Detoxification of Lignocellulosic Hydrolysates for Improved Bioethanol Production

Lignocelluloses are the most abundant raw materials on Earth comprised of cellulose, hemicelluloses and lignin. After cellulose, hemicellulose is the principal fraction of the plant cell wall that could serve as a potential substrate for the production of value-added products under optimized conditions [Chandel & Singh, 2011; Chandel et al., 2010a; Hahn-Hagerdal et al., 2007; Saha, 2003]. Largely, the secondary cell wall of plants contains cellulose (40– 80%), hemicellulose (10–40%), and lignin (5–25%). The carbohydrate fraction of the plant cell wall can be converted into fermentable monomeric sugars through acidic and/or enzymatic (hemicellulase/cellulase) reactions, which have been exploited to produce ethanol, xylitol, n-butanol and 2, 3-butanediol via microbial fermentation processes [Sun, 2009.; Chandel et al., 2010a; Carvalheiro et al., 2005; Saha, 2003]. Until now the pretreatment is unavoidable necessity, which has been examined and employed extensively in the past [Moiser et al., 2005, Taherzadeh & Karimi 2007; Kumar et al., 2009; Chandel et al., 2010b]. The acidic pretreatment of lignocellulosics hydrolyzes the hemicellulose fraction, enabling subsequent enzymatic digestion of the cellulose in fermentation reaction [Kumar et al., 2009; Chandel et al., 2007a; Chandel et al., 2007b; Chandel et al., 2007c]. However, the non-specificity of acidic treatment led to the formation of complex sugars and compounds inhibitory to the microorganisms for ethanol production [Parawira & Tekere, 2011]. The depolymerization of hemicellulose by chemical process yields xylose as the major fraction and arabinose, mannose, galactose, and glucose in smaller fractions in addition to potential microbial inhibitors [Chandel et al., 2010a; Gírio et al., 2010; Chandel et al., 2009; Chandel et al., 2007a]. These inhibitors can be divided into three major groups (Fig. 1), i.e. organic acids (acetic, formic and levulinic acids), ii. furan derivatives [furfural and 5hydroxymethylfurfural (5-HMF)], iii. phenolic compounds [Chandel et al., 2010a; Chandel et al, 2007b; Mussatto and Roberto, 2004; Palmqvist and Hahn-Hagerdal, 2000a], affecting overall cell physiology and often result in decreased viability, ethanol yield, and productivity [Chandel et al, 2007a; Palmqvist & Hahn-Hagerdal, 2000a]. The ethanologenic microorganisms have ability to degrade some of the inhibitors; however, the toxicity of hydrolysate was determined by the aggregate effect of compounds [Mussatto and Roberto, 2004; Zaldivar et al., 2001]. Progress has been made to achieve higher levels of sugars by diminishing the overall impact of fermentative inhibitors which in-turn improves the fermentability of lignocellulosic hydrolysates [Alriksson et al., 2011; Sun & Liu, 2011;