(235a) Biomass Digestion By a Clostridium Thermocellum Model Consolidated Bioprocess Compared to Free Enzyme Systems

Renewable fuels are essential to environmental sustainability, economic competitiveness, air and water quality, and national energy security for a transportation sector that is almost totally dependent on petroleum. Cellulosic biomass is the only natural resource from which liquid organic fuels can be made sustainably on a large scale due to its abundance, widespread geographic availability, and low cost. In order to reduce the initial capital investment of cellulosic biorefineries and associated operating costs, consolidated bioprocess (CBP) has been identified as a powerful biotechnology application that combines the conventionally separate steps of enzyme production, cellulose and hemicellulose saccharification, and fermentation of the resulting pentose and hexose sugars to ethanol or other products into a single unit operation by using microorganisms capable of both enzyme production and product fermentation. To capitalize on the added cost benefits of CBP, both hexose and pentose sugars must be conserved and utilized. Although much research has been devoted to obtaining biological systems and microorganisms which can achieve high sugar conversion, conservation of the total sugars available in cellulosic biomass proves to be an equal challenge in light of the overall process, specifically, during the preceding process step of pretreatment. Pretreatment conditions that favor high biomass solids digestibility for free enzyme hydrolysis systems are often the same conditions in which overall sugar degradation becomes important during pretreatment, therefore decreasing the product yields. With these points in mind, the objective of this study was to compare the influence of pretreatment severity on overall sugar release from a Clostridium thermocellum model CBP system against free enzyme systems to determine whether CBP offers improvements over conventional enzymatic hydrolysis by enhancing total sugar conservation through reduced severity pretreatments while achieving equivalent or improved biomass solubilization. In this study, both poplar (Populous trichocarpa) and switchgrass (Panicum virgatum) were pretreated at a variety of conditions and incubated with either C. thermocellum or conventional enzyme systems. Raw and pretreated biomass solids were tested against a pure microcrystalline cellulose control, Avicel.  Biomass solubilization, residual solids composition, and total sugar release from the combined operations of pretreatment and saccharification were employed to measure performance with changing pretreatment severity. Mass balances were determined for each processing step as well as the overall process. In addition, substrate material properties were characterized to gain insight into mechanisms leading to increased digestibility.