Integration of Clostridium thermocellum Consolidated Bioprocessing With Thermochemical Pretreatments for Fuel Ethanol Production From Switchgrass
There is an urgent need to replace petroleum-based transportation fuels with renewable and sustainable fuels to reduce the deteriorating impact of greenhouse gas emissions on climate change. Biofuels would not only provide a sustainable energy source but also help countries reduce their dependence on imported petroleum. Ethanol made from corn starch and cane sugar is presently the largest biotechnology-based product and commands a large share of the alternative fuels market. However, it is important to move towards making ethanol from lignocellulosic biomass that, unlike corn starch and cane sugar, does not have an important alternative use as food. However, biological conversion of this plentiful material suffers from high enzyme costs that stymie competitiveness. Clostridium thermocellum is a multifunctional ethanol producer capable of enzyme production, enzymatic saccharification, and fermentation that is fundamental to the consolidated bioprocessing (CBP) approach of ethanol production from lignocellulosic biomass. CBP eliminates the supplementation of expensive enzymes that are required in the traditional approach of ethanol production. However, the recalcitrance of lignocellulosic biomass is still a hindrance to effective ethanol production. C. thermocellum is unable to achieve complete biomass digestion and sugar release without pretreatment of lignocellulosic biomass. This work focuses on extensive process development for effective integration of CBP with four different thermochemical pretreatments of switchgrass. First, cellulose loading for C. thermocellum flask fermentations was optimized to understand the impacts of substrate structural features on digestion by C. thermocellum under non-inhibitory conditions. Next, the impact of various cellulose properties including, but not limited to, crystallinity, surface area, pore size, and degree of polymerization, was studied on C. thermocellum digestion of model cellulosic substrates compared to fungal enzymatic hydrolysis. With an extensive understanding of C. thermocellum fermentations on model substrates the interdependency of switchgrass structural features with thermochemical and biological digestion was studied. Process configurations to achieve complete cellulose solubilization and total sugar release from switchgrass were finally defined. The comprehensive nature of integration of four different thermochemical and two different biological approaches in this work is unparalleled and could provide a platform to systematically develop cost effective ethanol production from lignocellulosic biomass in the future.