14-01: The pretreatment conundrum: How do we recover all of the biomass components (hemicelluose/lignin/etc) in a useable form while producing a readily hydrolyzed cellulosic substrate?



It is widely accepted that pretreatment is a prerequisite to ensure effective enzymatic hydrolysis of cellulose. Although several pretreatment technologies are advocated by various groups, challenges with producing cellulosic substrates susceptible to hydrolysis at low enzyme loadings while recovering hemicelluloses and lignin in a usable form remains an ongoing conundrum. The influences of varying pretreatment conditions (e.g. time, temperature, chemical loading) on the resulting substrate characteristics have yet to be fully elucidated and are frequently difficult to measure. We will discuss the compromise between pretreatment, enzymatic hydrolysis and sugar recovery and the pretreatment induced formation and measurement of substrate characteristics such as surface area, charge, swelling, crystallinity and degree of polymerization(DP) by presenting examples from two very different pretreatment strategies- organosolv and steam. For example, organosolv pretreatment of softwoods usually results in a clean separation into longer fibers (~1.5mm) and decreases in cellulose  DP while increasing substrate accessibility allowing hydrolysis yields of >70% at enzyme loadings of 5FPU/g. However, the recovery of the hemicelluloses from the pretreatment liquor for subsequent fermentation remains a challenge. In contrast, steam pretreatment of softwoods results in smaller substrate particles (~0.3mm), the solubilization and recovery of most of the hemicellulose, redistribution of lignin on the fiber surface as measured by X-ray photoelectron spectroscopy, and a less accessible cellulose surface area measured by water retention value and Simons staining. However, post-treatments can considerably improve the effectiveness of both pretreatment strategies. The potential benefits and remaining challenges of both steam and organosolv pretreatments will be discussed.