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The microbial consortium of wine fermentations is highly dependent upon winemaking decisions made at crush, including the decision to inoculate and the decision to add sulfur dioxide (SO2) to the must. To investigate this, Chardonnay grape juice was subjected to two inoculation treatments (uninoculated and pied de cuve inoculation) as well as two SO2 addition concentrations (0 and 40 mg/L). The bacterial communities, fungal communities and Saccharomyces populations were monitored throughout fermentation using culture-dependent and culture-independent techniques. After fermentation, the wines were evaluated by a panel of experts. When no SO2 was added, the wines underwent alcoholic fermentation and malolactic fermentation simultaneously. Tatumella bacteria were present in significant numbers, but only in the fermentations to which no SO2 was added, and were likely responsible for the malolactic fermentation observed in these treatments. All fermentations were dominated by a genetically diverse indigenous population of Saccharomyces uvarum, the highest diversity of S. uvarum strains to be identified to date; 150 unique strains were identified, with differences in strain composition as a result of SO2 addition. This is the first report of indigenous S. uvarum strains dominating and completing fermentations at a commercial winery in North America.

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Background and Aims: During winemaking, alcoholic and malolactic fermentations are usually conducted by strains of Saccharomyces cerevisiae and Oenococcus oeni, respectively. Although most commercial wines are produced via inoculation with both yeast and bacteria, uninoculated fermentations can increase microbial diversity, leading to more complex wines. In this study, Chardonnay must was subjected to a combination of inoculation treatments with S. cerevisiae and O. oeni in order to investigate how yeast and bacterial inoculation may impact the microbial communities of these wines. Methods and Results: Yeast and bacterial community dynamics, and S. cerevisiae and O. oeni populations, were monitored at different fermentation stages. Yeast inoculation practices altered the yeast community and S. cerevisiae strains; wines left uninoculated had a higher diversity of S. cerevisiae strains and contained an increased abundance of Torulaspora delbrueckii. The inoculated O. oeni strain dominated the treatments to which it was added, but the uninoculated wines contained unique O. oeni populations, including potentially indigenous dominant strains. Conclusions: Uninoculated wines contained a greater diversity of yeast and bacteria than inoculated wines, including more indigenous/non-commercial species and strains. Significance of the Study: These results highlight the importance of identifying the entire microbial community in fermentations, at both the species and strain level.

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