The microbial conversion of wood to sugars, liquid fuels and chemicals is highly dependant on the pretreatment method used. In the present report we have studied the pretreatment method of steam explosion as it pertains to the one-step or two-step conversion of wood to ethanol. Steam explosion was found to render the hemicellulose soluble in water, depolymerize the lignin and make the cellulose component more accessible to cellulase enzymes. A dilute presoaking of the aspenwood chips with H2SO4 greatly increased the solubility of the hemicellulose and selective extraction of steam exploded aspenwood produced substrates containing over 90% cellulose. The one-step coversion of cellulose toethanol was found to have major drawbacks notably the low ethanol and low substrate tolerance of Clostridium thermocellum. Attempts to overcome the first drawback by the adaptation of C. thermocellum to tolerate high ethanol levels was successful but did not result in increased ethanol yields. Attempts at increasing ethanol yields through the co-culture of C. thermocellum with either C. thermohydrosulfuicum or C. thermosaccharolyticum resulted in a 2 fold increase in ethanol levels. However, further attempts at increasing the substrate concentration above 1% were only partially successful and only extremely low levels of ethanol (<1%) could be produced from pretreated aspenwood using the one-stage process.
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