1. Introduction
The most abundant feedstock for microbial fermentation is lignocellulosic biomass, comprising cellulose, hemicellulose, and lignin [1]. Cellulose is a linear polymer of glucose units linked by β-1,4-glucosidic bonds. It constitutes 35–50% dry weight in agricultural lignocellulosic biomass [2]. Hemicelluloses, unlike cellulose are heterogenous polymers of primarily pentoses (xylose, arabinose), hexoses (mannose, glucose, galactose) and sugar acids [3]. Hemicelluloses constitute 20–35% dry weight in lignocellulosic biomass, with d-xylose as the major sugar after hydrolysis [2] and [3]. The ability of microorganisms to ferment both glucose and xylose in biomass to ethanol is of importance for an economically feasible process [4]. Native strains of Saccharomyces cerevisiae are unable to utilize xylose.