Effect of polyols on thermostability of xylanase from a tropical isolate of Aureobasidium pullulans and its application in prebleaching of rice straw pulp

In an attempt to find a thermostable xylanase enzyme for potential application in the pretreatment prior to H2O2 bleaching of paper pulp for industry, an extracellular xylanase from Aureobasidium pullulans CBS 135684 was purified 17.3-fold to apparent homogeneity with a recovery yield of 13.7%. Its molecular mass was approximately 72 kDa as determined by SDS-PAGE. The optimal pH and temperature for activity of the purified enzyme were pH 6.0 and 70°C, respectively. The enzyme was relatively stable at 50°C, retaining more than half of its original activity after 3-h incubation. The thermostability of the enzyme was improved by the addition of 0.75 mM sorbitol prolonging the enzyme's activity up to 10-fold at 70°C. When the potential of using the enzyme in pretreatment of rice straw pulp prior to bleaching was evaluated, the greatest efficiency was obtained in a mixture containing xylanase and sorbitol. Treatment of the rice straw pulp with xylanase prior to treatment with 10% (v/v) H2O2 and production of hand sheets increased the ISO sheet brightness by 13.5% and increased the tensile and tear strengths of the pulp by up to 1.16 and 1.71-fold, respectively, compared with pulps treated with H2O2 alone. The results suggested the potential application of the enzyme before the bleaching process of paper pulp when the maintenance of high temperature and enzyme stability are desirable.