Oxygen and sulphur dioxide additions to Sauvignon blanc : effect on must and wine composition

[摘要] ENGLISH ABSTRACT: Sauvignon blanc wines have become increasingly popular in South Africa as it is a cultivar that can beeasily manipulated in the vineyard and cellar to produce a range of wine styles. These wines are usuallygiven aroma descriptors such as green pepper, grassy and asparagus; while other more tropical aromasinclude passion fruit and guava. These aromas are thought to be mainly caused by methoxypyrazinesand volatile thiols. These compounds are known to be character impacting compounds of Sauvignonblanc and are present in the grapes in the aromatic form (methoxypyrazines) or as non‐aromaticprecursors (thiols) that can be released by the yeast during fermentation. Other aroma compounds suchas esters, higher alcohols, fatty acids and monoterpenes are compounds that could potentially influencethe aroma bouquet of a wine significantly. These aroma compounds exist either as precursors in thegrapes (monoterpenes) or arise due to yeast metabolism during fermentation (esters, higher alcohols,fatty acids) and often display fruity, floral and pleasant aromas.In the cellar, winemaking practices can be manipulated to a certain extent to achieve the desired winestyle. Winemaking tools such as temperature, skin contact, pressing conditions, oxygen (O2), sulphurdioxide (SO2) and yeast strain are only a few factors influencing the outcome of a wine. In general, SouthAfrican winemakers maintain a very reductive environment during Sauvignon blanc wine production byusing inert gasses, thereby causing the production costs to increase. There is sufficient evidence tosupport the reductive handling of white wine, however there seems to be a lack of information as towhy the must should be treated reductively before fermentation. The over all goal of this study was thusto investigate the effect of different O2 and SO2 additions to Sauvignon blanc must before settling,specifically focussing on the typical aroma compounds often found in these wines.Chapter 2 gives an overview of the oxidation reactions occurring in must (enzymatic oxidation) and wine(chemical oxidation). This chapter also reports the origin of the specific Sauvignon blanc aromacompounds and their reaction to different must and wine treatments with a focus on oxidation. Chapter3 reports research results focussing on the effect of the different must treatments on the characterimpacting compounds of Sauvignon blanc wines, specifically the methoxypyrazines and the volatilethiols. The effect of the treatments on the polyphenols and glutathione content in the must and winewas also investigated. Oxidation in the absence of SO2 led to a decrease in glutathione and certainphenolic compounds in the must. In general, volatile thiols were protected against oxidation by SO2,even when O2 was present in the must. Methoxypyrazines concentrations were not significantlyinfluenced by the treatments. Chapter 4 elucidates the effect of the treatments on other yeast andgrape derived aroma compounds often found in Sauvignon blanc wines, such as the esters, higheralcohols, fatty acids and monoterpenes. In general, the effect of SO2 seemed to have the greatestinfluence on the produced aroma compounds.The results reported in this thesis could possibly change the way South African Sauvignon blanc mustsare handled in future during the winemaking process. It is clear that O2 and SO2 management in thecellar is of critical importance for the winemaker to produce wines of high quality. Future work isimportant to fully understand the mechanisms and evolution of important aroma compounds ofSauvignon blanc wines during the winemaking process.

[发布机构] Stellenbosch University