BREEDING OF A USEFUL STRAIN OF SAKE YEAST FOR GINJO-SAKE BREWING

A useful sake-fermenting yeast for brewing ginjo-sake was bred by protoplast fusion. First, 61 native strains were isolated from ginjo-sake mash. From these isolates, two strains, YNS50 and YNS59, were selected. These two strains possessed many of the characteristics required for ginjo-sake brewing in a complementary manner. Strain YNS50 produced high amounts of the flavor components isoamyl acetate and ethyl caproate and low amounts of acids. Strain YNS59, on the other hand, exhibited good growth and fermentation at low temperatures (approximately 10-degrees-C) and had high resistance to ethyl alcohol. Next, a lysine auxotrophic mutant (YDH17) of YNS50 and rho-mutant (YDH8) of YNS59 with a marker useful for the selection of the fusants were constructed. Finally, prototrophs arising from the complementation of each marker by protoplast fusion were obtained. A strain (strain YNF74) that stably maintained all of the useful characteristics derived from each wild type strain was screened from 120 fusants. In a sub-industrial scale sake brewing test, we demonstrated that strain YNF74 exhibited its superior characteristics in brewing ginjo-sake. The sake produced by strain YNF74 achieved high evaluation in a tasting test.