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°C, a concentration of ethanol of 106g/ 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°C to 33°C and then to 35°C had a very detrimental effect of S. cerevisiae IR-2, even in a molasses medium with 20% (w/v) sugar. The specific growth rate (μ) and the specific production rate of ethanol ( q) could be represented in terms of ethanol concentration as follows: μ=μ 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°C, respectively. The repeated-batch fermentation process using S. cerevisiae IR-2 gave good results at 30°C, but it did not proceed well at higher temperatures.