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 × 10 7 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 −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.