Continuous high-ethanol fermentation from cane molasses by a flocculating yeast

Abstract

Repeated-batch fermentation by a flocculating fusant, Saccharomyces cerevisiae HA 2, was done in a molasses medium that contained 20% (w/v) total sugar, at 30°C in an automatically controlled fermentor, and the effects of ethanol concentration on the specific growth rate and the specific production rate of ethanol were studied. Both the specific growth rate and the specific production rate of ethanol fell with increase of ethanol concentration, and there was a linear correlation between each rate and the concentration of thanol. The maximum specific growth rate ( μ max) and the maximum specific production rate of ethanol ( q max) were 0.12 h −1 and 0.1 g ethanol/10 9 cells·h, respectively. The specific growth rate and the specific production rate of ethanol fell to zero at ethanol concentration of 89 g/ l and 95 g/ l, respectively. The number of viable cells, calculated from the linear inhibition equation, was 1.3 × 10 9 cells/ml for production of 85 g/ l ethanol at a dilution rate ( D 1) of 0.2 h −1. Based on this estimation, a laboratory-scale continuous fermentation, using two fermentors in series, was done. In the second fermentor, 85 g/ l ethanol was produced at a dilution rate ( D 1) of 0.2 h −1 by the active feedig of the fermented mash from the first fermentor into the second fermentor by pumping (hereafter called active feeding). To maintain the number of viable cells above 10 9 cells/ml in the second fermentor, a active feeding ratio of more than 23% was required. Under these conditions, 81 g/ l ethanol was produced in the second fermentor at a dilution rate ( D t) of 0.25 h −1, and the high ethanol productivity of 20.3 g/ l·h could be achieved. A bench-scale continuous fermentation, using two fermentors in series, with a active feeding ratio of 25% was done. An ethanol concentration of 84 g/ l in the second fermentor at a dilution rate ( D t) of 0.25 h −1 was achieved, just as it was in the laboratory-scale fermentation test.