Optimization of Agitation and Aeration for Very High Gravity Ethanol Fermentation from Sweet Sorghum Juice by Saccharomyces cerevisiae Using an Orthogonal Array Design

Optimization of three parameters: agitation rate (A; 100, 200 and 300 rpm), aeration rate (B; 0.5, 1.5 and 2.5 vvm) and aeration timing (C; 2, 4 and 6 h), for ethanol production from sweet sorghum juice under very high gravity (VHG, 290 g L-1 of total sugar) conditions by Saccharomyces cerevisiae NP 01 was attempted using an L-9 (3(4)) orthogonal array design. The fermentation was carried out at 30 degrees C in a 2-L bioreactor and the initial yeast cell concentration was approximately 2 x 10(7) cells mL(-1). The results showed that the optimum condition for ethanol fermentation should be A(2)B(3)C(2) corresponding to agitation rate, 200 rpm; aeration rate, 2.5 vvm and aeration timing, 4 h. The verification experiments under the optimum condition clearly indicated that the aeration and agitation strategies improved ethanol production. The ethanol concentration (P), productivity (Q(p)) and ethanol yield (Y-p/s) were 132.82 +/- 1.06 g L-1, 2.55 +/- 0.00 g L(-1)h(-1) and 0.50 +/- 0.00, respectively. Under the same condition without aeration (agitation rate at 200 rpm), P and Q(p) were only 118.02 +/- 1.19 g L-1 and 2.19 +/- 0.04 g L(-1)h(-1), respectively while Y-p/s was not different from that under the optimum condition.