i s p o s t u l a t e d t o form t h r o u g h t h e r e a c t i o n o f 2 , 4 - d e c a d i e n a l and ammonia ( 2 0 ) . The r e a c t i o n o f 2 , 4 - d e c a d i e n a l w i t h e i t h e r c y s t e i n e o r g l u t a t h i o n e (γ-glu-cys-gly) i n a q u e o u s s o l u t i o n a t h i g h temperature (180°C) y i e l d e d 2 - p e n t y l p y r i d i n e as t h e major p r o d u c t (Zhang, Y. and Ho, C T., Rutgers u n i v e r s i t y , u n p u b l i s h e d d a t a ) . The amount o f 2 - p e n t y l p y r i d i n e generated i n the 2,4-decadienal/glutathione system d i d n o t d i f f e r s i g n i f i c a n t l y from t h a t i n t h e 2 , 4 - d e c a d i e n a l / c y s t e i n e system. I t i s p o s s i b l e t h a t t h e amino group from amino a c i d s , o r p e p t i d e , condenses d i r e c t l y w i t h t h e a l d e h y d i c group o f 2 , 4 - d e c a d i e n a l and i s then f o l l o w e d by an e l e c t r o c y c l i c r e a c t i o n and a r o m a t i z a t i o n t o form 2-pentylpyridine (Figure 2 ) . 2 - I s o b u t y l - 3 , 5 - d i i s o p r o p y l p y r i d i n e was i d e n t i f i e d i n f r i e d c h i c k e n and has a r o a s t e d c o c o a - l i k e aroma ( 2 1 ) . F i g u r e 3 shows t h e mechanism f o r t h e f o r m a t i o n o f t h i s compound as proposed by Shu e t
FLAVOR CHEMISTRY: TRENDS AND CH -(CH ) -CH=CH-C:'=CH-CHO 3
2
+
4
H
2
DEVELOPMENTS
N - C H - R COOH
H 0 2
^ C H - C H ^ C H
3
- ( C H
2
)
R
C H >
Jf
H
N = CH
I R
COOH
CH -(CH ) -^J> 3
2
4
I CH R
COOH CH: COOH
c
M
CH
C H
0fO
CH
3-( 0rO KJ
Figure
2.
'
Mechanism f o r the f o r m a t i o n
of 2-pentylpyridine.
OH R
Aldol 2
R - C H
2
- C H O
CHO
VH II
Condensation
I CH
R NH
y
V
VH
A
NH
2
II
CH
CH
\
CH
CH
2
I
2
R
R
RCH CHO 2
,CH,-R " ^ C H ^ H ^ C H ^ ' V C H
2
R
[Q]
^ C H
L
- 2 H
2
Q
^CH R K
OH
^ CH-CH R 2
F i g u r e 3. pyridine.
Mechanism f o r the f o r m a t i o n
of 2,3,5-trialkyl-
8.
H O AND CARLIN
Aroma Characteristics ofHeterocyclic Compounds 97
a l . (22). I t i n v o l v e s t h e r e a c t i o n o f a l d e h y d e and ammonia a t h i g h temperatures and i s known as t h e C h i c h i b a b i n c o n d e n s a t i o n . 2-Isob u t y l - 3 , 5 - d i i s o p r o p y l p y r i d i n e has a l s o been i d e n t i f i e d i n a g l u c o s e / l e u c i n e model system p o s s e s s i n g a c o c o a - l i k e aroma ( 2 3 ) . 2-Pentyl3 , 5 - d i b u t y l p y r i d i n e was i d e n t i f i e d i n f r e n c h f r i e d p o t a t o f l a v o r and presumably formed through a C h i c h i b a b i n r e a c t i o n i n v o l v i n g ammonia and h e x a n a l , an abundant t h e r m a l o x i d a t i v e d e c o m p o s i t i o n p r o d u c t o f lipids. Thiazoles T h i a z o l e s a r e a c l a s s o f compounds p o s s e s s i n g a five-membered r i n g w i t h s u l f u r and n i t r o g e n i n t h e 1 and 3 p o s i t i o n s , r e s p e c t i v e l y . The p o t e n t i a l f o r t h i a z o l e d e r i v a t i v e s as f l a v o r a n t s i s e v i d e n t from t h e work o f S t o l l e t a l . (24) who found t h e s t r o n g n u t - l i k e odor o f a cocoa e x t r a c t t o be due t o a t r a c e amount o f 4 - m e t h y l - 5 - v i n y l t h i a zole. S i n c e t h e n , numerous t h i a z o l e s have been i d e n t i f i e d i n food flavors. The e x a c t o r i g i n o f t h i a z o l e s remains a m y s t e r y . They might form through the t h e r m a l d e g r a d a t i o n o f c y s t i n e o r c y s t e i n e (25, 2 6 ) , o r by t h e i n t e r a c t i o n o f s u l f u r - c o n t a i n i n g amino a c i d s and c a r b o n y l compounds (27, 28). T h i a z o l e s have been i d e n t i f i e d as v o l a t i l e components o f t h e r m a l l y degraded t h i a m i n e ( 2 9 ) . Aroma p r o p e r t i e s o f some a l k y l t h i a z o l e s have been r e v i e w e d ( 3 0 ) . The most t y p i c a l a l k y l t h i a z o l e i s p r o b a b l y 2 - i s o b u t y l t h i a z o l e . T h i s compound was i s o l a t e d from tomato f l a v o r and was d e s c r i b e d as h a v i n g a s t r o n g g r e e n odor r e s e m b l i n g t h a t o f tomato l e a f ( 3 1 ) . When added to canned tomato puree o r p a s t e a t l e v e l s o f 20 t o 50 ppb, 2 - i s o b u t y l t h i a z o l e d e v e l o p s an i n t e n s e f r e s h t o m a t o - l i k e f l a v o r . Most o f the a l k y l t h i a z o l e s a r e d e s c r i b e d as g r e e n , n u t t y and v e g e t a b l e - l i k e (32, 3 3 ) . Table I I l i s t s a l k y l t h i a z o l e s i d e n t i f i e d i n f r i e d chicken f l a v o r (19) and f r e n c h f r i e d p o t a t o f l a v o r ( 7 ) . Several o f the a l k y l t h i a z o l e s i d e n t i f i e d i n french f r i e d potato f l a v o r , such as 2 , 4 - d i m e t h y l - 5 - p r o p y l t h i a z o l e , 2,4-dimethyl-5-pentylt h i a z o l e , 2 - b u t y l - 4 - m e t h y l - 5 - e t h y l t h i a z o l e and 2 - b u t y l - 4 - p r o p y l t h i a z o l e have a s t r o n g c h a r a c t e r i s t i c sweet, s u l f u r y and g r e e n aroma (33). T h i s aroma c h a r a c t e r i s t i c i s q u i t e d i s t i n c t i v e and i s p r e s e n t i n a l a r g e number o f t h e f r a c t i o n s g e n e r a t e d from t h e gas chromatographic f r a c t i o n a t i o n o f the french f r i e d potato f l a v o r i s o l a t e . I t i s p r o b a b l y an important p a r t o f t h e t o t a l f r e n c h f r i e d p o t a t o f l a vor. 2 - P e n t y l - 4 - m e t h y l - 5 - e t h y l t h i a z o l e has a s t r o n g p a p r i k a pepper f l a v o r and 2 - h e p t y l - 4 , 5 - d i m e t h y l t h i a z o l e has a s t r o n g s p i c y f l a v o r . 2 - 0 c t y l - 4 , 5 - d i m e t h y l t h i a z o l e has a sweet f a t t y aroma ( 3 3 ) . They a r e p r o b a b l y important c o n t r i b u t o r s t o t h e f l a v o r o f f r i e d f o o d s . These t h i a z o l e s and o t h e r t h i a z o l e s i d e n t i f i e d have l o n g - c h a i n a l k y l subs t i t u t i o n s on t h e t h i a z o l e r i n g . The involvement o f f r y i n g f a t o r f a t d e c o m p o s i t i o n p r o d u c t s i n t h e f o r m a t i o n o f these compounds i s again suggested. T h i a z o l i n e s , a reduced form o f t h i a z o l e s , have a l s o been r e p o r t e d t o o c c u r i n f o o d s , m a i n l y cooked b e e f . 2,4,5-Trimethyl-2t h i a z o l i n e was i d e n t i f i e d i n b e e f b r o t h ( 3 4 ) . 2,4-Dimethyl-3-thiaz o l i n e found i n cooked b e e f aroma was r e p o r t e d t o have a n u t t y , r o a s t e d and v e g e t a b l e aroma ( 3 5 ) .
98
FLAVOR CHEMISTRY:
Table
II.
TRENDS AND
Alkylthiazoles Identified i n Fried and F r e n c h F r i e d P o t a t o F l a v o r
Compound Thiazole 2-Methylthiazole 2,4,5-Trimethylthiazole 2-Methyl-4-ethylthiazole 2.4- D i m e t h y l - 4 - e t h y l t h i a z o l e 2-Ethyl-4,5-dimethylthiazole 2-Isopropyl-4-methylthiazole 2-Propyl-4,5-dimethylthiazole 2-Isopropyl-4,5-dimethylthiazole 2.5- D i m e t h y l - 4 - b u t y l t h i a z o l e 2-Isopropyl-4-ethyl-5-methylthiazole 2-Butyl-4,5-dimethylthiazole 2-Isobutyl-4,5-dimethylthiazole 2,4-Dimethyl-5-pentylthiazole 2,4-Diethyl-5-propylthiazole 2-Butyl-4-methyl-5-ethylthiazole 2-Butyl-4-propylthiazole 2-Pentyl-4,5-dimethylthiazole 2-Butyl-4-propyl-5-methylthiazole 2-Pentyl-4-methyl-5-ethylthiazole 2-Pentyl-5-propylthiazole 2-Hexyl-4,5-dimethylthiazole 2-Heptyl-4,5-dimethylthiazole 2-Heptyl-4-ethyl-5-methylthiazole 2-Octyl-4,5-dimethylthiazole 2-Octyl-4-methyl-5-ethylthiazole
Fried Chicken + + + + +
+ + + +
DEVELOPMENTS
Chicken
French-Fried Potato
+
+ + + + + + + + + +
+
+ +
+
+ + + +
+ + + +
+
+
+
+
+
Oxazoles O x a z o l e s a r e c h a r a c t e r i z e d by p o s s e s s i n g a five-membered r i n g w i t h oxygen and n i t r o g e n i n the 1 and 3 p o s i t i o n s . The o c c u r r e n c e o f o x a z o l e s i n food f l a v o r has been reviewed ( 3 6 ) . R e c e n t l y twentyf o u r a l k y l o x a z o l e s were i d e n t i f i e d i n the v o l a t i l e compounds from f r e n c h - f r i e d p o t a o t e s ( 3 7 ) . T h i s r e p r e s e n t s the l a r g e s t number o f o x a z o l e s r e p o r t e d i n a food system. L i k e t h e a l k y l t h i a z o l e s , s e v e r a l o f the a l k y l o x a z o l e s i d e n t i f i e d , such as 2 - p e n t y l - 4 , 5 - d i m e t h y l o x a z o l e , 2 - p e n t y 1 - 4 - m e t h y 1 - 5 - e t h y l o x a z o l e , 2-hexy1-4,5-dimethyloxaz o l e and 2-hexy1-4-methy1-5-ethyloxazole, have l o n g - c h a i n a l k y l subs t i t u t i o n s a t t h e 2 - p o s i t i o n o f the o x a z o l e r i n g . L i p i d s or l i p i d d e c o m p o s i t i o n p r o d u c t s c o u l d be i n v o l v e d i n t h e f o r m a t i o n o f these long-chain a l k y l substituted oxazoles.
8.
HO AND
CARLIN
Aroma Characteristics ofHeterocyclic Compounds 99
Oxazoles have a wide range o f aroma c h a r a c t e r i s t i c s . As an ex ample, T a b l e I I I l i s t s the aroma c h a r a c t e r i s t i c s o f some a l k y l o x a z o l e s i d e n t i f i e d i n f r i e d bacon f l a v o r (Lee, Κ. Ν., Rutgers U n i v e r s i ty, unpublished data). T a b l e I I I . Odor D e s c r i p t i o n o f A l k y l o x a z o l e s I d e n t i f i e d i n F r i e d Bacon F l a v o r Oxazole 5-butyloxazole
2,5-dimethyl-5-propyloxazole 2-methyl-5-pentyloxazole 2,5-dimethyl-4-butyloxazole 2,5-dimethyl-5-butyloxazole 2-isopropyl-4-ethyl-5-methyloxazole 2-pheny 1-5-ethy l o x a z o l e
Odor D e s c r i p t i o n v e r y s t r o n g , bacon f a t t y , A n i m a l i c , aged meat like g r e e n , f l o w e r y , sweet sweet, s t r o n g f l o r a l , fatty-waxy f r e s h a c i d i c green, pickle-like dry h e r b a l , seasonal herbal sweet, f r u i t y indole-like, phenolic
I t i s i n t e r e s t i n g t o note t h a t 5 - b u t y l o x a z o l e has a v e r y d i s t i n c t b a c o n - f a t t y aroma and c o u l d be an important f l a v o r c o n s t i t u e n t of f r i e d bacon. 5 - P e n t y l o x a z o l e a l s o p o s s e s s e s a s i m i l a r aroma characteristic. Both 5 - b u t y l o x a z o l e and 5 - p e n t y l o x a z o l e have no a l k y l group on carbon 2 o r 4 o f the o x a z o l e r i n g . When a m e t h y l group i s s u b s t i t u t e d on carbon-2 ( e . g . , 2 - m e t h y l - 5 - p e n t y l o x a z o l e ) , the f a t t y aroma d e c r e a s e s and a s w e e t - f l o r a l aroma becomes more c h a r acteristic. The s w e e t - f l o r a l c h a r a c t e r i s f u r t h e r enhanced by a d d i t i o n a l m e t h y l s u b s t i t u t i o n on carbon-4 ( 3 8 ) . 2- P e n t y l - 4 - m e t h y l - 5 - e t h y l o x a z o l e was i d e n t i f i e d i n f r e n c h - f r i e d p o t a t o f l a v o r and has a s t r o n g b u t t e r y , sweet and l a c t o n e - l i k e f l a v o r . I t i s p r o b a b l y an important c o n t r i b u t o r t o the f r i e d food a s p e c t o f f r e n c h - f r i e d potato f l a v o r (37). Oxazoles and t h i a z o l e s p o s s e s s i n g comparable a l k y l groups were r e p o r t e d t o have s i g n i f i c a n t aroma s i m i l a r i t i e s ( 3 3 ) . B u t t e r y e t a l . r e p o r t e d t h a t some 4 , 5 - d i a l k y l t h i a z o l e s p o s s e s s e d p o t e n t b e l l pepper aroma ( 3 9 ) . The most p o t e n t one, 4 - b u t y l - 5 - p r o p y l t h i a z o l e , was r e p o r t e d t o have a f l a v o r t h r e s h o l d o f 0.003 ppb i n water. Ho and T u o r t o (40) s y n t h e s i z e d s e v e r a l 4 , 5 - d i a l k y l o x a z o l e s and found them t o have a g r e e n , v e g e t a b l e - l i k e aroma. 4-Butyl-3-propyloxazole has a s t r o n g b e l l pepper aroma and a f l a v o r t h r e s h o l d o f 0.1 ppm i n water. 5 - P e n t y l t h i a z o l e a l s o had s t r o n g f a t t y and sweet aromas reminiscent of 5-pentyloxazole. 2 - P e n t y l - 5 - m e t h y l t h i a z o l e was judged t o have a fermented v e g e t a b l e - l i k e aroma. The c o r r e s p o n d i n g 2 - p e n t y l - 5 - m e t h y l o x a z o l e was d e s c r i b e d as a c i d i c and sweet w i t h a flowery a f t e r n o t e (38). F i g u r e 4 shows a proposed mechanism f o r the f o r m a t i o n o f 2,4,5t r i m e t h y l o x a z o l e and 4 , 5 - d i m e t h y l o x a z o l e from the S t r e c k e r degrada t i o n o f c y s t e i n e w i t h 2,3-butanedione ( 4 1 ) . 3- O x a z o l i n e s , the reduced form o f o x a z o l e s , a l s o have important s e n s o r y p r o p e r t i e s . The f i r s t r e p o r t o f a 3 - o x a z o l i n e was made by
100
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Chang e t a l . ( 4 2 ) . They i s o l a t e d and i d e n t i f i e d 2,4,5-trimethyl-3oxazoline i n b o i l e d beef. T h i s compound was d e s c r i b e d as h a v i n g a " c h a r a c t e r i s t i c b o i l e d b e e f aroma". M u s s i n a n e t a l . (35) i d e n t i f i e d o x a z o l i n e s and no o x a z o l e s i n t h e i r b e e f system. P e t e r s o n e t a l . (43) r e p o r t e d on the v o l a t i l e s o f canned b e e f stew. Both 2,4,5t r i m e t h y l o x a z o l e and 2 , 4 , 5 - t r i m e t h y l - 3 - o x a z o l i n e were p r e s e n t . The r e l a t i v e c o n c e n t r a t i o n o f 2 , 4 , 5 - t r i m e t h y l o x a z o l e was medium w h i l e f o r 2 , 4 , 5 - t r i m e t h y l - 3 - o x a z o l i n e was e x t r a h i g h . Lee e t a l . (44) i d e n t i f i e d 2-methyl-3-oxazoline, 2,4-dimethyl-3-oxazoline and 2,4,5t r i m e t h y l - 3 - o x a z o l i n e i n the v o l a t i l e s o f r o a s t e d p e a n u t s . The l a t t e r two 3 - o x a z o l i n e s were a l s o i d e n t i f i e d i n the v o l a t i l e s o f f r i e d chicken (19). Trithiolanes T r i t h i o l a n e s have r e c e i v e d i n c r e a s i n g a t t e n t i o n s i n c e the i d e n t i f i c a t i o n o f d i a s t e r e o m e r i c 3 , 5 - d i m e t h y l - l , 2 , 4 - t r i t h i o l a n e i n the v o l a t i l e s of b o i l e d beef (42). The parent 1 , 2 , 4 - t r i t h i o l a n e i s a component o f S h i i t a k e mushrooms (45) and r e d a l g a e ( 2 7 ) . In a d d i t i o n t o 3 , 5 - d i m e t h y l - l , 2 , 4 - t r i t h i o l a n e , Kubota e t a l . (46) i d e n t i f i e d 3m e t h y l - 5 - e t h y l - l , 2 , 4 - t r i t h i o l a n e and 3 , 5 - d i e t h y l - l , 2 , 4 - t r i t h i o l a n e i n b o t h c i s and t r a n s forms i n b o i l e d A n t a r c t i c G u l l s . Both compounds were d e s c r i b e d as g a r l i c k y . Flament e t a l . (47) r e p r o t e d the i d e n t i f i c a t i o n o f 3 - m e t h y l - 5 - e t h y l - l , 2 , 4 - t r i t h i o l a n e and 3-methyl-5i s o p r o p y l - l , 2 , 4 - t r i t h i o l a n e i n a commercial b e e f e x t r a c t . 3,5-Diisobutyl-l,2,4-trithiolane was i d e n t i f i e d i n the v o l a t i l e s i s o l a t e d from f r i e d c h i c k e n ( 2 1 ) . T h i s compound has been r e p o r t e d t o p o s s e s s r o a s t e d , r o a s t e d - n u t , c r i s p b a c o n - l i k e and pork r i n d - l i k e aromas and f l a v o r s ( 4 8 ) . In a d d i t i o n t o 3,5-dimethyl-l,2, 4 - t r i t h i o l a n e and 3 , 5 - d i i s o b u t y l - l , 2 , 4 - t r i t h i o l a n e , two l o n g - c h a i n a l k y l s u b s t i t u t e d t r i t h i o l a n e s , namely, 3 - m e t h y l - 5 - b u t y l - l , 2 , 4 - t r i t h i o l a n e and 3 - m e t h y l - 5 - p e n t y l - l , 2 , 4 - t r i t h i o l a n e , were r e p o r t e d t o be p r e s e n t i n f r i e d c h i c k e n f l a v o r ( 4 9 ) . Along with 3,5-dimethyl1,2,4-trithiolane, 3-methyl-5-ethy1-1,2,4-trithiolane, 3-methyl-5p r o p y l - 1 , 2 , 4 - t r i t h i o l a n e and 3 - m e t h y l - 5 - b u t y l - l , 2 , 4 - t r i t h i o l a n e a r e r e p o r t e d t o be important f l a v o r components o f C h i n e s e stewed pork (50). A mechanism has been r e p o r t e d f o r the f o r m a t i o n o f t r i t h i o l a n e from the r e a c t i o n o f a l d e h y d e s w i t h hydorgen s u l f i d e ( 5 1 ) . The i d e n t i f i c a t i o n o f 3 - m e t h y l - 5 - b u t y l - l , 2 , 4 - t r i t h i o l a n e and 3-methyl-5p e n t y 1 - 1 , 2 , 4 - t r i t h i o l a n e i n food f l a v o r s u g g e s t s t h a t p e n t a n a l and h e x a n a l were i n v o l v e d i n the f o r m a t i o n o f these compounds ( F i g u r e 5). P e n t a n a l and h e x a n a l a r e major t h e r m a l and o x i d a t i v e decomposit i o n products of l i p i d s . Summary H e t e r o c y c l i c compounds, e s p e c i a l l y those which c o n t a i n n i t r o g e n and s u l f u r atoms, p o s s e s s p o t e n t s e n s o r y q u a l i t i e s at low c o n c e n t r a tions. They a r e formed i n foods by t h e r m a l d e c o m p o s i t i o n and i n t e r a c t i o n o f food components. The i d e n t i f i c a t i o n o f many l o n g - c h a i n a l k y l s u b s t i t u t e d h e t e r o c y c l i c compounds s u g g e s t s t h a t t h e i r format i o n mechanisms d i r e c t l y i n v o l v e l i p i d s o r l i p i d d e c o m p o s i t i o n products.
H O AND CARLIN
Aroma Characteristics ofHeterocyclic Compounds
CH
3
H
Ç=0
+
N
-
2
C
H
_ H SH C
2
HOOC
Ç=0 CH 3
H C
/
(£H SH
3
H Cv— 3
2
HX
N
H a c Ï O ^ C CI H S H 2
-H S 2
ΗΪ
°Ί^Ν
H C^cA H 3
F i g u r e 4. oxazole.
C
3
Mechanism f o r the f o r m a t i o n o f 2 , 4 , 5 - t r i m e t h y l -
CH3CHO
+
C H C H C H C H C H C H O -f- 2 H S 3
2
2
2
2
2
CH -CH-S-CH-CHXH CH CH CH 3
2
SH
2
2
3
SH
H CyS^CHXH CH CH CH 3
2
S
2
2
3
S
F i g u r e 5. Mechanism f o r the f o r m a t i o n o f 3 - m e t h y l - 5 - p e n t y l 1,2,4-trithiolane.
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FLAVOR CHEMISTRY:
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Acknowledgments New J e r s e y A g r i c u l t u r a l Experiment S t a t i o n P u b l i c a t i o n No. D-102053-88 s u p p o r t e d by S t a t e Funds and R e g i o n a l P r o j e c t NE-116. We thank Mrs. J o a n B. Shumsky f o r h e r s e c r e t a r i a l h e l p .
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8.
23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49.
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103
Hartman, G. J . ; Scheide, J. D.; Ho, C.-T. Perfumer & Flavorist 1983, 8(6), 81-86. Stoll, M.; Dietrich, P.; Sundte, E.; Winter, M. Helv. Chim. Acta 1967, 50, 2065. Shu, C.-K.; Hagedorn, M. L.; Mookherjee, B. D.; Ho, C.-T. J. Agric. Food Chem. 1985, 33, 438-442. Shu, C.K.; Hagedorn, M. L.; Mookherjee, B. D.; Ho, C.-T. J. Agric. Food Chem. 1985, 33, 442-446. Ohloff, G.; Flament, I. Fortschr. Chem. Org. Naturst 1978, 36, 231-283. Hartman, G. J . ; Ho, C.-T. Lebensm.-Wiss. u. -Technol. 1984, 17, 171-174. Hartman, G. J . ; Carlin, J. T.; Scheide, J. D.; Ho, C.-T. J. Agric. Food Chem. 1984, 32, 1015-1018. Fors, S. ACS Symp. Ser. 1983, 215, 183-286. Viani, R.; Bricout, J . ; Marion,J. P.; Muggler-Chavan, F.; Reymond, D.; Egli, R. H. Helv. Chim. Acta 1969, 52, 887-891. Pittet, A.O.; Hruza, D. E. J. Agric. Food Chem. 1974, 22, 264-269. Ho, C.-T.; Jin, Q. Z. Perfumer & Flavorist 1984, 9(6), 15-18. Tonsbeek, C. H.T.; Copier, H.; Plancken, A. J. J. Agric. Food Chem. 1971, 19, 1014-1016. Mussinan, C. J . ; Wilson, R. Α.; Katz, I.; Hruza, Α.; Vock, M. H. ACS Symp. Ser. 1976, 26, 133-145. Maga, J. A. Crit. Rev. Food Sci. Nutr. 1981, 12, 295-307. Carlin, J. T.; Jin, Q. Z.; Huang, T.-Z.; Ho, C.-T.; Chang, S. S. J. Agric. Food Chem. 1986, 34, 621-623. Jin, Q. Z.; Hartman, G. J . ; Ho, C.-T. Perfumer & Flavorist 1984, 9(4), 25-29. Buttery, R. G.; Guadagni, D.; Lundin, R. J. Agric. Food Chem. 1976, 24, 1-6. Ho, C.-T.; Tuorto, R. M. J. Agric. Food Chem. 1981, 29, 1306-1308. Ho, C.-T.; Hartman, G.J. J. Agric. Food Chem. 1982, 30, 793-794. Chang, S. s.; Hirai, C.; Reddy, B. R.; Herz, K. O.; Kato, Α.; Sipma, G. Chem. Ind. 1968, 1639-1640. Peterson, R. J . ; Izzo, H. J . ; Jungermann, E.; Chang, S. S. J. Food Sci. 1975, 40, 948-954. Lee, M.-H.; Ho, C.-T.; Chang, S. s. J. Agric. Food Chem. 1981, 29, 684-686. Chen, C.-C.; Ho, C.-T. J. Agric. Food Chem. 1986. 34, 830-833. Kubota, K.; Kobayashi, Α.; Yamanishi, T. Agric. Biol Chem. 1980, 44, 2677. Flament, I.; Willhalm, B.; Ohloff, G. In Flavor of Foods and Beverages; Charalambous, G.; Inglett, G., Eds.; Academic: New York, 1978, p. 15. Shu, C.-K.; Mookherjee, B. D.; Vock, M. H. U. S. Patent 4,263,331, 1981. Hwang, S.-S.; Carlin, J. T.; Bao, Y.; Hartman, G. J. J. Agric. Food Chem. 1986, 34, 538-542.
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50. Chou, C.C.; Wu, C.-M. FIRDI Research Report No. 285, 1983. 51. Takken, H. J.; Van der lInde, L.M.; De Valois, P. J . ; Van Dort, H. M.; Boelen, M. ACS Symp. Ser. 1976, 26, 114-121. RECEIVED August 5, 1988
