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 Saccharomyces cerevisiae NP01 under very high gravity (VHG) fermentation (280 g·L of total sugar) conditions. The supplemented DSY and DSYH concentrations were 11, 16 and 21 g·L, corresponding to a yeast extract nitrogen content of 6, 9 and 12 g·L, respectively. The initial yeast cell concentration for ethanol fermentation was approximately 5 × 10 cells·mL. 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) was also performed as control treatments. The results showed that DSY at 21 g·Lgave the highest ethanol concentration (PE, 107 g·L) and yield (Yp/s, 0.47 g·g). The use of DSYH at the same DSY concentration improved ethanol productivity (Qp), but not PE and Yp/s. The ethanol production efficiencies of the juice under DSY and DSYH supplementations were markedly higher than those without nutrient supplementation. However, the PE and Qp values of the juice OPEN ACCESS Energies 2013, 6 1619 containing 21 g·L of DSY was approximately 7 g·L and 0.62 g·L·h lower than those under the presence of yeast extract (9 g·L), 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. The results showed that the system could be carried out at least 20 successive batches with the average PE, Yp/s and Qp of 95 g·L, 0.46 g·g and 1.45 g·L·h, respectively.