Surviving in the presence of sulphur dioxide: strategies developed by wine yeasts.

Sulphur dioxide has been used as a common preservative in wine since at least the nineteenth century. Its use has even become essential to the making of quality wines because of its antioxidant, antioxidasic and antiseptic properties. The chemistry of SO₂ in wine is fairly complex due to its dissociation into different species and its binding to other compounds produced by yeasts and bacteria during fermentation. The only antiseptic species is the minute part remaining as molecular SO₂. The latter concentration is both dependent on pH and concentration of free bisulphite. However, certain yeast species have developed cellular and molecular mechanisms as a response to SO₂ exposure. Some of these mechanisms are fairly complex and have only been investigated recently, at least for the molecular mechanisms. They include sulphite reduction, sulphite oxidation, acetaldehyde production, sulphite efflux and the entry into viable but not culturable state, as the ultimate response. In this review, the chemistry of SO₂ in wine is explained together with the impact of SO₂ on yeast cells. The different defence mechanisms are described and discussed, mostly based on current knowledge available for Saccharomyces cerevisiae.