Biological Conversion of Hemicellulose Extract into Value-Added Fuels and Chemicals

The research presented in this thesis describes the fermentation of hemicellulose extracts to produce ethanol and lactic acid. Hardwood hemicellulose extracts made at a variety of conditions using the green liquor and hot water extraction processes were characterized and then fermented by Escherichia coli K011 and Bacillus coagulans MXL-9. Hemicellulose extracts were found to contain a dilute amount of xylo-oligosaccharides and acetic acid as the major components, and many minor components including other organic acids, lignin-derived phenolics, and sugar degradation products. In order to generate ethanol economically, the fermentation must produce at least a 4% ethanol solution, which requires a starting sugar concentration of at least 8%. Since hemicellulose extracts did not contain high enough sugar titers, methods for concentrating the extracts prior to fermentation were investigated. Evaporation was found to be an ineffective method of concentrating sugars because it also concentrates such chemicals as acetic acid and sodium which inhibit fermentation. Ultrafiltration was then examined as a possible method of concentrating the oligomeric fraction of the extracts without effecting the concentrations of smaller molecules such as acetic acid and sodium. It does however concentrate the lignin fraction, which causes inhibition of fermentation organisms. Both E. coli and B. coagulans were found to consume all of the principal monosaccharides found in lignocellulosic biomass, including xylose and arabinose which cannot be utilized by traditional industrial yeast strains. These bacteria had higher tolerance to the inhibitors found in hemicellulose extracts than any other organism tested. The highest product titers achieved were 1% ethanol and 3% lactic acid.