Metabolism volatile sulfur compounds

Free amino acids are further catabolized into several volatile flavor compounds. However, the pathways involved are not fully known. A detailed summary of the various studies on the role of the catabolism of amino acids in cheese flavor development was published by Curtin and McSweeney (2004). Two major pathways have been suggested (1) aminotransferase or lyase activity and (2) deamination or decarboxylation. Aminotransferase activity results in the formation of a-ketoacids and glutamic acid. The a-ketoacids are further degraded to flavor compounds such as hydroxy acids, aldehydes, and carboxylic acids. a-Ketoacids from methionine, branched-chain amino acids (leucine, isoleucine, and valine), or aromatic amino acids (phenylalanine, tyrosine, and tryptophan) serve as the precursors to volatile flavor compounds (Yvon and Rijnen, 2001). Volatile sulfur compounds are primarily formed from methionine. Methanethiol, which at low concentrations, contributes to the characteristic flavor of Cheddar cheese, is formed from the catabolism of methionine (Curtin and McSweeney, 2004 Weimer et al., 1999). Furthermore, bacterial lyases also metabolize methionine to a-ketobutyrate, methanethiol, and ammonia (Tanaka et al., 1985). On catabolism by aminotransferase, aromatic amino acids yield volatile flavor compounds such as benzalde-hyde, phenylacetate, phenylethanol, phenyllactate, etc. Deamination reactions also result in a-ketoacids and ammonia, which add to the flavor of... 

Table 8D.3 Volatile sulfur compounds produced in wine by yeast metabolism ...

Landaud, S., Helinck, S., Bonnarme, P. (2008). Formation of volatile sulfur compounds and metabolism of methionine and other sulfur compounds in fermented food. Applied Microbiology Biotechnology, 77(6), 1191-1205. 

Volatile sulfur compounds formed in wine and beer production originate from methionine and are by-products of the microorganism s metabolism. The compounds formed are methional (I), methionol (II) and acetic acid-3-(methylthio)-propyl ester (III, cf. Reaction 5.33). 

The choice of the appropriate material is decisive for resistance against microbially influenced corrosion. This means that before the choice of material can be made, what kind of impacts is has to resist needs to be considered. Microbial influencing factors must also be considered. Accordingly, in the presence of volatile sulfur compounds, e.g., in sewage pipelines, it is recommended not to use materials like unprotected concrete which may be destroyed by the end product of the microbial degradation process (in this case, sulfuric acid formed by Thiobacilli). Another example would be the choice of a stainless steel or of an alloy that cannot be attacked under the conditions of a biofllm and the complex metabolic processes occurring underneath it. If, for instance, a material has to be chosen for static reasons, this material has to be protected by a coating or a liner made of an inert material. All these examples are based on the consideration that all attack factors have been identified by a complete inventory. 

Methionine and cysteine are metabolized into volatile sulfur compounds. The O. oeni species is particularly active in converting cysteine into hydrogen sulfide and 2-sulfanyl ethanol, and methionine into dimethyl disulfide, 3-(metha-sulfanyl) propanol, 3-(methasulfanyl) propan-l-ol and 3-(methasulfanyl) propionic acid. The most interesting of these compounds from a sensory point of view is 3-(methasnlfanyl) propionic acid, with its earthy, red-berry fruit nuances (Pripis-Nicolau, 2002). 

Martinez-Cuesta, M.D., Pelaez, C., and Requena, T. (2013) Methionine metabolism major pathways and enzymes involved and strategies for control and diversification of volatile sulfur compounds in cheese. Crit Rev Food Sci Nutr 53,366-385. 

The exclusion of selenium from the proteins of accumulator plants is thought to be the basis for their selenium tolerance. Their selenium metabolism is based mainly on water-soluble nonprotein forms such as selenium methylselenomethionine (Jacobs, 1989). The garlic odor characteristic of selenium-accumulator plants reflects the volatile organic compounds dimethylselenide and dime-thyldiselenide. Plants can suffer selenium toxicity as a result of selenium competition with essential metabolites for biochemical sites, replacement of essential ions by selenium, mainly major cations, selenate occupation of the sites of essential groups such as phosphate and nitrate, or selenium substimtion in essential sulfur compounds. 

Methionine metabolism Sulfur compounds, responsible for aroma in wine and typically related to the grape variety, are released by yeast during the AF. In addition, the metabolism of the sulfur-containing amino acid methionine has an impact on wine aroma. Lact. brevis, Lact. plantarum, and O. oeni strains, using a pathway similar to dairy LAB, catabolize methionine producing light volatile sulfur molecules such as methanethiol and dimethyldisulfide, and heavy volatile molecules such as 3-(methylsulphanyl) propan-l-ol and 3-(methylsulphanyl) propionic acid (Pripis-Nicolau et al. 2004 Weimer et al. 1999 VaUet et al. 2008). In wine, O. oeni strains produce more heavy compounds, mainly 3-(methylsulphanyl) propionic acid, than lactobacilli. In water 3-(methyl-sulphanyl) propionic acid descriptors are chocolate and roasted but these notes are not found in wine where they are replaced by red fruit and earthy odors probably because of interactions with other wine components (Pripis-Nicolau et al. 2004). 

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