Optimization of Consolidated Bioprocessing Fermentation of Uncooked Sweet Potato Residue for Bioethanol Production by Using a Recombinant Amylolytic Saccharomyces cerevisiae Strain via the Orthogonal Experimental Design Method

An amylolytic industrial yeast strain named 1974-GA-temA, developed previously by our research team by coexpressing the α-amylase and glucoamylase genes, combines enzyme production, sweet potato residue (SPR) hydrolysis, and glucose fermentation into ethanol in a one-step process. This consolidated bioprocessing (CBP) method has great application potential in the commercial production of bioethanol from SPR, but important fermentation parameters should be optimized to further increase the ethanol concentration and yield. In this study, the effects of the initial fermentation pH, solid-to-liquid ratio, inoculation volume, addition of exogenous enzyme, and supplementation with metal ions were systemically investigated. Single-factor experiments revealed that the optimal pH was 4.0. In the solid-to-liquid ratio test, an increase in the solid-to-liquid ratio corresponded with a gradual increase in the ethanol concentration, peaking at 1:5. However, the ethanol yield gradually decreased, with the optimal solid-to-liquid ratio identified as 1:5. The ethanol concentration and yield reached 9.73 g/L and 5.84%, respectively. Additionally, an increase in the inoculum size resulted in increased ethanol concentration and yield, with the optimal inoculum level determined to be 10%. An ethanol concentration of 7.87 g/L was attained under these specified conditions, equating to an ethanol yield of 4.72%. Further analysis was conducted to assess the effects of exogenous cellulase, hemicellulase, and pectinase, both individually and in combination, on ethanol concentration and yield. The results indicated that pectinase had a particularly significant effect. The highest ethanol concentration was observed when all three enzymes were administered concurrently, yielding 27.27 g/L ethanol. Then, the role of metal ions in SPR fermentation was evaluated. The metal ions did not significantly affect the process, with the exception of copper ions. The addition of copper ions at a specific concentration of 0.2 g/100 g SPR increased the ethanol concentration. However, concentrations exceeding 0.2 g/100 g SPR inhibited yeast cell growth. Finally, orthogonal optimization was employed to determine the optimal combination of factors: pH, 4.0; solid-to-liquid ratio, 1:6; inoculation volume, 10%; cellulase and pectinase addition; and the absence of Cu2+ addition. Under these conditions, strain 1974-GA-temA produced 34.83 ± 0.62 g/L ethanol after 8 days of fermentation, corresponding to a 20.90% ± 0.37% ethanol yield. This value markedly exceeds the outcomes of all the conducted orthogonal experiments. The fermentation optimization experiments in this study are expected to increase ethanol production during the CBP fermentation of SPR.