Potential of thermostable cellulases in the bioprocessing of switchgrass to ethanol

Switchgrass (Panicum virgatum), a perennial grass native to North America, is a promising energy crop for bioethanol production. The aim of this study was to optimize the enzymatic saccharification of thermo-mechanically pretreated switchgrass using a thermostable cellulase from Geobacillus sp. in a three-level, four-variable central composite design of response surface methodology. Different combinations of solids loadings (5 to 20%), enzyme loadings (5 to 20 FPU g-1 DM), temperature (50 to 70 oC), and time (36 to 96 h) were investigated in a total of 30 experiments to model glucose release from switchgrass. All four factors had a significant impact on the cellulose conversion yields with a high coefficient of determination of 0.96. The use of higher solids loadings (20%) and temperatures (70 oC) during enzymatic hydrolysis proved beneficial for the significant reduction of hydrolysis times (2.67-times) and enzyme loadings (4-times), with important implications for reduced capital and operating costs of ethanol production. At 20% solids, the increase of temperature of enzymatic hydrolysis from 50 oC to 70 oC increased glucose concentrations by 34%. The attained maximum glucose concentration of 23.52 g L-1 translates into a glucose recovery efficiency of 46% from the theoretical yield. Following red yeast fermentation, a maximum ethanol concentration of 11 g L-1 was obtained, accounting for a high glucose to ethanol fermentation efficiency of 92%. The overall conversion efficiency of switchgrass to ethanol was 42%.