Observation on the refermentation of sweet Botrytis-affected wines

Abstract

Botrytis-affected wines are microbiologically unstable. Fermentation can occasionally occur during maturing time or bottle-ageing. However, wines which undergo refermentations seem to be free of yeasts. This work deals with microbial aspects and ecology of wines during alcoholic fermentation, then during refermentation. Yeast survival after stopping the alcoholic fermentation by sulphur dioxide addition is considered. Results suggest that most yeasts could survive in sweet wine under the viable but non-culturable (VBNC) state. The increase of acetaldehyde these wines in response to sulfiting is known for long time but not the conditions of its formation. In this work we show that it is probably the key of the refermentation phenomenon, through the increase of the binding power. During maturing successive cycles occur: acetaldehyde consumption, revival, death and/or new enter into VBNC state accompanied by acetaldehyde formation which increased the binding power. Then as molecular SO2 became too low, yeasts could start a new fermentation. In this work, an ecology study was also undertaken, to understand the effect of SO2 on yeast biodiversity. Most of the fermentative yeasts could survive especially Saccharomyces cerevisiae and Zygosaccharomyces sp., together with other yeast genera. In regard to Saccharomyces cerevisiae only which is often responsible for the refermentation phenomenon, this work concluded on the decrease of strain diversity to only one or two strains, in spite of the high degree of diversity during the alcoholic fermentation. The intra-species selection probably conducted to sulfite-resistant strains.