INDUCTION OF FREEZE-SENSITIVE MUTANTS FROM A FREEZE-TOLERANT YEAST, TORULASPORA-DELBRUECKII

Freeze-sensitive strains of yeast were induced from a freeze-tolerant yeast Torulaspora delbrueckii by incubation with ethylmethane sulfonat e as a mutagen. A maximum ratio of mutation was attained by the incuba tion at 30 degrees C for 75 min. One-hundred and fifty strains of free ze-sensitive yeast were selected by plating-culture for the first scre ening. The freeze-tolerance ratio of each strain was examined based on the fermentative activity before and after freezing in liquid medium and dough. Strain 60B3 showed the highest freeze-sensitivity in a pre- fermented frozen dough (pre-fermented at 30 degrees C for 2h, and froz en at -20 degrees C for 7 days) among eight strains finally selected.