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

Abstract

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 L9 (34) orthogonal array design. The fermentation was carried out at 30 °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 × 107 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 (Yp/s) and productivity (Qp) 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, Yp/s and Qp were only 93.45 ± 0.45 g·L−1, 0.49 ± 0.00 and 1.30 ± 0.01 g·L−1·h−1, respectively.