Abstract:
Volatile sulfur compounds (VSCs) are an important family of aroma molecules found in wine and other fermented foods. Most possess a very low sensory threshold and strong unpleasant sensory descriptors that can influence consumer acceptance. However, there is a remarkable information gap regarding the origin of these compounds in the yeast Saccharomyces cerevisiae, including their possible precursors, their metabolic pathways, and even the time in which they are produced during fermentation. In this study, headspace solid phase microextraction coupled with gas chromatography mass spectrometry (HS-SPME/GC-MS) was used to analyse the biosynthesis of 12 VSCs. Laboratory yeast strains in defined media were used as initial standards, with specific observations later confirmed using commercial wine strains in Sauvignon blanc juice. First, a time course analysis revealed considerable differences in the timing and evolution of VSC production during fermentation, with significant differences linked to the yeast strains and fermentation media employed. Then, a study into the production of hydrogen sulfide (H2S) during fermentation revealed evidence of a direct link between this yeast metabolite and the formation of three VSCs with strong onion aroma descriptors. Additionally, a previously unrecognised role for the yeast glyoxylase I gene, GLO1, was identified, linking it to reduced activity of the O-acetyl homoserine sulfhydrylase, Met17p, and subsequent H2S generation. Moreover, short-term additions of exogenous VSCs to exponentially growing fermentations allowed for the elucidation of two distinct, simultaneously occurring mechanisms for VSC formation: biological reactions requiring live yeast cells, and chemical reactions occurring in the wine matrix. Finally, the addition and tracing of isotopically labelled sulfur sources allowed the clustering of specific VSC species by possible common origins, as well as the elucidation of multiple biosynthetic routes for some compounds. Collectively, these findings will be useful for designing novel strategies to prevent the accumulation of sulfurylated off-flavours in winemaking.