Thermal Decomposition of Lipids - ACS Symposium Series (ACS

Chapter 9

Thermal Decomposition of Lipids

Downloaded by STANFORD UNIV GREEN LIBR on September 22, 2012 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch009

A n Overview Wassef W. Nawar Department of Food Science and Nutrition, University of Massachusetts, Amherst, MA 01003

The chemistry of lipid decomposition in foods at elevated temperatures is complicated. Multiple reactions and interactions can occur rapidly and competitively. The rates and pathways of these reactions are influenced significantly by temperature, reaction time, other constituents in the surrounding environment, physical state, and molecular organization. The b a s i c mechanism o f a u t o x i d a t i o n a t e l e v a t e d temperatures i s s i m i l a r t o t h a t o f room-temperature o x i d a t i o n , i . e . , a f r e e r a d i c a l c h a i n r e a c t i o n i n v o l v i n g t h e f o r m a t i o n and d e c o m p o s i t i o n o f hydroperoxide intermediates. Although r e l a t i v e proportions o f the i s o m e r i c h y d r o p e r o x i d e s , s p e c i f i c f o r o l e a t e , l i n o l e a t e and l i n o l e n a t e , v a r y w i t h o x i d a t i o n temperatures i n the range 25°C 80°C, t h e i r q u a l i t a t i v e p a t t e r n i s t h e same (_1). L i k e w i s e , the major d e c o m p o s i t i o n p r o d u c t s i s o l a t e d from f a t s o x i d i z e d over wide temperature ranges a r e those r e f l e c t i n g a u t o x i d a t i o n o f t h e i r c o n s t i t u e n t f a t t y a c i d s (2 - 6 ) . The mechanisms and p r o d u c t s o f l i p i d o x i d a t i o n have been e x t e n s i v e l y s t u d i e d . The r e a d e r i s r e f e r r e d t o the numerous monographs, r e v i e w s and r e s e a r c h a r t i c l e s a v a i l a b l e i n the l i t e r a t u r e (1,4,7,8,9,10,11). The b u l k o f our knowledge r e g a r d i n g t h e r m a l o x i d a t i o n has been d e r i v e d from s t u d i e s w i t h model systems o f f a t t y a c i d s and t h e i r d e r i v a t i v e s , o r w i t h i n d i v i d u a l n a t u r a l o i l s (2,3,6,12,13,14,15,16). However, i n b i o l o g i c a l systems as complex as f o o d , l i p i d s u s u a l l y e x i s t i n a c o m p l i c a t e d environment markedly d i f f e r e n t from t h a t o f the s i n g l e phase model system. I n c e l l membranes, f o r example, t h e l i p i d molecules are h i g h l y ordered, r e l a t i v e l y r e s t r i c t e d i n distance and m o b i l i t y , and c l o s e l y a s s o c i a t e d w i t h d i f f e r e n t n e i g h b o r i n g m o l e c u l e s , e.g., o t h e r l i p i d s , p r o t e i n , c h o l e s t e r o l , w a t e r , p r o - and a n t i o x i d a n t s . What i n f l u e n c e does such an environment have on the o x i d a t i o n o f t h e l i p i d s a t e l e v a t e d temperature? Even i n l e s s o r g a n i z e d systems, e.g., depot f a t from a n i m a l o r p l a n t , the l i p i d s 0097-6156/89/0409-0094$06.00/0 ο 1989 American Chemical Society In Thermal Generation of Aromas; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

9.

NAWAR

Thermal Decomposition ofLipids

95

come i n c o n t a c t w i t h o t h e r f o o d c o n s t i t u e n t s f o r extended p e r i o d s a t r e l a t i v e l y h i g h t e m p e r a t u r e s , as i n the case o f f r y i n g , b r o i l i n g and baking. I n v i e w o f t h e above I have chosen t o emphasize, i n t h i s a r t i c l e , t h r e e areas which i n my o p i n i o n m e r i t a d d i t i o n a l f o c u s : (1) unique a s p e c t s o f h i g h - t e m p e r a t u r e o x i d a t i o n ; (2) i n t e r a c t i v e e f f e c t s ; (3) influence of p h y s i c a l s t a t e .


UNIQUE ASPECTS OF HIGH-TEMPERATURE

OXIDATION

S t u d i e s i n which l i p i d o x i d a t i o n a t d i f f e r e n t temperatures was compared a r e l i m i t e d . However, c e r t a i n s p e c i f i c d i f f e r e n c e s between h i g h and l o w temperature o x i d a t i o n s have been o b s e r v e d . The f o l l o w i n g a r e some examples: Hydroperoxide L e v e l s . I n t h e r m a l l y o x i d i z e d f a t s hydroperoxides are u s u a l l y v e r y low. A t h i g h e r t e m p e r a t u r e s , o x i d a t i o n proceeds r a p i d l y and the r a t e o f h y d r o p e r o x i d e d e c o m p o s i t i o n exceeds t h a t o f h y d r o p e r o x i d e f o r m a t i o n (17,18). F o r example, when e t h y l l i n o l e a t e was o x i d i z e d a t 70°C, p e r o x i d e c o n t e n t reached a maximum o f 1777 meq/kg a f t e r 6 h r then d e c r e a s e d t o 283 meq/kg a f t e r 70 h r . At 250°C, on the o t h e r hand, p e r o x i d e v a l u e reached a maximum o f o n l y 198 meq/kg a f t e r 10 min, and was zero a f t e r 30 min. Q u a n t i t a t i v e A s p e c t s . A l t h o u g h the major p r o d u c t s i s o l a t e d from heated f a t s a r e l a r g e l y t y p i c a l o f l i p i d a u t o x i d a t i o n , and i n s p i t e of the f a c t t h a t the t o t a l amount o f t h e s e p r o d u c t s i n c r e a s e s s i g n i f i c a n t l y w i t h i n c r e a s e d temperatures o f h e a t i n g , the f l a v o r q u a l i t i e s o f heated and o x i d i z e d f a t s a r e markedly d i f f e r e n t . This i s most l i k e l y due t o q u a n t i t a t i v e d i f f e r e n c e s i n t h e d e c o m p o s i t i o n p r o d u c t p a t t e r n s r a t h e r than t o the presence o r absence o f i n d i v i d u a l compounds. I t s h o u l d be r e a l i z e d t h a t the amount o f a g i v e n d e c o m p o s i t i o n p r o d u c t a t any g i v e n time d u r i n g the c o u r s e o f o x i d a t i o n i s determined by t h e n e t b a l a n c e between complex r e a c t i o n s o c c u r r i n g s i m u l t a n e o u s l y and c o m p e t i t i v e l y . S i n c e t h e r a t e s o f these r e a c t i o n s a r e i n f l u e n c e d d i f f e r e n t l y by t e m p e r a t u r e , t h e q u a n t i t a t i v e p a t t e r n o f t h e r e s u l t i n g p r o d u c t s w i l l o b v i o u s l y be d i f f e r e n t a t d i f f e r e n t temperatures. Aldehyde F o r m a t i o n . S e v e r a l i n v e s t i g a t o r s observed a marked dominance o f h e x a n a l i n the v o l a t i l e p r o d u c t s o f low-temperature o x i d a t i o n . A t the h i g h e r t e m p e r a t u r e s , however, 2 , 4 - d e c a d i e n a l was the major aldehyde formed (19,20,21). Both aldehydes a r e t y p i c a l s c i s s i o n p r o d u c t s o f l i n o l e a t e h y d r o p e r o x i d e s . Swoboda and Lea (20) e x p l a i n e d t h i s d i f f e r e n c e on t h e b a s i s o f a s e l e c t i v e f u r t h e r o x i d a t i o n o f the d i e n a l a t the h i g h e r t e m p e r a t u r e , w h i l e Kimoto and Gaddis (19) s p e c u l a t e d t h a t t h e carbon-carbon bond between the c a r b o n y l group and the double bond (Type B) i s the most v u l n e r a b l e t o c l e a v a g e under moderate c o n d i t i o n s o f a u t o x i d a t i o n , w h i l e s c i s s i o n a t the carbon-carbon bond away from the o l e f i n i c l i n k a g e (Type A) i s f a v o r e d under s t r e s s such as heat o r a l k a l i .

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

T H E R M A L GENERATION OF AROMAS

96

A Β -CH-j-CH-fCH=CHο ·


é

The 13-hydroperoxide o f l i n o l e a t e would thus produce more h e x a n a l a t l o w e r t e m p e r a t u r e s w h i l e 2 , 4 - d e c a d i e n a l from the 9-hydroperoxide isomer predominates a t e l e v a t e d t e m p e r a t u r e s . A l t h o u g h our q u a n t i t a t i v e work w i t h p r o p y l l i n o l e a t e (21) s u p p o r t s t h i s r a t i o n a l e , i . e . , a temperature-dependent p r e f e r e n t i a l c l e a v a g e , t h e p a t t e r n was not as c l e a r when l i n o l e a t e was o x i d i z e d a t t h r e e d i f f e r e n t t e m p e r a t u r e s ( 1 8 ) . The more u n s a t u r a t e d s u b s t r a t e o x i d i z e d much f a s t e r and many o f the o x i d a t i o n p r o d u c t s , themselves p o l y u n s a t u r a t e d , r e a d i l y underwent f u r t h e r d e c o m p o s i t i o n . C o n s e q u e n t l y some o f the p r e d i c t e d compounds c o u l d n o t be d e t e c t e d w h i l e o t h e r s were formed w h i c h were n o t e x p e c t e d from d i r e c t a l k o x y r a d i c a l c l e a v a g e . F u r t h e r m o r e , t h e amounts o f v o l a t i l e s p r e s e n t v a r i e d markedly w i t h h e a t i n g t i m e . I t i s n o t s u r p r i s i n g , t h e r e f o r e , t h a t a t h i g h t e m p e r a t u r e s , and p a r t i c u l a r l y w i t h p o l y u n s a t u r a t e d systems, i t has been e x t r e m e l y d i f f i c u l t t o e s t a b l i s h the n a t u r e and extent of p r e f e r e n t i a l hydroperoxide s c i s s i o n . Secondary D e c o m p o s i t i o n and P o l y m e r i z a t i o n . R e a c t i o n s which u s u a l l y o c c u r i n the l a t e r s t a g e s o f a u t o x i d a t i o n a t room temperature may assume i n c r e a s e d s i g n i f i c a n c e , and t h e i r consequences may be e n c o u n t e r e d much more r a p i d l y , a t e l e v a t e d t e m p e r a t u r e s . F o r example, the s e r i e s o f s h o r t c h a i n e s t e r s , o x o - e s t e r s , and d i c a r b o x y l i c a c i d s - u s u a l l y formed i n o n l y t r a c e amounts i n room-temperature o x i d a t i o n o f o l e a t e , l i n o l e a t e and l i n o l e n a t e - can be found i n s i g n i f i c a n t amounts a f t e r h e a t i n g f o r 1 h r a t 180°C. P o s s i b l e r e a c t i o n s l e a d i n g t o t h e f o r m a t i o n o f such compounds a r e g i v e n below.

R-CfC-C-(C) -COOR 5

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ester

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5

C Oxoester, CQ D i c a r b o x y l i c CO2+C7 e s t e r

\a-cleavage «C- (C) -COOR—• C

8

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7

ester

monester, Jox Joxidn.

^

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«OC-(C) -COOR s

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9.

NAWAR


97


The C8 aldehyde e s t e r may be produced by c l e a v a g e of the 9-hydroperoxide o f e t h y l l i n o l e a t e f o l l o w e d by t e r m i n a l hydroperoxidation. F u r t h e r o x i d a t i o n would produce the c o r r e s p o n d i n g d i c a r b o x y l i c a c i d which upon d e c a r b o x y l a t i o n would g i v e r i s e to e t h y l h e p t a n o a t e . The 8-alkoxy r a d i c a l may a l s o decompose to g i v e the C7 a l k y l r a d i c a l , which would y i e l d e t h y l heptanoate or form a t e r m i n a l h y d r o p e r o x i d e , and so on. P o l y m e r i z a t i o n , b o t h i n t r a - and i n t e r m o l e c u l a r , i s a l s o a major r e a c t i o n i n h i g h temperature o x i d a t i o n . C o m b i n a t i o n of a l k y l , a l k o x y , and peroxy r a d i c a l s y i e l d s a v a r i e t y of d i m e r i c and p o l y m e r i c compounds w i t h C-O-C or C-O-O-C crosslinks. C y c l i c Compounds. C e r t a i n c y c l i c compounds i d e n t i f i e d i n heated p o l y u n s a t u r a t e d e s t e r s c o u l d be e x p l a i n e d o n l y by i n v o k i n g mechanisms w h i c h i n v o l v e a l l y l i c p r o t o n a b s t r a c t i o n s o u t s i d e the 1,4-pentadiene systems ( 2 2 ) . Thermolytic Reactions. At the h i g h e r t e m p e r a t u r e s , n o n o x i d a t i v e r e a c t i o n s a l s o o c c u r as f o r example the f o r m a t i o n of t e t r a s u b s t i t u t e d c y c l o h e x e n e s v i a D i e l s - A l d e r r e a c t i o n , or the f o r m a t i o n of dehydrodimers and mono- or p o l y c y c l i c dimers v i a c o m b i n a t i o n of a l k y l f r e e r a d i c a l s or f r e e r a d i c a l a t t a c k on double bonds. D e c o m p o s i t i o n of S a t u r a t e d F a t t y A c i d s . Saturated f a t t y a c i d s which are e x t r e m e l y s t a b l e a t low temperatures and r a r e l y c o n t r i b u t e to the o x i d a t i v e d e c o m p o s i t i o n p a t t e r n , can p l a y a s i g n i f i c a n t r o l e when temperatures h i g h e r than 150°C are used. N o n - o x i d a t i v e r e a c t i o n s of t r i a c y l g l y c e r o l s are r e s p o n s i b l e f o r the f o r m a t i o n of a l k a n e and 1-alkene s e r i e s , propene- and p r o p a n e d i o l d i e s t e r s , o x o p r o p y l e s t e r s , symmetric k e t o n e s and f a t t y a c i d s (12,16,23). I f heated i n a i r a t h i g h temperature the s a t u r a t e d f a t t y a c i d s undergo o x i d a t i v e d e c o m p o s i t i o n . I t i s b e l i e v e d t h a t the p r i n c i p a l mechanism i n v o l v e s the f o r m a t i o n of monohydroperoxides, and t h a t oxygen a t t a c k may o c c u r a t a l l methylene groups of the f a t t y a c i d c h a i n w i t h l o c a t i o n s near the e s t e r c a r b o n y l f a v o r e d . P r o d u c t s of the s a t u r a t e d f a t t y a c i d s i n c l u d e m e t h y l k e t o n e s , a l d e h y d e s , l a c t o n e s , and hydrocarbons (12,16). F o r m a t i o n of the odorous l a c t o n e s and m e t h y l ketones t y p i c a l of d a i r y p r o d u c t s are b e l i e v e d to o r i g i n a t e from the s a t u r a t e d hydroxy f a t t y a c i d s and b e t a - k e t o a c i d s , r e s p e c t i v e l y , which n a t u r a l l y occur i n m i l k f a t . THERMAL INTERACTIONS OF LIPIDS AND

PROTEINS

The n a t u r e of p r o t e i n - l i p i d complexes and the consequences of i n t e r a c t i o n between l i p i d s and p r o t e i n s i n b i o s y s t e m s have been e x t e n s i v e l y s t u d i e d . The r e a d e r i s r e f e r r e d to the e l e g a n t r e v i e w by K a r e l (24). I t i s generally recognized that l i p i d - p r o t e i n i n t e r a c t i o n s are i n v o l v e d i n a v a r i e t y of p h y s i c a l and c h e m i c a l changes w h i c h are i m p o r t a n t to food q u a l i t y , as f o r example i n the a g i n g of meat o r the f r o z e n s t o r a g e of f i s h . The r e a c t i o n of p r o t e i n s w i t h p e r o x i d i z i n g l i p i d s and/or t h e i r breakdown p r o d u c t s r e c e i v e d p a r t i c u l a r a t t e n t i o n (25-29). However, i n s p i t e of the f a c t t h a t food i s f r e q u e n t l y s u b j e c t e d t o heat a t numerous s t a g e s i n the c o u r s e of i t s p r o c e s s i n g and p r e p a r a t i o n , r e p o r t s on l i p i d - p r o t e i n


98

THERMAL GENERATION OF AROMAS

i n t e r a c t i o n s a t e l e v a t e d temperatures a r e s c a r c e and t h e s p e c i f i c r o l e o f temperature i n such i n t e r a c t i o n s i s i m p e r f e c t l y u n d e r s t o o d . We c a n o n l y g i v e some examples. N - H e t e r o c y c l i c s . The r e a c t i o n o f p r i m a r y amines w i t h t h e c a r b o n y l p r o d u c t s d e r i v e d from l i p i d o x i d a t i o n i s a major pathway i n l i p i d - p r o t e i n i n t e r a c t i o n s . F o r m a t i o n o f S c h i f f s base i n t e r m e d i a t e s f o l l o w e d by c y c l i z a t i o n and rearrangement c a n y i e l d i m i n e s , p y r i d i n e s and p y r r o l e s (5,15,30,31). F o r example, 2 - p e n t y l p y r i d i n e may r e s u l t from t h e r e a c t i o n o f ammonia w i t h 2 , 4 - d e c a d i e n a l , one o f the p r i n c i p l e aldehydes from t h e a u t o x i d a t i o n o f l i n o l e a t e (_5) . Downloaded by STANFORD UNIV GREEN LIBR on September 22, 2012 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch009

f

CH (0Η )ι+ CH=CH-CH=CH-CHO 3

2

+

ι

NH

3

CH (0Η )ι+ CH=CH-CH=CH-CH=NH 3

2

CH s C C H ^ - k ^ L i k e w i s e , 2 - o c t e n a l would y i e l d 2 - b u t y l p y r r o l e NH CH (CH )4CH=CH-CH0 • C H ( C H ) i CH=CH-CH=NH—¥ C H ( C H )

|p

3

3

2

3

2

+

3

2

3

Amides and N i t r i l e s . P r i m a r y amines o r ammonia, from t h e t h e r m a l d e c o m p o s i t i o n o f amino a c i d s , c a n be a c y l a t e d by c a r b o x y l i c a c i d s t o produce amides ( 3 0 ) .

The

RCOOH

+

NH

RCOOH

+

R*NH

*

3

RC0NH

+

2

H 0 2

1

2

• RCONHR

amides c a n decompose f u r t h e r t o g i v e n i t r i l e s RC0NH

2

•

RCN

+

H 0 2

S u b s t r a n t i a l y i e l d s o f N - s u b s t i t u t e d amides were observed by Sims and F i o r i t i (32) when s a f f l o w e r o i l o r m e t h y l e s t e r s were heated w i t h s i m p l e alpha-amino a c i d s a t temperatures above 150°C. The r e a c t i o n i s b e l i e v e d t o i n v o l v e d e c a r b o x y l a t i o n o f t h e amino a c i d and d i s p l a c e m e n t o f t h e a l c o h o l m o i e t y o f t h e f a t t y e s t e r by the amine w h i c h i s formed. These a u t h o r s concluded t h a t t h e e s t e r c a r b o n y l group may p a r t i c i p a t e i n t h e d e c a r b o x y l a t i o n r e a c t i o n s i n c e no C 0 was e v o l v e d and m e t h i o n i n e c o u l d be r e c o v e r e d q u a n t i t a t i v e l y when i t was heated i n m i n e r a l o i l a t 200°C f o r p r o l o n g e d p e r i o d s . 2

I n t e r a c t i o n s w i t h H i s t i d i n e . I m i d a z o l e l a c t i c a c i d and i m i d a z o l e a c e t i c a c i d were i d e n t i f i e d as breakdown p r o d u c t s when h i s t i d i n e was reacted w i t h methyl l i n o l e a t e , methyl l i n o l e a t e hydroperoxide or h e x a n a l f o r 3 weeks a t 25°C and 51°C ( 3 3 ) . I t was p o s t u l a t e d t h a t these compounds were formed v i a f r e e r a d i c a l r e a c t i o n s . Two o t h e r p r o d u c t s were a l s o produced w h i c h y i e l d e d h i s t i d i n e upon a c i d h y d r o l y s i s . These were thought t o be S c h i f f ' s base compounds a r i s i n g


9.

NAWAR


from c o n d e n s a t i o n of the h i s t i d y l alpha-amino group w i t h the groups of aldehydes from p e r o x i d a t i o n of the l i n o l e a t e .

99 carbonyl


I n t e r a c t i o n s w i t h Hexanal and P e n t a n a l . V a r i o u s compounds 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 , e.g., 2 - h e x y l - 4 , 5 - d i m e t h y l o x a z o l e and 2- p e n t y l - 3 , 5 - d i b u t y l p y r i d i n e , and i n f r i e d c h i c k e n f l a v o r , e.g., 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 (34,35) are b e l i e v e d to a r i s e from the i n t e r a c t i o n of h e x a n a l or p e n t a n a l , b o t h major components of l i p i d o x i d a t i o n , w i t h p r o d u c t s of p r o t e i n d e c o m p o s i t i o n . Lipid-Protein-Carbohydrate I n t e r a c t i o n s . Evidence f o r such complex i n t e r a c t i o n was r e c e n t l y r e p o r t e d by Huang et a l (36) who observed t h a t the a d d i t i o n of c o r n l i p i d s to z e i n and c o r n c a r b o h y d r a t e s enhanced the f o r m a t i o n of a l k y l p y r a z i n e s , i n d i c a t i n g t h a t l i p i d - d e r i v e d f r e e r a d i c a l s may a c c e l e r a t e the r a t e of M a i l l a r d r e a c t i o n s . Two of the a l k y l p y r a z i n e s , i d e n t i f i e d i n such m i x t u r e s a f t e r h e a t i n g f o r 30 minutes a t 180°C, have 5-carbon a l k y l s u b s t i t u t i o n at the p y r a z i n e r i n g and c o u l d o n l y be e x p l a i n e d by i n t e r a c t i o n of l i p i d 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 . These a u t h o r s suggested t h a t c o n d e n s a t i o n of amino k e t o n e s , formed by protein-carbohydrate i n t e r a c t i o n , may y i e l d 3 , 6 - d i h y d r o p y r a z i n e which would i n t u r n r e a c t 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 , to form 2,5-dimethyl-3-pentylpyrazine. I n t e r a c t i o n s w i t h Membrane Components. I n aqueous systems m i l k f a t g l o b u l e membrane l i p i d s and the n o n - l i p i d membrane s o l i d s were found to a c c e l e r a t e the o x i d a t i o n of m i l k f a t a t 50°C, but e x h i b i t e d a n t i o x i d a n t e f f e c t s a t 95°C (Chen, Z. Y.; Nawar, W. W., U n i v e r s i t y of M a s s a c h u s e t t s a t Amherst, u n p u b l i s h e d d a t a ) . Cholesterol Oxidation. I n our r e c e n t s t u d i e s on i n t e r a c t i o n e f f e c t s i n c h o l e s t e r o l o x i d a t i o n , we o b s e r v e d t h a t v a r i o u s amino a c i d s and p h o s p h o l i p i d s i n h i b i t , w h i l e t r i a c y l g l y c e r o l s a c c e l e r a t e , the o x i d a t i o n of c h o l e s t e r o l at 180°C. Some p h o s p h o l i p i d s were p r o t e c t i v e w h i l e o t h e r s a c t e d as p r o o x i d a n t s a t 130°C, and some e x h i b i t e d a c c e l e r a t i o n a t the b e g i n n i n g of h e a t i n g f o l l o w e d by p r o t e c t i o n (Kim, S. K.; L i , Y. G. ; Nawar, W. W., U n i v e r s i t y of M a s s a c h u s e t t s a t Amherst, u n p u b l i s h e d d a t a ) . EFFECTS OF PHYSICAL STATE To g a i n i n s i g h t i n t o the e f f e c t of p h y s i c a l s t a t e and/or m o l e c u l a r o r g a n i z a t i o n on l i p i d o x i d a t i o n , a v a r i e t y of model systems have been used. These i n c l u d e d i s p e r s i o n s , l i p o s o m e s or v e s i c l e s (37,38), monolayers adsorbed on s i l i c a (39,40,41), and r e d b l o o d c e l l ghosts (42). I n most of these s t u d i e s , o x i d a t i o n was conducted a t r e l a t i v e l y low t e m p e r a t u r e s , i . e . , 20 - 40°C. Very l i t t l e i n f o r m a t i o n i s a v a i l a b l e on the e f f e c t s of p h y s i c a l s t a t e on h i g h temperature o x i d a t i v e r e a c t i o n s or i n t e r a c t i o n s of l i p i d s . M o l e c u l a r Order. The a u t o x i d a t i o n of l i n o l e i c a c i d i n monolayers a t 60°C d i f f e r e d from t h a t i n b u l k i n t h a t i t i n v o l v e d no l a g p e r i o d s , was c o n s i d e r a b l y f a s t e r , and e x h i b i t e d f i r s t o r d e r k i n e t i c s i m p l y i n g t h a t the o v e r a l l r e a c t i o n i s not a u t o c a t a l y t i c ( 4 1 ) . More r e c e n t l y



100


we compared monolayer and b u l k a u t o x i d a t i o n a t 60°C and 180°C ( 3 9 ) . As e x p e c t e d , the r a t e o f o x i d a t i o n was much f a s t e r a t the h i g h e r temperature f o r b o t h t h e adsorbed and t h e b u l k samples. A t 60°C e t h y l l i n o l e a t e o x i d i z e d f a s t e r when adsorbed on s i l i c a than when i n b u l k . On the o t h e r hand, o x i d a t i o n o f t h e b u l k samples was f a s t e r a t 180°C than t h a t o f the monolayers ( F i g . 1 ) . The f a s t e r o x i d a t i o n o f l i n o l e a t e i n monolayers a t 60°C i s p r o b a b l y due t o t h e l a r g e r s u r f a c e and t h e more f a c i l e exposure t o oxygen. T h i s , however, appears t o be o f f s e t by the h i g h e r m o b i l i t y o f the s u b s t r a t e m o l e c u l e s and the f a s t e r r a d i c a l t r a n s f e r i n b u l k a t the h i g h e r t e m p e r a t u r e . The p r o d u c t s o f l i p i d o x i d a t i o n i n monolayers were a l s o s t u d i e d . Wu and coworkers (41) c o n c l u d e d t h a t e p o x i d e s r a t h e r than h y d r o p e r o x i d e s might be t h e major i n t e r m e d i a t e s i n t h e o x i d a t i o n o f u n s a t u r a t e d f a t t y a c i d s adsorbed on s i l i c a , presumably because o f t h e p r o x i m i t y o f the s u b s t r a t e c h a i n s on the s i l i c a s u r f a c e . I n o u r work w i t h e t h y l o l e a t e , l i n o l e a t e and l i n o l e n a t e which were t h e r m a l l y o x i d i z e d on s i l i c a , the major d e c o m p o s i t i o n p r o d u c t s found were those t y p i c a l o f h y d r o p e r o x i d e d e c o m p o s i t i o n ( 3 9 ) . However, the d e c o m p o s i t i o n p a t t e r n s i n monolayers were s i m p l e r and q u a n t i t a t i v e l y d i f f e r e n t from those o f b u l k samples. F o r example, b u l k samples produced s i g n i f i c a n t l y more e t h y l o c t a n o a t e than those o f s i l i c a , whereas s i l i c a samples produced more e t h y l 9-oxononanoate than those of b u l k . T h i s t r e n d was c o n s i s t e n t r e g a r d l e s s of t e m p e r a t u r e , h e a t i n g p e r i o d o r degree o f o x i d a t i o n . The f a c t t h a t the same p a t t e r n of v o l a t i l e s was found a t b o t h 60°C and 180°C i m p l i e s t h a t t h e same mode o f d e c o m p o s i t i o n o c c u r s over t h i s temperature range. Another d i f f e r e n c e between b u l k and monolayer o x i d a t i o n s was observed i n t h e amounts o f a l k a n e s and a l k e n e s produced. In bulk, the a l k a n e s i n e t h y l p a l m i t a t e heated f o r 1 h r a t 180°C were g r e a t e r than the 1-alkenes, i n d i c a t i n g t h a t hydrogen a b s t r a c t i o n i s t h e p r e f e r r e d r o u t e f o r t e r m i n a t i o n o f the a l k y l r a d i c a l s which a r e formed by h y d r o p e r o x i d e s c i s s i o n . The r e v e r s e was t r u e i n the o r d e r e d s t a t e . R e s t r i c t i o n o f the a l k y l r a d i c a l s l i m i t s a b s t r a c t i o n of hydrogen from o t h e r s u b s t r a t e m o l e c u l e s t o form a l k a n e s thus f a v o r i n g t h e p r o d u c t i o n o f 1-alkenes (Hau, L. B.; Nawar, W. W. U n i v e r s i t y of M a s s a c h u s e t t s , Amherst, u n p u b l i s h e d d a t a ) . A l t h o u g h we c o n f i r m e d t h e f o r m a t i o n o f e p o x i d e s i n t h e case o f monolayers, we suggested t h a t t h e i r f o r m a t i o n may be t h e r e s u l t o f a c a t a l y t i c e f f e c t o f s i l i c a , r a t h e r than t h a t o f an i n t e r a c t i o n between t h e r i g i d l y o r i e n t e d n e i g h b o r i n g m o l e c l u e s as e x p l a i n e d by Mead's group. P o s s i b l y , h y d r o p e r o x i d e i n t e r m e d i a t e s a r e the major p r i m a r y p r o d u c t s i n the adsorbed phase as w e l l , and the a c i d i c n a t u r e of s i l i c a f a v o r s a s e l e c t i v e h e t e r o l y t i c c l e a v a g e as proposed by Kimoto and Gaddis (19). E f f e c t o f M e l t i n g . Below i t s m e l t i n g p o i n t pure c h o l e s t e r o l i s r e s i s t a n t t o o x i d a t i o n as l o n g as i t remains s o l i d . Thus, a t 130°C i t remains s t a b l e up t o a p o i n t which c o i n c i d e s w i t h v i s i b l e m e l t i n g when a sudden r i s e i n o x i d a t i o n r a t e o c c u r s (Kim, S. K.; Nawar, W.W., U n i v e r s i t y o f M a s s a c h u s e t t s a t Amherst, u n p u b l i s h e d d a t a ) . T h i s phase t r a n s i t i o n d u r i n g o x i d a t i o n a t 130°C i s b e l i e v e d t o r e f l e c t t h e i n f l u e n c e o f the t r a c e amounts o f o x i d e s formed on t h e m e l t i n g p o i n t .


9.

NAWAR

101



180°C

8

16

24

HOURS e

Figure 1. Rate of ethyl linoleate oxidation on silica and in bulk at 60 C and 180 *C. (Reprinted with permissionfromref. 39. Copyright 1988 American Oil Chemistry Society.)




102

Dry v s . Aqueous S t a t e s , I n t h e d r y s t a t e , m i l k f a t g l o b u l e membrane, the membrane l i p i d s and the n o n - l i p i d membrane s o l i d s i n h i b i t e d t h e o x i d a t i o n o f m i l k f a t a t b o t h 50°C and 95°C. I n t h e presence o f w a t e r , however, a l l t h r e e enhanced t h e o x i d a t i o n a t 50°C b u t remained p r o t e c t i v e a t 95°C. Moreover, v a r i o u s amino a c i d s w h i c h i n h i b i t e d the o x i d a t i o n o f m i l k f a t a t 95°C i n t h e d r y system a c t e d as p r o o x i d a n t s when water was p r e s e n t (Chen, Ζ. Y.; Nawar, W. W., U n i v e r s i t y o f M a s s a c h u s e t t s a t Amherst, u n p u b l i s h e d d a t a ) . The e f f e c t s o f amino a c i d s on l i p i d a u t o x i d a t i o n i n e m u l s i o n s , i n o i l s , a n d i n f r e e z e - d r i e d systems were a l s o i n v e s t i g a t e d (43,44). The amino a c i d s w h i c h were t e s t e d were a l l p r o o x i d a n t s ( w i t h t h e e x c e p t i o n o f c y s t e i n e ) i n t h e d r i e d systems. Farag and Osman (44) a t t r i b u t e d t h e p r o o x i d a n t e f f e c t o f amino a c i d s i n aqueous media p r e d o m i n a n t l y t o t h e presence o f t h e p r o t o n a t e d amino n i t r o g e n . P r o o x i d a n t / a n t i o x i d a n t e f f e c t s v a r i e d w i t h type o f e m u l s i f i e r s used, pH, and presence o f added s u g a r . Sims e t a l (45) suggested t h a t t h e p r o t e c t i v e e f f e c t o f added sugar i s n o t due t o an a c t u a l a n t i o x i d a n t e f f e c t b u t r a t h e r t o an improved r e s i s t a n c e t o phase s e p a r a t i o n r e s u l t i n g i n a lower c o n c e n t r a t i o n o f oxygen i n t h e aqueous phase and a s l o w e r d i f f u s i o n o f t h e gas t h r o u g h t h e o i l - w a t e r i n t e r f a c e . P h y s i c a l B a r r i e r s . Wu e t a l (46) observed t h a t t h e i n c l u s i o n o f p a l m i t i c a c i d o r c h o l e s t e r y l a c e t a t e i n l i n o l e i c a c i d monolayers on s i l i c a , e x e r t e d a p r o t e c t i v e e f f e c t a g a i n s t o x i d a t i o n . They suggested t h a t these compounds a c t as a s p a c e r k e e p i n g t h e l i n o l e i c a c i d molecules f a r t h e r apart while being only slowly o x i d i z e d t h e m s e l v e s . S i m i l a r l y , o u r r e c e n t work w i t h c h o l e s t e r o l o x i d a t i o n appears t o i n d i c a t e t h a t c a r b o h y d r a t e s do n o t change t h e pathway o f c h o l e s t e r o l o x i d a t i o n b u t r a t h e r a c t as a p h y s i c a l b a r r i e r a g a i n s t the m i g r a t i o n o f r e a c t i v e s p e c i e s . Conclusion Space does n o t a l l o w a complete c i t i n g o f t h e l i t e r a t u r e on t h e r m a l d e c o m p o s i t i o n o f l i p i d s . Nor was i t i n t e n d e d t o l i s t t h e hundreds o f p r o d u c t s which were i d e n t i f i e d i n heated f a t s and r a t i o n a l i z e mechanisms f o r t h e i r f o r m a t i o n . The above d i s c u s s i o n i s an attempt to i d e n t i f y c e r t a i n f a c t o r s w h i c h i n f l u e n c e t h e f a t e o f l i p i d s i n complex b i o l o g i c a l systems when s u b j e c t e d t o h i g h t e m p e r a t u r e . I t s h o u l d be o b v i o u s t h a t t h e p o t e n t i a l f o r t h e m u l i t i p l e r e a c t i o n s and i n t e r a c t i o n s which c o u l d take p l a c e i n such systems i s enormous. Those w h i c h do o c c u r , t h e i r r a t e s , and r o u t e s , depend on a c o m p l i c a t e d b a l a n c e between t h e i n f l u e n c e s o f many f a c t o r s . Major among t h e s e a r e t e m p e r a t u r e , r e a c t i o n time, o t h e r c o n s t i t u e n t s i n t h e environment, p h y s i c a l s t a t e , and m o l e c u l a r o r g a n i z a t i o n . Indeed, s t u d i e s w i t h model systems a r e i n v a l u a b l e , however, p e r f e c t s i m u l a t i o n o f the n a t u r a l s i t u a t i o n i s p r a c t i c a l l y i m p o s s i b l e . The model and t h e r e a l systems a r e s t i l l f a r a p a r t and much more r e s e a r c h i s r e q u i r e d w i t h b o t h t o b e t t e r u n d e r s t a n d t h e wide gap i n between.

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9. NAWAR


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Thermal Decomposition of Lipids - ACS Symposium Series (ACS

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