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 °C in a 2-L bioreactor and the initial yeast cell concentration was approximately 2 × 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.