Organosolv process involves the use of an organic or aqueous organic solvent mixture (methanol, ethanol, acetone, ethylene glycol, triethylene glycol, tetrahydrofurfuryl alcohol, glycerol, aqueous phenol, aqueous я-butanol) and water, with or without ad­dition of an acid (oxalic, salicylic, and acetyl salicylic acid) or base as catalyst agents at high temperature (150-200 °C) for the lignin degradation and partial solubilization of hemicellulose sugars [10]. Solvents used during the pretreatment can be elimi­nated by evaporation and condensation and recycled to reduce the operational costs of the process. This step allows avoiding the presence of inhibitory compounds, which mayreduce the rate of enzymatic hydrolysis [14,27]. During organosolv, low boiling point alcohols such as methanol, ethanol seems more effective due to low cost and easy recovery of solvents. Lignocellulosic substrates after organosolv pretreatment have high crystallinity and increased surface area for better cellulolytic enzymes ac­tion. However, organosolv pretreatment methods are expensive at large scale. Zhao et al. [67] comprehensively reviewed the various organosolv pretreatment methods (alcohol mediated, ethanol, acetone, per acetic acid, per formic acid, etc). They further recommended developing the pretreatment strategies based on continuous processes in reactors with fewer loads of organic solvents with high solid biomass and exploration of the increased applications of by-products generated during the organosolv pretreatment.

Mesa et al. [23] described a combination of a dilute-acid pretreatment followed by the organosolv pretreatment with NaOH (60 min, 195 °C, ethanol 30 % v/v) to fractionate the SB which resulted in 67.3 % (w/w) glucose released.

16.3.1 Biological Pretreatment

The selective delignifying microorganisms usually applied to SB/SL for the degra­dation of lignin. This process can also be referred as in-situ microbial delignification (ISMD). Generally white-rot fungi are used for the degradation of lignin and hemi- celluloses fraction of SB/SL. The myco-SB/SL (SB/SL after fungal growth) can be used subsequently for enzymatic hydrolysis which in turn yields high amount of fermentable sugars [13, 68]. Figure 16.2 shows the mechanistic demonstration of biological pretreatment applied to SB/SL.

This is the low energy and capital intensive process where mild environmen­tal conditions are required [68]. Selective lignin degraders such as Pycnoporous cinnabarinus, and Ceriporiopsis subvermispora are more useful in ISMD, having affinity toward lignin breakdown than cellulose and hemicelluloses [13]. SB was pre­treated with the C. subvermispora for 30 days of incubation. The results of chemical analysis and mass component loss showed that C. subvermispora was selective to lignin degradation. Pretreated SB after soda/anthraquinone pulping showed improved pulp yields, kappa number, and viscosity pretreated SB [69].