A plating medium which can discriminate some nonfoaming mutants from the parent Kyokai No.7

Abstract

Some non-foaming mutants have been isolated from Kyokai No.7, a superior strain of sake yeasts, and one of them has been used commercially. One of the merits using the non-foaming mutant is to be able to ferment a comparatively large amounts of mash in a tank, because of the lack of high froth head formation. If the mash are contaminated with some froth-forming yeasts, however, materials of the mash will be overflowed with the froth from the tank. Therefore, prevention against the yeast contamination is very important when the mash is fermented with a non-foaming mutant.
Any method discriminating non-foaming mutant from the parent has been never developed.
In this paper, some trials to discriminate the non-forming mutant from the parent on a plate culture were carried out, depending upon the difference in galactose fermentability between the parent and the mutant.
Yeasts were grown at 30°C for 4 days on a medium consisting of 1% galactose, 0.2% polypeptone, 0.15% yeast extract, 0.1% KH₂PO₄, 0.04% MgSO4·7 H₂O, and 2.5% agar. Then, a medium containing 0.5% galactose, 0.05% triphenyltetrazolium chloride (TTC) and 1.5% agar was overlayed on the yeast colonies.
Colonies of parental strain were colored in red, but colonies of the non-foaming mutant, which do, not ferment galactose, remained white. Strains of Kyokai Nos. 6, 8, and many other sake yeasts were also turned into red by this galactose-TTC-overlay method. There was a yeast which was hardly distinguished from the mutants by this method, but the strain could bediscriminated from the non-foaming mutant by a β-alanine method.