Process development for combined pentose and hexose fermentation

Second-generation ethanol from lignocellulose is a sustainable alternative that can partially replace fossil fuels. To be competitive with first generation ethanol from sugar and starch crops and fossil fuels, the conversion efficiency and ethanol yields of second-generation ethanol conversion processes must be improved. Improving the performance of the fermenting microorganism and efficiently convert both glucose and xylose in lignocellulosic biomass is imperative to achieve these targets. This thesis addresses means to improve the performance of the biochemical steps of the lignocellulose-to-ethanol process. The main focus has been on enhancing the xylose utilization of xylose-fermenting Saccharomyces cerevisiae by adapting the yeast to lignocellulosic hydrolysates during propagation and developing novel co-fermentation strategies that promote xylose utilization. Co-fermentation strategies based on separate hydrolysis and co-fermentation (SHCF) and simultaneous saccharification and co-fermentation (SSCF) were investigated.

Furthermore, scale-up of co-fermentation strategies and the use of multiple and blended feedstocks in the conversion process were investigated. The findings show that adaptation of the yeast to the conditions in fermentation during propagation provides a broad adaptive response that improves fermentation performance of xylose-fermenting S. cerevisiae. Co-fermentation designs that take the xylose consumption patterns of xylose-fermenting S. cerevisiae into consideration can further enhance the xylose utilization and ethanol yields. Furthermore, feedstocks with similar attributes and blends thereof could be concurrently pretreated and co-fermented, eliciting comparable ethanol yields of the whole range of feedstocks and feedstock blends. This suggests that feedstocks with similar attributes can be used interchangeably to improve supply efficiency and hedge economic and technologic risks. Scale-up experiments show that the advanced co-fermentation strategies can be scaled-up from lab scale to process development and demonstration scale and maintain comparable ethanol yields, thus bringing the lab-scale process improvements closer to implementation at commercial scale.