Optimization of medium composition for cellulase-free xylanase production by solid-state fermentation on corn cob waste by Aspergillus niger DX-23

Economical and higher production of cellulase-free xylanase(s) is needed for their wider application in paper and pulp industries. For enzyme production at industrial scale, the growth medium contributes up to 40% of the total production cost. The large quantity of xylan-rich corn cob biomass generated worldwide from corn-processing industries can be used as a cheaper substrate for the production of xylanase. In the present study, the optimization of the production of xylanase by Aspergillus niger DX-23 was studied under solid-state fermentation (SSF) using the Plackett-Burman design (PBD) and response surface methodology (RSM). Among eleven factors investigated (each at two levels) in PBD, corn cob powder, KH2PO4, yeast extract, Tween 20, FeSO4.7H(2)O, MgSO4.7H(2)O, CoCl2, MnSO4.H2O and ZnSO4 center dot H2O significantly (p < 0.05) influenced the production of xylanase by A. niger DX-23. Subsequently, RSM involving central composite design (CCD) was adopted to determine the optimum levels of corn cob powder, NaNO3 and KH2PO4 and FeSO4. According to ANOVA for xylanase production, for CCD-I although the quadratic model developed was significant (p = 0.026465), the model also showed significant lack-of-fit (p = 0.004056) and low predicted R-2. Hence, CCD-II which included two variables, i.e. corn cob powder and NaNO3, was carried out. For CCD-II, the quadratic model developed was significant (p = 0.007753), lack-of-fit value was non-significant (p = 0.677031) and the predicted R-2 was 0.51. Based on CCD-II, the optimum levels of corn cob powder and NaNO3 in the medium were found to be 150.0 g/l and 5.1 g/l, respectively. To confirm the accuracy of the model, when xylanase production by A. niger DX-23 was studied in an optimized medium under SSF conditions, the xylanase yield reached 306.12 +/- 7.4 U/g after 10 days of growth which agreed fairly well with the predicted value (290.15 U/g). Hence, the quadratic model created could be considered to be accurate and reliable for predicting the production of xylanase by A. niger DX-23 under SSF. Moreover, when the effect of pH and inoculum concentration on xylanase production was investigated, in an optimized medium, the maximum production of xylanase was obtained at pH 5.0 and at an inoculum level of 5.0 x 10(6) spores/ml. When the time course of the fermentation was followed, A. niger DX-23 produced maximum xylanase (496.9 +/- 3.0 U/g) after 6 days of fermentation.