Thermal Generation of Aromas - ACS Publications - American

Chapter 10

Downloaded by OHIO STATE UNIV LIBRARIES on September 18, 2013 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch010

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

106


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

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107


108


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



10.

HO ET AL.


109

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 .



110

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).



HO ET AL.

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+

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ι Downloaded by OHIO STATE UNIV LIBRARIES on September 18, 2013 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch010

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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 .


112


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, 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 .

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