Contribution of cysteine and glutathione conjugates to the formation of the volatile thiols 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA) during fermentation by 'Saccharomyces cerevisiae'

Abstract

Background and Aims: 3-Mercaptohexan-1-ol (3MH) and its ester 3-mercaptohexyl acetate (3MHA) are potent aromatic thiols that substantially contribute to varietal wine aroma. During fermentation, non-volatile 3MH conjugates are converted by yeast to volatile 3MH and 3MHA. Two types of 3MH conjugates have been identified, S-3-(hexan-1-ol)-L-cysteine (Cys-3MH) and S-3-(hexan-1-ol)-glutathione (GSH-3MH). Yeast-driven formation of 3MH from these precursors has been previously demonstrated, while the relationship between 3MHA and GSH-3MH remains to be established. This paper aims to investigate yeast conversion of GSH-3MH to 3MH and 3MHA, and to assess the relative contribution of each individual conjugate to the 3MH/3MHA pool of finished wines. Methods and Results: Fermentation experiments were carried out in model grape juice containing Cys-3MH and GSH-3MH. We found 3MH formation from GSH-3MH to be significantly less efficient than that of Cys-3MH. Conversely, esterification of 3MH to 3MHA was higher when 3MH was formed from GSH-3MH. Additional 'in vitro' assays for measuring enzyme cleavage activity suggest the involvement of a different mechanism in 3MH conversion for the two precursors. Conclusions: These results indicate that although both 3MH conjugates can be converted by yeast, the type of precursor affects the rate of formation of 3MH and 3MHA during fermentation. Significance of the Study: Management of the pool of aromatic thiols during fermentation can depend on relative proportions of different 3MH conjugates.