Steam explosion can be considered as one of the most promising techniques of fractionation of SB/SL. In the process, hydrolysis of the hemicellulose is accelerated by the contact of the biomass through steam at high temperature (160-240 °C) and high pressure (between 0.7 and 4.8 MPa) followed by quick decompression [16], which leads to breakdown of biomass [33]. Singh et al. [34] reported the steam explosion of SB which eventually showed the enzymatic hydrolysis efficiency of 100 % after 24 h of incubation by using the cellulases from Penicillium sp. Steam explosion disrupts the compactness of Abrils, thus increasing the surface area for better enzymatic action [16].

Sendelius [35] investigated steam explosion in SB at varying temperatures (180, 190, and 205 °C) for different time periods (5 and 10 min) using different impreg­nating agents (water, 2 % SO2, and 0.25 g sulfuric acid (H2SO4) per 100 g dry matter). These optimization studies showed 80 % theoretical hydrolysis yield at SO2-impregnation-180 °C-5min conditions. One of the major advantages of the steam explosion process is the rare or no use of chemicals, eventually reducing the operational costs and minimum production of potential inhibitors [17].

16.3.2.3 CO2 Explosion

Carbon dioxide (CO2) explosion is based on the utilization of CO2 as supercritical fluid leading to lignin removal. CO2 molecules at high pressure are able to penetrate small pores of SB disrupting the chemical nature of the substrate [36, 37]. Carbonic acid is formed in aqueous CO2 solution penetrating into the small pores of lignocel — lulosic material under high pressure. This method is considered environment friendly due to minimum by-products generation [14]. CO2 in presence of water forms car­bonic acid which allows the depolymerization of holocellulose eventually increasing the surface area of substrate. Zheng et al. [38] pretreated SB using CO2 as super­critical fluid which showed an increase in glucose yield by 50 % after enzymatic hydrolysis.