Winemaking practices affecting glutathione concentrations in white wine

[摘要] ENGLISH ABSTRACT:Glutathione (GSH), a tripeptide consisting of glutamate, cysteine and glycine, is the most ubiquitousnon‐protein intracellular thiol in a large variety of organisms, including plants, animals and fungi. Thethiol moiety of the cysteine residue confers unique redox and nucleophilic properties. In plant cells,GSH fulfils an indispensible role in the antioxidant system, sulphur metabolism and detoxification ofxenobiotics.Upon grape crushing, GSH is extracted into the juice where it exerts several protective effectsduring the vinification process. In must, it reacts with oxidized phenolic compounds to form thecolourless grape reaction product (GRP) which limits must browning to a certain extent. During wineageing, GSH impedes the decrease of important aroma compounds, including certain esters, terpenesand volatile thiols, while at the same time preventing the development of atypical ageing off‐flavours.GSH may also inhibit the yellowing of wine during ageing. It is thus evident that elevated GSH levels inwine, in particular white wine which is more sensitive to oxidation, may be highly valuable for winequality.The reductive crushing and pressing of white grape varieties, which limits oxidation and theconsequent incorporation of GSH into GRP, promotes higher GSH levels in the juice. The reductivehandling of juice also limits the formation of oxidized glutathione (GSSG). However, during alcoholicfermentation and maturation, levels generally decrease as a result of assimilation by the yeastSaccharomyces cerevisiae, and inevitable oxidation that takes place during the vinification process. Theprincipal focus of this study was to gain a better understanding of the fate of glutathione duringalcoholic fermentation and to establish whether certain oenological applications could result in elevatedwine GSH levels. The application studied, included choice of yeast strain, extended lees contact,nitrogen supplementation and supplementation with glutathione enriched inactive dry yeastpreparations (GHS‐IDYs). In addition, the need for a rapid analytical method for the simultaneousquantification of both GSH and GSSG in must and wine which does not involve derivatization or requireextensive sample preparation, led to the development of a novel UPLC‐MS/MS method. The methodwas also employed to determine intracellular GSH and GSSG contents of the yeast S. cerevisiae and wasstudied for the first time in winemaking conditions.It was shown that the GSH levels fluctuated during alcoholic fermentation, suggesting the uptakeand release by yeast. At the end of alcoholic fermentation, levels were generally lower than thoseinitially present in grape juice, but in some cases, concentration increases were also observed. Thisfinding indicates that, in some cases, endogenously‐produced GSH may be secreted into must duringalcoholic fermentation, contributing to higher GSH levels in wine. Albeit small, significant differences inGSH content could be seen in wines fermented with different yeast strains, implying that yeast strainmay to a certain extent influence wine GSH levels. While the effects of lees ageing and nitrogensupplementation seem to be insignificant in contributing to higher GSH levels in wine, thesupplementation of must with GSH‐IDYs could result in increased wine GSH levels, provided thesupplementation is done early during fermentation. This study has broadened our knowledge of severaloenological factors, influencing GSH levels in wine and provided a new baseline for future researchstudies.

[发布机构] Stellenbosch University