Distribution of varietal thiol precursors in the skin and the pulp of Melon B. and Sauvignon Blanc grapes

In order to examine varietal thiol precursors in both the skin and the pulp of grapes, two grape varieties, Melon B. and Sauvignon Blanc, were considered. We found that cysteinylated and glutathionylated precursors of 3-mercapto-1-hexanol and 4-methyl-4-mercapto-2-pentanone were preferentially in skin. In the Sauvignon Blanc variety, precursors were detected both in the skin and the pulp, while in Melon B., only S-3-(1-hexanol)glutathione was detected in pulp, any other precursors being exclusively found in skin. During an industrial pressing cycle, extraction of thiol precursors was enhanced at the end of the cycle (highest pressures), thus producing more varietal thiols in the resulting wines. Cold prefermentation skin contact did not influence the concentration of the measured precursors in Sauvignon Blanc must. Nevertheless, the release of thiol in resulting wines increased significantly between 1 and 7 days of cold prefermentation skin contact. This increase could be due to the formation of other thiol precursors, such as S-3-(1-hexanal)glutathione or S-3-(1-hexanal)cysteine, during prefermentative operations.