Optimization of Nitrogen and Metal Ions Supplementation for Very High Gravity Bioethanol Fermentation from Sweet Sorghum Juice Using an Orthogonal Array Design

Optimization of four parameters, i.e., zinc (Zn2+), magnesium (Mg2+), manganese (Mn2+) and yeast extract for bioethanol production from sweet sorghum juice by Saccharomyces cerevisiae NP 01 under very high gravity (VHG, 270 g.L-1 of total sugar) conditions was performed using an L-9 (3(4)) orthogonal array design. The fermentation was carried out at 30 degrees C in 500-mL air-locked Erlenmeyer flasks at the agitation rate of 100 rpm and the initial yeast cell concentration in the juice was approximately 5 x 10(7) cells.mL(-1). The results showed that the order of influence was yeast extract > Mn2+ > Zn2+ > Mg2+ and the optimum nutrient concentrations for the ethanol fermentation were Zn2+, 0.01; Mg2+, 0.05; Mn2+, 0.04; and yeast extract, 9 g.L-1. The verification experiments under the optimum condition clearly indicated that the metals and nitrogen supplementation improved ethanol production efficiency under the VHG fermentation conditions. The ethanol concentration (P), yield (Y-p/s) and productivity (Q(p)) were 120.58 +/- 0.26 g.L-1, 0.49 +/- 0.01 and 2.51 +/- 0.01 g.L-1.h(-1), respectively, while in the control treatment (without nutrient supplement) P, Y-p/s and Q(p) were only 93.45 +/- 0.45 g.L-1, 0.49 +/- 0.00 and 1.30 +/- 0.01 g.L-1.h(-1), respectively.