The role of yeasts in grape flavor development during fermentation: The example of Sauvignon blanc

The development of the aromas of certain grapes varieties during fermentation has not been greatly investigated in enology until now. Recent works on sulfur compounds that participate in the aroma of Sauvignon blanc wines offer an example of grape flavor enhancement by yeasts. Several very odorous volatile thiols have been identified as constituents of the particular aroma of Sauvignon blanc wines: 4-mercapto-4-methylpentan-2-one (4MMP), 4-mercapto-4-methylpentan-2-ol, 3-mercaptohexanol, 3-mercaptohexyl acetate. These different thiols are practically absent in grapes or musts. They appear in wines during the alcoholic fermentation. This increase in varietal flavor due to yeast metabolism can be explained by the degradation of S-cysteine conjugates in the grape, leading to the formation of corresponding volatile thiols. The ability of Saccharomyces cerevisiae strains to augment Sauvignon blanc aromas related to these sulfur-containing aroma compounds is variable. The same Sauvignon must fermented by pure cultures of different yeast strains results in wines with different levels of volatile sulfur aromas. Certain yeast strains (EG8, VL3c) which winemakers have empirically noted produce Sauvignon wines with pronounced varietal character, release the largest quantities of 4MMP. The differences in behavior between S. cerevisiae strains also appears during the fermentation of model mediums added with S-4-(4-methylpentan-2one)-L-cysteine. Comparing the S. cerevisiae strains commonly used in winemaking, certain wild varieties of Saccharomyces bayanus have been proved particularly suited to freeing the Sauvignon sulfur aromas. Such strains have been isolated in spontaneous fermentation in Sancerre. Natural hybrids between S. cerevisiae and S. bayanus var. uvarum have also been found. They may also be obtained in the laboratory by breeding. The interest in the interspecific hybrids for the vinification of certain grape varieties must be examined.