抄録
Single step bioconversion of raw starch to ethanol has raised considerable interest in recent years. We have developed yeast strains displaying α-amylase and glucoamylase on yeast cell surface for this purpose. The advantage of this system is that the displayed enzymes can be recovered easily with the yeast after fermentation compared to the case that these amylolytic enzymes are secreted into medium. However, display of α-amylase on cell surface may reduce the apparent association rate of the enzyme to the substrate because both of them are huge molecules, resulting a lower apparent conversion rate. The recovery of the yeast cells together with the amylolytic enzymes becomes easier when a flocculating yeast strain is used as the host strain. However, flocculation of the arming yeast may again reduce the apparent conversion rate. To establish an industrial ethanol production process with a good cost performance ratio, the advantages and disadvantages of each strategy should be evaluated quantitatively. Therefore, from a viewpoint of kinetics, we compared the performance of the fermentation systems in the case that α-amylase is displayed on the cell surface or secreted in the medium, and in the cases that a flocculating or non-flocculating strain is used as the host strain. A kinetic model has been developed to simulate the direct ethanol fermentation from raw corn starch by arming yeast. The simulation results were found agree well with experimental data obtained from direct ethanol fermentation.
