CELF Pretreatment Improves Ethanol Titers from High Solids SSF of Hardwood Poplar

Woody biomass is not only abundant and sustainably available but also contains more glucan than most agricultural feedstocks. However, its structural strength and density make it more recalcitrant to enzymatic breakdown and limit SSF performance, especially at higher solids loadings needed to achieve desired ethanol titers. Because Co-Solvent Enhanced Lignocellulosic Fractionation (CELF) highly enriches glucan contents in pretreated solids by removing and recovering most of the hemicellulose and lignin, we hypothesized that the greater glucan in SSF at solids loadings that inhibit mixing than possible for other pretreatments would make it possible to achieve higher ethanol titers and yields. Application of a fractal kinetic model to data from enzymatic hydrolysis of CELF pretreated poplar indicated that CELF solids could sustain high cellulase enzyme activity over the entire course of hydrolysis with minimal enzyme deactivation. Model parameters further suggested that increasing enzyme loadings beyond 15 mg protein/g glucan in raw poplar would not increase saccharification performance. Based on these predictions, SSF was applied to solids produced by CELF pretreatment of poplar using Cellic® CTec2 cellulase at a loading of 15 mg-protein/ g-glucan in combination with S. cerevisiae D5A. At CELF solids loadings of 13, 17, and 20 wt%, SSF realized ethanol titers of 60, 78, and 87 g/L and yields of 87, 84, and 79% of theoretical in respective orders of increasing solids loadings. Furthermore, sugar release was sustained throughout SSF, irrespective of solids loading, suggested that yeast ethanol tolerance and metabolic capability and not glucan digestibility limited SSF performance.