Physical methods involve breakdown of biomass size by coarse size reduction, chipping, shredding, grinding, and milling in order to increase the available specific surface area and reduce the degree of polymerization, enhancing the digestibility of lignocellulosic biomass (Agbor et al., 2011; Brodeur et al., 2011). However, it has been shown that further reduction of biomass parti­cle size below 0.4 mm has little effect on the rates and yields of biomass hydrolysis (Agbor et al., 2011).

Chipping reduces heat and mass transfer limitations; grinding and milling are more effective at reducing the particle size and cellulose crystallinity than chipping, probably as result of the shear forces generated during milling. Vibratory ball milling has been used with more effective results in reducing cellulose crystallinity than ordinary ball milling. Also, disk milling, which pro­duces fibers, has been reported as more efficient in enhancing cellulose hydrolysis than hammer milling, which produces finer bundles (Agbor et al., 2011). Stir­ring ball milling also could significantly damage the structure of biomass, resulting in the variation of surface morphology, the increase in amorphous region ratio and hydrogen bond energy, and the decrease in crystallinity and crystalline size (Liao et al., 2011).

The energy requirements for physical pretreatments are dependent on the biomass characteristics, final par­ticle size and reduction in crystallinity, for example, hardwoods require more energy input than agricultural residues (Agbor et al., 2011). Taking into account the high-energy requirement of milling on an industrial scale and the rise in energy demands, this method is not economically feasible and likely will not be used in a full-scale process (Agbor et al., 2011; Saritha and Lata, 2011). In most cases where the only option avail­able for pretreatment is physical, the required energy is higher than the theoretical energy content available in the biomass (Brodeur et al., 2011).

The pretreated biomasses by physical methods are subjected to heating, mixing and shearing resulting in physical and chemical modifications (Karunanithy and Muthukumarappan, 2011; Lamsal et al., 2010; Saritha and Lata, 2011). Also, Agbor et al., in 2011, suggested that the materials can be milled after chemical pretreat­ment with significantly reduction of (1) milling energy consumption, (2) reduce cost of solid liquid separation because the pretreated chips can be easily separated,

(3) eliminate energy-intensive mixing of pretreatment slurries, (4) liquid to solid ratio and (5) did not result in the production of fermentation inhibitors.

Another physical method is extrusion that disrupts the lignocellulose structure and increases the accessi­bility of carbohydrates to enzyme attack. This method has reported the improvement on sugar recovery up to 63.5% (Karunanithy and Muthukumarappan, 2011). Other physical pretreatments involve the use of gamma rays that cleave the b-1,4 glycosidic bonds, thus giving a larger surface area and lower crystallinity. This method will undoubtedly be very expensive on a large scale with huge environmental and safety concerns (Agbor et al., 2011). Proton beam irradiation has also been tested reporting a glucose conversion of 68% of the theoretical maximum at 72 h (Kim et al., 2011b).

The effect of microwave and microwave-chemical pre­treatments on densification characteristics and physical quality of pellets has also been investigated showing that microwave pretreatment was significantly able to disintegrate the lignocellulosic structure of wheat and barley straw grinds (Kashaninejad and Tabil, 2011). These pretreatments also have been tested on barley husks, sweet sorghum bagasse, bamboo, coconut husk and gar­den biomass (Choudhary et al., 2012; Ding et al., 2012; Gabhane et al., 2011; Jackowiak et al., 2011; Janker — Obermeier et al., 2012; Roos et al., 2009; Wu et al., 2012). However, some restrictions on energy consumption must be accomplished to obtain a positive energy balance with these pretreatments. Ultrasounds and ultrasound — assisted alkaline pretreatment also has been reported (Sun et al., 2002; Velmurugan and Muthukumar, 2012b). On the other hand, some continuous system includes a wet disk milling of rice straw (Hideno et al., 2012) and pulsed electric field of wood chip and switchgrass (Kumar et al., 2011).