Direct and efficient production of ethanol by fermentation from raw corn starch was achieved by using the yeast Saccharomyces cerevisiae codisplaying Rhizopus oryzae glucoamylase and Streptococcus bovis ␣-amylase by using the C-terminal-half region of ␣-agglutinin and the flocculation functional domain of Flo1p as the respective anchor proteins. In 72-h fermentation, this strain produced 61.8 g of ethanol/liter, with 86.5% of theoretical yield from raw corn starch.

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Four types of cell-surface-engineered yeast Saccharomyces cerevisiae displaying glucoamylase, namely, systems A, B, C, and D, were constructed to evaluate their performance in direct ethanol fermentation from raw corn starch. Systems A and B were glucoamylase-displaying nonflocculent yeast (YF237) types that secrete alpha-amylase into the culture medium and codisplay alpha-amylase on the cell surface, respectively. Systems C and D were flocculent yeast counterparts (YF207) for systems A and B, respectively. In batch fermentations, the specific ethanol production rates of systems A, B, C, and D were 0.18, 0.06, 0.06, and 0.04 g (g cell)(-1) h(-1), respectively. In repeated fermentations, the specific ethanol production rate of system A decreased with the number of repetitions, whereas, that of system B was maintained. In all systems, the rate-limiting step was the conversion of starch to oligosaccharide because oligosaccharide and glucose were not accumulated throughout the fermentations.