Rapid ethanol fermentation of cellulose hydrolysate. II. Product and substrate inhibition and optimization of fermentor design

High concentrations of both ethanol and sugar in the fermentation broth inhibit the growth of yeast cells and the rate of product formation. Inhibitory effects of ethanol on the yeast strain Saccharomyces cerevisiae NRRL-Y-132 were studied in batch and continuous chemostat cultures. Growth was limited by either glucose or ethanol. Feed medium was supplemented with different ethanol concentrations. Ethanol was found to inhibit growth and the activity of yeast to produce ethanol in a noncompetitive manner. A linear kinetic pattern for growth and product formation was observed according to μ = μ_m (1 – P/P_m) and v = v_m (1 – P/P_m′), where μ_m is the maximum specific growth rate at P = 0 (hr^?1); P_m is the maximum specific product formation rate at P = 0 (hr^?1); P_m is the maximum ethanol concentration above which cells do not grow (g/liter); P_m′ is the maximum ethanol concentration above which cells do not produce ethanol (g/liter). Substrate inhibition studies were carried out using short-time experimental techniques under aerobic and anaerobic condition. The degree of substrate inhibition was found to be higher than that has been reported for ethanol fermentation of pure sugar. The kinetic relationships thus obtained were used to compute growth, substrate utilization, and alcohol production patterns and have been discussed with reference to batch and continuous fermentation of enzymatically produced bagasse hydrolysate.