A variety of nonbiological pretreatment methods have been extensively reviewed. These include physical, chemical, physicochemical and other combinations of procedures (Alvira et al., 2010; Chandra et al., 2007; da Costa Sousa et al., 2009; Hendriks and Zeeman, 2009; Sun and Cheng, 2002; Taherzadeh and Karimi, 2008). Based on their effects on biomass structure, pretreat­ments can be divided into different categories: those that increase enzyme accessibility to crystalline cellulose by decreasing the fiber’s degree of polymerization or by facilitatinghemicellulose and/or lignin removal to create pores in the cellulose fibrils. Since hemicellulose and lignin are the two main protective coats surrounding cellulose, they have to be removed or altered in order to achieve fast enzymatic hydrolysis of the biomass. How­ever, to obtain high sugar yield for both hexoses and pen­toses, an ideal pretreatment procedure should efficiently remove or modify lignin and also hydrolyze hemicellu — lose, but not degrade these hemicellulose sugars (Ohgren et al., 2007). Some of the most widely investigated proce­dures are briefly described.

Physical Pretreatments

These include mechanical methods to chip, grind and mill the biomass to reduce particle size and, potentially, the crystallinity and degree of polymerization of ligno — cellulose in order to maximize the downstream enzyme hydrolysis process (Tassinari et al., 1980). Recently, a novel extrusion method was developed where the biomass materials are subjected to heating, mixing and shearing to cause both physical and chemical modifica­tions to the material in order to increase cellulose acces­sibility (Karunanithy and Muthukumarappan, 2010a, b; Karunanithy et al., 2012).

Chemical Pretreatments

These are mainly alkali and acid pretreatments. Alkali pretreatments increase cellulose digestibility by enhancing lignin solubilization and decreasing cellulose crystallinity. This method is more effective on agricul­tural biomass than on wood material (Kumar et al., 2009; Playne, 1984). Acid pretreatment, mostly diluted acid pretreatments, increase cellulose accessibility mainly by solubilizing hemicellulose. It can be used as either a pretreatment or a direct hydrolysis process but leads to toxic degradation products that inhibit down­stream fermentation (Alvira et al., 2010). On the con­trary, ozonolysis uses the powerful oxidant ozone to delignify lignocellulosic materials at room temperature and does not form inhibitory compounds, yet it is economically unviable due to large amounts of ozone consumed (Sun and Cheng, 2002). On the other hand, organosolv process can efficiently remove lignin and result in minimal cellulose loss. This is a promising pro­cess if economic solvents are available at commercial scales (Wood and Saddler, 1988; Zhao et al., 2009).