MECHANICAL COMMINUTION

The objective is to cut the particle size and crystal­linity of lignocellulosic biomass in order to increase the surface area and reduce the degree of polymeriza­tion. Methods like chipping, grinding and milling can be used to improve the further enzymatic hydrolysis. However, this process is not economically feasible due to the high energy requirement (Tassinari et al., 1980). During comminution, vibratory ball milling is found to be more efficient in breaking down the cellu­lose molecules of spruce and aspen chips and improving the digestibility of the biomass than ordi­nary ball milling (Sun and Cheng, 2002). The power requirement of mechanical comminution of agricultural materials depends on the final particle size and the waste biomass characteristics.

STEAM EXPLOSION

It is a hydrothermal pretreatment in which the ligno — cellulose is subjected to pressurized water vapors for few seconds to several minutes, and then suddenly dep­ressurized. In this process, combination with the partial hydrolysis of hemicelluloses and solubilization, the lignin is redistributed and removed up to certain level from the material (Pan et al., 2005). Although this tech­nique is cost-effective, it generates toxic by-products and the hemicelluloses degradation is partial (Saritha et al., 2012).

ULTRASONIC PRETREATMENT

This technique is extensively used for the treatment of sludge from wastewater treatment plants. An experi­ment on carboxyl methyl cellulose with irradiation increased the rate of enzymatic hydrolysis up to 200% approximately (Imai et al., 2004). The mechanism of ac­tion in ultrasonic treatment remains unknown. One guess is that, the hydrogen bonds in the crystalline cel­lulose structures were broken due to irradiation energy, whose energy is higher than the hydrogen bond energy (Bochek, 2003).

EXTRUSION

This process disrupts the crystal structure of lignocel — lulose and increases the accessibility of carbohydrates to enzyme. In this case, materials are subjected to heating, mixing and shearing resulting in physical and chemical modifications in biomass structure (Karunanithy et al.,

2008) . However, the process is novel and not widely applied.