Isolation of Hemicellulose from Corn Fiber by Alkaline Hydrogen Peroxide Extraction

Approximately one-half of the lignin and most of the hemicellulose present in agricultural residues such as wheat straw and corn stover are solubilized when the residue is treated at 25 degrees C in an alkaline solution of hydrogen peroxide. The delignification reaction is most efficient when the ratio of hydrogen peroxide to substrate is at least 0.25 (w/w) and the pH is 11.5. The supernatant fraction from a given pretreatment, after addition of makeup peroxide and readjustment of the pH, can be recycled to treat at least six additional batches of substrate, resulting in a substantial concentration of hemicellulose and soluble lignin degradation products. Hydrolysis of the insoluble fraction with Trichoderma reesei cellulase after alkaline peroxide treatment yields glucose with almost 100% efficiency, based upon the cellulose content of the residue before treatment. These data indicate that alkaline peroxide pretreatment is a simple and efficient method for enhancing the enzymatic digestibility of lignocellulosic crop residues to levels approaching the theoretical maximum.

Alkaline solutions of hydrogen peroxide partially delignify wheat straw and other lignocellulosic materials, leaving a cellulosic residue that is highly susceptible to enzymatic digestion by cellulase. The delignification reaction is strongly dependent upon the pH of the reaction mixture, with an optimum at pH 11.5-11.6, pKa for the dissociation H(2)O(2) right harpoon over left harpoon H(+) + HOO(-). The data are consistent with a mechanism in which H(2)O(2) decomposition products such as .OH and O(2) (-)., rather than H(2)O(2) or HOO(-), are the primary lignin oxidizing species. During the course of the delignification reaction, O(2) is evolved from the reaction mixture indicating active H(2)O(2) decomposition. At a given concentration of H(2)O(2), the rate of O(2) evolution is proportional to the amount of lignocellulosic substrate present in the reaction mixture. However, the total amount of O(2) evolved is inversely proportional to the amount of substrate present, indicating that some of the peroxide oxygen becomes incorporated into lignin degradation products. The amount of peroxide oxygen incorporated can range as high as 2 O(2) per lignin C(9) unit, depending upon the initial concentration of lignocellulosic substrate.