Native (indigenous) cellulose fractions of cellulosic materials are recal­citrant to enzymatic breakdown, so a pretreatment step is required to render them amenable to enzymatic hydrolysis to glucose. A number of pretreatment processes have been developed in laboratories, including:

■ Physical pretreatment—mechanical comminution, irradiation, and

pyrolysis

■ Physicochemical pretreatment—steam explosion or autohydrolysis, ammonia fiber explosion (AFEX), SO2 explosion, and CO2 explosion

■ Chemical pretreatment—ozonolysis, dilute-acid hydrolysis, alkaline hydrolysis, organosolvent process, and oxidative delignification

■ Biological pretreatment

However, not all of these methods may be technically or economically feasible for large-scale processes. In some cases, a method is used to increase the efficiency of another method. For instance, milling could be applied to achieve better steam explosion by reducing the chip size. Furthermore, it should be noticed that the selection of pretreatment method should be compatible with the selection of hydrolysis. For exam­ple, if acid hydrolysis is to be applied, a pretreatment with alkali may not be beneficial [18]. Pretreatment methods have been reviewed by Wyman [2] and Sun and Cheng [12].

Among the different types of pretreatment methods, dilute-acid, SO2, and steam explosion methods have been successfully developed for pre­treatment of lignocellulosic materials. The methods show promising results for industrial application. Dilute-sulfuric acid hydrolysis is a favorable method for either pretreatment before enzymatic hydrolysis or conversion of lignocellulose to sugars.