The Maillard Reaction in Foods and Nutrition - ACS Publications

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11 The Maillard Reaction and Meat Flavor MILTON E . BAILEY

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University of Missouri, Department of Food Science and Nutrition, Columbia, M O 65211

Many d e s i r a b l e meat f l a v o r v o l a t i l e s are synthesized by heating water-soluble precursors such as amino acids and carbohydrates. These l a t t e r c o n s t i t u e n t s i n t e r a c t to form intermediates which are converted to meat f l a v o r compounds by o x i d a t i o n , decarboxylat i o n , condensation and c y c l i z a t i o n . O-, N-, and S­ - h e t e r o c y c l i c s i n c l u d i n g furans, furanones, pyrazines, thiophenes, t h i a z o l e s , t h i a z o l i n e s and cyclic p o l y s u l f i d e s c o n t r i b u t e s i g n i f i c a n t l y to the o v e r a l l d e s i r able aroma impression o f meat. The M a i l l a r d r e a c t i o n , i n c l u d i n g formation of Strecker aldehydes, hydrogen s u l f i d e and ammonia, is important i n the mechanism of formation o f these compounds.

Raw meat has l i t t l e d e s i r a b l e f l a v o r , but each type of meat has a c h a r a c t e r i s t i c f l a v o r due to the animal species and the temperature and type o f cooking. Both water-soluble and l i p i d - s o l u b l e f r a c t i o n s c o n t r i b u t e to meat f l a v o r and the water-soluble components i n c l u d e precursors which upon heating are converted to v o l a t i l e compounds described as "meaty. There are two approaches to studying meat f l a v o r ; one i s concerned with i d e n t i f i c a t i o n of n o n v o l a t i l e precursors of f l a v o r components and the other i n v o l v e s i s o l a t i o n and i d e n t i f i c a t i o n of v o l a t i l e f l a v o r molecules. Water-soluble meat f l a v o r precursors encompass a number of d i f f e r e n t organic c l a s s e s of compounds, i n c l u d i n g ; n u c l e i c a c i d s , n u c l e o t i d e s , nucleosides, peptides, amino a c i d s , f r e e sugars, sugar amines, glycogen, and amines. Bender and co-workers (1,_2) were perhaps the f i r s t to thoroughly examine contents of water-soluble e x t r a c t s of b o i l e d beef, which they b e l i e v e d contained precursors of meaty f l a v o r . These i n v e s t i g a t o r s reported the presence of many low-molecular-weight n i t r o g e n compounds. Kramlich and Pearson (3) reported that pressed f l u i d from raw beef produced meat f l a v o r upon heating. Hornstein and Crowe (4) found that f l a v o r precursors of meat were e x t r a c t a b l e with cold water and demonstrated that l y o p h i l i z e d 11

0097-6156/83/0215-0169$06.00/0 © 1983 American Chemical Society In The Maillard Reaction in Foods and Nutrition; Waller, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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d i f f u s a t e from c o l d water e x t r a c t s of raw beef and pork produced meaty odors upon h e a t i n g . Wood (5) demonstrated that heating a t 100°C r e s u l t e d i n i n c r e a s e i n inorganic phosphate and a decrease i n organic phosphate and reducing substances. This was accompanied by pronounced meat odor. Heating mixtures of amino a c i d s i n the absence of reducing sugars produced n e i t h e r browning nor meaty f l a vor. To o b t a i n meaty aromas s y n t h e t i c a l l y , a mixture of a l l compounds i d e n t i f i e d i n beef e x t r a c t was r e q u i r e d . Wood concluded, " i t i s shown beyond reasonable doubt that development of the brown c o l o r and meaty f l a v o r c h a r a c t e r i s t i c s of these e x t r a c t s i s a r e ^ s u i t of the M a i l l a r d r e a c t i o n . Macy, et a l . (6-10) studied the water-soluble precursors of beef, pork and lamb i n c l u d i n g the d i a l y z a b l e low-molecular-weight c o n s t i t u e n t s , amino a c i d s , carbohydrates, n u c l e o t i d e s , and nucleos i d e s . They studied the i n f l u e n c e of heating on these c o n s t i t u e n t s . The predominant amines i n the d i a l y z a b l e d i f f u s a t e were a l a n i n e , anserine-carnosine and t a u r i n e , and these decreased c o n s i d e r a b l y during h e a t i n g . Other amino a c i d s decreasing during heating i n eluded m e t h y l h i s t i d i n e , i s o l e u c i n e , l e u c i n e , methionine, c y s t i n e , s e r i n e , l y s i n e , g l y c i n e , and glutamic a c i d . Ribose-5-phosphate, r i b o s e , glucose and f r u c t o s e decreased e x t e n s i v e l y i n c o n c e n t r a t i o n during heating at 100°C f o r one h. Ribose and ribose-5-phosphate decreased most r e a d i l y under these c o n d i t i o n s . S i m i l a r r e s u l t s were published by Wasserman and S p i n e l l i (11), who examined the amino a c i d s and sugars i n beef d i f f u ~ sate a f t e r heating f o r one h at 125°C. They a l s o studied changes i n amino a c i d s and sugars during heating i n model systems. Wasser^ man (12) a l s o concluded that M a i l l a r d browning was important f o r the formation of d e s i r a b l e f l a v o r compounds during heating of meat but that the r e l a t i o n s h i p between compounds formed by t h i s r e a c t i o n and meat f l a v o r was unknown. These statements concerning the M a i l l a r d r e a c t i o n are a s s o c i ated with the formation of l-amino-l-deoxy-2-ketose a d d i t i o n products during heating of reducing sugars and amines. These a d d i ^ t i o n products can then degrade by s e v e r a l pathways to form f l a v o r compounds. The a d d i t i o n product f o r one pathway e n o l i z e s at C-2 - C-3 and e l i m i n a t e s the amine from C - l to form methyl d i c a r bonyl intermediates (13,14), which f u r t h e r r e a c t to give f i s s i o n products such as C-methyl reductones, keto aldehydes, d i c a r b o n y l compounds and reductones (15)· Important v o l a t i l e f l a v o r produci n g compounds i n c l u d e pyruvaldehyde, d i a c e t y l and hydroxyacetone. Another pathway begins with 1,2-enediol form of the a d d i t i o n product with subsequent deamination at C - l y i e l d i n g 3-deoxyhexosone (16) . Dehydration r e s u l t s i n the formation of important f l a v o r compounds such as 2-furaldehydes ( f u r f u r a l ) , a f o r m y l p y r r o l e , and s i m i l a r compounds. Another pathway p o s s i b l y beginning with the 2 3 e n e d i o l i n v o l v e s the e n o l i c form of a l-deoxy-2,3-dicarbonyl i n t e r mediate or an e q u i l i b r i u m e n o l i c form which leads to formation of f l a v o r compounds such as methylfuranones, i s o m a l t o l , and m a l t o l (17) . The a c i d i t y of the r e a c t i o n medium and the b a s i c i t y of the 11

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In The Maillard Reaction in Foods and Nutrition; Waller, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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amine c o n s t i t u t i n g the a d d i t i o n product (Amadori compound) are irn^portant v a r i a b l e s f o r the formation of these v a r i o u s f l a v o r v o l a ­ tiles. Two branches of the above r e a c t i o n pathways provide a c t i v e reagents f o r the degradation of α-amino a c i d s to aldehydes and ke­ tones of one l e s s carbon atom (Strecker degradation), which i s an^ other arm of the M a i l l a r d r e a c t i o n . Strecker aldehydes from these r e a c t i o n s are important f l a v o r compounds (18). Further evidence of the importance of the M a i l l a r d r e a c t i o n i n the formation of v o l a t i l e f l a v o r a n t s from meat p r e c u r s o r s i s gleaned by examining i n g r e d i e n t s i n r e a c t i o n mixtures patented as s y n t h e t i c meat c o n s t i t u e n t s . Ching (19) examined 128 patents o£ meat f l a v o r and found that 55 s p e c i f i e d use of both amino a c i d s and sugars. Amino a c i d s were the predominant components of these mixtures, and among the amino a c i d s , s u l f u r - c o n t a i n i n g c y s t e i n e and/or c y s t i n e were used i n 39 mixtures. Glutamic a c i d was a l s o used i n 39 such mixtures. In t h e i r recent comprehensive review of n a t u r a l and s y n t h e t i c meat f l a v o r s , MacLeod and Seyyedain^-Ardebili (20) l i s t e d 80 patents d e s c r i b i n g " r e a c t i o n products" procedures that produced meat^like f l a v o r s upon h e a t i n g . Approximately one-half of these precursor mixtures included amino acids, and reducing sugars. Most of the mixtures d e s c r i b e d i n patented procedures f o r s y n t h e t i c meat f l a ~ vor are modeled a f t e r i n g r e d i e n t s found i n the water-soluble d i a l y zable f r a c t i o n of f r e s h meat. These c o n s t i t u e n t s serve as reagents for M a i l l a r d r e a c t i o n s . Many i n v e s t i g a t i o n s have been made of the v o l a t i l e f l a v o r com­ pounds r e s u l t i n g from meat cookery or heating of mixtures simula­ t i n g meat f l a v o r precursors i n model systems. These have been r e ­ viewed by Herz and Chang (21), Dwivedi (22), Ching (19) and Mac­ Leod and S e y y e d a i n - A r d e b i l i (20). I t has been estimated that approximately 600 v o l a t i l e s have been i d e n t i f i e d from meat or sim­ u l a t e d meat precursors (23). These v o l a t i l e s are not only from pre-formed v o l a t i l e s i n meat but l a r g e l y from changes of precur­ sors by o x i d a t i o n , d e c a r b o x y l a t i o n , fragmentation, recombination, rearrangement, condensation, and c y c l i z a t i o n . Meat aroma i s not the r e s u l t of one chemical c o n s t i t u e n t but the sum of the sensory e f f e c t s of many of these v o l a t i l e s . Over 90% of the volume of v o l a t i l e c o n s t i t u e n t s from f r e s h l y roasted beef i s from l i p i d , but approximately 40 percent of the v o l a t i l e s from the aqueous f r a c t i o n are thought to be h e t e r o c y c l i c compounds, many r e s u l t i n g from M a i l l a r d r e a c t i o n products or t h e i r i n t e r a c ­ t i o n s with other i n g r e d i e n t s . H e t e r o c y c l i c compounds as meat v o l a t i l e s have been reviewed r e c e n t l y by O h l o f f and Flament (23) and by Shibamoto (24). More comprehensive coverage of these c o n s t i t u e n t s as food f l a v o r a n t s i s c u r r e n t l y being published (Vernin, 25). H e t e r o c y c l i c compounds c o n t r i b u t e s i g n i f i c a n t l y to the over­ a l l aroma impression of meat. They i n c l u d e 0-, N-, and S-heteroc y c l i c s t r u c t u r e s . Meat f l a v o r h e t e r o c y c l i c s i n c l u d e furans,

In The Maillard Reaction in Foods and Nutrition; Waller, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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furanones, p y r a z i n e s , thiophenes, t h i a z o l e s , t h i a z o l i n e s , oxazol i n e s and c y c l i c p o l y s u l f i d e s . These compounds can a l l be formed by M a i l l a r d type i n t e r a c t i o n s . Furans and Furanones H e t e r o c y c l i c s , p a r t i c u l a r l y the S-containing ones, are extremely important c o n t r i b u t o r s to "meaty f l a v o r s . Ching (19) i d e n t i f i e d 11 furans and 7 furanones from r e a c t i o n mixtures cont a i n i n g p r e c u r s o r s r e s p o n s i b l e f o r beef f l a v o r . F u r f u r a l d e r i v a t i v e s were obtained by h e a t i n g s e v e r a l r e a c t i o n mixtures i n c l u d i n g low-molecular-weight w a t e r - e x t r a c t a b l e components from beef and simple amino acid-sugar mixtures. 4-Hydroxy-2,5-dimethyl-3(2H)furanone, 4-hydroxy-5-methyl-3-(2H)-furanone, and four s i m i l a r compounds were i d e n t i f i e d by Ching (19) from beef by d i a l y z i n g water-soluble m a t e r i a l s and concentrating the d i f f u s i b l e s o l u t e s . The diffusâtes were then heated with or without sugars a t 130°C f o r 2 h. Furanones can be formed by Amadori compound pathways (17, 18)· Some of these compounds were o r i g i n a l l y i s o l a t e d and i d e n t i f i e d from beef by Tonsbeek, e t a l . (26), who stated they might be formed through i n t e r a c t i o n of ribose-5-phosphate and t a u r i n e during h e a t i n g . Hexoses form 5-methylfufurals and 4-hydroxy-2,5dimethyl-3(2H) furanone, while pentoses produce f u r f u r a l and 4hydroxy-5-methyl-3(2H) furanone. Hicks and Feather (27) and H i c k s , et a l . (28) synthesized the l a t t e r compound by h e a t i n g amines with x y l o s e , r i b o s e , ribose-5-phosphate, or gluconic a c i d . Although exact mechanisms have not been described f o r the f o r ~ mation of other f u r a n o i d compounds through amine-carbohydrate i n t e r a c t i o n s , probably many of the 32 furans described by O h l o f f and Flament (23) from meat aroma mixtures a r e from t h i s source. Shibamoto (24) described many of the same components from mixtures producing meat odors. Herz and Chang (21) examined s e v e r a l furan compounds which had a wide v a r i e t y of aromas, but none of them were meaty, Furans that do not c o n t a i n s u l f u r are u s u a l l y f r u i t y , nutty, and c a r a m e l - l i k e i n odor. The furanones described above have burnt pineapple and roasted c h i c o r y odors, but these c o n t r i b u t e to o v e r a l l f l a v o r imp r e s s i o n of meat and important Ν and S meat f l a v o r compounds might be formed from them during cooking,

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Thiophenes MacLeod and S e y y e d a i n - A r d e b i l i (20) l i s t e d 36 thiophene d e r i ­ v a t i v e s as having been found during v a r i o u s i n v e s t i g a t i o n s of meat or meat c o n s t i t u e n t s . Ching (19) found 18 and Shibamoto (24) l i s t e d 29. Thiophenes a r e extremely important i n f l a v o r and are r e s p o n s i ­ b l e f o r the m i l d s u l f u r o u s odor of cooked meat. Numerous other thiophenes have been i d e n t i f i e d during heating of meat or meat con­ s t i t u e n t s . The s u l f u r i n thiophene may be derived from amino acids ( c y s t e i n e , c y s t i n e , methionine) or from v i t a m i n Β ·

In The Maillard Reaction in Foods and Nutrition; Waller, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Probably the most important r e a c t a n t i n the formation of v o l a ­ t i l e meat f l a v o r compounds i s hydrogen s u l f i d e . I t can be formed by s e v e r a l pathways during meat cookery, but one mechanism i s Strecker degradation of c y s t e i n e i n the presence of a diketone as e s t a b l i s h e d by Kobayashi and Fujimaki (29). The c y s t e i n e conden­ ses with the diketone and the product i n turn decarboxylates to amino carbonyl compounds that can be degraded to hydrogen s u l f i d e , ammonia and acetaldehyde. These become very r e a c t i v e v o l a t i l e s f o r the formation of many f l a v o r compounds i n meat and other foods. Shibamoto (30) formed α-thiophenecarboxaldehyde by r e a c t i n g f u r f u r a l and hydrogen d i s u l f i d e . This means that there probably was exchange between the S and 0 i n the furan r i n g during h e a t i n g . van den Ouweland and Peer (31) made a s i g n i f i c a n t c o n t r i b u ­ t i o n to the e l u c i d a t i o n of meat f l a v o r when they demonstrated that 4-hydroxy-5-methyl-3(2H)-furanone formed a number of "meat-like" mercapto-substituted furan and thiophene d e r i v a t i v e s when heated i n the presence of hydrogen s u l f i d e . These authors p o s t u l a t e d that the i n i t i a l stage i n forming the thiophene d e r i v a t i v e s i n v o l v e s a p a r t i a l s u b s t i t u t i o n of the r i n g oxygen by s u l f u r to give the thio analog. Compounds having a meaty odor included 3-mer cap t o u ­ rne thy 1-2,3-dihydr ο thiophene, 3-mercapto-4-hydroxy-2-methyl-2,3dihydrοthiophene, 4-mercapto-2-methyltetrahydrothiophene, and 3mercapto-2-methyltetrahydrothiophene. The proposed (31,32) r e a c ­ t i o n s f o r the formation of some of these "meaty" compounds i s sum­ marized i n F i g u r e 1. Although these authors assumed that the d i hydrofuranone was derived from ribose-5-phosphate v i a a dephosphorylation-dehydration r e a c t i o n (31), there i s evidence which i n ­ d i c a t e s that the furanone can be formed by a r e a c t i o n of aldose sugars with amines to produce Amadori products which subsequently dehydrate with amine e l i m i n a t i o n (17,28,33). The amine could be taurine,which i s i n ample supply i n muscle (7) and was mentioned by Tonsbeek, et^ a l . (26) i n t h e i r o r i g i n a l d i s c o v e r y of these com­ pounds. None of these compounds have been i d e n t i f i e d from meat e x t r a c t s , but Ching (19) i d e n t i f i e d 4-mercapto-2-methyl-4,5-dihydrothiophene from s y n t h e t i c beef d i f f u s a t e . The mercaptothiophenes are undoubtedly formed at low concentrations i n meat. Evers, ejt a l . (34) i d e n t i f i e d s e v e r a l S - s u b s t i t u t e d furans having meaty aroma i n c l u d i n g 3-mercapto-2-methylfuran and 3-mercapto-2,5-dimethylfuran from M a i l l a r d r e a c t i o n mixtures. These compounds were r e a d i l y o x i d i z e d to s u l f i d e s , some of which r e ­ tained meaty odors. A l l furans having the s u l f u r atom bound to the β-carbon had meaty aromas, whereas those with s u l f u r bound to the α-carbon had hydrogen s u l f i d e - l i k e odors. M a i l l a r d r e a c t i o n products formed by i n t e r a c t i o n of reducing sugar and amino a c i d s such as ot-dicarbonyl compounds, aldehydes, hydrogen s u l f i d e , and ammonia can r e a c t f u r t h e r to form d e r i v a ­ t i v e s that have been i d e n t i f i e d from meat or i t s components during h e a t i n g . Important reviews of s u l f u r compounds that might be pro­ duced by these r e a c t i o n s have been published by Schutte (35) and

In The Maillard Reaction in Foods and Nutrition; Waller, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Figure 1. Reaction of 4-hydroxy-5-methy1-3(2H)-juranone with hydrogen sulfide to form mercaptofurans and mercaptothiophenes having beef-like odors (31,).

In The Maillard Reaction in Foods and Nutrition; Waller, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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by Takken, ej. a l . 136)_. Some of the important s u l f u r - c o n t a i n i n g compounds i n c l u d e t h i a z o l e s , t h i a z o l i n e s , a,nd poly s u l f i d e hetero­ cyclics.

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T h i a z o l e s and

Thiazolines

Takken, et^ a l . (36) found that these compounds could be formed by combining butane-2,3-dione ( d i a c e t y l ) , pentane-2,3-dione, or pyruvaldehyde with acetaldehyde, hydrogen s u l f i d e , and ammonia. Some of the r e a c t i o n pathways suggested f o r t h i a z o l e and t h i a z o l i n e formation f o r these r e a c t i o n i n g r e d i e n t s are o u t l i n e d i n F i g ~ ure 2. T h i a z o l e s have a l s o been i d e n t i f i e d i n v o l a t i l e s r e s u l t i n g from heating c y s t e i n e and c y s t i n e with glucose and pyruvaldehyde at 160°C (37) and by Mulders (38) who heated a s i m i l a r system at 125°C and pH 5.6 f o r 24 h. P o s s i b l e pathways f o r these r e a c t i o n s were discussed by Schutte (35). Corresponding t h i a z o l i n e s and t h i a z o l e s f r e q u e n t l y c o e x i s t due to the dehydration of t h i a z o l i n e s during heating (39). MacLeod and S e y y e d a i n - A r d e b i l i (20) l i s t e d 12 t h i a z o l e s and t h i a z o l i n e s that had been i d e n t i f i e d i n beef samples by d i f f e r e n t investigators. Some patented compounds l i s t e d as having meaty f l a v o r are 4-methyl-5-(2-hydroxyethyl)thiazole, 4-ethyl-5-(3-acet ox y p r o p y l ) t h i a z o l e , 2 - a c e t y l - 2 - t h i a z o l i n e and 2 - a c e t y l - 5 - p r o p y l 2 - t h i a z o l i n e . Ching (19) i d e n t i f i e d 12 t h i a z o l e d e r i v a t i v e s i n her s t u d i e s of the v o l a t i l e produced by heating components of meat. Important t h i a z o l e s i d e n t i f i e d i n these s t u d i e s i n c l u d e d 2 and 4 - a c e t y l d e r i v a t i v e s . I t was p o s t u l a t e d by Tonsbeek, et_ a l . (40) that 2 - a c e t y l - 2 - t h i a z o l i n e i s formed by Strecker degradation o f c y s t e i n e with methylglyoxal or s i m i l a r compounds followed by o x i d a t i v e c y c l i z a t i o n as shown i n Figure 3. This compound was i s o l a t e d from 225 kg of cooked beef and had a strong odor of f r e s h l y baked bread. Other t h i a z o l e s have been i s o l a t e d from meat or meat c o n s t i t u e n t s which have undergone M a i l l a r d - t y p e r e a c t i o n s (Wilson, et_ a l . , 41) . Polysulfide Heterocyclics Wilson, et a l . (41) a l s o confirmed the presence of p o l y s u l f u r h e t e r o c y c l i c s i n meat i n c l u d i n g t h i a l d i n e (5,6-dihydro-2,4,6-trim e t h y l - l , 3 , 5 - d i t h i a z i n e ) and t r i t h i o a c e t o n e (2,2,4,4,6,6-hexamet h y l - l , 3 , 5 - t r i t h i a n e ) . Wilson (42) l a t e r discussed the p o s s i b l e routes of formation of some of these compounds from c y s t e i n e . T h i a l d i n e was found by Brinkman, ejt a l . (43) i n the headspace v o l a t i l e s of beef b r o t h . These workers a l s o i d e n t i f i e d 3,5-dimethyl-1,2,4 - t r i t h i o l a n e from the same source. Both c i s and trans isomers of t h i s compound had p r e v i o u s l y been i d e n t i f i e d as f l a v o r components of b o i l e d beef by Chang, et^ a l . (44) and Herζ (45). Chicken has a l s o been found to be a source of p o l y s u l f i d e h e t e r o c y c l i c s . Pippen and Mecchi (46) t h e o r i z e d that 2 , 4 , 6 - t r i -

In The Maillard Reaction in Foods and Nutrition; Waller, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

MAILLARD

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REACTIONS

2 , 4 , 5 - TRIMETHYLTHIAZOLINE

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Reaction pathways for formation of thiazoles and thiazolines by heating Maillard degradation products of meat precursors (36).

In The Maillard Reaction in Foods and Nutrition; Waller, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Figure 3. Formation of 2-acetyl-2-thiazoline by Strecker degradation of cysteine. (Reproduced from Ref. 40. Copyright 1971, American Chemical Society.)

In The Maillard Reaction in Foods and Nutrition; Waller, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

MAILLARD

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REACTIONS

m e t h y l - l , 3 , 5 - t r i t h i a n e might be formed i n chicken by i n d i r e c t com­ b i n a t i o n s of hydrogen s u l f i d e and acetaldehyde i n f a t . The same h e t e r o c y c l i c was l i s t e d as a f l a v o r v o l a t i l e i n chicken by Wilson and Katz (47) i n t h e i r review of chicken f l a v o r . These same wor­ kers (48) a l s o i d e n t i f i e d t h i s compound i n l e a n ground beef. The formation of 5,6-dihydro-2,4,6-trimethyl-l,3,5-dithiazine, 2,4,6-trimethyl-1,3,5-trithiane, and 3 , 5 - d i m e t h y l - l , 2 , 4 - t r i t h i o l a n e by heating of acetaldehyde, hydrogen s u l f i d e , and ammonia was out­ l i n e d by Takken and coworkers (36) and i s summarized i n Figure 4. Under o x i d a t i v e c o n d i t i o n s , d i a l k y l t r i t h i o l a n e s are formed; a t low pH there i s conversion to t r i a l k y l t r i t h i a n e s ; a t elevated tempera­ ture i s o m e r i z a t i o n i n t o t r i s u l f i d e s occurs, which compounds d i s p r o ­ p o r t i o n a t e i n t o d i and t e t r a s u l f i d e s ; and i n the presence of ammo­ n i a , d i t h i a z i n e s are formed. These compounds and the c o n d i t i o n s f o r t h e i r formation are of extreme importance f o r the production of d e s i r a b l e meat f l a v o r s . 1-(Me t h y l t h i o ) e t h a n e t h i o l This important f l a v o r compound was i d e n t i f i e d i n the headspace v o l a t i l e s of beef broth by Brinkman, e_t a l . (43) and although i t has the odor of f r e s h onions, i t i s b e l i e v e d to c o n t r i b u t e to the f l a v o r of meat. This compound can be formed q u i t e e a s i l y from Strecker degradation products. Schutte and Koenders (49) conclud­ ed that the most probable precursors f o r i t s formation were ethan a l , methanethiol and hydrogen s u l f i d e . As shown i n Figure 5, these immediate precursors are generated from a l a n i n e , methionine and c y s t e i n e i n the presence of a Strecker degradation d i c a r b o n y l compound such as pyruvaldehyde. These same precursors could a l s o i n t e r a c t under s i m i l a r c o n d i t i o n s to give dimethyl d i s u l f i d e and 3,5-dimethyl-l,2,4-trithiolane previously discussed. Pyrazines The formation of pyrazines i n foods has been reviewed exten­ s i v e l y by Mega and S i z e r (50). Temperature and pH are very impor­ tant f a c t o r s i n the formation of s p e c i f i c p y r a z i n e s . Forty-two pyrazines have been i d e n t i f i e d i n meat from v a r i o u s sources by these authors. MacLeod and S e y y e d a i n - A r d e b i l i (20) l i s t e d 49 pyra­ zines found i n beef by v a r i o u s i n v e s t i g a t o r s . Ching (19) i d e n t i ­ f i e d 28 pyrazines i n her s t u d i e s of sugar-amine r e a c t i o n s simula­ t i n g beef f l a v o r . Several mechanisms have been reported f o r pyrazine formation by M a i l l a r d r e a c t i o n s (21,52,53). The carbon skeletons of pyra­ zines come from α-dicarbonyl (Strecker) compounds which can r e a c t with ammonia to produce α-amino ketones as described by Flament, et a l . (54) which condense by dehydration and o x i d i z e to pyrazines (Figure 6 ) , or the d i c a r b o n y l compounds can i n i t i a t e Strecker de­ gradation of amino a c i d s to form α-amino ketones which are hydrolyzed to carbonyl amines, condensed and are o x i d i z e d to s u b s t i t u t e d

In The Maillard Reaction in Foods and Nutrition; Waller, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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