Methanethiol yeasts

Examination of 37 basidiomycetous yeasts indicated formation of several sulfur volatiles 3-(methylthio)-l-propanol, methanethiol (MT), S-methyl thio-acetate, dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS), allyl methyl sulfide and 4,5-dihydro-3(2//)-thiophenone. The component produced in the largest amounts, 40 100 mg L-1, was 3-(methylthio)-l-propanol29 Cheeseripening yeasts are considered later (Section 11.1.2.4.5). 

This enzyme [EC 4.2.99.10], also referred to as O-acetyl-homoserine sulfhydrylase, catalyzes the reaction of O-acetylhomoserine with methanethiol to generate methionine and acetate. The enzyme can also act on other thiols or H2S, producing homocysteine or thioethers. The enzyme isolated from baker s yeast will also catalyze the reaction exhibited by O-acetylserine (thiol)-lyase [EC 4.2.99.8], albeit more slowly. 

MGL catalyzes the o ,7-elimination reaction of methionine to a-ketobutyrate, methanethiol, and ammonia. MGL has been isolated from a number of bacteria, including Pseudomonas putida, Aeromonas sp., Clostridium sporogenes, P. taetrolens, and Brevibacterium linens, from the primitive protozoa Entamoeba histolytica and Trichomonas vaginalis, but is not believed to be present in yeast, plants, or mammals. " " Two MGL isoforms have been isolated from T. vaginali and Entamoeba histolytica, which differ in substrate specificity, overall charge, and catalytic properties. They show a high degree of sequence identity to MGL from Pseudomonas putida. MGL has demonstrated antitumor efficacy in a number of methionine-dependent cancer cell lines. ... 

Hydrogen sulfide and methanethiol are directly produced by yeast metabolism. The production of H2S during alcoholic fermentation is controlled by the enzymes responsible for reducing sulfates and biosynthesizing certain sulfur amino acids (cysteine and methionine) (Figure 8.19). Methanethiol is synthesized by yeast from methionine (De Mora et al., 1986). 

It is quite true, however, that the lees gradually lose their capacity to reduce sulfur derivatives. This is probably due to the inactivation of the enzyme responsible for reducing sulfites to H2S (sulfite reductase). It is then possible for methanethiol and ethanethiol to fix on fresh yeasts, with even more serious consequences (Lavigne and Dubourdieu, 1996). Disulfide cross-bonds are formed between cysteine from the yeast wall mannoproteins and the SH group of the sulfur derivatives (Figure 8.21). The copper adsorbed by the yeast lees is involved to a large extent in the formation of disulfide cross-bonds between the free thiols and the cysteine remains of the... 

Besides charcoal, ancient enology treatises mention other products likely to eliminate unpleasant smells and off-flavors toasted barley or wheat, mustard flour, oil, milk, etc. All these have practically disappeared from use. Fresh yeast lees are permitted in treating wine and are effective in eliminating a number of olfactory defects. This has already been mentioned in connection with fixing certain thiols, such as methanethiol (Section 8.6.2). This treatment is also recommended for adsorbing chloroanisoles in moldy wines (Section 8.5.2). 

Juice turbidity also influences the production of volatile sulfur-containing compounds by yeasts (Volume 2, Section 8.6.2) H2S, methanethiol. 

Yeasts, taken at the end of fermentation and added to a model solntion containing methanethiol and ethanethiol, are capable of adsorbing these volatile thiols. They are fixated by the yeast cell wall mannoproteins. Dnring aeration, a disulfur bond is formed between the cysteine of the cell wall mannoproteins and the thiols from the wine. 

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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