Sunday, April 29, 2007

Bioethanol production from uncooked raw starch by surface-engineered yeast cells

Kuo-Wei Wu and Jyh-Ping Chen. Department of Chemical and Materials Engineering, Chang Gung University, 259 Wen Hwa 1st Rd., Kwei-San, Taoyuan, Taiwan

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 raw 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 α-amylase and glucoamylase on its surface and could also be conveniently immobilized within loofa sponge at high cell density. By using free cells at 50 g/L cell concentration and with 200 g/L uncooked raw starch as substrate, ethanol concentration could reach 56 g/L and the ethanol concentration could be maintained for 7 consecutive fed-batch fermentation cycles. For the immobilized cell system, the concentration of ethanol could reach 42 g/L in a recirculating packed-bed bioreactor in 3 d. However, ethanol concentration gradually decreased during repeated batch fermentation with concomitant decrease of cell viability.