Isolation of sake yeast strains which produce no urea was attempted without mutagenesis treatment, for the purpose of producing sake containing no urea, the main precursor of ethyl carbamate, a suspected carcinogen in sake. The isolation was made to obtain arginase-deficient mutants, because urea is produced by the action of arginase in yeasts. Thus, Kyokai sake yeast No.9 and No.10 were cultured on CAO medium, on which arginase-deficient mutants can grow but wild type sake yeasts can not. Strains grown on CAO medium were inoculated to media containing arginine or ornithine, to obtain mutants which can grow on ornithine but not on arginine. Laboratory scale sake brewings carried out with these isolated arginase-deficient sake yeasts gave as good fermentability and quality of sake as those with their parent strains.
The resulting sake contained no urea and no production of ethyl carbamate was observed after heat treatment.
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