Major: Medical Sciences
Faculty Advisor: Paul A. Rowley
Killer yeasts as a biological control of diastatic yeast contamination
Diastatic yeast contamination is a problem that many craft-brewing companies struggle with as it can prolong fermentation after packaging, increasing gravity, off flavors, and the explosion of cans and bottles. Diastatic yeasts are a strain of Saccharomyces cerevisiae that contain the STA1 gene that allows these strains to hydrolyze residual carbohydrate polymers of dextrin and starch. Killer yeasts have the unique ability to release protein toxins that can inhibit the growth of competing yeast. In this study, we tested 8 canonical killer yeast against 38 diastatic strains and discovered that the killer toxin K1 was effective at inhibiting 89.7% and K2 inhibited 55% of these spoilage yeasts. Four killer toxin-resistant strains of diastatic yeasts were found to secrete killer toxins that were identified as K2 by reverse transcriptase PCR. The production of K2 by diastatic yeasts would possibly explain the reason why these strains were resistant to K2. These four resistant strains were tested against a collection of 200 unknown killer yeast and were susceptible to ten or more of the novel killer yeast. Based on their spectrum of activity it appears these unknown killer toxins spectrum of activity are similar to K2 toxin, this indicates that these novel killer yeast strains may produce a variant K2 that is effective against strains that are resistant to the canonical K2. Fermentation trials have shown that K1 killer yeasts are also effective in inhibiting the growth diastatic yeast in a simulated contamination event, without affecting the gravity of the beer. Therefore, killer yeasts appear to be an effective intervention to prevent the spoilage of craft beers by diastatic yeasts. Continued collaboration with Rhinegeist breweries will enable the further commercialization of this novel biological control.