Most biofuel processes currently under development require pretreatment followed by hydrolysis to produce monomeric sugars. The goal of any pretreatment is to separate the polysaccharide matrix of cellulose and hemicellulose from lignin and to loosen the structure enabling sites for chemical or enzymatic catalysis. The polysaccharide matrix may be hydrolyzed using chemical routes (such as acid hydrolysis) or by biological enzymatic routes (Huber et al. 2006). The sugars derived from hemicellulose and cellulose are fermented to produce a wide range of biofuels such as ethanol, hydrogen, biodiesel via lipid biosynthesis and butanol (Chandrakant and Bisaria 1998; Kim et al. 2008; Alvira et al. 2010; Panagiotopoulos et al. 2010; Wu et al. 2011a, b).

Many possible methods of chemical pretreatment have been reported, including steam explosion, dilute acid hydrolysis, concentrated acid hydrolysis, supercritical CO2 explosion and extraction, alkaline pretreatment (sodium hydroxide, potassium hydroxide, lime), ionic liquids, soaking in aqueous ammonia (SAA), ammonia recycles percolation (ARP) and ammonia fiber explosion (AFEX) pretreatment (Alizadeh et al. 2005; Mosier et al. 2005; Huber et al. 2006; Kim et al. 2006, 2007, 2008; Isci et al. 2009; Singh et al. 2009; Alvira et al. 2010; Panagiotopoulos et al. 2010; Wu et al. 2011a, b). In many of these pretreatments, the formation of inhibitory compounds at high temperatures (such as furfural and 5-hydroxymethylfurfural (HMF)) are one of the main constraints (Mosier et al. 2005).

Lignocellulosic biomass is highly recalcitrant to fermentation due to natural resistance mechanisms. Moreover, woody biomass such as pinewood has an even greater microbial recalcitrance than herbaceous biomass due to a tightly bound structure with high lignin content (Galbe and Zacchi

2002) . To enhance the release of polysaccharides from the lignocellulosic biomass, upstream processing (including size reduction and pretreatment) is a necessary step in biofuel production. For the production process to be economically feasible, total energy consumption in the size reduction and the pretreatment steps should be minimized as much as possible (Zhu et al. 2010).

Physical pretreatment involves size reduction to increase the available surface area and enhance enzyme hydrolysis of plant polysaccharides. Chemical and biological pretreatment methods are designed to liberate the convertible polysaccharide from the protective lignin casing, as well as to reduce the crystallinity of the cellulose so as to make the polysaccharides available to the hydrolyzing microorganisms (Hendriks and Zeeman 2009; Harmsen et al. 2010; Alvira et al. 2010). Selecting a pretreatment process is dependent on the particle size, moisture content and lignin content of the lignocellulosic biomass.