Improvement of ethanol production from sweet sorghum juice under high gravity and very high gravity conditions: Effects of nutrient supplementation and aeration

Ethanol production by Saccharomyces cerevisiae NP 01 from sweet sorghum juice was conducted under nutrient supplementation and/or aeration conditions. Yeast extract (YE) and dried spent yeast (DSY) were used as nitrogen supplements. Under high gravity (HG, ∼200 g l−1 of total sugar) conditions, the addition of either 6 g l−1 of YE or 9 g l−1 of DSY did not increase the ethanol concentration (P), but it promoted the rate of ethanol production or ethanol productivity (Qp). The Qp value of the control treatment (no supplement) was 1.96 g l−1 h−1. The P (93.4-94.0 g l−1) and Qp (3.89-3.92 g l−1 h−1) values under the YE and DSY supplementation were comparable, indicating that DSY could be used to replace YE in ethanol production. Under very high gravity (VHG, ∼280 g l−1 of total sugar) conditions, the supplementation of nitrogen and trace elements (Zn2+, Mg2+ and Mn2+) coupled with the aeration supply were required to improve the ethanol production efficiency. Under the supplementation (in g l−1) of DSY (13.5), Zn2+ (0.01), Mg2+ (0.05) and Mn2+ (0.04) in the presence of aeration supply at 0.05 vvm for 12 h, the P and Qp values were 126.3 g l−1 and 2.11 g l−1 h−1, respectively. The P and Qp values under the same supplementation without aeration were 114.8 g l−1 and 1.91 g l−1 h−1, respectively, and these values under no supplementation and no aeration were 108.0 g l−1 and 1.50 g l−1 h−1, respectively. In addition, glycerol (the main by-product during ethanol fermentation) under aeration conditions was ∼3 g l−1 lower than that without aeration.