mercaptans, or thiol compounds, are a group of usually potent and often foul-smelling chemical compounds formed by yeast metabolism during alcoholic fermentation or by yeast reacting with sulfur in the lees after fermentation. If not removed from the new wine (which can usually be achieved by simple aeration, by prompt racking, for example) less volatile and even more unpleasant compounds tend to be formed.
At extremely low concentration, some mercaptans can have less undesirable and even pleasant odours. An example of this phenomenon is to be found in the mercaptans recently discovered as important contributors to the varietal aroma of Sauvignon Blanc and other wines, see flavour compounds and flavour precursors. Professor Denis dubourdieu and his group at the University of bordeaux, who have extensively investigated the role of mercaptans in wine aroma, have also found that benzyl mercaptan contributes to the empyreumatic aroma, i.e. reminiscent of smoke or ‘gun flint’, of Chardonnay, as well as some Sauvignon Blanc and Sémillon wines. Even oak flavour is affected by mercaptans, and furfuryl mercaptan, a compound with a powerful coffee aroma, can be released into wines during maturation in barrels with toasted staves. While the detailed chemical and biological mechanisms responsible for the formation of mercaptans in wines are not known in every case, Professor Alain Maujean has reviewed and discussed the known chemistry of sulfur in winemaking.
