REPEATED-BATCH FERMENTATION PROCESS USING A THERMOTOLERANT FLOCCULATING YEAST CONSTRUCTED BY PROTOPLAST FUSION

A thermotolerant flocculating yeast, Saccharomyces cerevisiae KF-7 (KF-7), was constructed by protoplast fusion of the flocculating yeast S. cerevisiae IR-2 (IR-2) and the thermotolerant yeast S. cerevisiae EP-1 (EP-1). In repeated-batch fermentation with KF-7 at 30-degrees-C in a molasses medium containing 20% (w/v) total sugar, stable fermentations could be performed continuously and an ethanol productivity of 5 g/l.h was obtained. Even at 35-degrees-C a productivity of 3.6 g/l.h was achieved, which was 1.7 times higher than that achieved with IR-2. In a flask-cultivation test, the maximum growth rate and the decay constant of KF-7 were not influenced by an increase in temperature, unlike the results for IR-2. Thus, the fusant KF-7 had acquired thermotolerance. However, KF-7 may be inferior to IR-2 in terms of osmotic tolerance.