Flavor Formation in Meat-Related Maillard Systems Containing

Chapter 41

Flavor Formation in Meat-Related Maillard Systems Containing Phospholipids 1

2

Donald S. Mottram and Linda J . Salter

Downloaded by UNIV OF ARIZONA on March 27, 2017 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch041

1

Department of Food Science and Technology, University of Reading, Whiteknights, Reading RG6 2AP, United Kingdom Bristol Laboratory, AFRC Institute of Food Research, Langford, Bristol BS18 7DY, United Kingdom

2

In heated foods the main reactions by which flavors are formed are the Maillard reaction and the thermal degradation of lipids. These reactions follow complex pathways and produce reactive intermediates, both volatile and non-volatile. It has been demonstrated that lipids, in particular structural phospholipids, are essential for the characteristic flavor development in cooked meat and that the interaction of lipids with products of the Maillard reaction is an important route to flavor. When model systems containing amino acids and ribose were heated in aqueous buffer, the addition of phospholipids had a significant effect on the aroma and on the volatile products. In addition a number of heterocyclic compounds derived from lipid - Maillard interactions were found. The extent of the interaction depends on the lipid structure, with phospholipids reacting much more readily than triglycerides.

The c h a r a c t e r i s t i c f l a v o r s a s s o c i a t e d w i t h cooked foods such as meat have proved p a r t i c u l a r l y d i f f i c u l t t o c h a r a c t e r i z e , b o t h f o r t h e s e n s o r y a n a l y s t and the f l a v o r c h e m i s t . L a r g e numbers o f v o l a t i l e s c o m p r i s i n g a l i p h a t i c , a r o m a t i c and h e t e r o c y c l i c compounds have been i d e n t i f i e d i n cooked f o o d s , and meat, w i t h a l m o s t 1000 v o l a t i l e s , c o n t a i n s the l a r g e s t number o f compounds i d e n t i f i e d i n any food o r beverage. Despite t h i s considerable research i n t e r e s t , the key aroma component(s) o f meat remain e l u s i v e and i t i s p r o b a b l e t h a t no s i m p l e c o m b i n a t i o n o f compounds i s r e s p o n s i b l e f o r meat aroma ( 1 , 2 ) . Meat f l a v o r i s t h e r m a l l y d e r i v e d , s i n c e uncooked meat has l i t t l e o r no aroma and o n l y a b l o o d - l i k e t a s t e . The major p r e c u r s o r s o f meat f l a v o r can be d i v i d e d i n t o two c a t e g o r i e s : water s o l u b l e components (amino a c i d s , p e p t i d e s , s u g a r s , n u c l e o t i d e s , t h i a m i n e , e t c ) and components o f l i p i d s ( p r i n c i p a l l y the f a t t y a c i d s ) . The two main types of f l a v o r - f o r m i n g r e a c t i o n s which o c c u r d u r i n g t h e c o o k i n g o f meat a r e t h e M a i l l a r d r e a c t i o n between amino compounds and r e d u c i n g s u g a r s , and the t h e r m a l d e g r a d a t i o n o f l i p i d s . 0097-6156/89/0409-0442S06.00/0 o 1989 American Chemical Society Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

41.

M O T T R A M AND SALTER

Flavor in Meat-Related Maillard Systems

443

The M a i l l a r d R e a c t i o n i n Meat F l a v o r The importance of the M a i l l a r d r e a c t i o n i n meat f l a v o r has l o n g been r e c o g n i z e d and i t has formed the b a s i s o f the l a r g e number o f p a t e n t s f o r " r e a c t i o n p r o d u c t s i m u l a t e d meat f l a v o r i n g s . R e s e a r c h on cooked meat i n the 1950s had shown t h a t d u r i n g c o o k i n g t h e r e were s i g n i f i c a n t d e p l e t i o n s i n the l e v e l s of r e d u c i n g sugars and amino a c i d s , i n p a r t i c u l a r c y s t e i n e ( 3 , 4 ) . T h i s l e d to the c l a s s i c p a t e n t o f Morton et a l i n 1960 w h i c h i n v o l v e d the f o r m a t i o n of a m e a t - l i k e f l a v o r by h e a t i n g a m i x t u r e of c y s t e i n e and r i b o s e (J>). Practically a l l the subsequent p r o p o s a l s f o r m e a t - l i k e f l a v o r i n g s from M a i l l a r d systems i n v o l v e d s u l f u r , u s u a l l y as c y s t e i n e o r o t h e r s u l f u r c o n t a i n i n g amino a c i d s . H e t e r o c y c l i c compounds are dominant among the aroma compounds produced i n the M a i l l a r d r e a c t i o n , and s u l f u r - c o n t a i n i n g h e t e r o c y c l i c s have been shown to be p a r t i c u l a r l y i m p o r t a n t i n m e a t - l i k e f l a v o r s . I n a r e c e n t r e v i e w , MacLeod (6) l i s t e d 78 compounds w h i c h have been r e p o r t e d i n the l i t e r a t u r e as p o s s e s s i n g meaty aromas; seven a r e a l i p h a t i c s u l f u r compounds, the o t h e r 71 a r e h e t e r o c y c l i c of w h i c h 65 c o n t a i n s u l f u r . The S t r e c k e r d e g r a d a t i o n of c y s t e i n e by d i c a r b o n y l s i s an e x t r e m e l y i m p o r t a n t r o u t e f o r the f o r m a t i o n of many h e t e r o c y c l i c s u l f u r compounds; hydrogen s u l f i d e and mercaptoacetaldehyde are formed by the d e c a r b o x y l a t i o n and d e a m i n a t i o n of c y s t e i n e and p r o v i d e r e a c t i v e i n t e r m e d i a t e s f o r i n t e r a c t i o n w i t h other M a i l l a r d products.


11

L i p i d - d e r i v e d V o l a t i l e s i n Meat F l a v o r The v o l a t i l e s from cooked meat c o n t a i n l a r g e numbers of a l i p h a t i c compounds i n c l u d i n g a l d e h y d e s , a l c o h o l s , k e t o n e s , hydrocarbons and a c i d s . These a r e d e r i v e d from l i p i d s by thermal d e g r a d a t i o n and o x i d a t i o n (_7) and many may c o n t r i b u t e to d e s i r a b l e f l a v o r . In a d d i t i o n , the a l d e h y d e s , u n s a t u r a t e d a l c o h o l s and ketones produced i n t h e s e r e a c t i o n s , as w e l l as the p a r e n t u n s a t u r a t e d f a t t y a c i d s , a r e r e a c t i v e s p e c i e s and under c o o k i n g c o n d i t i o n s c o u l d be expected to i n t e r a c t w i t h i n t e r m e d i a t e s of the M a i l l a r d r e a c t i o n to produce o t h e r f l a v o r compounds. Meat c o n t a i n s n e u t r a l t r i g l y c e r i d e l i p i d s i n the f a t depots and i n the i n t r a - m u s c u l a r f a t c e l l s . I n a d d i t i o n , the p o l a r phosphol i p i d s are i m p o r t a n t s t r u c t u r a l components of a l l t i s s u e s and, a l t h o u g h they comprise l e s s than 1% o f m u s c l e , the p h o s p h o l i p i d s c o n t a i n a much h i g h e r p r o p o r t i o n of p o l y u n s a t u r a t e d f a t t y a c i d s than t r i g l y c e r i d e l i p i d s and t h e r e f o r e p r o v i d e a s i g n i f i c a n t s o u r c e o f autoxidation products. The r o l e of p h o s p h o l i p i d s i n the development of o f f - f l a v o r s i n reheated meats (warmed-over f l a v o r ) has l o n g been r e c o g n i z e d ( 8 ) , but u n t i l r e c e n t l y t h e i r p o s s i b l e r o l e i n the development of d e s i r a b l e f l a v o r s has been l a r g e l y u n e x p l o r e d . L i p i d s and the M a i l l a r d R e a c t i o n I n an e x a m i n a t i o n of the c o n t r i b u t i o n w h i c h l i p i d s make to the development of aroma d u r i n g the h e a t i n g o f meat, the p h o s p h o l i p i d s were shown to be p a r t i c u l a r i l y i m p o r t a n t . Consumer and t a s t e p a n e l s t u d i e s had f a i l e d to show any r e l a t i o n s h i p between the meaty f l a v o r of l e a n meat and the l e v e l of f a t on the c a r c a s s , a p p a r e n t l y

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


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THERMAL GENERATION OF AROMAS

c o n f i r m i n g e a r l y s t u d i e s on meat f l a v o r w h i c h a s s o c i a t e d meatiness w i t h water s o l u b l e f l a v o r p r e c u r s o r s and s p e c i e s c h a r a c t e r i s t i c s w i t h the f a t ( 9 , 1 0 ) . However, the v o l a t i l e s of cooked meat a r e dominated by l i p i d - d e r i v e d compounds and i t i s d i f f i c u l t to b e l i e v e t h a t such compounds p l a y no r o l e i n meat f l a v o r . The c o n t r i b u t i o n these v o l a t i l e s of l i p i d o r i g i n make to meat aroma was i n v e s t i g a t e d by o b s e r v i n g the changes i n aroma w h i c h o c c u r r e d when l i p i d s were e x t r a c t e d from the meat p r i o r to c o o k i n g ( 1 1 ) . The i n t e r - and i n t r a - m u s c u l a r t r i g l y c e r i d e f a t s were removed from l e a n muscle by s o l v e n t e x t r a c t i o n w i t h hexane, and the m a t e r i a l was s u b s q u e n t l y c o o k e d . The r e s u l t i n g aroma was v e r y s i m i l a r to t h a t from the u n t r e a t e d m a t e r i a l ; b o t h p r e p a r a t i o n s were judged to be meaty and c o u l d not be d i f f e r e n t i a t e d by aroma i n s e n s o r y t r i a n g l e t e s t s . However, when a more p o l a r s o l v e n t ( c h l o r o f o r m methanol) was used to e x t r a c t a l l the l i p i d s - p h o s p h o l i p i d s and t r i g l y c e r i d e s - a h i g h l y s i g n i f i c a n t d i f f e r e n c e i n aroma r e s u l t e d ; the meaty aroma was r e p l a c e d by a r o a s t e d , b i s c u i t - l i k e aroma. E x a m i n a t i o n of the aroma v o l a t i l e s showed t h a t both the c o n t r o l and the m a t e r i a l w i t h o u t t r i g l y c e r i d e s had s i m i l a r p r o f i l e s dominated by a l i p h a t i c aldehydes and a l c o h o l s , w h i l e removal of p h o s p h o l i p i d s as w e l l as t r i g l y c e r i d e s gave a markedly d i f f e r e n t p r o f i l e ; the l i p i d o x i d a t i o n products were l o s t , but t h e r e was a l a r g e i n c r e a s e i n the amounts of a l k y l p y r a z i n e s . T h i s i n c r e a s e i n a l k y l p y r a z i n e s , v o l a t i l e p r o d u c t s of the M a i l l a r d r e a c t i o n , i m p l i e d t h a t i n normal meat the l i p i d s o r t h e i r d e g r a d a t i o n p r o d u c t s i n h i b i t the f o r m a t i o n of c e r t a i n h e t e r o c y c l i c compounds by p a r t i c i p a t i n g i n the M a i l l a r d reaction. There i s i n c r e a s i n g e v i d e n c e t h a t the i n t e r a c t i o n of l i p i d s w i t h the M a i l l a r d r e a c t i o n i s r e l e v a n t to the g e n e r a t i o n of f l a v o r i n many cooked f o o d s . F o r i n s t a n c e , the removal of l i p i d s from coconut has been shown to cause f l a v o r changes i n the r o a s t m a t e r i a l ( 1 2 ) . Uncooked coconut c o n t a i n e d s i g n i f i c a n t amounts of l a c t o n e s as the main aroma components; on r o a s t i n g p y r a z i n e s , p y r r o l e s and furans were a l s o found i n the aroma v o l a t i l e s which added a s t r o n g n u t - l i k e aroma to the sweet aroma of the unroasted c o c o n u t . When ground coconut was d e f a t t e d and then r o a s t e d , the sweet aroma due to l a c t o n e s d i s a p p e a r e d and the p r o d u c t possessed a b u r n t , n u t - l i k e aroma. A marked i n c r e a s e i n the number and amount of M a i l l a r d r e a c t i o n p r o d u c t s , i n p a r t i c u l a r p y r a z i n e s , was f o u n d . I t has been known f o r many years t h a t M a i l l a r d R e a c t i o n p r o d u c t s can behave as a n t i o x i d a n t s i n food systems ( 1 3 , 1 4 ) , and they have been shown to i n h i b i t warmed-over f l a v o r development i n cooked meat w h i c h i s caused by the a u t o x i d a t i o n of l i p i d s , e s p e c i a l l y p h o s p h o l i p i d s . There has been a s i g n i f i c a n t amount o f r e s e a r c h examining the M a i l l a r d r e a c t i o n p r o d u c t s and i n t e r m e d i a t e s from model systems w h i c h may have a n t i o x i d a t i v e p r o p e r t i e s . However, the f o r m a t i o n of v o l a t i l e aroma compounds from the i n t e r a c t i o n o f M a i l l a r d i n t e r m e d i a t e s w i t h l i p i d - d e r i v e d compounds has r e c e i v e d l i t t l e a t t e n t i o n . I n some r e c e n t r e s e a r c h on f l a v o r f o r m a t i o n d u r i n g d e e p - f a t f r y i n g a t Rutgers U n i v e r s i t y , a number of h e t e r o c y c l i c compounds w i t h 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 s were found the v o l a t i l e s of f r i e d c h i c k e n (15) and f r i e d p o t a t o ( 1 6 ) . These i n c l u d e d p y r i d i n e s , t h i a z o l e s , o x a z o l e s , t r i t h i o l a n e s and a p y r a z i n e . Only the i n v o l v e ment of l i p i d s or l i p i d d e g r a d a t i o n p r o d u c t s i n the f o r m a t i o n of


41.

MOTTRAM AND SALTER



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 . I n a s t u d y o f v o l a t i l e s from a heated m i x t u r e of c o r n components ( z e i n , c o r n s t a r c h and c o r n o i l ) two a l k y l s u b s t i t u t e d p y r a z i n e s ( 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 - m e t h y l - 3 ( o r 6 ) - p e n t y l p y r a z i n e ) were i d e n t i f i e d ( 1 7 ) ; a g a i n t h e s e compounds must a r i s e from a l i p i d - p r o t e i n - c a r b o h y d r a t e i n t e r a c t i o n . The i n t e r a c t i o n o f s i m p l e t r i g l y c e r i d e s w i t h amino a c i d s a t h i g h temperatures (270 C) produced f a t t y a c i d amides ( 1 8 ) , and when beef f a t was heated to 200 C w i t h g l y c i n e , b u t y l - and p e n t y l p y r i d i n e s were found among the l i p i d d e g r a d a t i o n p r o d u c t s ( 1 9 ) . The c o n d i t i o n s p r e v a i l i n g i n a l l t h e s e systems were e s s e n t i a l l y low m o i s t u r e and h i g h l i p i d , w h i l e i n meat much h i g h e r water ( c a . 7 5%) and l o w e r l i p i d (2-3%) l e v e l s e x i s t , and t h i s c o u l d r e s u l t i n d i f f e r e n t r e a c t i o n pathways to v o l a t i l e compounds. R e a c t i o n of Amino A c i d s , R i b o s e and P h o s p h o l i p i d s . The r e d u c t i o n i n l e v e l s of v o l a t i l e M a i l l a r d r e a c t i o n p r o d u c t s observed i n d e f a t t e d cooked meat l e d to an i n v e s t i g a t i o n of t h e e f f e c t of p h o s p h o l i p i d s on the v o l a t i l e p r o d u c t s from aqueous model systems c o n t a i n i n g amino a c i d s and sugars ( 2 0 - 2 2 ) . S e v e r a l amino a c i d s were used i n c l u d i n g g l y c i n e , the s i m p l e s t amino a c i d , and c y s t e i n e , a s u l f u r c o n t a i n i n g amino a c i d . R i b o s e was employed as the r e d u c i n g sugar because of i t s r e c o g n i z e d r o l e i n the p r o d u c t i o n of cooked meat f l a v o r and i t s h i g h r e a c t i v i t y i n M a i l l a r d s y s t e m s . I n i n i t i a l w o r k , L - a - p h o s p h a t i d y l c h o l i n e ( l e c i t h i n ) from egg y o l k was s e l e c t e d as the p h o s p h o l i p i d , and l a t e r s t u d i e s compared o t h e r p h o s p h o l i p i d s and l i p i d e x t r a c t s from meat. As the study o r i g i n a t e d from i n v e s t i g a t i o n s of cooked meat f l a v o r , the model system r e a c t i o n s were c a r r i e d out i n aqueous s o l u t i o n b u f f e r e d w i t h phosphate a t an i n i t i a l pH of 5.7 and c o n c e n t r a t i o n s of the r e a c t a n t s were s e l e c t e d to approximate t h e i r r e l a t i v e c o m p o s i t i o n s i n mammalian m u s c l e . The r e a c t i o n s were c a r r i e d o u t under p r e s s u r e a t 140 C f o r 1 hour i n s e a l e d g l a s s ampoules. A t the end of the r e a c t i o n s v o l a t i l e s were c o l l e c t e d by headspace entrainment on Tenax GC and analysed by c a p i l l a r y gas chromatography - mass s p e c t r o m e t r y . V o l a t i l e s from Amino A c i d + R i b o s e a l o n e . A l l the r e a c t i o n systems produced complex m i x t u r e s of v o l a t i l e s , c o n t a i n i n g l a r g e numbers of h e t e r o c y c l i c compounds, many of w h i c h have not been r e p o r t e d p r e v i o u s l y i n M a i l l a r d systems. I n the absence o f p h o s p h o l i p i d 74 compounds were separated and i d e n t i f i e d from the g l y c i n e system ( 2 1 ) ; f u r f u r a l s and f u r a n y l k e t o n e s dominated and the t h r e e l a r g e s t peaks i n the chromatogram were 2 - f u r f u r a l [ 1 ] , d i m e t h y l f u r f u r a l [2] and e t h y l m e t h y l f u r f u r a l [ 3 ] .

2

R =R =CH

3

R =CH

4

R =H

1

2

1

1

3

3

R =C H 2

R =CH 2

2

5

5

R=CH

6

R= C H 2

3

5

7

"=1

8

n = 2

3


445

446


Other furans i n c l u d e d m e t h y l - 2 - f u r f u r a l [ 4 ] , 2 - a c e t y l f u r a n [ 5 ] , 2 - p r o p i o n y l f u r a n [ 6 ] , l - ( 2 - f u r a n y l ) - 2 - p r o p a n o n e [7] and l - ( - 2 - f u r a n y l ) - 3 - b u t a n o n e [ 8 ] . S e v e r a l a l k y l p y r a z i n e s were a l s o f o u n d , w i t h the 2 , 5 - d i m e t h y l [9] and 2 - m e t h y l - 5 - e t h y l [10] t h e most abundant d e r i v a t i v e s , and some 2 - a c y l - l - m e t h y l p y r r o l e s of w h i c h o n l y the f o r m y l [11] and a c e t y l [12] d e r i v a t i v e s were c o n c l u s i v e l y identified.


CH

3

0

When c y s t e i n e was used i n s t e a d of g l y c i n e the r e a c t i o n v o l a t i l e s became dominated by s u l f u r c o n t a i n i n g h e t e r o c y c l i c s ( 2 2 ) ; the a l k y l f u r f u r a l s and f u r a n y I k e t o n e s were reduced to t r a c e amounts a n d , a l t h o u g h a s e r i e s of a l k y l p y r a z i n e s were found, the l e v e l s were much lower than i n the g l y c i n e s y s t e m . A t o t a l o f 118 v o l a t i l e compounds were i s o l a t e d from the c y s t e i n e + r i b o s e r e a c t i o n ; the major components were 2 - m e t h y l - 3 - f u r a n t h i o l [ 1 3 ] , f u r a n m e t h a n e t h i o l [14], 2-thiophenethiol [15], 2-methyl-3-thiophenethiol [16], d i h y d r o - 2 - m e t h y l - 3 ( 2 H ) - t h i o p h e n o n e [17] and 3-mercapto-2-pentanone [18]. A number of o t h e r f u r a n t h i o l s , t h i o p h e n e t h i o l s and thiophenones were a l s o found i n c l u d i n g a s e r i e s of a c y l thiophenes analogous to the f u r f u r a l s and f u r a n y l k e t o n e s found i n the g l y c i n e r e a c t i o n . Other 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 c l u d e d s e v e r a l a l k y l t h i a z o l e s , t h r e e i s o m e r i c t h i e n o t h i o p h e n e s [19-21] and t h e i r m e t h y l - and d i h y d r o - d e r i v a t i v e s , as w e l l as a l k y l - s u b s t i t u t e d l,2-dithiolan-4-ones [22-24], 1,2-dithian-4-ones [25-27], 1 , 2 , 4 - t r i t h i o l a n e s [28] and 1 , 2 , 4 - t r i t h i a n e s [29].

13

14

15

O

SH

16

17

18

19

20

21



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MOTTRAM AND SALTER

R =H

R =CH

22 23

R =R =CH

24

R = CH

1

1

2

2

28

3

3

R =C2H$ 2

3


25 26 27

447

R = R = H 1

2

R =R =CH 1

R =H 1

2

3

R = CH 2

3

29

E f f e c t of P h o s p h o l i p i d s on R e a c t i o n V o l a t i l e s . As would be e x p e c t e d , the i n c l u s i o n o f p h o s p h o l i p i d s i n the r e a c t i o n m i x t u r e s produced many v o l a t i l e s d e r i v e d from l i p i d d e g r a d a t i o n ; t h e s e i n c l u d e d h y d r o c a r b o n s , a l k y l f u r a n s , s a t u r a t e d and u n s a t u r a t e d a l c o h o l s , aldehydes and k e t o n e s . However, two o t h e r i m p o r t a n t o b s e r v a t i o n s were made. F i r s t , the c o n c e n t r a t i o n s o f most o f the h e t e r o - c y c l i c s , formed by the amino a c i d + r i b o s e M a i l l a r d r e a c t i o n , were r e d u c e d . For most of the major v o l a t i l e s t h i s r e d u c t i o n was of the o r d e r of 40 - 50%, but i n the case of t h i o p h e n e t h i o l and m e t h y l - f u r a n t h i o l the r e d u c t i o n was over 65%. T h i s appears to support the f i n d i n g s t h a t i n meat and c o c o n u t , l i p i d s e x e r t a quenching e f f e c t on the amount of h e t e r o c y c l i c compounds formed i n M a i l l a r d r e a c t i o n s d u r i n g heat t r e a t m e n t (11,12). Second, and perhaps more i m p o r t a n t , the a d d i t i o n of p h o s p h o l i p i d to the r e a c t i o n m i x t u r e s r e s u l t e d i n the p r o d u c t i o n o f l a r g e amounts of compounds d e r i v e d from the i n t e r a c t i o n of the l i p i d or i t s degradation products 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 . I n the g l y c i n e r e a c t i o n , 2 - p e n t y l p y r i d i n e [30] was formed. T h i s compound has been r e p o r t e d i n the v o l a t i l e s from heated lamb f a t ( 2 3 ) , f r i e d c h i c k e n (15) and f r e n c h f r i e d p o t a t o ( 1 6 ) , and was a major product i n the thermal 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 i c a c i d ( 2 4 ) . I t i s p o s t u l a t e d to be formed by the r e a c t i o n of d e c a d i e n a l and ammonia, and l a r g e c o n c e n t r a t i o n s o f 2 , 4 - d e c a d i e n a l were found i n the v o l a t i l e s of l e c i t h i n when heated a l o n e o r i n the M a i l l a r d m i x t u r e s . The a d d i t i o n o f p h o s p h o l i p i d to the c y s t e i n e - r i b o s e r e a c t i o n system gave o v e r 10 times more 2 - p e n t y l p y r i d i n e than the c o r r e s p o n d i n g g l y c i n e r e a c t i o n . I n the S t r e c k e r d e g r a d a t i o n of c y s t e i n e by d i c a r b o n y l compounds, ammonia i s known to accompany the f o r m a t i o n of hydrogen s u l f i d e ( 2 5 , 2 6 ) and t h i s w i l l p r o v i d e a ready source of ammonia f o r r e a c t i o n w i t h d i e n a l s from l i p i d breakdown. W i t h g l y c i n e , ammonia i s not formed so r e a d i l y at the s l i g h t l y a c i d i c pH employed i n the r e a c t i o n m i x t u r e and hence the lower c o n c e n t r a t i o n s of 2 - p e n t y l p y r i d i n e . Among the most abundant components of the c y s t e i n e + r i b o s e + l e c i t h i n r e a c t i o n m i x t u r e were the 2 - p e n t y l p y r i d i n e and 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 h i o p h e n e s : 2 - p e n t y l - and 2 - h e x y l t h i o p h e n e s [ 3 1 , 3 2 ] , t o g e t h e r w i t h a r e l a t e d compound w i t h m o l e c u l a r formula and


448



mass spectrum v e r y s i m i l a r to 2 - h e x y l t h i o p h e n e but w i t h a l a t e r r e t e n t i o n t i m e . T h i s h e x y l t h i o p h e n e isomer was t e n t a t i v e l y i d e n t i f i e d as 2 - p e n t y l - 2 H - t h i a p y r a n [ 3 3 ] . C l o s e r e x a m i n a t i o n of the GC-MS a n a l y s i s r e v e a l e d a s e r i e s o f 2 - a l k y l t h i o p h e n e s w i t h n - a l k y l s u b s t i t u e n t s between C3 and C8 i n c l u s i v e . Each had an i s o m e r i c compound w i t h v e r y s i m i l a r mass spectrum e l u t i n g c l o s e to the n e x t h i g h e r 2 - a l k y l t h i o p h e n e . Other l i p i d d e r i v e d compounds i n c l u d e d 2 - h e x e n y l t h i o p h e n e [ 3 4 ] , 1 - h e p t a n e t h i o l and l ^ o c t a n e t h i o l . A l l these compounds a r e p r o b a b l y formed by the r e a c t i o n o f l i p i d breakdown p r o d u c t s , such as d i e n a l s , w i t h hydrogen s u l f i d e o r mercaptoacetaldehyde o b t a i n e d i n the S t r e c k e r d e g r a d a t i o n o f cysteine.

C Hn

^

5

30

^

31

R=C Hn

32

R=CeHi3

34

R= CH=CH-C H

C,H„ 33

5

4

9

Hydrogen s u l f i d e i s a l s o a key component i n the f o r m a t i o n of s u l f u r - c o n t a i n i n g compounds i n the M a i l l a r d r e a c t i o n ( 2 7 ) . Thiophenones may be formed by the a c t i o n of hydrogen s u l f i d e on s u g a r d e g r a d a t i o n p r o d u c t s such as d e h y d r o r e d u c t o n e s , f u r f u r a l (28) o r furanones ( 2 9 ) , w h i l e t h i a z o l e s can be produced by the r e a c t i o n of hydrogen s u l f i d e w i t h d i c a r b o n y l s and ammonia o r an amino a c i d ( 3 0 ) . The s m a l l amount of f u r f u r a l i n the c y s t e i n e m i x t u r e compared w i t h t h a t c o n t a i n i n g g l y c i n e may be due to i t s r e a c t i o n w i t h hydrogen s u l f i d e to g i v e f u r a n m e t h a n e t h i o l . I n the r e a c t i o n m i x t u r e s c o n t a i n i n g l e c i t h i n , the d e c r e a s e i n c o n c e n t r a t i o n s o f many of the s u l f u r - c o n t a i n i n g h e t e r o c y c l i c compounds d e r i v e d from the b a s i c c y s t e i n e + r i b o s e r e a c t i o n can be e x p l a i n e d i n terms o f c o m p e t i t i o n f o r hydrogen s u l f i d e by l i p i d d e g r a d a t i o n compounds r e s u l t i n g i n o t h e r s u l f u r compounds such as the a l k y l t h i o p h e n e s and alkanethiols. Aroma of R e a c t i o n M i x t u r e s The aroma of the g l y c i n e + r i b o s e r e a c t i o n m i x t u r e a f t e r h e a t i n g was d e s c r i b e d as " c a r a m e l , " s w e e t " , " b u r n t s u g a r " w h i l e the a d d i t i o n of l e c i t h i n added " o i l y " and " c h i c k e n - l i k e " n o t e s . The o v e r a l l odor of the c y s t e i n e c o n t a i n i n g r e a c t i o n m i x t u r e s was d e s c r i b e d as " s u l f u r o u s , r u b b e r y " but t h e r e was a d i s t i n c t u n d e r l y i n g meaty aroma. T h i s "cooked meat", "beef" aroma was much more pronounced i n the m i x t u r e c o n t a i n i n g the p h o s p h o l i p i d . When the aromas e l u t i n g from the GC column were e v a l u a t e d , the c y s t e i n e + r i b o s e + l e c i t h i n system gave a t l e a s t s i x a r e a s where meaty aromas c o u l d be d e t e c t e d . Only one o f t h e s e was d e t e c t e d i n the absence of the p h o s p h o l i p i d and t h i s was a t t r i b u t e d to 2 - m e t h y l - 3 - t h i o p h e n e t h i o l ; none of the o t h e r meaty odors c o u l d be a s s i g n e d to p a r t i c u l a r compound i d e n t i f i c a t i o n s . The a l k y l thiophenes and the o t h e r i d e n t i f i e d p r o d u c t s of the l e c i t h i n M a i l l a r d i n t e r a c t i o n d i d not have m e a t - l i k e aromas. 11


41.

MOTTRAM AND SALTER


449


Comparison of D i f f e r e n t L i p i d s The o r i g i n a l work on d e f a t t e d meat suggested t h a t p h o s p h o l i p i d s r a t h e r than t r i g l y c e r i d e s were i m p o r t a n t i n d e t e r m i n i n g meat f l a v o r and the p a r t i c i p a t i o n of p h o s p h o l i p i d s i n the M a i l l a r d r e a c t i o n between amino a c i d s and r i b o s e has been c l e a r l y demonstrated i n the model s y s t e m s . I n mammalian t i s s u e the s t r u c t u r a l p h o s p h o l i p i d s c o n t a i n s i g n i f i c a n t amounts o f p o l y u n s a t u r a t e d f a t t y a c i d s , i n p a r t i c u l a r a r a c h i d o n i c ( 2 0 : 4 ) , d o c o s a p e n t a e n o i c ( 2 2 : 5 ) and docosahexaenoic ( 2 2 : 6 ) . Beef t r i g l y c e r i d e f a t c o n t a i n s l e s s than 2% polyunsaturated f a t t y a c i d s , mainly l i n o l e i c (18:2) and, although pork c o n t a i n s h i g h e r l e v e l s of l i n o l e i c a c i d , the t r i g l y c e r i d e l i p i d s of the d i f f e r e n t meat s p e c i e s do not c o n t a i n s i g n i f i c a n t amounts o f C20 o r C22 p o l y u n s a t u r a t e d f a t t y a c i d s . D i f f e r e n c e s i n f a t t y a c i d c o m p o s i t i o n may be one f e a t u r e of p h o s p h o l i p i d s w h i c h w i l l r e s u l t i n a d i f f e r e n t b e h a v i o u r from t r i g l y c e r i d e s i n M a i l l a r d s y s t e m s , w h i l e the o t h e r f e a t u r e i s the p o l a r n a t u r e of the phosphatidyl group. The o b s e r v a t i o n s on the aromas from c y s t e i n e + r i b o s e r e a c t i o n m i x t u r e s have been extended to compare the e f f e c t o f d i f f e r e n t l i p i d s : t r i g l y c e r i d e s and p h o s p h o l i p i d s e x t r a c t e d from b e e f , and commercial egg l e c i t h i n ( p h o s p h a t i d y l c h o l i n e ) and egg c e p h a l i n ( p h o s p h a t i d y l e t h a n o l a m i n e ) ( L . J . S a l t e r ; D.S Mottram, u n p u b l i s h e d data). The i n c l u s i o n of the beef t r i g l y c e r i d e s (TG) d i d not appear to have any e f f e c t on the aroma of the c y s t e i n e + r i b o s e r e a c t i o n m i x t u r e , w h i c h was s u l f u r o u s w i t h an u n d e r l y i n g m e a t i n e s s . However, when beef p h o s p h o l i p i d s ( P L ) were used the meaty aroma i n c r e a s e d m a r k e d l y . S i m i l a r i l y , a d d i t i o n of egg l e c i t h i n ( L E C ) o r egg c e p h a l i n (CEPH) to the c y s t e i n e + r i b o s e r e a c t i o n m i x t u r e gave i n c r e a s e d m e a t i n e s s , w i t h the c e p h a l i n - c o n t a i n i n g m i x t u r e b e i n g judged to have the most meaty c h a r a c t e r . The e f f e c t of t h e s e d i f f e r e n t l i p i d s on the f o r m a t i o n of s e l e c t e d v o l a t i l e s has a l s o been e v a l u a t e d ( T a b l e I ) . Many compounds Table I .

R e l a t i v e c o n c e n t r a t i o n s o f some h e t e r o c y c l i c compounds from c y s t e i n e + r i b o s e M a i l l a r d r e a c t i o n m i x t u r e s containing different l i p i d s

Compound

No Lipid

Beef TG

Beef PL

2-Furanmethanethiol 2-Methyl-3-furanthiol 2-Thiophenethiol

1 1 1

0.67 0.40 0.32

0.63 0.15 0.03

2-Pentylpyridine 2-Pentylthiophene 2-Hexylthiophene 2-Pentylthiapyran

0 0 0 0

0.09 0 0.15 0.01

1 1 1 1

a

tentative

a

Egg LEC

Egg CEPH

0.62 0.27 0.46

0.72 0.24 0.10

18.65 23.86 6.63 11.02

1.54 10.86 2.38 3.97

identification


450


d e r i v e d from the b a s i c c y s t e i n e + r i b o s e system were reduced i n the presence of l i p i d s , w i t h the p h o s p h o l i p i d s i n g e n e r a l h a v i n g more e f f e c t than the t r i g l y c e r i d e s . The a l k y l t h i o p h e n e s and o t h e r compounds r e s u l t i n g from the i n t e r a c t i o n o f l i p i d s w i t h M a i l l a r d i n t e r m e d i a t e s were o n l y found i n t r a c e amounts i n the presence of t r i g l y c e r i d e s , w h i l e c o n s i d e r a b l y l a r g e r amounts were found i n a l l the p h o s p h o l i p i d - c o n t a i n i n g m i x t u r e s .


Conclusion L i p i d s p l a y an i m p o r t a n t p a r t i n the development of aroma i n cooked f o o d s , such as meat, by p r o v i d i n g a source of r e a c t i v e i n t e r m e d i a t e s w h i c h p a r t i c i p a t e i n the M a i l l a r d r e a c t i o n . P h o s p h o l i p i d s appear to be more i m p o r t a n t than t r i g l y c e r i d e s . The a d d i t i o n of p h o s p h o l i p i d to aqueous amino a c i d + r i b o s e m i x t u r e s l e a d s to r e d u c t i o n s i n the c o n c e n t r a t i o n s of h e t e r o c y c l i c compounds formed i n the M a i l l a r d r e a c t i o n . T h i s e f f e c t c o u l d be due to l i p i d o x i d a t i o n p r o d u c t s r e a c t i n g w i t h s i m p l e M a i l l a r d i n t e r m e d i a t e s , such as hydrogen s u l f i d e and ammonia, to g i v e compounds not n o r m a l l y found i n the M a i l l a r d r e a c t i o n . The p r e c i s e n a t u r e of the o d o r i f e r o u s p r o d u c t s o b t a i n e d from l i p i d - M a i l l a r d i n t e r a c t i o n s i s d i c t a t e d by the l i p i d s t r u c t u r e and may depend on the f a t t y a c i d c o m p o s i t i o n and the n a t u r e of any p o l a r group a t t a c h e d to the l i p i d .

Literature Cited 1. MacLeod, G.; Seyyedain-Ardebili, M. CRC Crit. Rev. Food Sci. Nutr. 1981, 14, 309. 2. Shahidi, F; Rubin, L.J.; D'Souza, L.A. CRC Crit. Rev. Food Sci. Technol. 1986, 24, 141. 3. Macy, R.L.; Naumann, H.D.; Bailey, M.E. J. Food Sci. 1964, 29, 142. 4. May, C.G. Food Trade Rev. 1974, 44, 7. 5. Morton, I.D.; Akroyd, P.; May, C.G. Brit. Patent 836694, 1960. 6. MacLeod, G. In Developments in Food Flavors; Birch, G.C.; Lindley, M.G., Eds; Elsevier: London, 1986; p 191. 7. Mottram, D.S. Food Sci. Technol. Today 1987, 1, 159. 8. Pearson, A.M.; Love, J.D.; Shorland, F.B. Adv. Food Res. 1977, 23, 1. 9. Hornstein, I.; Crowe, P.F. J. Agric. Food Chem. 1963, 8, 494. 10. Mottram, D.S.; Edwards, R.A.; MacFie, H.J.H. J. Sci. Food Agric. 1982, 33, 934. 11. Mottram, D.S.; Edwards, R.A. J. Sci. Food Agric. 1983, 34, 517. 12. Saittagaroon, S; Kawakishi, S; Namiki, M. Agric. Biol. Chem. 1984, 48, 2301 13. Hodge, J.E.; Rist, C.E. J. Am. Chem. Soc. 1953, 75, 316. 14. Bailey, M.E. Food Technol. 1988, 42, 123. 15. Tang, J.; Jin, Q.Z.; Shen, G.H.; Ho, C.-T.; Chang, S.S. J. Agric. Food Chem. 1983, 31, 1287. 16. Carlin, J.T.; Jin, Q.Z.; Huang, T.Z.; Ho. C.-T.; Chang, S.S. J. Agric. Food Chem. 1986, 34, 621.



41. MOTTRAM AND SALTER


17. Huang, T.-C.; Bruechert, L.J.; Hartman, T.G.; Rosen, R.T.; Ho, C.-T. J.Agric. Food Chem. 1987, 35, 985. 18. Lien, Y.C.; Nawar, W.W. J. Food Sci. 1974, 39, 917. 19. Ohnishi, S.; Shibamoto, T. J. Agric. Food Chem. 1984, 32, 987. 20. Whitfield, F.B.; Mottram, D.S.; Brock, S; Puckey, D.J.; Salter, L.J. J. Sci. Food Agric. 1987, 42, 261. 21. Salter, L.J.; Mottram, D.S.; Whitfield, F.B. J. Sci. Food Agric. 1988, in press. 22. Salter, L.J.; Mottram, D.S.; Whitfield, F.B. J. Sci. Food Agric. 1989, in press. 23. Buttery, R.G.; Ling, L.C.; Teranishi,R.; Mon, T.R. J. Agric. Food Chem. 1977, 25, 1227. 24. Henderson, S.K.; Nawar, W.W. J. Am. Oil Chem. Soc. 1981, 58, 632. 25. Schutte, L. Crit. Rev. Food Technol. 1974, 4, 457. 26. Gruenwedel, D.W.; Patnaik, R.K. J. Agric. Food Chem. 1971, 19, 775. 27. Vernin, G.; Parkanyi,C. In Chemistry of Heterocyclic Compounds in Flavors and Aromas; Vernin, G., Ed.; Ellis Horwood: Chichester, 1982; p 151. 28. T. Shibamoto, T. J. Agric. Food Chem. 1977, 25, 206. 29. van der Ouwelend, G.A.M.; Peer, H.G. J. Agric. Food Chem. 1975, 23, 501. 30. Takken, H.J.; van der Linde, L.M.; de Valois, P.J.; Dort, H.M.; Boelens, M. In Phenolic, Sulfur and Nitrogen Compounds in Food Flavors; Charalambous, G.; Katz I., Eds; American Chemical Society: Washington, DC, 1976; p. 114. RECEIVED

January 26, 1989


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Flavor Formation in Meat-Related Maillard Systems Containing

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