REPEATED-BATCH ETHANOL FERMENTATION BY A FLOCCULATING YEAST, SACCHAROMYCES-CEREVISIAE IR-2

Repeated-batch fermentation by a flocculating yeast, Saccharomyces cerevisiae IR-2, was set up in a molasses medium using an automatically controlled system, and the effect of temperature and mash density were studied. At 30-degrees-C, a concentration of ethanol of 106 g/l with a productivity of 2.5 g/l.h, which was 2.5 times higher than that of the fed-batch fermentation process used commercially in Japan, and a fermentation efficiency of 83% were obtained in a molasses medium that initially contained 25% (w/v) sugar. However, raising the temperature from 30-degrees-C to 33-degrees-C and then to 35-degrees-C had a very detrimental effect on S. cerevisiae IR-2, even in a molasses medium with 20% (w/v) sugar. The specific growth rate (mu) and the specific production rate of ethanol (q) could be represented in terms of ethanol concentration as follows: mu = mu-max (1 - P/P(g)), and q = q(max) (1 - P/P(e)), where P represents the concentration of ethanol. Increasing the initial concentration of sugar had little effect on the values of P(g) (the concentration of ethanol above which yeast cells do not grow) and P(e) (the concentration of ethanol above which yeast cells do not produce ethanol), which were about 78 and 104 g/l, respectively. Raising the temperature caused a decrease in P(g) and P(e), which fell to 32 and 65 g/l at 35-degrees-C, respectively. The repeated-batch fermentation process using S. cerevisiae IR-2 gave good results at 30-degrees-C, but it did not proceed well at higher temperatures.