Abstract
Two selection methods of non-foaming mutants of sake yeasts (a kind of cell wall mutants lacking the ability to form froth head in sake mash) are described. The mutants, being different in both the affinity to gas bubble and in the agglutinability from the parent, were concentrated, by removing the wild type cells with froth in froth flotation method and by removing them by agglutination caused by lactobacillus cells in cell agglutination method. Spontaneous non-foaming mutants of Kyokai No. 7 strain were isolated from the concentrates after 9 successive trials of each selection procedure at the rates of 50% by the former and 81% by the latter. The UV-induced mutants were also isolated from the concentrates after the 7 successions at the rates of 80% and 100%, respectively, by the former and by the latter. There were two types among the non-foaming mutants with respect to the agglutinability; the one was non- or almost non-agglutinable type (type 1) and the other was weakly-agglutinable one (type 2). The spontaneous mutants isolated by the froth flotation method were all of type 2, while 54% of those isolated by the cell agglutination method was of type I and the rest was type 2. On the other hand, only type I was found with the UV-induced mutants. It is inferred from the concentration rate determined by the froth flotation method in a model experiment that the mutation would occur spontaneously at a rate of 10-8 and be stimulated about 100 fold by the UV-irradiation in Kyokai No. 7 strain. The usefulness of the non-foaming mutants are discussed from a practical point of view for sake production.
