First quantification of glut-3SH-SO₃ and glut-3SH-al in juice and wine

Abstract

3-Sulfanylhexan-1-ol (3SH) is a key impact odorant of white wines such as Sauvignon Blanc.[1] In particular, the varietal characters of Sauvignon Blanc, especially from Marlborough New Zealand, are strongly influenced by the concentrations of 3SH.[2,3] 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 fully understood.[1,4] The polyfunctional varietal thiols can be 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.[4]

One precursor of particular interest is the glutathione conjugate to the aldehyde form of 3SH, 3S-glutathionylhexanal (glut-3SH-al). The presence 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. Additionally, the bisulfite adduct of glut-3SH-al (glut-3SH-SO3) has been identified in wine samples.[5,6] The interconversion of glut-3SH-al and glut-3SH-SO3 is of great interest as this equilibrium 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.[6]

A method for the extraction and quantification of glut-3SH-al and glut-3SH-SO3 has been developed, using previously synthesised deuterated analogues of these compounds to ensure reliable quantification.[7] The compounds are separated using solid phase extraction (SPE), followed by oxime derivatisation and MRM analysis on an LC-QqQ. This method has been validated using standard addition of synthetic glut-3SH-al and was found to be linear up to 1000 ppb.

Using this method, we have analysed the glut-3SH-al and glut-3SH-SO3 content of laboratory scale synthetic grape media samples before, during, and after fermentation, as well as a selection of commercial wines and grape juices. With the SPE and LC-QqQ analysis described here, the glut-3SH-al and glut-3SH-SO3 content of a wide range of grape derived samples can be measured, a valuable piece of the puzzle in elucidating 3SH biogenesis.