Simultaneous production of high activities of thermostable endoglucanase and β-glucosidase by the wild thermophilic fungus Thermoascus aurantiacus

Abstract

The culture-medium composition was optimised, on a shake-flask scale, for simultaneous production of high activities of endoglucanase and β-glucosidase by Thermoascus aurantiacus using statistical factorial designs. The optimised medium containing 40.2 g l⁻¹ Solka Floc as the carbon source and 9 g l⁻¹ soymeal as the organic nitrogen source yielded 1130 nkat ml⁻¹ endoglucanase and 116 nkat ml⁻¹β-glucosidase activities after 264 h as shake cultures. In addition, good levels of β-xylanase (3479 nkat ml⁻¹) and low levels of filter-paper cellulase, β-xylosidase, α-l-arabinofuranosidase, β-mannanase, β-mannosidase, α-galactosidase and β-galactosidase were detected. Batch fermentation in a 5-l laboratory fermentor using the optimised medium allowed the production of 940 nkat ml⁻¹ endoglucanase and 102 nkat ml⁻¹β-glucosidase in 192 h. Endoglucanase and β-glucosidase showed optimum activity at pH 4.5 and pH 5, respectively, and they displayed optimum activity at 75 °C. Endoglucanase and β-glucosidase showed good stability at pH values 4–8 and 4–7, respectively, after a prolonged incubation (48 h at 50 °C). Endoglucanase had half-lives of 98 h at 70 °C and 4.1 h at 75 °C, while β-glucosidase had half-lives of 23.5 h at 70 °C and 1.7 h at 75 °C. Alkali-treated bagasse, steam-treated wheat straw, Solka floc and Sigmacell 50 were 66, 48.5, 33.5 and 14.4% hydrolysed by a crude enzyme complex of T. aurantiacus in 50 h.