Vidhya Ramakrishnan and Linda Bisson*  
*Department of Viticulture and Enology, University of California, Davis, Davis, CA 95616 (lfbisson@ucdavis.edu)

Instances of problematic fermentations arise periodically during wine production in spite of careful fermentation management practices. The initial microbial load of grape must is one factor known to impact yeast fermentation. We are focused on understanding the mechanism underlying yeast fermentation performance in the presence of lactic and acetic acid bacteria in the grape must. Although only some Lactobacillus species, including Lactobacillus kunkeei, are associated with fermentation arrest, both lactic and acetic acid bacteria have been isolated from problematic fermentations. Our results show that many of these bacteria can induce the [GAR+] prion in yeast. The [GAR+] prion alters yeast metabolism, reducing fermentation rate and decreasing the ability of the yeast to dominate the fermentation. Acetic acid, a common bacterial metabolite, was found to induce the [GAR+] prion. However, in commercial samples the simultaneous presence of acetic acid bacteria and L. kunkeei is often observed, suggesting that acetic acid production alone is not the sole factor leading to arrest. Therefore a metabolomics analysis was performed, comparing the spent media of inducing and non-inducing acetic acid bacteria. Higher levels of caprylic acid and oxidized glucose intermediates were found in the medium of the inducing bacteria. None of these compounds induced the prion state in yeast. Acetic acid levels did not vary across inducing and non-inducing strains of bacteria. The presence of glucose derivatives known to bind to SO2 may explain the fermentation problems that arise even when SO2 is added and SO2-sensitive inhibitory bacteria are also present. Thus, the microbial community plays an important role in enabling inhibition of yeast fermentation by specific community members.

Funding Support: American Vineyard Foundation