P26: Repeated batch-fermentation of rice straw hydrolysate to ethanol using metabolically engineered Saccharomyces cerevisiae strains

To exploit lignocellulosic materials for fuel ethanol production, the improvement of tolerance to fermentation inhibitors present in the hydrolysate is required for Saccharomyces cerevisiae strains to be used. Acetic and formic acids released during the pretreatment of lignocelluloses negatively affect microbial growth and ethanol production. However, genetic engineering strategies focusing on the tolerance of the yeast to the weak acids are rare, even though the concentrations of these weak acids in the hydrolysate are often higher than other inhibitors such as furan derivatives and phenolics. We report herein a successful development of a recombinant xylose-fermenting S. cerevisiae strain that demonstrated higher ethanol production in the present of both acetate and formate through metabolic pathway engineering. Moreover, crossbreeding of the metabolically engineered strains improved ethanol productivity in the presence of the weak acids. The recombinant diploid strain successfully produced ethanol from lignocellulosic hydrolysate that had not undergone any detoxification processes. In addition, batch fermentation of the hydrolysate was repeated five times without any loss of fermentation ability. The repeated-batch fermentation process is considered to be a promising method for cost effective ethanol production, due to the reduction in the time and costs associated with inoculum preparation. To our knowledge, this is the first report of repeated-batch fermentation of lignocellulosic materials to produce ethanol.