WINETECH Technical Yearbook 2021

OENOLOGY RESEARCH | SEPTEMBER 2021

Adding antioxidants to Sauvignon blanc musts (PART 1)

Other than protecting the already present volatile thiols from oxidation reactions, GSH can potentially also contribute to the formation of 3MH and 3MHA. GSH may be a potential source of hydrogen sulphide during the early stages of fermentation, which, in turn, can react with C6 compounds, particularly trans-2- hexenol and trans-2-hexenal resulting in the formation of 3MH and 3MHA (Araujo et al ., 2017; Harsch, et al ., 2013). GSH can also react with C6 compound, trans-2- hexenal to form thiol precursors (Clark & Deed, 2018). The International Organisation of Vine and Wine (OIV) advises adding GSH to the musts or at the start of alcoholic fermentation while ensuring that, prior to and during fermentation, the assimilable nitrogen level is sufficient to avoid the metabolism of GSH by the yeast. They also recommend that the GSH be in a reduced form and comply with the prescriptions of the International Oenological Codex while not exceeding a dosage of 20 mg/L (OIV, 2015). According to the Liquor Products Act 60 of 1989, the addition of pure GSH is not currently permitted in South Africa (it is RECOMMENDATIONS AND REGULATIONS

CARIEN COETZEE: Basic Wine, Stellenbosch KEYWORDS: Antioxidants, Sauvignon blanc, volatile thiols.

the oxidation compounds, thereby protect- ing the fruity volatile thiols (Herbst-John- stone et al ., 2011).

THE ADDITION OF ANTIOXIDANTS TO Sauvignon blanc must was investigated in two trials conducted by New Zealand researchers. The main findings of this study are reported in this two-part article. INTRODUCTION Sauvignon blanc aromas are often charac- terised by attributes such as tropical fruit, passion fruit, grapefruit, guava and citrus. The aroma compounds mainly responsible for these attributes are the fruity vola- tile thiols (or polyfunctional mercaptans), particularly 3-mercaptohexanol (3MH) and 3-mercaptohexyl acetate (3MHA) and winemakers often adjust winemaking pro- cesses to maximise the formation and pres- ervation of these compounds. The volatile thiols are sensitive to oxidation and the de- crease in these compounds has been linked to polyphenol oxidation, catalysed by met- als. The addition of antioxidants can thus be valuable by preferentially reacting with

HOW CAN GLUTATHIONE CONTRIBUTE TO THE

PRESERVATION AND FORMATION OF THE FRUITY VOLATILE THIOLS? Glutathione (GSH) is tripeptide with its biological significance mostly related to its free sulfhydryl moiety which confers unique redox and nucleophilic properties (Penninckx, 2000). During winemaking, GSH plays an integral role in the oxidation of musts, where it traps o- quinones, formed during oxidation, to limit the amount of browning pigments and loss of aroma in wine (Salgues et al ., 1986; Singleton et al ., 1985; Du Toit et al ., 2017; Tirelli et al ., 2010). GSH, therefore, competes with the fruity volatile thiols to bind the o- quinones and serves as a sacrificial compound.

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