The Influence of Novel Species of Bacteria On Yeast Fermentation

Sluggish or stuck fermentations are a chronic problem in winemaking. One cause of stuck fermentation is a negative impact of bacteria arising from surface biota of the grapes present early in the fermentation. We isolated fourteen novel bacteria belonging to the genera Acetobacter, Gluconobacter and Lactobacillus from stuck fermentation wine samples. Previous research has shown that acetic acid bacteria can be directly inhibitory of yeast or induce the yeast [ GAR+] prion which is associated with a decrease in fermentative capacity resulting in sluggish or stuck fermentations.;Our research has shown that all the Gluconobacter we tested had the ability to induce [GAR+] prion in at least two different yeast strains. Three out of five Lactobacillus we tested had the ability to induce [GAR+] prion in at least two different yeast strains. Three out of four Acetobacter we tested had the ability to induce [GAR +] prion in at least one yeast strain.;Four bacteria were chosen to test their sensitivity to sulfur dioxide. Gluconobacter cerinus, a [GAR+] prion inducer, was found to still grow at addition levels of 50 ppm of SO2 , while the other grape acetic acid bacteria were inhibited by this concentration under the same conditions. This preliminary result led us to hypothesize that the survival of these bacteria may cause a sluggish fermentation due to the induction of the [GAR+] prion.;To test the induction of the prion, a series of side by side bacterial induction trials were performed with different bacteria isolated from problematic fermentation wine samples. To test the influence of bacteria on yeast fermentation, a side by side fermentation trial was performed with UCD 932 yeast in co-cultivation with these bacteria. Dialysis tubing created two relative independent growing environments and was used to separate the yeasts and bacteria physically but not chemically. We performed the fermentation in a synthetic juice media, in order to provide yeasts and bacteria a relative simple and controlled growth environment. The co-fermentation was compared to the fermentation without the influence of bacteria.;To confirm the ability of bacteria to induce [GAR +] prion in liquid living environment. The yeasts were inoculated into conditioned medium which contained [GAR+]-inducing factors. These yeasts were then incubated and isolated from the conditioned medium, and screened for the [GAR+] prion tests and compared to yeasts from control medium in the absence of bacteria. We aim to elucidate the possible cause of stuck fermentations related to microbial contamination.