Abstract
Varietal thiols (3-sulfanylhexan-1-ol, 3SH and its acetate, 3SHA) are powerful aromas that give grapefruit and passion fruit notes to certain wines. These aromatic notes and their power make the typicality of Côtes de Gascogne wines. In this appellation, the evolution of practices towards organic viticulture has highlighted technical obstacles to the production of these aromas. This work aims to understand the effect of the growing method on the production of these thiols in the wines of two emblematic grape varieties of the region, Colombard and Gros Manseng. To do this, the effect of copper was studied through the different stages of the wine-making process, from the plot to the finished wine.The monitoring of a network of plots within the appellation enabled us to characterize precisely the thiol-type aromatic potential of these grape varieties, thus enabling us to classify them among the most concentrated in thiol precursors, along with Sauvignon blanc. These observation data from the three vintages also showed a negative effect of organic practice on the concentration of thiol precursors in the grapes of both varieties, with Gros Manseng appearing to be the most impacted. The cellar study did not reveal any particular behavior of the organic matrices compared to the conventional ones. Regardless of the management method and the type of grape variety, the extraction of thiol precursors was lower than that of Sauvignon blanc, which raised the need to optimize the extraction steps for these two grape varieties. Copper was not affected by the pre-fermentation stages, whereas it decreased significantly during alcoholic fermentation. During this stage, we identified several effects of copper on the development of varietal thiols. Firstly, there was a significant oxidative effect, regardless of the grape variety. This oxidation considerably reduced the fraction of reduced (odorous) thiols in the finished wine. Secondly, and only for Gros Manseng, there was an increase in total thiol production during fermentation. However, this increase did not compensate for the losses due to copper-related oxidation. Finally, we identified a new S-conjugated and N-acetylated intermediate in the interconversion of precursors during fermentation, N-acetyl-S-(1-hydroxyhexan-3-yl)-cysteine (NAC3SH). This compound produced only by the yeast in the presence of copper during fermentation completed our knowledge of this degradation pathway and brought it a little closer to the detoxification pathway of xenobiotics. NAC3SH, when produced by yeast, also acted as a precursor and allowed the release of 3SH and 3SHA in synthetic matrix. Similar to the other precursors the release of NAC3SH to 3SH appeared to be diastereomer dependent, with the (R,S) isomer being converted more efficiently than the (R,R) by X5 yeast.