Switchgrass polysaccharides deconstruction by Clostridium thermocellum consolidated bioprocessing compared to fungal enzymes for solids produced by co-solvent enhanced lignocellulosic fractionation, hydrothermal, and dilute acid pretreatments

Conventional production of fuel ethanol from lignocellulosic biomass involves the primary steps of pretreatment, enzyme production, enzymatic hydrolysis, fermentation, and product recovery. Consolidated bioprocessing (CBP) eliminates the separate and expensive enzyme production step by using a single organism such as the thermophilic anaerobe Clostridium thermocellum for enzyme production, biomass saccharification, and carbohydrate fermentation. However, pretreatment may still be required to achieve high product yields. Low pH pretreatments, such as dilute acid and hydrothermal, remove/relocate a portion of lignin and solubilize most of the hemicellulose sugars. Whereas, co-solvent enhanced lignocellulosic fractionation (CELF), a novel pretreatment technology invented at the University of California, Riverside, which makes use of tetrahydrofuran and water mixtures as solvent, removes almost all of the hemicellulose sugars and about 85-90% of the lignin. As opposed to enzymatic hydrolysis, it is not known how CBP performance is impacted by physical and compositional characteristics of solids produced by different kinds of pretreatment. Hence, the objective of this study was to vary operating conditions of the three pretreatments when applied to switchgrass to determine the effect of removal of different amounts of hemicellulose and lignin on total sugar release by C. thermocellum CBP compared to fungal enzymes. Hydrothermal pretreatment was operated at 180-200°C for 10-40 min, whereas, CELF and dilute acid pretreatments were performed at 150°C and 140-180°C respectively for 10-30 min with identical dilute sulfuric acid loadings. CBP was run at 60°C and 180 rpm and enzymatic hydrolysis at 50°C and 150 rpm, both at 5 g/L glucan loadings.