Stability of Varietal Thiols in Commercial Sauvignon blanc Wines

Abstract

The intense passion fruit-type aroma of New Zealand Sauvignon blanc wines, attributed to high concentrations of the varietal thiols 3-mercaptohexan-1-ol (3-MH) and 3-mercaptohexan-1-ol acetate (3-MHA), is known to diminish rapidly over just one year in the bottle. It is thus important to understand the processes by which the aroma compounds are lost during storage to improve the shelf life of these wines. The loss of varietal thiols has been linked to polyphenol oxidation, catalyzed by metals, a process that can be inhibited by antioxidants such as sulfur dioxide (SO₂), ascorbic acid, and glutathione. The evolution of 3-MH and 3-MHA was monitored in commercially bottled New Zealand Sauvignon blanc wines, stored in the dark at 15°C for 7 months, together with changes in polyphenol content and small molecule antioxidants. The acetate ester 3-MHA was the least stable of the varietal thiols and declined steadily in concentration following first-order kinetic decay, whereas 3-MH barely decreased over the first 3 months of bottle storage, and was instead followed by an increase in concentration after a further 4 months. These results pointed to hydrolysis of 3-MHA to 3-MH as the major loss mechanism in the commercial wines during the initial months of storage. Concomitantly, the flavan-3-ols exhibited a greater susceptibility to oxidation and polymerization reactions compared with the more stable hydroxycinnamic acids. Glutathione concentrations also declined steadily over the first year and would thus only provide protection against oxidative thiol losses up to that point. Free SO₂ remained relatively stable in the commercially bottled Sauvignon blanc wines after an initial drop in concentration associated with oxygen entry at bottling.