Chiang Mai Journal of Science

This research aimed to increase ethanol production efficiency from sweet sorghum juice (SSJ) in terms of ethanol productivity (Q_p) and concentration (P_E) by Saccharomyces cerevisiae. The growth rate of two ethanol-producing yeast strains, i.e., S. cerevisiae NP01 and SSJ01KKU was determined. The specific growth rate and maximum cell counts of SSJ01KKU in SSJ containing 100 g L^-1 of total sugars were higher than those of NP01. Urea addition to the SSJ medium did not promote the growth of SSJ01KKU, and its optimum growth temperature was 32 °C. Then, central composite design (CCD) in conjunction with response surface methodology (RSM) was used to optimize the initial sugar and urea concentrations in SSJ for the highest Q_p and P_E values. Ethanol fermentations were carried out in batch mode at 32°C, with an initial cell count of ~5´10⁷ SSJ01KKU cells mL^-1. The results showed that the optimum conditions for the maximum predicted Q_p (2.82 g L^‑1 h^‑1) were 176 g L^-1 of sugar and 2.68 g L^-1 of urea (condition A); whereas the optimum conditions for the maximum predicted P_E value (100.04 g L^-1) were sugar and urea concentrations of 227 and 2.94 g L^-1, respectively (condition B). When the verification experiments under both conditions were done, the experimentally derived P_E and Q_p values were very closed to the predicted values. When the initial cell counts for the ethanol fermentations were increased to ~1´10⁸ cells mL^-1, the P_E values were unchanged, but the Q_p values increased to 3.65 and 2.93 g L^-1 h^-1 for conditions A and B, respectively.