Enhanced Enzymatic Hydrolysis of Lignocellulose in Bioethanol Production Substrate Interactions with Surfactants and Polymers
Abstract: Ethanol produced from cellulose is a promising future alternative fuel. The production process has three main steps: (i) pretreatment of raw material to increase degradability; (ii) enzymatic hydrolysis of cellulose to produce glucose; (iii) fermentation of glucose into bioethanol. This thesis contains studies with the aim to improve the enzymatic hydrolysis of cellulose from steam pretreated spruce. Improvements have been made by additions of surface active additives such as non-ionic surfactants or poly(ethylene glycol) polymers (PEG). It was found that that these surface active additives increased the enzymatic hydrolysis by reduction of non-productive adsorption of cellulases on the lignin part of the substrate. As a result the enzyme consumption could be significantly lowered with retained cellulose conversion. With addition of PEG a higher hydrolysis temperature (50 C) could be used; reduced deactivation of enzymes is due to PEG adsorption on lignin surfaces. By increased temperature the conversion rate was significantly improved. PEG was found to interact with lignin by hydrophobic interaction. Lignin adsorbed PEG polymers excludes enzymes from the lignin surface, which will increase the amount enzymes available for cellulose hydrolysis. A study on the adsorption of cellulase modules showed difference in lignin affinity for the cellulose binding modules of the enzymes Cel7A and Cel7B. This was suggested to be caused by surface exposed hydrophobic aromatic residues. Simultaneous saccharification and fermentation of spruce lignocellulose with Tween 20 additions resulted in increased ethanol yields and shorter residence times. Enzymes in a new efficient cellulase system from Penicillium brasilianum has been purified and characterised. Activity studies showed that these enzymes were two cellobiohydrolases, three endoglucanases and one xylanase. Two of the endoglucanase enzymes were found to belong to glycoside hydrolase family 5 and 12. In production of cellulases, degradation of produced enzymes by proteases is not uncommon. An unidentified protease from the widely used cellulase producer Hypocrea jecorina has been purified and characterised. The new protease was found to be a trypsin-like serine protease.
This dissertation MIGHT be available in PDF-format. Check this page to see if it is available for download.