High Level Ethanol Production by Nitrogen and Osmoprotectant Supplementation under Very High Gravity Fermentation Conditions

Optimization of nutrient supplements i.e., yeast extract (1, 3 and 5 g·L ？1), dried spent yeast (DSY: 4, 12 and 20 g·L ？1) and osmoprotectant (glycine: 1, 3 and 5 g·L ？1) to improve the efficiency of ethanol production from a synthetic medium under very high gravity (VHG) fermentation by Saccharomyces cerevisiae NP 01 was performed using a statistical method, an L 9 (3 4) orthogonal array design. The synthetic medium contained 280 g·L ？1 of sucrose as a sole carbon source. When the fermentation was carried out at 30 °C, the ethanol concentration ( P), yield ( Yp/s) and productivity ( Qp) without supplementation were 95.3 g·L ？1, 0.49 g·g ？1 and 1.70 g·L ？1·h ？1, respectively. According to the orthogonal results, the order of influence on the P and Qp values were yeast extract > glycine > DSY, and the optimum nutrient concentrations were yeast extract, 3; DSY, 4 and glycine, 5 g·L ？1, respectively. The verification experiment using these parameters found that the P, Yp/s and Qp values were 119.9 g·L ？1, 0.49 g g ？1 and 2.14 g·L ？1·h ？1, respectively. These values were not different from those of the synthetic medium supplemented with 9 g·L ？1 of yeast extract, indicating that DSY could be used to replace some amount of yeast extract. When sweet sorghum juice cv. KKU40 containing 280 g·L ？1 of total sugar supplemented with the three nutrients at the optimum concentrations was used as the ethanol production medium, the P value (120.0 g·L ？1) was not changed, but the Qp value was increased to 2.50 g·L ？1·h ？1.