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-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 x 10(7) cells.mL(-1). The fermentation was carried out in single batch mode at 30 degrees 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(-1)gave the highest ethanol concentration (P-E, 107 g.L-1) and yield (Y-p/s, 0.47 g.g(-1)). The use of DSYH at the same DSY concentration improved ethanol productivity (Q(p)), but not P-E and Y-p/s. The ethanol production efficiencies of the juice under DSY and DSYH supplementations were markedly higher than those without nutrient supplementation. However, the P-E and Q(p) 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 P-E, Y-p/s and Q(p) of 95 g.L-1, 0.46 g.g(-1) and 1.45 g.L-1.h(-1), respectively.