Yeast population diversity on grapes during on-vine withering and their dynamics in natural and inoculated fermentations in the production of icewines

In this study, attention has been focused on microbial ecology, during an on-vine withering period of 120 days, of Mondeuse grapes, an autochthonous variety cultivated in the Western Alps in Europe, and on their fate during alcoholic fermentation. Laboratory fermentations have been followed in order to describe the yeast dynamics in both spontaneous and inoculated fermentations by means of culture-dependent and -independent methods (PCR-DGGE). The ability of Saccharomyces cerevisiae to take over the fermentation has also been assessed by means of minisatellite analysis. The chemical composition has been evaluated on the final products. A biodiversity of the grapes has been detected during the withering period, despite the predominance of Aureobasidium pullulans and Rhodotorula glutinis thus empathising that withering conditions could affect yeast populations. The spontaneous laboratory fermentation was characterized by a predominance of Hanseniaspora uvarum, Metschnikowia fructicola and S. cerevisiae. In the inoculated ones, counts of S. cerevisiae, belonging to the inoculated starter culture, became predominant after 7 days, even though the PCR-DGGE analysis showed the starter profile for the entire fermentation period. Finally, when the S. cerevisiae isolates were characterized by the minisatellite analysis, it was determined that the inoculated culture was responsible for the alcoholic fermentation. As far as spontaneous fermentation is concerned, the autochthonous S. cerevisiae showed a good ethanol production yield, but left a high concentration of residual sugars.