Dried Spent Yeast and Its Hydrolysate as Nitrogen Supplements for Single Batch and Repeated-Batch Ethanol Fermentation from Sweet Sorghum Juice

Dried spent yeast (DSY) and its hydrolysate (DSYH) were used as low-cost nitrogen supplements to improve ethanol production from sweet sorghum juice by<i> Saccharomyces cerevisiae </i>NP01 under very high gravity (VHG) fermentation (280 g·L⁻¹ of total sugar) conditions. The supplemented DSY and DSYH concentrations were 11, 16 and 21 g·L⁻¹, corresponding to a yeast extract nitrogen content of 6, 9 and 12 g·L⁻¹, respectively. The initial yeast cell concentration for ethanol fermentation was approximately 5 × 10⁷ cells·mL⁻¹. The fermentation was carried out in single batch mode at 30 °C in 1-L air-locked bottles with an agitation rate of 100 rpm. Ethanol production from the juice with and without yeast extract (9 g·L⁻¹) was also performed as control treatments. The results showed that DSY at 21 g·L⁻¹gave the highest ethanol concentration (<i>P_E</i>, 107 g·L⁻¹) and yield (<i>Y_p/s</i>, 0.47 g·g⁻¹). The use of DSYH at the same DSY concentration improved ethanol productivity (<i>Q_p</i>), but not <i>P_E </i>and <i>Y_p/s</i>. The ethanol production efficiencies of the juice under DSY and DSYH supplementations were markedly higher than those without nutrient supplementation. However, the <i>P_E</i> and <i>Q_p</i>values of the juice containing 21 g·L⁻¹ of DSY was approximately 7 g·L⁻¹ and 0.62 g·L⁻¹·h⁻¹ lower than those under the presence of yeast extract (9 g·L⁻¹), respectively. At the end of the single batch fermentation under the optimum DSY concentration, the sugar consumption was approximately 80%. Therefore in the repeated-batch fermentation, the initial total sugar was reduced to 240 g·L⁻¹. The results showed that the system could be carried out at least 20 successive batches with the average <i>P_E</i>, <i>Y_p/s</i> and <i>Q_p</i> of 95 g·L⁻¹, 0.46 g·g⁻¹ and 1.45 g·L⁻¹·h⁻¹, respectively.