The effects of pH, high concentration of glucose and the initial ethanol content on the fermentation process of ethanol with three strains of Zymomonas mobilis were investigated and the strain of ATCC 29191 was chosen for the next study. The optimal parameters for the ethanol fermentation were studied using the sweet potato raw material as feedstock with an orthogonal experimental design. It was found that the condition for ethanol production was optimized to be pH 4, substrate concentration of 20%, inoculum size of 7.5% and time of fermentation of 24 h, resulting in ethanol yield of 66.4 g/L, productivity of 2.77 g/L/h and fermentation efficiency of 93.5%, respectively. In addition, the inoculum size was identified to be the main factor for efficient ethanol production. By adopting the optimized fermentation condition, high concentration fermentation using sweet potato as sole feedstock was achieved with Z. mobilis ATCC 29191. The ethanol yield and fermentation efficiency were obtained with 99.1 g/L and 92.4%, respectively, in the presence of 400 g/L of initial content of sweet potato. This work demonstrates that the low-cost sweet potato is a feasible feedstock for ethanol fermentation with Z. mobilis.Key words: Ethanol, Zymomonas mobilis, sweet potato, fermentation, orthogonal experimental design
