Drivers of Sauvignon blanc aroma at harvest: C6 compounds, antioxidants, and sulfur

Abstract

Important volatile varietal thiol compounds contribute significantly to the aroma of Sauvignon blanc wines. They impart pleasant tropical, passion fruit, gooseberry, and grape fruit notes to wine, among other aromas. The formation of these compounds has been the subject of much research, mainly focused on the release of varietal thiols from conjugated precursors. An alternative pathway was shown to have immense potential to form 3-mercaptohexanol (3MH) via the addition of H2S to C6 compounds. However, both pathways have limitations to their relevance in oenological conditions and were not able to fully explain the high varietal thiol content in Sauvignon blanc wine. The practices undertaken at the early stages of winemaking were found to be important in modulating the level of varietal thiols in wine. The method of harvesting and extent of grape maceration, the addition of antioxidants at harvest, and the extent of grape pressing, all had a significant impact on the formation of varietal thiols. The aim of this thesis was to investigate how the conditions of grape harvesting and processing impact the aroma of Sauvignon blanc wines. To this end, experiments were undertaken to examine the effects of antioxidant additions to machine harvested grapes on the chemical and sensory profile of the wines. Attention was also paid to the formation of C6 compounds in juice and the relationship of their levels to the formation of varietal thiols. The role of elemental sulfur residues in juice in the formation of 3MH became a particular focus as the research developed. The addition of SO2 to machine-harvested Sauvignon blanc grapes in the field produced wines with higher levels of varietal thiols, compared to wines made with no antioxidant protection. Despite large differences in the chemical composition of the wines, the sensory profiles were generally similar between samples from a specific site. High levels of 3MH and 3MHA, together with some reductive sulfur compounds, were observed in wines with a marked flinty character. Another antioxidant compound tested was chitosan. Unlike SO2, the addition of chitosan early in the grape processing stages was severely detrimental to the formation of varietal thiols. However, it was not possible to identify the exact means by which chitosan affected thiol formation, although some impact on enzyme systems was suggested. The investigations regarding the production of C6 compounds during grape processing revealed that the concentrations of these compounds were not correlated with the levels of varietal thiols in wine. Moreover, the addition of leaves to grapes did not increase the levels of C6 compounds. Further, an inconsistent effect of leaves additions on the formation of varietal thiols was observed and related to extrinsic factors rather than a direct contribution of the leaves themselves. Finally, the important role of elemental sulfur in the formation of varietal thiols was demonstrated. Varietal thiols levels were proportional to the levels of elemental sulfur added to the juice prior to fermentation. Unwanted reductive sulfur compounds also remained in the wines if high levels of sulfur were present during fermentation. New insights into the mechanism of the formation of 3MH, via the reduction of elemental sulfur into H2S in grape juice, are proposed.