Improvement of very-high-gravity ethanol fermentation from sweet sorghum juice by controlling fermentation redox potential

• Sweet sorghum juice was used as substrate for ethanol production.
• Redox potential control was implemented to improved ethanol production.
• Improved sugar utilization was achieved with proper redox potential control during ethanol production.

Effects of fermentation redox potential control (no control, −100 mV and −150 mV) on the improvement of ethanol productivity during very-high-gravity (VHG) fermentation from sweet sorghum juice (300 g/l of total sugar) were investigated. Results showed that redox potential controlled at −150 mV gave the highest ethanol production efficiencies (ethanol concentration or P value, 134.35 ± 1.67 g ethanol 1/l and ethanol productivity or Qp value, 2.80 ± 0.03 g ethanol 1/l h) at the fermentation time of 48 h. Under the same condition without the redox potential control, the P and Qp values were 115.34 ± 2.01 g/l and 2.14 ± 0.05 g/l h, respectively at the fermentation time of 54 h. To complete sugar utilization under redox potential controlled at −150 mV, 16 mM urea was supplemented into the VHG medium. Results showed that the P and Qp values were 140.23 ± 1.88 g/l and 2.92 ± 0.01 g/l h, respectively corresponding to sugar utilization of 96%. When the initial total sugar in the juice was reduced to 250 g/l, no urea addition was required in order to complete sugar utilization, and the P and Qp values were 121.54 ± 2.23 g/l and 3.38 ± 0.04 g/l h, respectively at the fermentation time of 36 h. These results clearly indicated that redox potential control coupling with supplementation of assimilable nitrogen levels significantly improved the ethanol production efficiency from the sweet sorghum juice under the VHG fermentation by Saccharomyces cerevisiae NP 01.