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Substrate properties that influence the enzymatic hydrolysis of organosolv-pretreated softwoods at low enzyme loadings Del Rio, Luis Fernando

Abstract

Lignocellulosic biomass is a potential source of sugars for the production of fuels and chemicals. However, its resistance to chemical and biological degradation poses a significant challenge. Consequently, a pretreatment is required to increase the accessibility of cellulose to cellulases. The organosolv process is one of the few pretreatments that can process softwoods to generate substrates that are readily hydrolyzed by cellulases. However, because the residual lignin and hemicelluloses can restrict cellulose accessibility, obtaining significant cellulose conversion at low enzyme loadings (< 5 FPU g-¹ cellulose) remains a challenge. As complete delignification is not economically viable, we hypothesize that other substrate properties could be altered to reduce the recalcitrance of organosolv-pretreated softwoods. The initial work reported in this thesis compared the effects of pretreatment severity on the physico-chemical properties and enzymatic hydrolysis of organosolv-pretreated lodgepole pine and hybrid poplar (a representative hardwood). As expected, hybrid poplar was less recalcitrant than lodgepole pine. However, unlike lodgepole pine, which remained unaffected by changes in pretreatment severity, the recalcitrance of hybrid poplar increased with pretreatment severity. Interestingly, it was found that this increased recalcitrance was due to non-productive binding between lignin and cellulases. In contrast the inhibitory effect of lignin on the enzymatic hydrolysis of lodgepole pine was due to a combination of restricted cellulose accessibility and hydrophobic interactions between lignin and cellulases. Subsequent studies showed that rather than a single substrate characteristic such as lignin content, particle size or cellulose degree of polymerization, the overall cellulose accessibility is the key substrate characteristic that governs the susceptibility of the organosolv-pretreated substrates to enzymatic hydrolysis. Surprisingly, attempts to increase cellulose accessibility through PFI-mill refining were unsuccessful. However, increasing the hydrophilicity of the residual lignin via neutral sulphonation resulted in significant increases in enzymatic hydrolysis at 5 and 2.5 FPU g-¹ cellulose (from 80% to 95% and from 35% to 80%, respectively). This was likely due to decreased nonspecific interactions between the sulphonated lignin and cellulases. Overall, the results suggest that increasing the hydrophilicity of the residual lignin could be a viable strategy to reduce the recalcitrance of organosolv-pretreated softwoods.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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