UBC Theses and Dissertations
The influence of lignin on the enzymatic hydrolysis of pretreated biomass substrates Nakagame, Seiji
During the “enzymatic hydrolysis of the cellulose” component of the overall lignocelluloses-to-bioethanol process, lignin has been shown to be a very influential factor, acting as both a physical barrier and limiting hydrolysis through the adsorption of cellulases. Although hydrophobic, electrostatic and hydrogen bonding interactions between lignin and cellulases have been suggested to influence the hydrolysis efficiency, comparative studies using isolated lignins from different types of biomass which have been pretreatment in different ways have not been done. To gain a better understanding of the effects of lignin on enzymatic hydrolysis, six different substrates: steam and organosolv pretreated softwood (lodgepole pine), hardwood (poplar) and an agricultural residue (corn stover), were prepared. Lignin was isolated from the pretreated substrates by two methods. The lower lignin yields obtained with corn stover when compared to poplar and lodgepole pine suggested that the hydrophobicity of the corn stover derived lignin was lower than the lignin from poplar and lodgepole pine. The characterization of the physical and chemical properties of the isolated lignins showed that the carboxylic acid present in the isolated lignin had a significant influence on the enzymatic hydrolysis yields when lignin was added to pure cellulose. Dehydrogenative polymers (DHP) from ferulic acid adsorbed lower amounts of cellulases and did not decrease hydrolysis yields when compared to the DHP from coniferyl alcohol, showing that the increased carboxylic acid content of the lignin alleviated the non-productive binding of cellulases and increased the enzymatic hydrolysis of the cellulose. Douglas-fir was next steam pretreated at different severities and the lignin was isolated from the water insoluble fraction. The lower hydrolysis yields obtained with the substrates pretreated at 190⁰C when compared with those treated at 200 and 210⁰C was attributed to the lower accessible surface area of the substrate pretreated at 190⁰C rather than lignin-enzyme interactions. Isoelectric focusing analysis after incubation of cellulases with the lignin showed that the positively charged cellulases were preferentially adsorbed, indicating that electrostatic interaction was involved in cellulase adsorption onto the lignin. It was also apparent that the hydrophobicity of the lignin also played a role in the adsorption of cellulases.
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