Central role of the vacuole in nutrient modulation of volatile sulfur compound formation during fermentation by Saccharomyces cerevisiae

Abstract

Volatile sulfur compounds (VSCs) are a key component of wine aroma, contributing both attractive ('fruity') and repulsive ('rotten eggs') odours. These compounds largely develop in wine by yeast metabolic activity, as yeast consume the sulfur nutrients available in the grape juice and release a range of VSCs. This thesis studies the formation of VSCs by the wine yeast Saccharomyces cerevisiae and its modulation through nutrient supplementation, focusing on organic sulfur nutrient sources- cysteine, glutathione (GSH) and their polyfunctional thiol conjugates. The thesis first objective was to characterise the effect of an organic nutrient preparation on the resultant sulfur aroma composition. Using metabolite profiling we demonstrated two distinct effects; concentration of the 'fruity' polyfunctional thiols 3-mercaptohexan-1-ol (3MH) and 3- mercaptohexyl acetate (3MHA), was increased while the concentration of the 'rotten egg' aroma compound, hydrogen sulfide (H2S), was decreased. Nutrient supplementation also changed the kinetics of H2S production during fermentation by advancing its onset. Transcriptomic analysis revealed that this was not due to expression changes within the sulfate assimilation pathway, which is known to be a major contributor to H2S production. This is the first genome-wide study aimed to elucidate the cellular factors affecting cysteine catabolism. Foundations laid here support the use of S. cerevisiae as a model organism to study cysteine catabolism and may provide insights into the underlying cause of cysteine accumulation and H2S generation in eukaryotes. In conclusion, this thesis demonstrates the effective use of nutrients supplementation as a means for wine aroma management. Results and techniques obtained here significantly contribute to our understanding of volatile sulfur compounds formation and provide tools that can be implemented in winemaking for better modulation of sulfur aroma compounds in wine.

Date of Award	2012
Original language	English