(11b) Production of Bioethanol From Saccharomyces Pastorianus and Escherichia Coli Using a Two-Stage Fermentation Process

Non-renewable energy sources are being consumed at an alarming rate and are estimated to be depleted over the next 25 to 30 years. Hence there is a great need for an alternative source of energy. Bioethanol is a biofuel that is produced from lignocellulosic biomass, which primarily consists of plant wastes, forestry residues, paper waste etc. Lignocellulosic biomass is rich in cellulose and hemicellulose, the majority of which are converted into glucose and xylose via pretreatment and hydrolysis processes. Yeast can easily convert glucose into ethanol; however, converting xylose to ethanol is a major hurdle in producing economic levels of bioethanol. This major hurdle is primarily due to the lack of appropriate xylose metabolizing pathways in yeast, of the genus Saccharomyces. Previously we showed that xylose isomerase could improve ethanol productivity. Additionally other microorganisms can metabolize xylose; including Escherichia coli. In this study, a novel two-stage fermentation process was proposed to improve the bioethanol productivity. In the first stage, glucose in the hydrolysate was converted to ethanol by S. pastorianus. After the fermentation was completed, the mixture of ethanol, biomass and unconverted xylose was separated. In the second stage, the xylose separated was used as a feed for E. coli to grow. The E. coli cells obtained were killed by heating and recycled back to first stage yeast fermentation. Overall the combination of xylose isomerase and killed E. coli extract had significantly higher ethanol productivity.