Phenotypes and brewing characteristics of sake yeast Kyokai no. 7 mutants resistant to valproate

Screening of drug‐resistant mutants of sake yeast strains has been a major method for creation of superior strains. We attempted to create a valproic acid (VPA)‐resistant mutant strain from sake yeast Kyokai No. 7 (K7). VPA is a branched‐chain fatty acid and is an inositol synthesis inhibitor in mammals and yeast. We succeeded in isolating a mutant of strain K7 that can survive long‐term in a VPA‐containing medium. This strain, K7‐VPA^LS, is significantly more resistant to not only VPA‐induced cell death but also ethanol in comparison with the parent strain. Further experiments showed that the new strain is likely to have a deficiency in inositol and/or phosphatidylinositol synthesis. The major characteristics of sake brewed by strain K7‐VPA^LS (compared with K7) were lower amino acidity, higher isoamyl acetate content without an increase in the isoamyl alcohol level and changes in constituent organic acids, particularly higher malate and succinate but lower acetate concentrations. In addition, taste sensor analysis revealed that K7‐VPA^LS‐brewed sake has milder sourness and higher saltiness or richness than K7‐brewed sake does. High isoamyl acetate production may be related to a deficiency in phosphatidylinositol because this compound directly inhibits alcohol acetyltransferase, an enzyme responsible for isoamyl acetate synthesis. Strain K7‐VPA^LS grew more rapidly than the parental strain did in a medium containing acetate as a sole carbon source, indicating that K7‐VPA^LS effectively assimilates acetate and converts it to malate and succinate through the glyoxylate cycle. Thus, strain K7‐VPA^LS shows improved characteristics for brewing of high‐quality sake. Copyright © 2017 The Institute of Brewing & Distilling