Biological pretreatments
Biological pretreatment of rice straw is a safe and environmental friendly process. It has low chemical and energy requirement, higher yield of desired product and greater substrate and reaction specificity [79]. Generally, white-rot fungi belong to Basidiomycetes class is employed for biological pretreatment [80]. White rot fungi produces lignolytic enzymes such as Manganese peroxidase (MnP), lignin peroxidase (LiP) and laccase [81] and [82] and in the presence of Mn (III) and veratryl alcohol (VA), MnP and LiP are oxidized by H2O2 respectively, the oxidized form of these enzymes causes the oxidation of lignin. Laccase catalyzes the oxidation of phenolic compounds of lignin in the presence of lignin-oxidizers such as 2,2′-azinobis-(3)-ethylbenzylthiazoline-6-sulfonate (ABTS) and 3-hydroxyanthranilic acid (HA) or laccase-oxidizing precursors such as O2[83]. Several researchers have reported that white rot fungi such as Phanerochaete chrysosporium, Ceriporiopsis subvermispora, Phlebia subserialis and Pleurotus ostreatus can competently remove lignin [79] and [84]. Dichomitus squalens used in biological pretreatment of rice straw increases the enzymatic digestibility of lignocelluloses and promote cellulose hydrolysis [85].