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, <em>i.e.</em>, zinc (Zn²⁺), magnesium (Mg²⁺), manganese (Mn²⁺) and yeast extract for bioethanol production from sweet sorghum juice by <em>Saccharomyces cerevisiae </em>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 (<em>P</em>), yield (<em>Y_p/s</em>) and productivity (<em>Q_p</em>) 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) <em>P</em>, <em>Y_p/s</em>and <em>Q_p </em>were only 93.45 ± 0.45 g·L⁻¹, 0.49 ± 0.00 and 1.30 ± 0.01 g·L⁻¹·h⁻¹, respectively.