Strecker degradation of cysteine

Probably the most important reactant in the formation of volatile meat flavor compounds is hydrogen sulfide. It can be formed by several pathways during meat cookery, but one mechanism is Strecker degradation of cysteine in the presence of a diketone as established by Kobayashi and Fujimaki (29). The cysteine condenses with the diketone and the product in turn decarboxylates to amino carbonyl compounds that can be degraded to hydrogen sulfide, ammonia and acetaldehyde. These become very reactive volatiles for the formation of many flavor compounds in meat and other foods. 

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

Heterocyclic compounds are dominant among the aroma compounds produced in the Maillard reaction, and sulfur-containing heterocyclics have been shown to be particularly important in meat-like flavors. In a recent review, MacLeod (6) listed 78 compounds which have been reported in the literature as possessing meaty aromas seven are aliphatic sulfur compounds, the other 71 are heterocyclic of which 65 contain sulfur. The Strecker degradation of cysteine by dicarbonyls is an extremely important route for the formation of many heterocyclic sulfur compounds hydrogen sulfide and mercaptoacetaldehyde are formed by the decarboxylation and deamination of cysteine and provide reactive intermediates for interaction with other Maillard products. 

Figure A shows a proposed mechanism for the formation of 2,A,5-trlmethyloxazole and A,5-dlmethyloxazole from the Strecker degradation of cysteine with 2,3-butanedlone (Al).

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 ... 

Thiophenes are formed by different reactions. The general reaction is that of furan derivatives with hydrogen sulfide (Figure 8.72). This mechanism is assumed, for example, in the formation of some thiophene derivatives from 4-hydroxy-2//-furan-3-ones. Alkylth-iophenes substituted in positions C-2 and C-3 may arise in reactions of 2-mercaptoethanal with unsaturated aldehydes, such as acrolein or but-2-enal (Figure 8.73). 2-Mercaptoethanal is a product of Strecker degradation of cysteine. 

Thiazole itself is smelling like pyridine, but substituted thiazoles, such as 2-acetylthiazole and 2,4-dimethyl-5-vinylthiazole, generally have a desirable odour, frequently described as nutty, green, roasted and vegetable-like. For example, the Strecker degradation of cysteine by methylglyoxal yields 2-acetylthiazole in thermally processed foods (Figure 8.83). A possible intermediate of this reaction is 2-acetyl-2-thiazoline with an intense aroma of fresh bread crust, which is similar to the odour of 2-acetyl-... 

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. 

Thiols are another key group of odor compounds in coffee, these are formed due to Strecker degradation of amino acids like mediionine to give mediional and die reaction of H2S (formed by the degradation of Cysteine) with fiiraldehydes, namely the formation of 2-furylthiol (62). 

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. 

Ho C.T. and Hartman G.J. (1982) Formation of oxazolines and oxazoles in Strecker degradation of DL-alanine and L-cysteine with 2,3-butanedione. J. Agric. Food Chem. 30, 793-4. 

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]. 

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. 

Decarboxylation of amino acids can also proceed as a non-enzymatic reaction. Analogously to the enzyme catalysed decarboxylations, amines are formed as byproducts of the Strecker degradation of amino acids and by thermal decarboxylation of amino acids, especially of sulfur amino acids, hydroxyamino acids and aromatic amino acids. For example, thermal decarboxylation and subsequent reactions of cysteine and cystine produce ammonia... 

Hydrogen sulphide is a key intermediate in the formation of many heterocyclic sulphur compounds. It is produced from cysteine by hydrolysis or by Strecker degradation ammonia, acetaldehyde and mercaptoacetaldehyde are also formed (Scheme 12.4). All of these are reactive compounds, providing an important source of reactants for a wide range of flavour compounds. Scheme 12.6 summarises the reactions between hydrogen sulphide and other simple intermediates formed in other parts of the Maillard reaction. 

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. 

Probably the most important reactant in the formation of volatile meat flavor compounds is hydrogen sulfide. It can be formed as a Strecker degradation product of cysteine in the presence of a diketone (37). 

Cysteine can undergo the Strecker degradation, transamination, and -elimination, as shown by Tressl et al.247 using [1- or 6-13C]glucose (equimolar aqueous solution, 160 °C, 1.5 h). 2-Furylmethanethiol (T 0.005 ppb), very important in providing the aroma of roasted coffee and roasted meat, is formed as shown in Scheme 5.14 from [l-13C]glucose via the 3-deoxy-l,2-dicarbonyl, which loses... 

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

ThQ formation pathways to furfurylmercaptans and sulfides (Scheme 3) include the incorporation of sulfur during Strecker degradation as an important step. The mass spectrometric fragmentation of [2- C]-21j formed in [l-i3c] ) giucose/L-cysteine model experiments, clearly indicates the intact carbon chain by M(0%) M+l(100%) M+2(0%) and an unlabeled acetyl group by mi/z=43 and 112 (COCHs and M-COCH3). 

Fig. 5.17. Cysteine decomposition by a Strecker degradation mechanism formation of H2S (I) or 2-mer-captoethanal (II)...

Dawes and Edwards (1966) isolated it from the volatiles obtained by heating a mixture of D-fructose and glycine or L-3-phenylalanine. Wang et al. (1969) presented a model reaction in which the product of condensation of pyruvaldehyde with any amino acid degraded by a Strecker reaction can form an aminoketone which by subsequent steps of self-condensation and oxidation formed 2,5-dimethylpyrazine (Wang et al., 1969 Manley et al., 1974). It was found in a heated cysteine/glucose model system (Sheldon et al., 1986). 

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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