Useful cryophilic killer strains of Saccharomyces cerevisiae were bred by back-crossing KL-88, which is a wild killer sake yeast, with a repetitive parent WL-7and is one of the best cryophilic wine yeasts. The hybrids maybe aid in producing good wines in pure cultures at relatively low temperature (1315°C).
A haploid of KL-88 was crossed with haploids of WL-7to yield first-generation hybrids. Haploids possessing both killing activity and SO₂ tolerance were selected from the hybrids and back-crossed with a haploid of WL-7 to yield second-generation hybrids. Several strains of killer hybrids were comparable to WL-7in fermentation ability at 15°C, SO₂ tolerance, TTC stain, and growth on β-alanine medium at 35°C. One strain (2HYL-2) aided in producing wine as good as with WL-7, in fermentation at 15°C on pilot-plant scale.
The hybrid killed only Saccharomyces yeasts in grape must. In the last-stage of fermenting
Koshu grape must with the killer hybrid (2HYL-2), no sensitive yeasts were detected, because they were killed by the killer yeast. Moreover, most of the neutral yeasts, such as Kloeckera and Torulopsis, that were abundant in grape juice before the addition of the starter yeast, disappeared in the last-stage. On the other hand, many sensitive pseudo-film-forming yeasts of the genus Saccharomyces were detected in the last-stage of fermenting must with WL-7strain. These results show the possibility of fermenting musts with only a single strain of cryophilic killer yeasts at a relatively low temperature without contaminating other yeast strains.

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