Hemicellulose consists mainly of xylose and is, next to cellulose, the most abundant renewable carbon source. Efficient
bioconversion of xylose to ethanol could make hemicellulose a valuable raw material (2). The hexose sugars (glucose, mannose, and galactose) are relatively easily fermented to ethanol, whereas the pentose sugars (xylose and arabinose) are not (3). Xylose is abundant in the hydrolysates of hemicellulose, for about 10-40% of the total carbohydrates (4). Hence, fuel ethanol production from pentose-rich sources, such as hardwood or agricultural residues will only be economically viable
if xylose is fermented to ethanol (5, 6).