Efficient ethanol production from starch through development of novel flocculent yeast strains displaying glucoamylase and co-displaying or secreting α-amylase

To develop novel yeasts with high starch-to-ethanol productivity, we constructed two cell-surface-engineered flocculent Saccharomyces cerevisiae strains; one co-displaying glucoamylase and α-amylase on the cell surface and the other displaying glucoamylase and secreting α-amylase into the culture medium. With starch as the carbon source, both yeast strains grew faster under aerobic conditions than strains displaying only glucoamylase. In fed-batch fermentation of ethanol, these recombinant yeasts co-expressing sequential amylolytic enzymes also showed higher starch decomposition and ethanol production abilities than yeast cells displaying only glucoamylase, with the concentration of ethanol produced reaching 60 g/l after approximately 100 h fermentation under anaerobic conditions. Both co-display and secretion of α-amylase are thus effective in improving ethanol production from starchy materials in glucoamylase-displaying yeast cells.