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Cellulose and xylan are the two major polysaccharides found in plant fiber and represent a vast feedstock source for microbial ethanol production. To render the cellulose amenable to bioconversion, xylan is removed by a dilute sulfuric acid pretreatment, and at present, not utilized. However, the recent development of a recombinant Zymomonas moblis capable of co-fermenting glucose and xylose has renewed interest in fermentation of xylan to ethanol. For fermentation, the xylan must first be converted to xylose; enzymatic hydrolysis is a promising approach. Since xylan is a highly branched polymer, it is degraded by multiple debranching and depolymerizing enzymes. Acetyl xylan esterase (AXE), which cleaves acetyl esters from the xylan backbone, is required early in this process. Chapter One describes the purification and biochemical properties of an AXE from Aspergillus niger. The protein has a molecular weight of 32.1 kDa, an isoelectric point of 3.5, and is 10.4% glycosylated. The pH and temperature optima are 6.0 and 55C respectively. The Kusd\rm\sb{m}usd values for AXE are 0.298 g/L acetylated xylan and 0.023 Mp-nitrophenyl acetate; the Vusd\rm\sb{max}usd values are 3692 and 46,603 units/mg protein using acetylated xylan and p-nitrophenyl acetate respectively. N-terminal sequencing showed a single sequence homology, an AXE from A. niger DS16813. In addition to the AXE work, which has long-term potential to improve biomass conversion, studies focused on more immediate advances were undertaken. Dilute sulfuric acid pretreatment is corrosive and produces waste CaSO4. However, dilute phosphoric acid is less corrosive and can be neutralized with ammonia, generating ammonium phosphate, a fermentation nutrient. Chapter Two describes a pretreatment using dilute phosphoric acid in both aqueous and organosolv (70% methanol or ethanol) solutions. Fermentation of corn stover following pretreatment with either phosphoric or sulfuric acid resulted in similar ethanol yields. In addition, fermentation of switchgrass pretreated with phosphoric acid in 70% ethanol produced similar ethanol yields in comparison to dilute sulfuric acid pretreated switchgrass. Hence, phosphoric acid pretreatment is as effective as sulfuric acid pretreatment, but is less corrosive and yields a beneficial neutralization product.
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