Optimization of culture conditions for production of cellulases and xylanases by Scytalidium thermophilum using Response Surface Methodology

Summary Scytalidium thermophilum type culture Humicola insolens MTCC 4520 isolated from composting soil was optimized for production of cellulolytic and hemicellulolytic enzymes (endoglucanase, Avicel-adsorbable endoglucanase, FPase, β-glucosidase, xylanase and mannanase) by solid-state fermentation (SSF). Initial experiments showed that culture medium containing rice straw and wheat bran (1:3) as carbon source prepared in a synthetic basal medium supported maximal enzyme production at 45 °C. Further optimization of enzyme production was carried out using Box-Behnken design of experiments to study the influence of process variables (inoculum level, (NH₄)₂SO₄ and pH) on enzyme production. The response surface plots revealed the conditions for obtaining optimal enzyme levels. The models computed for R ² value ranged between 95% and 98.7% indicating they are appropriate and can be useful to predict the effect of inoculum level, (NH₄)₂SO₄ and pH on enzyme production. Under optimized conditions 62.5 ± 0.50, 23.0 ± 0.58, 3.0 ± 0.50, 151.00 ± 8.194, 196 ± 5.033 and 4.9 ± 0.32 (units/g substrate) of endoglucanase (EG), Avicel-adsorbable endoglucanase (AAEG), FPase, β-glucosidase, xylanase and mannanase were produced, respectively. Isoelectric focusing (IEF) of the crude extract showed that S. thermophilum produced six different EG isoforms, of which the EG corresponding to pI values of 8.4, 7.9 and 6.5 showed affinity for Avicel, thereby indicating the presence of a cellulose-binding domain (CBD). Furthermore, seven isoforms of β-glucosidase and ten multiple forms of xylanase distributed over a wide range of pI were also detected.