Contribution of Lipids to the Formation of Heterocyclic Compounds in

Oct 3, 1989 - Heterocyclic compounds are primarily formed through nonenzymatic browning reactions. Recent studies of deepfat fried food flavors led to...
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Chapter 10

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Contribution of Lipids to the Formation of Heterocyclic Compounds in Model Systems Chi-Tang Ho, Linda J . Bruechert, Yuangang Zhang, and Ε-Mean Chiu

1

Department of Food Science, New Jersey Agricultural Experiment Station, Cook College, Rutgers, The State University of New Jersey, New Brunswick, NJ 08903

Heterocyclic compounds are primarily formed through non­ -enzymatic 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 sub­ stitutions. Model systems were used to study the par­ ticipation of lipids in the formation of pyrazines, pyridines, thiophenes and cyclic polysulfides. More than 10,000 compounds have been i d e n t i f i e d as v o l a t i l e s o f f o o d s . Of t h e s e , h e t e r o c y c l i c compounds are an i m p o r t a n t c l a s s , be­ cause o f 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 and/or N) i n r i n g s o r 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 are formed through 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 M a i l l a r d r e a c t i o n s . 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 and 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 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 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 . Recent s t u d i e s i n our l a b o r a t o r y show 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 the M a i l l a r d r e a c t i o n i n the 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 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 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 model s t u d i e s w h i c h i n d i c a t e t h a t 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 such as 2 , 4 - d e c a d i e n a l and h e x a n a l may r e a c t w i t h M a i l l a r d r e a c t i o n i n t e r m e d i a t e s t o form h e t e r o c y c l i c compounds. Current address: Development Center for Biotechnology, 81 Chang Hsing Street, Taipei, Taiwan, Republic of China 0097-6156/89/0409-0105$06.00/0 c 1989 American Chemical Society In Thermal Generation of Aromas; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

THERMAL GENERATION OF AROMAS

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L i p i d D e g r a d a t i o n P r o d u c t s and P y r a z i n e s 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 i m p o r t a n t 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 the M a i l l a r d r e a c t i o n . I n t e r e s t i n the i n f l u e n c e o f l i p i d s on p y r a z i n e f o r m a t i o n has r e c e n t l y been g e n e r a t e d by the i d e n t i f i c a t i o n o f 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 i n foods and i n model systems. P y r a z i n e s i n t h i s c a t e g o r y i n c l u d e 2 - h e p t y l p y r a z i n e i s o l a t e d from f r e n c h f r i e d p o t a t o f l a v o r ( 7 ) , and 2-methyl-3(or 6 ) - p e n t 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 s o l a t e d from e x t r u d e d z e i n / c o r n a m y l o p e c t i n / c o r n o i l systems ( 8 , 9 ) . Only the 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 the 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 t h e 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 the pyrazine ring. The r e l a t i o n s h i p between l i p i d d e g r a d a t i o n p r o d u c t s and l o n g c h a i n a l k y l s u b s t i t u e n t o f p y r a z i n e s was i n v e s t i g a t e d by examining the r e a c t i o n p r o d u c t s g e n e r a t e d from the t h r e e model systems, acetol/ammonium a c e t a t e , acetol/ammonium a c e t a t e / p e n t a n a l and acetoi/ammonium a c e t a t e / h e x a n a l . C o n f i r m i n g the r e s u l t o f R i z z i ( 1 0 ) , 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 were the major p r o d u c t s i n the r e a c t i o n o f a c e t o l w i t h ammonium a c e t a t e . I n the acetol/ammonium a c e t a t e / p e n t a n a l system, 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 were t h e major p r o d u c t s , and i n the acetol/ammonium a c e t a t e / h e x a n a l system, 2 , 5 - d i m e t h y l - 3 h e x y l p y r a z i n e and 2 , 6 - d i m e t h y l - 3 - h e x y l p y r a z i n e were the major p r o d u c t s . T h i s r e s u l t suggests t h a t 2 , 5 - d i h y d r o p y r a z i n e i n t e r m e d i a t e s e i t h e r undergo d e h y d r o g e n a t i o n t o form p y r a z i n e s o r they r e a c t w i t h aldehydes t o generate p y r a z i n e s w i t h c o r r e s p o n d i n g a l k y l s u b s t i t u e n t s . The c o m p e t i t i o n o f p e n t a n a l o r h e x a n a l f o r 2,5- o r 2,6d i m e t h y l d i h y d r o p y r a z i n e might a l s o account f o r t h e diminishment o f 2,5- o r 2 , 6 - d i m e t h y l p y r a z i n e i n acetol/ammonium a c e t a t e systems when p e n t a n a l o r h e x a n a l i s i n c l u d e d . P o s s i b l e f o r m a t i o n pathways o f 2 , 5 - d i m e t h 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 the acetol/ammonium a c e t a t e / p e n t a n a l r e a c t i o n a r e shown i n F i g u r e 1. 2 , 5 - D i m e t h y l - 3 - h e x y l p y r a z i n e and 2 , 6 - d i m e t h y l - 3 - h e x y l p y r z i n e were a l s o i d e n t i f i e d when a s o l u t i o n o f h e x a n a l and pyruvaldehyde was heated w i t h v a r i o u s amino a c i d s , i n c l u d i n g g l y c i n e , a l a n i n e , v a l i n e and l e u c i n e , a t 180°C. I n t e r a c t i o n o f 2,4-Decadienal

and C y s t e i n e

The t h e r m a l i n t e r a c t i o n between 2 , 4 - d e c a d i e n a l and c y s t e i n e was s e l e c t e d as a model f o r l i p i d - p r o t e i n i n t e r a c t i o n . 2,4-Decadienal i s the major d e g r a d a t i o n p r o d u c t o f l i n o l e i c a c i d and c y s t e i n e i s a s u l f u r - c o n t a i n i n g amino a c i d i n f o o d s . Some h e t e r o c y c l i c compounds i d e n t i f i e d i n the r e a c t i o n m i x t u r e o f 2 , 4 - d e c a d i e n a l and c y s t e i n e a r e l i s t e d i n Table I . A c o n s i d e r a b l y l a r g e number o f 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 were d e t e c t e d i n t h i s i n t e r a c t i o n system.

In Thermal Generation of Aromas; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

Lipids and the Formation of Heterocyclic Compounds

HO ET AL.

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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 - d i m e t h 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 .

In Thermal Generation of Aromas; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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THERMAL GENERATION OF AROMAS

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T a b l e I . Some H e t e r o c y c l i c Compounds I d e n t i f i e d from Thermal I n t e r a c t i o n o f 2,4-Decadienal and C y s t e i n e

Compounds , Furans 2-butylfuran 2-pentylfuran 2-hexylfuran

Amount Produced (mg/mol) 12.8 6.4 t

Thiophenes thiophene tetrahydrothiophene-3-one 2-butylthiophene 2-formyl-3-methylthiophene 2-pentylthiophene 2-hexylthiophene 2-heptylthiophene 2- f o r m y l - 5 ( o r 3 ) - p e n t y l t h i o p h e n e

3.5 10.5 57.2 29.8 13.1 42.0 1-8 15.6

Thiazoles thiazole 3- m e t h y l i s o t h i a z o l e 2- a c e t y l t h i a z o l e

25.6 2.0 2.2

Cyclic Polysulfides 3 , 5 - d i m e t h y l - l , 2 , 4 - t r i t h i o l a n e (isomer) 3 , 5 - d i m e t h y l - l , 2 , 4 - t r i t h i o l a n e (isomer) 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 2,4,6-trimethylperhydro-l,3,5-thiadiazine 2,4,6-trimethylperhydro-l,3,5-dithiazine 2,4-dimethyl-6-pentylperhydro-l,3,5-dithiazine 2-pentyl-4,6-dimethylperhydro-l,3,5-dithiazine

122.8 18.2 14.3 828.5 284.2 18.9 28.7

Pyridine 2-pentylpyridine

501.5

In Thermal Generation of Aromas; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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10.

HO ET AL.

Lipids and the Formation of Heterocyclic Compounds

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A c c o r d i n g t o the f o r m a t i o n mechanism o f h e t e r o c y c l i c compounds from a model system o f a l d e h y d e s , hydrogen s u l f i d e and ammonia ( 1 1 ) , a l dehydes such as a c e t a l d e h y d e and h e x a n a l must have been i n v o l v e d i n 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 i d e n t i f i e d i n t h i s s t u d y . As proposed by Josephson and L i n d s a y ( 1 2 ) , under w a t e r - m e d i a t e d cond i t i o n s , 2 , 4 - d e c a d i e n a l undergoes r e t r o - a l d o l i z a t i o n t o g i v e a c e t a l d e h y d e and h e x a n a l . The most i n t e r e s t i n g compounds i d e n t i f i e d i n the r e a c t i o n o f 2 , 4 - d e c a d i e n a l and c y s t e i n e are c y c l i c p o l y s u l f i d e s such as t r i t h i o l a n e s , d i t h i a z i n e s and t h i a d i a z i n e s . 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 of diastereomeric 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 o f b o i l e d beef ( 1 3 ) . The p a r e n t 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 (14) and red a l g a e ( 1 5 ) . 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 . (16) i d e n t i f i e d 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 , 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 syn and a n t i 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 and co-workers (17) r e p o r 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 - m e t h y l - 5 - i 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 beef extract . 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 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 , the 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 , 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 1 , 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 ( 1 1 ) . A l o n g w i t h 3 , 5 - d i m e t h y l - l , 2 , 4 - t r i t h i o l a n e , 3-methyl5-ethy1-1,2,4-trithiolane, 3-methyl-5-propyl-l,2,4-trithiolane 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 are r e p o r t e d to be i m p o r t a n t f l a v o r components o f C h i n e s e stewed pork ( 1 8 ) . I n the p r e s e n t s t u d y o f the i n t e r a c t i o n between 2 , 4 - d e c a d i e n a l and c y s t e i n e , h e x a n a l i s i n v o l v e d i n 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 - l , 2 , 4 - t r i t h i o l a n e . 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 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 c h i c k e n may form t h r o u g h a s i m i l a r pathway. I n the p r e s e n t s t u d y , the 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 d i thiazines, 2,4-dimethyl-6-pentylperhydro-l,3,5-dithiazine and 2penty1-4,6-dimethylperhydro-l,3,5-dithiazine were i d e n t i f i e d i n add i t i o n to 2,4,6-trimethylperhydro-l,3,5-dithiazine. 2,4,6-Trimethylp e r h y d r o - l , 3 - 5 - d i t h i a z i n e , a l s o known as t h i a l d i n e , has been r e p o r t e d a cooked f l a v o r component o f foods (19, 20, 21). Recently, d i t h i a z i n e s c o n t a i n i n g p r o p y l and b u t y l s u b s t i t u e n t s such as 2-prop y 1 - 4 , 6 - d i m e t h y l p e r h y d r o - 1 , 3 , 5 - d i t h i a z i n e and 4-butyl-2,6-dimethylp e r h y d r o - l , 3, 5 - d i t h i a z i n e have been i d e n t i f i e d i n v o l a t i l e compon e n t s from cooked S a k u r a e b i ( 2 2 ) . I t i s a l s o i n t e r e s t i n g t o note t h a t t h e r e was no l o n g - c h a i n s u b s t i t u t e d t h i a d i z i n e formed i n the r e a c t i o n o f 2 , 4 - d e c a d i e n a l and c y s t e i n e a l t h o u g h the amount o f 2,4,6-trimethylperhydro-l,3,5-thiad i a z i n e was much h i g h e r than t h a t o f 2 , 4 , 6 - t r i m e t h y l p e r h y d r o - 1 , 3 , 5 dithiazine. I t was a l s o found t h a t 2,4,6-trimethylperhydro-l,3,5t h i a d i a z i n e was u n s t a b l e and t h a t i t d i s a p p e a r e d from the gas c h r o matogram a f t e r two weeks, even when the sample was s t o r e d i n a -40°C f r e e z e r . The o b s e r v e d absence o f l o n g - c h a i n s u b s t i t u t e d t h i a d i a z i n e s suggests t h a t the l o n g - c h a i n s u b s t i t u e n t makes t h e s e compounds even more u n s t a b l e .

In Thermal Generation of Aromas; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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T H E R M A L GENERATION OF AROMAS

A l s o i d e n t i f i e d i n t h i s system are 2 - a l k y l f u r a n s . 2 - A l k y l f u r ans are known t h e r m a l and a u t o x i d a t i o n p r o d u c t s o f u n s a t u r a t e d f a t t y a c i d s ( 2 3 ) . 2 - A l k y l f u r a n s i d e n t i f i e d i n t h i s study a r e presumably d e r i v e d by the t h e r m a l o x i d a t i o n o f 2 , 4 - d e c a d i e n a l . Another i n t e r e s t i n g group o f h e t e r o c y c l i c compounds formed i n the r e a c t i o n o f 2 , 4 - d e c a d i e n a l and c y s t e i n e i s the t h i o p h e n e s . Thiophenes h a v i n g b u t y l t o h e p t y l groups s u b s t i t u t e d a t the 2 - p o s i t i o n were i d e n t i f i e d . Mechanisms have been suggested f o r the p r o ­ d u c t i o n o f t h i o p h e n e d e r i v a t i v e s by the a c t i o n o f hydrogen s u l f i d e on 1 , 4 - d i c a r b o n y l compounds ( 2 4 ) . The e x a c t mechanism f o r the f o r ­ m a t i o n o f t h i o p h e n e s i n t h i s system i s not c l e a r and deserves more study. Mechanism f o r the F o r m a t i o n o f

2-Pentylpyridines

2 - P e n t y l p y r i d i n e has been 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 p o t a t o 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 the major p r o d u c t i n the v o l a t i l e s gener­ 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 5 ) . I t i s p o s t u l a t e d t o form through the 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 5 ) . I n our s t u d y , i t was found t h a t at h i g h temperature (180°C) 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 or g l u t a t h i o n e (γ-glu-cys-gly) i n aqueous s o l u t i o n y i e l d e d 2 - p e n t y l p y r i d i n e as the major p r o d u c t . The q u a n t i t i e s o f f i v e major v o l a ­ t i l e components, i n c l u d i n g 2 - p e n t y l p y r i d i n e , g e n e r a t e d f o r t h e s e two systems are l i s t e d i n T a b l e I I . The amount o f 2 - p e n t y l p y r i d i n e g e n e r a t e d i n the 2,4-decadienal/ g l u t a t h i o n e system was g r e a t e r than t h a t i n the 2,4-decadienal/ c y s t e i n e system. I t i s known t h a t the f o r m a t i o n o f d i t h i a z i n e o r t h i a d i a z i n e r e q u i r e s the presence o f f r e e ammonia. The absence o f f o r m a t i o n o f d i t h i a z i n e o r t h i a d i a z i n e i n the 2,4-decadienal/glut a t h i o n e system i n d i c a t e s t h a t no f r e e ammonia i s a v a i l a b l e . T h i s s t r o n g l y suggests t h a t the f o r m a t i o n o f 2 - p e n t y l p y r i d i n e does not r e q u i r e the presence o f f r e e ammonia. I t i s p o s s i b l e t h a t the amino Table I I . Q u a n t i t y o f Some M a j o r V o l a t i l e Compounds Formed i n the I n t e r a c t i o n o f 2,4-Decadienal w i t h Cysteine or G l u t a t h i o n e Amount Produced (mg/mol) With Cysteine With G l u t a ­ thione Compounds 45.4 hexanal 278.6 368.5 3,5-dimethyl-l,2,4-trithiolane 140.9 2,4,6-trimethylperhydro-1,3,5-thiadiazine 828.5 -2,4,6-trimethylperhydro-l,3,5-dithiazine 284.2 1219.0 2-pentylpyridine 501.5 group from amino a c i d s or p e p t i d e s condenses d i r e c t l y w i t h the 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 - p e n t y l p y r i d i n e (Figure 2).

In Thermal Generation of Aromas; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

Lipids and the Formation of Heterocyclic Compounds

HO ET AL.

C H - (c H ) - C H = C H - C H = C H - C H O 3

+

2

H N —CH—R 2

COOH

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-

H0 2

^CH-^CH CH -(CH V-CH> 3

N

2

C H

N=CH

I

R

F i g u r e 2.

COOH

Mechanism f o r the f o r m a t i o n o f 2 - p e n t y l p y r i d i n e .

In Thermal Generation of Aromas; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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THERMAL GENERATION OF AROMAS

Summary

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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, possess p o t e n t s e n s o r y q u a l i t i e s a t low c o n c e n t r a t i o n s . 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 suggests t h a t t h e i r f o r m a t i o n mech­ anisms 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 p r o d u c t s . Model s t u d i e s i n d i c a t e d t h a t 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 such as 2 , 4 - d e c a d i e n a l and h e x a n a l can r e a c t w i t h M a i l l a r d r e a c t i o n i n t e r ­ mediates t o form h e t e r o c y c l i c compounds. Acknowledgement 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-102056-88 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. Joan B. Shumsky f o r h e r s e c r e t a r i a l h e l p .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Garnero, J. In The Chemistry of Heterocyclic Flavoring and Aroma Compounds; Vernin, G. Ed.; John Wiley & Sons: New York, 1982. Mottram, D. S.; Whitfield, F. B. In Flavor Science and Technology; Martens, M.; Dalen, G. Α.; Russwurm, Jr., H. Eds; John Wiley & Sons: New York, 1987. Maga, J. A. CRC Crit. Rev. Food Sci. Nutr. 1982, 16, 1-115. Rizzi, G. P. J. Agric. Food Chem. 1972, 20, 1081-1085. Shibamoto, T.; Bernhard, R. A. J. Agric. Food Chem. 1977, 25, 609-614. Wong, J. M.; Bernhard, R. A. J. Agric Food Chem. 1988, 36, 123-129. Carlin, J. T. Ph.D. Thesis, Rutgers University, New Jersey, 1983. Huang, T.-C.; Bruechert; L. J., Hartman; T. G.; Rosen, R. T.; Ho, C.-T. J. Agric. Food Chem. 1987, 35, 985-990. Bruechert, L. J. M. S. Thesis, Rutgers University, New Jersey, 1987. Rizzi, G. P. J. Agric. Food Chem. 1988, 36, 349-352. Hwang, S.-S.; Carlin, J. T.; Bao, Y.; Hartman, G. J.; Ho, C.-T. J. Agric. Food Chem. 1986, 34, 538-542. Josephson, D. B.; Lindsay, R. C. J. Food Sci. 1987, 52, 1186-1190. Chang, S. S.; Hirai, C.; Reddy. B. R.; Herz, K. O.; Kato, Α.; Sipma, G. Chem. Ind. 1968, 1639-1640.. Chen, C.-C. and Ho, C.-T. J. Agric. Food Chem. 1986, 34, 830-833. Ohloff, G.; Flament, I. Fortschr. Chem. Org. Naturst. 1978, 36, 231-283. Kubota, K.; Kobayashi, Α.; Yamanishi, T. Agric. Biol. Chem. 1980, 44, 2677-2682. Flament, I.; Willhalm, B.; Ohloff, G. In Flavor of Foods and Beverages; Charalambous, G.; Inglett, G., Eds.; Academic: New York, 1978.

In Thermal Generation of Aromas; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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18. Chou, C.-C.; Wu, C.-M. FIRDI Research Report No. 285, 1983. 19. Tang, J.; Jin, Q. Z.; Shen, G.-H.; Ho, C.-T.; Chang, S. S. J. Agric. Food Chem. 1983, 31, 1287-1292. 20. Kubota, K.; Shijimaya, H.; Kobayashi, A. Agric. Biol. Chem. 1986. 50, 2867-2873. 21. Choi, S. H.; Kobayashi, Α.; Yamanishi, T. Agric. Biol. Chem. 1983, 47, 337-342. 22. Kubota, K.; Watanabe, K.; Kobayashi, A. Agric. Biol. Chem. 1988, 52, 1537-1540. 23. Chang, S. S.; Peterson, R. J.; Ho, C.-T. J. Amer. Oil Chem. Soc. 1978, 55, 718-727. 24. Boelens, M.; van der Linde, L. M.; de Valois, P.J.; van Dort, J. M.; Takken, H. J. In Aroma Research; Maarse, H.; Groenen, P. J. Eds.; Center for Agricultural Publishing and Documenta tion, Wageningen, 1975. 25. Henderson, S. K.; Nawar, W. W. J. Amer. Oil Chem. Soc. 1981, 58, 632-635. RECEIVED May 11, 1989

In Thermal Generation of Aromas; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.