4. Pretreatment methodsA rough classification of pretreatment based on pH divides it into acidic, alkaline and neutral pretreatments (Galbe and Zacchi 2007). This classification is focused on chemical pretreatment only. It does not encompass other pretreatment methods such as physical or biological pretreatments. A generalized classification of pretreatment methods groups them into; physical, chemical, biological and multiple or combinatorial pretreatment. In combinatorial pretreatmentmethods, physical parameters such as temperature or pressure or a biological step are combined with chemical treatments and are termed physicochemical or biochemical pretreatment methods. AFEX is a good example of a physicochemical method (Sun and Cheng 2002), and Bioorganosolv is a good example of a biochemical method for biomass pretreatment (Itoh et al. 2003). Combinatorial pretreatment strategies are generally more effective in enhancing the biomass digestibility, and often employed in designing leading pretreatment technologies.
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