Production of a xylose-stimulated β-glucosidase and a cellulase-free thermostable xylanase by the thermophilic fungus Humicola brevis var. thermoidea under solid state fermentation

Humicola brevis var. thermoidea cultivated under solid state fermentation in wheat bran and water (1:2 w/v) was a good producer of β-glucosidase and xylanase. After optimization using response surface methodology the level of xylanase reached 5,791.2 ± 411.2 U g−1, while β-glucosidase production was increased about 2.6-fold, reaching 20.7 ± 1.5 U g−1. Cellulase levels were negligible. Biochemical characterization of H. brevis β-glucosidase and xylanase activities showed that they were stable in a wide pH range. Optimum pH for β-glucosidase and xylanase activities were 5.0 and 5.5, respectively, but the xylanase showed 80 % of maximal activity when assayed at pH 8.0. Both enzymes presented high thermal stability. The β-glucosidase maintained about 95 % of its activity after 26 h in water at 55 °C, with half-lives of 15.7 h at 60 °C and 5.1 h at 65 °C. The presence of xylose during heat treatment at 65 °C protected β-glucosidase against thermal inactivation. Xylanase maintained about 80 % of its activity after 200 h in water at 60 °C. Xylose stimulated β-glucosidase activity up to 1.7-fold, at 200 mmol L−1. The notable features of both xylanase and β-glucosidase suggest that H. brevis crude culture extract may be useful to compose efficient enzymatic cocktails for lignocellulosic materials treatment or paper pulp biobleaching.