Factor affecting the reaction kinetics of 3-MHA and 3-MH in Sauvignon blanc

Abstract

The passion fruit-type aroma of New Zealand Sauvignon blanc wines has been attributed to 3-mercaptohexan-1-ol (3-MH) and its acetate ester 3-mercaptohexan-1-ol acetate (3-MHA). 3-MH also makes an important contribution to the fruity aroma of many other wine styles. Latest research has shown that these varietal thiols are unstable throughout storage. Their loss has been ascribed to polyphenol oxidation, catalyzed by metals, a process which can be inhibited by antioxidants. In order to uncover the mechanism(s) leading to the loss of these passion fruit-type aroma notes, the effect of pro-oxidants (metals and polyphenols), anti-oxidants (SO2, glutathione, and ascorbic acid) as well as temperature and pH on 3-MHA and 3-MH stability was examined in a white wine matrix (Sauvignon blanc) at 45°C over a 28-day period. Sulfur dioxide (using flow injection analysis), glutathione, ascorbic acid, and polyphenols (using RP-HPLC coupled with an electrochemical detector and diode array detector) were monitored, in addition to changes in the concentrations of the varietal thiols using GC/MS. This study revealed that 3-MHA is the least stable varietal thiol, but to be largely unaffected by additions of pro-oxidants and anti-oxidants. Instead hydrolysis is suggested as the predominant loss mechanism for 3-MHA, accelerated by higher temperatures and lower pH conditions, thus causing the loss of the passion fruit-type character in New Zealand Sauvignon blanc wines during bottle storage. On the other hand, the stability of 3-MH was positively affected by the presence of antioxidants and negatively by pro-oxidants, indicating significant oxidative degradation of this thioalcohol. The antioxidants were capable of preventing the polyphenol-mediated degradation of varietal thiols by interfering in the polyphenol oxidation process via scavenging oxygen (thus inhibiting polyphenol autoxidation) and interacting with the polyphenol oxidation products, namely ortho-quinones and hydrogen peroxide, in place of varietal thiols.