Revisiting the evaluation strategy of varietal thiol biogenesis

The varietal thiols 3-mercaptohexan-1-ol (3MH), 3-mercaptohexyl acetate (3MHA), and 4-mercapto-4-methylpentan-2-one (4MMP) are key aroma compounds in wine due to the tropical notes they impart. They are released by yeast during alcoholic fermentation from different precursors. However, a large part of 3MH origin remains unknown. In this study, we focused on dipeptide forms arising from glutathione S-conjugates to 3MH and 4MMP. Using labelled tracers, we showed in spiked must the release of varietal thiols from 4 different compounds. We highlighted the interconversion between different forms of precursors under defined enological conditions. Cysteinyl-glycine S-conjugates are partially degraded into cysteine S-conjugates, contrary to γ-glutamyl-cysteine S-conjugates. Glutathione S-conjugate to 3MH can be partially degraded to γ-glutamyl-cysteine S-conjugate to 3MH. For the first time, all these labeled forms of precursors were found to release 3MH or 4MMP between 0.17 and 1% molar conversion yield. Two different yeasts were compared without any significant difference.