Direct production of bioethanol from raw starch by immobilized yeast cells surface engineered with amylolytic enzymes

The production of bioethanol from starch by fermentation has gained considerable interest recently due to global research interest in biomass conversion for renewable energy production. Traditionally, the bioethanol production process requires both amylolytic enzymes for catalyzing the starch liquefaction and saccharification steps and yeast cells for fermenting glucose to ethanol. The concept of cell immobilization provides a promising strategy for recovery and reuse of the yeast cells. However, the enzymes still need to be refurnished at the beginning of each fermentation cycle which will lead to increased production cost. When insoluble starch is used as the substrate, additional production costs will incur with the energy-consuming cooking process before adding the enzymes. In this study, we have developed a process for direct production of bioethanol from insoluble raw starch using surface-engineered Saccharomyces cereisiae. The flocculating, recombinant-yeast strain co-displayed a-amylase and glucoamylase on its surface and could be conveniently immobilized within loofa sponge at high cell density. With this immobilized cell system, the concentration of bioethanol could reach 47 g/L in a recirculating, packed-bed bioreactor in 3 d. The ethanol yield could be maintained for at least four consecutive batches of fermentation. This fermentation system can be easily scaled-up and will be more cost effective than traditional systems when employing raw starch from agricultural wastes as the substrate.