Comparison of sugar release by hydrolysis with fungal enzymes vs consolidated bioprocessing (CBP) with Clostridium thermocellum from solids produced by dilute acid and hydrothermal pretreatments of switchgrass

Consolidated bioprocessing (CBP) offers the potential of considerable cost savings by significantly simplifying biological conversion of cellulosic biomass into fuels through a combination of enzyme production, saccharification, and fermentation in a single vessel.  Clostridium thermocellum is a particularly promising CBP organism, but pretreatment can still be needed to achieve high yields.  However, because it is not known how well the varying physical and compositional characteristics of solids produced by different pretreatment technologies will be amenable to CBP, this research compared the effects of varying operating conditions for dilute acid and hydrothermal pretreatments of switchgrass on total sugar release by conventional enzymes and C. thermocellum. Hydrolysis performance of both routes was measured in terms of the time required for the release of sugars from solids produced by pretreatments at different severities (i.e., times, temperatures, and acid concentrations). Hydrothermal pretreatment was performed at 160-200°C for 10 to 60 min and dilute acid pretreatment was run at 140-160°C for 5-30 min in 0.5% and 1wt% sulfuric acid concentrations to prepare pretreated solids for biological conversion.  The resulting washed solids were then employed in C. thermocellum fermentations at 5 g/l glucan loadings in 50 mL bottle reactors incubated at 60˚C with a shaking speed of 180 RPM and fungal enzymes mediated enzymatic hydrolysis at 50°C, pH 5.0 and 150 rpm at low to high enzyme protein loadings.  From this data, pretreatment conditions were identified to give the highest possible sugar release in pretreatment (Stage 1) and biological conversion (Stage 2) steps for both systems.