Investigating biomass saccharification for the production of cellulosic ethanol
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The production of second generation biofuels -- cellulosic ethanol from renewable lignocellulosic biomass has the potential to lead the bioindustrial revolution necessary to the transition from a fossil fuel-based economy to a sustainable carbohydrate economy. Effective release of fermentable sugars through biomass pretreatment followed by enzymatic hydrolysis is among the most costly steps for emerging cellulosic ethanol biorefineries. In this project, two pretreatment methods (dilute acid, DA, and cellulose solvent- and organic solvent-lignocellulose fractionation, COSLIF) for corn stover were compared. It was found that glucan digestibility of the corn stover pretreated by COSLIF was much higher, along with faster hydrolysis rate, than that by DA- pretreated. This difference was more significant at a low enzyme loading. Quantitative measurements of total substrate accessibility to cellulase (TSAC), cellulose accessibility to cellulase (CAC), and non-cellulose accessibility to cellulase (NCAC) based on adsorption of a non-hydrolytic recombinant protein TGC were established to find out the cause. The COSLIF-pretreated corn stover had a CAC nearly twice that of the DA-pretreated biomass. Further supported by qualitative scanning electron microscopy images, these results suggested that COSLIF treatment disrupted microfibrillar structures within biomass while DA treatment mainly removed hemicelluloses, resulting in a much less substrate accessibility of the latter than of the former. It also concluded that enhancing substrate accessibility was the key to an efficient bioconversion of lignocellulose. A simple method for determining the adsorbed cellulase on cellulosic materials or pretreated lignocellulose was established for better understanding of cellulase adsorption and desorption. This method involved hydrolysis of adsorbed cellulase in the presence of 10 M of NaOH at 121oC for 20 min, followed by the ninhydrin assay for the amino acids released from the hydrolyzed cellulase. The major lignocellulosic components (i.e. cellulose, hemicellulose, and lignin) did not interfere with the ninhydrin assay. A number of cellulase desorption methods were investigated, including pH adjustment, detergents, high salt solution, and polyhydric alcohols. The pH adjustment to 13.0 and the elution by 72% ethylene glycol at a neutral pH were among the most efficient approaches for desorbing the adsorbed cellulase. For the recycling of active cellulase, a modest pH adjustment to 10.0 may be a low-cost method to desorb active cellulase. More than 90% of cellulase for hydrolysis of the pretreated corn stover could be recycled by washing at pH 10.0. This study provided an in-depth understanding of biomass saccharification for the production of cellulosic ethanol for cellulose hydrolysis and cellulase adsorption and desorption. It will be of great importance for developing better lignocellulose pretreatment technologies and improving cellulose hydrolysis by engineered cellulases.
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