Methionine Strecker degradation

This important flavor compound was identified in the head-space volatiles of beef broth by Brinkman, et al. (43) and although it has the odor of fresh onions, it is believed to contribute to the flavor of meat. This compound can be formed quite easily from Strecker degradation products. Schutte and Koenders (49) concluded that the most probable precursors for its formation were etha-nal, methanethiol and hydrogen sulfide. As shown in Figure 5, these immediate precursors are generated from alanine, methionine and cysteine in the presence of a Strecker degradation dicarbonyl compound such as pyruvaldehyde. These same precursors could also interact under similar conditions to give dimethyl disulfide and 3,5-dimethyl-l,2,4-trithiolane previously discussed. 

In roasted coffee similar reactions seem to be involved forming methylmercaptan by the Strecker degradation of free methionine and forming H S from peptide cysteine. Figure 8 presents additional flavor contributing constituents of roasted coffee. 3-Thiolanone 6 and 2-methyl-3-thiolanone 1 were identified by Stoll et al. (19) and patented as coffee flavors. The two thiolanones 6, T are formed as major constituents in erythrose and xylose/ cysteine model systems, respectively. 

A heterocyclic sulfur-containing compound, 2-methyl-thiophene, was identified in boiled crayfish tail meat and pasteurized crabmeat. Thiazole and 3-methylthiopropanal were identified in the crayfish hepatopancreas. Heterocyclic sulfur-containing compounds play important roles in generating meaty aromas in a variety of meat products and are considered important volatile aroma components of marine crustaceans (12— 14). The 2-methylthiophene could be an important flavor cemponent in boiled crayfish tail meat. Both thiazole find 3-methylthiopropanal were important contributors to the desirable meaty aroma associated with crayfish hepatopancreas. The 3-methyl-thiopropanal, identified in boiled crayfish hepatopancreas, is derived from Strecker degradation of methionine (15), and has been considered to be an important cemponent in basic meat flavor (16). Pyridine was detected in the headspace of the hepatopancreas from freshly boiled crayfish. Pyridine and 2-ethylpyridine have been previously reported as components in the atmospheric distillate from a sample of crayfish hepatopancreas frozen for three months (2). 

The latter authors also found it when serine and/or threonine reacted with sucrose. It was previously identified in other model reactions by treating furfural with hydrogen sulfide and ammonia (Shibamoto, 1977) or by heating rhamnose with ammonia (Shibamoto and Bernhart, 1978). Ho and Hartman (1982) proposed a plausible mechanism for the formation 2,4,5-trimethyloxazole from the reaction of dl-alanine or L-cysteine and 2,3-butanedione. Ho el al. (1982) also studied the formation of oxazoles and oxazolines in the Strecker degradation of dl-methionine and L-cysteine with 2,3-butanedione. In their proposed mechanisms, the authors suggested the formation of intermediate 3-oxazolines. 

Methional results from a Strecker degradation of methionine. It is at the origin of formation of methanethiol (Q.2), dimethyl disulfide (Q.14) and acrolein (C.16). 

Ballance P.E. (1961) Production of volatile compounds related to the flavour of foods from the Strecker degradation of DL-methionine. J. Sci. Food Agric. 12, 532-6. 

The Strecker degradation involves the oxidative deamination and decarboxylation of a a-amino acid in the presence of a dicarbonyl compound. The products formed from this reaction are an aldehyde containing one less carbon atom than the original amino acid and an a-aminoketone (Table 9.2). The Strecker degradation of methionine and cystein is a source of sulfur-containing intermediates (e.g hydrogen sulfide and 2-methylthiopropanal = methional) [48]. 

Strecker Degradation, fi-Eliminatton and TVansamination of Cysteine/ Methionine during Heating with [l(or 6)- C]-D-Glucose... 

During Strecker degradation of [l-i CJ-D-glucose with primary a-amino acids, pyrroles and pyridinols are formed as major products (6). 4-Aminobutyric acid and peptide bound lysine are transformed into [i3CHO]-2-formyl-5-hydroxymethylpyrroles (9). Amino acids like Val, He, Leu, Phe and Met are transformed into 2-[i3cjjO]-pyrrole lactones (70). Equimolar amounts of cysteine (methionine) and [l(or 6)- C]-D-glucose were heated for 1,5 h at 160°C in aqueous solution at pH 5. The volatiles were extracted with pentane/ether and analyzed as described (7). In Table I selected (unlabeled) Strecker degradation products from cysteine and methionine are summarized. Pyruvat (1), 2- and 3-mercaptopropionic acids (2, 3) from cysteine as well as 2-oxo-5-thiahexanoic acid (4) and 5-thiahexanoic acid (5) from methionine. 

Table I. Strecker Degradation Products from [l- C]-D-Glucose with Cysteine or Methionine...

Hydrogen sulhde and 2-mercaptoacetaldehyde are obtained during the course of the Strecker degradation of cysteine (Fig. 5.17). In a similar way, methionine gives rise to methional, which releases methanethiol by P-ehmination (Fig. 5.18). Dimethylsulhde is obtained by methylation during heating of methionine in the presence of pectin ... 

Strecker aldehydes are a frequent source of off-flavors in fermented products. Development of off-flavors in oxidized white wines typically marks the end of shelf life. Methional (3-methylthiopropionaldehyde) was identified as producing a cooked vegetables off-flavor character in a young white wine that had undergone spontaneous oxidation (101). Methional levels increased in wines spiked with methionol or methionine, suggesting its formation via direct peroxidation or Strecker degradation of methionine. Methional was recently demon-... 

Also MTP increases likely for a degradation of methionine via Strecker mechanism (Ribereau-Gayon et al., 1998). Furthermore, EtSH and DES show opposite changing in the time resulting for DEDS a similar increasing tendency as for DES thus a shift towards the oxidized form... 

The formation of furans, thiophenes, furanones, thiophenones etc. was investigated in a series of [l(or 6)- C]-glucose and [l- C]-arabinose/ cysteine and methionine model experiments. The labeled compounds were analyzed by capillary GC/MS and NMR-spectroscopy. From their structures the degradation pathways via different reactive intermediates (e.g. 3-deoxyaldoketose, 1-deoxydiketose) and fragmentations were evaluated. Besides the transformations to flavor compounds via identical labeled precursors, major differences in the flavor compounds result from specific Strecker reaction sequences. Major unlabeled compounds e.g. 3-mercaptopropionic acid from cysteine and 4-methylthiobutyric acid from methionine demonstrate transamination/reduction, and the formation of pyruvate and 2-mercaptopropionic acid from [l-i C]-glucose/cysteine indicates B-elimination. 

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