Methionine, sulfur-containing flavor

R. Tressl, E. Kersten, C. Nittka, and D. Rewicki, Formation of sulfur-containing flavor compounds from [13C]-labeled sugars, cysteine, and methionine, in Sulfur Compounds in Foods, C. J. Mussinan and M. E. Keelan (eds), American Chemical Society, Washington, DC, 1994, 224-235. 

Formation of Sulfur-Containing Flavor Compounds from [ C]-Labeled Sugars, Cysteine, and Methionine... 

These four compounds were also selected because they were commercially available in pure form and reasonably soluble in unbuffered solutions at room temperature. In addition, the radiation chemistry of glycine, glycylglycine, and methionine has been studied at room temperature (4, 5, 6, 12, 36, 49, 50, 51). Also, sulfur-containing amino acids have been suggested by chemical-irradiation flavor correlation studies (4, 21, 38, 44f 49, 50, 53) as being related to irradiation flavor. 

That summary is based on the reports of a well-conceived and carefully executed research program carried out by Rohan. Mohr et al. (7) extended these studies and was able to draw additional conclusions. First, without exception, free amino acids are much more sensitive to destruction in this system than the peptide-bound amino acids. Second, differences in the stability of amino acids under these conditions are not great —from 25% loss for isoleucine to 68.5% for lysine, over a relatively short period of time. In this system the reducing sugars must be the limiting factor, since the glucose and fructose are completely destroyed or removed. Third, neither cystine nor cysteine are reported to be present, and the only other sulfur-containing amino acid, methionine, is present at a much lower concentration than any other amino acid. Clearly, as we shall see later, cocoa would probably have a considerably different flavor if cysteine or cystine were present in the fermented beans. 

Two patents (42-43) claim the contribution to meatlike flavors made by thiamine when it is present in the standard pyrolytic mixture. Arnold et al. (44) have reported on the volatile flavor compounds produced by the thermal degradation of thiamine alone. It is generally agreed that the presence of methionine, the other sulfur-containing amino acid in the flavor-developing mixture, produces negative and/or undesirable results. However, one patent (45) specifies methionine in a standard Maillard procedure, and no cysteine. 

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). 

Volatile sulfur compounds are found in most cheeses and can be important flavor constituents. The origin of sulfur-containing compounds is generally thought to be the sulfur-containing amino acids methionine and cysteine (Law, 1987). As Cys is rare in the caseins (occurring at low levels only in Os2- and K-caseins, which are not extensively hydrolyzed in cheese), the origin of sulfur compounds must be primarily Met. Sulfur compounds formed from Met include H2S, dimethylsulfide, and methanethiol. The importance of methanethiol and related compounds in cheese aroma is discussed by Law (1987). 

Methionine may be degraded to yield numerous sulfur-containing aroma compounds, the most important of which is methional. Methional has a very low sensory threshold, and thus is found to contribute to the flavor of many foods. Methionine may also be degraded by microbial lyases to yield methanethiol. Methanethiol may make a contribution to flavor itself or through secondary reactions that yield various sulfides, disulfides, tetrasulfides, and thioesters (Figure 5.15). 

The protein (meal) of sesame has become increasingly important for human food due to the following unique properties (1) the presence of a high level of the sulfur-containing amino acids, methionine and cystine (2) its freedom from the trypsin inhibiting factor, an objectionable characteristic of soy protein and (3) its pleasant flavor. The meal is very palatable to humans. In India, it is used extensively in human foods and in India and Java, it is sometimes fermented for food. 

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