Competitive advantage and tolerance of selected shochu yeast in barley shochu mash

A shochu yeast strain, Saccharomyces cerevisiae BAW-6, was previously isolated from Kagoshima yeast strain Ko, and has since been utilized in shochu production. The BAW-6 strain carries pho3/pho3 homozygous genes in contrast to the heterozygous PHO3/pho3 genes in the parental Ko strain. However, absence of the PHO3 gene per se cannot explain the fermentation superiority of BAW-6. Here, we demonstrate the growth advantage of the BAW-6 strain over the Ko strain by competitive cultivation in barley shochu preparation, where alcohol yield and nihonshudo of the former strain were higher than those of the latter strain. In addition, the maximum growth rate of BAW-6 was less affected than that of Ko by high Brix values of barley koji medium, suggesting that BAW-6 is less sensitive to growth inhibitory compounds derived from barley or barley koji. The tolerance of BAW-6 to growth inhibitory compounds, cerulenin and diethylstilbestrol (an H⁺-ATPase inhibitor), was also higher than that of other yeast strains. Consistent with BAW-6's tolerance to diethylstilbestrol in the presence of 8% ethanol (pH 4.5), H⁺-ATPase activity, but not transcription of its gene, was higher in BAW-6 than in Ko. We conclude that the BAW-6 strain is associated with certain gene alterations other than PHO3, such that it can maintain cellular ion homeostasis under conditions of ethanol stress during the latter phase of fermentation.