Origin of unpleasant smelling sulphur compounds during wine fermentation

Abstract

The wine sector is undergoing considerable transformation, particularly as a result of climate change and increasing consumer expectations for quality products, in a globalised and increasingly competitive market. Therefore, the control of the sensory quality of wines is a major challenge that the actors of the sector have to overcome, promoting the formation of compounds with positive contribution while limiting the production of off-flavours. This requires a thorough understanding of the underlying mechanisms and the factors that can modulate these productions. Significant research efforts have been made to produce this information for positive compounds, but data on negative molecules remain very sparse.

Volatile sulphur compounds (VSCs) are considered as responsible for the reduction defect of wines, a regular issue for winemakers. These molecules, belonging mainly to the chemical families of thiols, thioesters, sulphides and disulphides, are formed during the fermentation and aging of wines. Their production involves the metabolism of yeast but also chemical reactions, as well responsible for many interconversions between these compounds. The main objective of our project was to provide a comprehensive view of the formation of VSCs during fermentation and its regulation by environmental factors.

To elucidate the metabolic and chemical molecular basis of these production and the better understand the regulatory mechanisms, two complementary lines of research were developed. A chemical approach spiking ongoing fermentation with sulphur compounds and incubating samples with and without cells was carried out. This enabled us to discriminate between enzymatic and chemical reactions within the VSCs formation network and to unravel the interconnections between compounds. Furthermore, the dynamics of formation of VSCs was monitored both in liquid samples and in the headspace of fermenters, directly connected to a gas chromatography device to detect extremely volatile compounds, as sulphide and methanethiol. The sequence of VSCs production, including the transient formation of some molecules, was therefore established. Our data clearly showed the involvement of methionine and cysteine as precursors for the biological formation of VSCs, as well as the key role of methanethiol as metabolic hub. In addition, our results highlighted the existence of chemical and biological interconversions between thiols, thioesters and disulphides that contribute to the VSC profile of wines. Overall, extending our knowledge on the VSCs formation and origin during wine fermentation, this study provided clues for the design of strategies to control the formation of these unpleasant smelling molecules during wine fermentation