Among sulphur compounds, three varietal thiols (4-mercapto-4-methylpentan-2-one (4MMP), 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexylacetate (A3MH) are key odorant compounds of wine. These thiols are released during the alcoholic fermentation (AF) from precursors present in grapes or musts. Different biogenetic pathways have been reported for the 4MMP and 3MH formation. The first two pathways involved cysteinylated and glutathionylated S-conjugates, cleaved by the yeast β-lyase activity during the AF. A third pathway has been finally identified involving (E)-2-hexenal and (E)-2-hexenol, even if the sulphur donor hasn’t been yet identified. Recently, some new derivatives (aldehyde forms, SO2-combined forms, glutathione derivatives) have been pointed out. The 3MH levels originating from those pathways could explain from 10 to 65% of the wine 3MH. Thus a large field of investigation is opened to well understand the thiols biogenesis. Some viticulture techniques favoured thiols precursors accumulation in grapes or musts, such as the vine water status and the nitrogen fertilization management (soil fertilizer or foliar spraying), or the oxidation mechanisms during must elaboration. However, fermentation conditions hugely influence the thiols release: yeast strain, temperature, nitrogen nutrition. If lot of studies focused on the yeast thiols release capacity, let’s keep in mind that the conversion yields never exceed 10%. Surprisingly, few studies were dedicated to the thiol precursors transport into the yeast. The General Amino acid Permease 1 (GAP1) is known to be the main transport system for cysteinylated precursors. Its regulation by the ammonium levels in must explain that the yeast nutrient composition (NH4+ / amino acids ration) and time of addition are two key parameters. For glutathionylated precursors transport into the yeast cell a recent study showed the importance of Opt1 transporters. The importance of the vacuolar y-glutamyltranspeptidase in the 3MH release was also pointed out even if further studies are needed to determine the entire mechanism.

Varietal thiols origins in wine: A review on their liberation mechanisms from the precursors present in grapes and musts

254th National Meeting (2017)

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