Unraveling the mystery of 3SH: Quantifying glut-3SH-al and its bisulfite adduct in a range of white grape juice and wine samples

Abstract

3-Sulfanylhexan-1-ol (3SH) is a key impact odorant of white wines such as Sauvignon Blanc. In particular, the varietal characters of Sauvignon Blanc, especially from Marlborough NZ, are strongly influenced by the concentrations of 3SH. Although only trace levels of 3SH are needed to impart perceptible aroma characters of passionfruit and grapefruit, the biogenesis of this compound during fermentation is not yet well understood. The polyfunctional varietal thiols are produced during fermentation by metabolism of non-volatile precursors such as glutathione and cysteine conjugates of 3SH, however the routes by which these precursors are metabolised are complex, and not fully elucidated. 1One precursor of particular interest is the glutathione conjugate to the aldehyde form of 3SH, 3S-glutathionylhexanal (glut-3SH-al). Retention of the aldehyde functional group drastically changes the reactivity of the precursor in wine-like systems. Recent work by this group has shown that this compound can exist as tautomers in solution, suggesting possible new reaction pathways for the metabolism of glut 3SH-al. 2 Additionally, the bisulfite adduct of glut-3SH-al (glut-3SH-SO3) has been identified in wine samples, as SO2 is widely present in wine media. 3,4 The interconversion of glut-3SH-al and glut-3SH-SO3 is of great interest as it is an equilibrium and will be influenced by the concentrations of both glut-3SH-al and free SO2 in the sample. As such, it is thought that glut-3SH-SO3 may exist in finished wines as a potential reservoir for the release of 3SH which could extend the life of the fruity characters which are so desirable in young white wines. 4 We have developed a method for the extraction and quantification of glut-3SH-al and glut-3SH-SO3, using previously synthesised deuterated analogues of these compounds to ensure reliable quantification.5 The compounds are separated using solid-phase extraction (SPE), followed by oxime derivatisation and MRM analysis on an LC-QqQ. This method enables determination of the glut-3SH-al and glut-3SH-SO3 content of a wide range of grape-derived samples, a valuable piece of the puzzle in elucidating 3SH biogenesis.

Using this method, we have analysed the glut-3SH-al and glut-3SH-SO3 content of a wide range of white wines from around NZ, investigating the impact of location, age, and noble rot or late harvest on the precursor content of these wines. This insight into factors impacting glut-3SH-al and glut-3SH-SO3 content in commercial wines, and the possible influence on the finished wine aroma, adds another piece to the puzzle of 3SH and 3SHA biogenesis in wines. Indeed, despite the contribution of these volatile thiols to the aroma of botrytised white wines, these wines contain significantly lower, or negligible, concentrations of glut-3SH-al and glut-3SH-SO3, something which will be explored further in future work