Impact of the [GAR+] Prion on Fermentation and Bacterial Community Composition with Saccharomyces cerevisiae UCD932

The efficiency and efficacy of alcoholic fermentation by yeast is crucial for the winemaking process. Sluggish or arrested fermentations can negatively affect winery operations and wine quality. Here, we present a novel mechanism by which problem fermentations can arise. Yeast can induce a prion known as [GAR(+)] that allows the cell to circumvent glucose repression of alternative carbon substrates. We have confirmed that Saccharomyces cerevisiae strain UCD932 can spontaneously generate the [GAR(+)] phenotype and that this phenotype exhibits the genetic traits of a prion. Differences were observed in the fermentative behavior of UCD932 wild-type [gar(-)] versus [GAR(+)] yeasts in laboratory-scale model juice fermentations. To further understand these differences, fermentations were performed in Chardonnay juice to study the interaction of the [GAR(+)] prion and presence of sulfur dioxide (SO2) on fermentation kinetics, bacterial community composition, and volatile compound production. Cells harboring the [GAR(+)] prion displayed reduced fermentation capacity, which was especially evident in the absence of SO2. Presence of SO2 and fermentation time had the most significant effects on the types of bacteria present in the fermentation. However, [GAR(+)] yeasts without added SO2 were especially sensitive to bacterial competition. This difference was also reflected in the bacterial and volatile profiles of the finished wine. We hypothesize that the bacterial induction of the [GAR(+)] prion by yeast during fermentation is another possible mechanism by which stuck or sluggish fermentations may become established.