Science
Thiols: passionfruit, grapefruit and box hedge
Sulfur-containing compounds released by yeast from odourless grape precursors — the chemistry behind Marlborough Sauvignon Blanc.
The short answer
Volatile thiols are sulfur compounds present in the grape as odourless bound precursors and released by yeast during fermentation. Three of them account for much of the passionfruit, grapefruit and boxwood character in Sauvignon Blanc.
Overview
Thiols are the best worked-out example in wine of an aroma that is created rather than merely revealed: the grape contains no smell, and the fermentation produces one.
They are also the clearest case where yeast strain choice has a demonstrable and reproducible aromatic consequence.
How it works
The mechanism, at the scope it has actually been established.
The grape accumulates cysteine- and glutathione-bound precursors that have no aroma of their own.
During fermentation, yeast enzymes cleave these bonds and release the volatile thiol. Strains differ substantially in how efficiently they do this, and strains selected for the property are widely used.
The three principal compounds are 3-mercaptohexan-1-ol (3MH, associated with passionfruit and grapefruit), its acetate ester 3MHA (associated with passionfruit more intensely still), and 4-mercapto-4-methylpentan-2-one (4MMP, associated with box hedge and blackcurrant bud).
Precursor concentration is influenced by vineyard factors including water status and nitrogen, and by harvest and pressing conditions — mechanical harvesting and skin contact are associated with higher precursor extraction.
Thiols are highly susceptible to oxidation and decline relatively quickly, which is a large part of why aromatic Sauvignon Blanc is handled reductively and drunk young.
The same compound family, at different concentrations and in different combinations, contributes to characters in Sémillon, Grenache rosé, Riesling and some beers.
Compounds named on this page
Where a compound comes from is chemistry; what it smells like is perception. These are kept apart deliberately.
- Volatile thiols
- Sulfur compounds released by yeast from odourless grape precursors. Associated with passionfruit, grapefruit, box hedge, blackcurrant bud. Caution: The same word covers compounds that are valued at low concentration and treated as reduction faults at higher ones. Context and quantity decide which is which.
Sources
Sources consulted
- Research on volatile thiols in Sauvignon Blanc — Peer-reviewed oenological literature
- The Australian Wine Research Institute — AWRI
- New Zealand Winegrowers annual report — New Zealand Winegrowers
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