Biological Pretreatment

Most pretreatment technologies require exclusive and expensive instruments or equipment that have high energy requirements, depending on the process. In particular, physical and thermochemical processes require abundant energy for biomass conversion. Biological treatment using various types of rot fungi, a safe and environmentally friendly method, is increasingly being advocated as a process that does not require high energy for lignin removal from a lignocellulosic bio­mass, despite extensive lignin degradation (Okano et al. 2005).

In biological pretreatment processes, microorganisms such as brown-, white-, and soft-rot fungi are used to degrade lignin and hemicellulose in waste materials (Galbe and Zacchi 2007). Brown rots mainly attack cellulose, whereas white and soft rots attack both cellulose and lignin. Lignin degradation by white-rot fungi occurs through the action of lignin-degrading peroxidases and laccase (Lee et al.

2007) . These enzymes are regulated by carbon and nitrogen sources. White-rot fungi are the most effective for biological pretreatment of lignocellulosic materials (Fan et al. 1987). An extensive range of studies has been carried out on the pretreatment of wheat straw by 19 white-rot fungi (Hatakka 1983).

The white-rot fungus P. chrysosporium produces lignin degrading enzymes, lignin peroxidases, and manganese-dependent peroxidases, during secondary metabolism, in response to carbon or nitrogen limitation (Boominathan and Reddy

1992) . Both enzymes have been found in the extracellular filtrates of many white — rot fungi for the degradation of wood cell walls (Kirk and Farrell 1987; Waldner et al. 1988). Singh et al. (2008) evaluated eight bioagents, including fungi and bacteria, for their pretreatment effects on sugarcane trash. They narrowed down the C/N ratio of trash from 108:1 to a varying range from approximately 42:1-60:1. The maximum drop in C/N ratio of 61 % was observed using Aspergillus terreus, followed by those obtained using Cellulomonas uda (52 %) and Trichoderma reesei and Zymomonas mobiliz (49 %). The C/N ratio is important for biomass pretreatment, because degradation of lignocellulosic material depends on the material’s C/N ratio. To degrade each molecule of carbon, a definite proportion of nitrogen is required by the microorganisms, and this varies with different kinds of micro flora. Fungi have a higher C/N ratio (30:1) as compared to bacteria (10:1); hence, fungi are more capable of degrading any lignocellulosic material, as their dependency on nitrogen is comparatively lower (Wichern et al. 2004). Report by Ravichandra et al. (2012) on the microbial and cell free hydrolysis of corncobs using P. chrysosporium has shown the efficient release of sugars (424.50 mg/2 g for 120 h and 395.15 mg/2 g for 48 h).

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