Biological pretreatment of lignocellulosic biomass changes the physico-chemical characteristic of biomass. Among the changes, lignin degradation is the most at­tractive and most studied. For example, lignin loss in wheat straw was found 25 % after 1 week [128]; lignin loss in corn straw was up to 54.6 % after 30 days pre­treatment with T. vericolor [129]; lignin loss increased from 75.67 % to 80 % when corn stalk treated with Irpex lacteus [130]; lignin extractability and glucose yield could be improved in canola straw with fungus strain T. vericolor and cellobiose dehydrogenase-deficient strain (m4D) [44]. Degradation of lignin by microbes is mainly due to a non-specific oxidative reaction, which leads to complete oxidation of lignin. Among bio-delignifier, white-rot fungus is one of the mostly studied microbes, as discussed earlier, which has unique capability to cleave carbon-carbon linkages of lignin and oxidizes with the help of various lignolytic enzymes. The changes in terms of the ratio between p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units of lignin were analyzed using pyrolysis-gas chromatography-mass spectrometry (Py — GC-MS) and concluded that the susceptibility of lignin units are in the following order: S > G > H. This order indicates that the biomass with S-rich lignin is more susceptible to fungal degradation than the biomass with other lignin units [32].

During fungal attack on biomass, hemicellulose and cellulose are also consumed and among biomass components, hemicellulose is easier to degrade. White-rot fungi such as P. chrysosporium [131], P. citrinopileatus and P. florida [132], Trametes ochracea and E. taxodii 2538 [77], C. subvermispora [133] have been found to degrade hemicellulose along with lignin loss (Table 1.1) and showed the multiple endoxylanase activity. This effect results in reduction of recalcitrance of lignocellu — lose but increases the risk of loss of cellulose or lowering the all sugar recovery in bioconversion process [130].

White-rot fungi also secrete cellulase enzyme with different specificity and synergistic characteristics during biological treatment of lignocellulose. Cellulase hydrolyzes P-1,4-linkage of cellulose to glucose and the hydrolyzed products are utilized by same fungi or other microbes. As mentioned earlier, non-selective white — rot fungi mineralize all lignocellulosic components equally. Selective white-rot fungi generally degrade negligible amount of cellulose and have promising role in the bi­ological pretreatment of lignocellulose. Cellulose loss can be analyzed by X-ray diffraction (XRD) method in terms of crystallinity index (CrI). When the corn stover was treated with brown-rot fungi Fomitopsis sp. IMER2, the crystallinity degree of treated biomass could be increased from 33.22 % to 46.06 % and crystalline por­tion from 59.96 % to 94.96 % [134]; and it was found that the crystalline change of the treated biomass is due to Fomitopsis sp. IMER2 preferential degradation of the amorphous region of cellulose. In contrast, crystallinity decreased from 68.4% to 64-65.9 % after the biological pretreatment of Japanese red pine (Pinus densiflora) with three white-rot fungi [5].

Further, Xu et al. [31] investigated the surface morphological changes dur­ing white-rot fungus I. lacteus CD2 attack on corn stover by scanning electron microscopy (SEM). SEM images showed some physical changes after biologi­cal treatment and resulted in irregular holes in the corn stover. The functional group changes and bond arrangement in the treated corn stover were analyzed by Fourier transform infrared (FTIR) spectroscopy [31], wheat straw biodegradation by P chrysosporium [131] and bamboo culms (Phyllostachys pubescence), which was treated by E. taxodii 2538 and T. versicolor G20 [135]. The various characterization results, obtained by distinguished researchers, indicate that biological treatment in­creases the pore volume, pore size and remarkably enhance the surface area of the lignocellulose. A more-defined surface area obtained from wheat straw treated by P chrysosporium supplemented with Tween 80 inorganic salts, indicating removal of lignin and making more accessible the surface of hemicellulose and cellulose [128]. Xu et al. [31] also indicated that biological treatment of corn stover with I. lacteus CD2 enhanced the pore size and pore volume of corn stover, resulted more accessible surface area for enzymatic saccharification.

Raw material Strain name Weight loss (%) Lignin loss (%) Cellulose loss (%) Hemicellulose loss (%) Reference Softwood, Pinus densiflom Ceriporia laceratalacerate 9.5 ±0.5 13.1 ±0.4 8.0 ±0.5 — [5] Polyporus bnnnalis 9.9 ±0.4 11.6 ±0.3 10.6 ±0.3 — Stereum hirsutum 10.7 ±0.7 14.5 ±0.4 7.8 ±0.3 — Sugarcane trashes Celhdomonas cartae 15.5 ±3.83 5.5 ±0.26 25.4 ±0.66 — [13] Celhdomonas и da 24.3 ±2.06 5.5 ±0.25 21.8 ±1.25 — Bacillus macerans 17.5 ±0.49 5.5 ±0.22 30.4 ±0.51 — Zymomonas mobdis 17.9 ±0.54 8 ±0.51 26.8 ±0.63 — Prosopis juliflora wood Pycnopoms cinnabarinus 18.87 ±1.11 8.87 ±0.22 4.06 ±0.18 — [136] Lantana camara wood Pycnopoms cinnabarinus 15.4 ±1.88 13.13 ±1.32 2.34 ± 0.54а — Chinese willow {Salbe baby- Echinodontium taxodii 32.5 ±1.7 45.6 ±2.0 26.7 ±0.2 50.8 ±1.8 [77] lonica, hardwood) China-fir (Cunninghmnia Echinodontium taxodii 24.1 ±0.9 39.8 ±1.2 12.6 ±0.1 31.4 ±2.7 lanceolata, softwood) Com stover Ceriporiopsis subvennispora 18.8 39.2 4.8 ±0.25 28 ±0.5 [137] Ceriporiopsis subvennispora 14.59 ±0.28 31.33 ±1.01 4.49 ±1.29 22.45 ±0.54 [138] Bamboo clurns Echinodontium taxodii 10.58 24.28 1.64 28.46 [135] Flammulina velutipes 2.27 3.14 3.88 4.82 Ganodenna lucidum 12.1 10.56 12.83 15.16 Trametes ochracea 15.21 18.63 10.79 29.22 Trichaptum bifonne 11.04 12.54 8.48 32.7 Water hyacinth Pleurotus citrinopileatus 31.9 ±0.2 19.1 ±0.4 30.1 ±0.5 37.5 ±0.1 [132] Pleurotus plorida 28.8 ±0.4 19.7 ±0.3 28.5 ±0.8 30.5 ±0.7 Moso Bamboo Irpex lacteus — 17.87 ±0.83 48.20 ±0.92 18.50 ±0.97 [139] Wheat straw Fames fomentarius — 35 ±1 45 ±1 51 ±27 [140] Com stover Auricularia polytricha, Irpex — 17.8 ±1.0 31.5 ±0.8 16.8 ±0.9 [141] lacteus

The effect of biological pretreatment of lignocellulose in terms of weight loss, cellulose, hemicellulose, and lignin losses is summarized in Table 1.1.