Identification of yeast genes involved in Sauvignon Blanc aroma ...

110 IDENTIFICATION OF YEAST GENES INFLUENCING VOLATILE THIOL PRODUCTION IN SB4.6 Supplementation of grape juice with sulfur andnitrogen compounds and their influence on volatilethiol releaseThe following sections describe the impact of additional sulfur and nitrogen on the releaseof volatile thiols during fermentation in SB grape juice. Three sulfur sources, cysteine,glutathione (GSH) and S-ethyl-L-cysteine (SEC), were added in the 1 mM range. Twonitrogen sources, diammonium phosphate (DAP) and urea, were added to provide additional120 mg/L nitrogen to the SB grape juice.4.6.1 CysteineThe preceding section showed that mutations in cysteine biosynthetic genes increasedthiols, raising the possibility that cysteine supply in yeast cells may affect thiol synthesis.Preliminary experiments indicated that cysteine influenced volatile thiol release in CYS3and CYS4 deletions (data not shown). There are several plausible mechanisms for thiseffect. Schneider et al. (2006) suggested that cysteine might be able to bind to E-2-hexenal, a compound commonly found in grape must, which could then be converted byyeast to 3MH. The proposed cysteinylated thiol precursor could possibly act as a sulfur ornitrogen source for the yeast cell. Since cysteine and methionine are the major productsof the sulfur metabolism in yeast, cysteine could play a role in regulating the uptake ofCys-3MH into the cell or its cleavage.To test the potential impact of cysteine on volatile thiol release, various amounts wereadded to grape juice, which was then fermented with BY4743 and three different commercialwine yeasts. The response to cysteine was also examined in single-gene deletionsof CYS3 and CYS4 in the laboratory strain background and in two wine strains.4.6.1.1 Effect of cysteine on cys3 and cys4 laboratory strains and two wild typestrainsThe response of volatile thiol release to increasing amounts of cysteine in the grape juicewas investigated in wine yeast EC1118 and laboratory yeast BY4743 including its cys3and cys4 deletion mutants.