Energy-saving direct ethanol production from low-temperature-cooked corn starch using a cell-surface engineered yeast strain co-displaying glucoamylase and α-amylase

Direct ethanol fermentation from corn starch cooked at low temperature (80 °C) was carried out using flocculent Saccharomyces cerevisiae strains co-expressing Rhizopus oryzae glucoamylase and Bacillus stearothermophilus α-amylase. The yeast strain YF207/pGA11/pAA12 co-displaying the two enzymes was able to produce 18 g/l of ethanol from low-temperature-cooked corn starch (50 g/l) after 36 h of fermentation, an ethanol concentration almost equal to that produced from corn starch cooked at conventional high temperature (120 °C). The yeast strain YF207/pGA11/pSAA11, displaying glucoamylase on the cell surface and secreting α-amylase, was similarly able to produce ethanol from low-temperature-cooked corn starch, but with a time lag in the initial phase of fermentation. With an initial corn-starch concentration of 90 g/l, which is easily prepared at low temperature and avoids gel formation by the starch, the former strain was able to produce 30 g/l of ethanol after 36 h of fermentation. The yeast strain co-displaying glucoamylase and α-amylase is thus very effective in ethanol production from corn starch cooked under energy-saving low-temperature conditions.