**2. Aerobic microbial pretreatment**

From both economic and environmental perspectives, fungal pretreatment with lignin-degrading microorganisms, preliminary white-rot fungi, has received renewed interest as an alternative to thermal/chemical pretreatment for biogas production [16]. The degradation of lignin by white-rot fungus is a process of biological oxidation. Under suitable conditions, the mycelium of white-rot fungus first secretes super fiber oxidase to dissolve the wax on the plant surface, and then the mycelium enters the plant interior and secretes the enzyme system to degrade lignin to complete the degradation of lignin. The results show that white-rot fungus can not only degrade lignin but also protect cellulose from damage, so as to improve the quality of lignocellulose and make it easier to be degraded by anaerobic bacteria. Ghosh and Bhattacharyya used *Phanerochaete chrysosporium* and *Polyporus ostreiformis* to treat rice straw, by which biogas and methane production was increased by about 34.73 and 46.19% in treated straw, respectively [17]. Taniguchi et al.

*Solid-State Anaerobic Microbial Ensilage Pretreatment DOI: http://dx.doi.org/10.5772/intechopen.92571*

compared the gas production effect of four kinds of white-rot fungi (*Trametes versicolor*, *Phanerochaete chrysosporiu*, *Ceriporiopsis subvermispora*, and *Pleurotus ostreatus*) on the pretreatment of rice straw. It was found that *P. ostreatus* selectively degraded the lignin fraction of rice straw rather than the holocellulose component [18]. Zhong et al. analyzed the pretreatment effect of *Pleurotus florida* on anaerobic fermentation of corn straw to produce biogas. AD experiments showed that the biogas productivity was increased by all the pretreatments and that the biogas production after NaOH pretreatment would be 20.07 and 16.58% higher than the raw corn straw and biologically pretreated ones, respectively [19]. Mackulak et al. used *Auricularia auricula-judae* to pretreat sweet chestnut leaves and hay at 37°C for 4–5 weeks, which had a 15% increase in biogas production compared with the untreated samples [20].

The advantages of this technology over thermochemical pretreatments include simple techniques, low energy requirements, no or reduced output of waste streams, reduced downstream processing costs, and no or reduced inhibitors to biogas fermentation. Despite the advantages, substantial holocellulose loss and long pretreatment time are the major issues associated with fungal pretreatment. The growth cycle of microorganism is long, such as white-rot fungus, which usually takes 7–15 days generally. White-rot fungi are sensitive to temperature, which grow faster at 26–32°C, and their growth will be inhibited if the temperature is too high or too low. Meanwhile, compared with chemical and physical methods, the biodegradation efficiency of lignocellulose by biological method is not high, while the pH value and material composition will also affect the growth of microorganisms. For example, white-rot fungus grows better under the condition of partial acid, and its growth and enzyme activity will be hindered with the increase in pH value. In addition, there are a few kinds of microorganisms that could degrade lignin, while the low enzyme activity is also an important factor limiting its application.
