Utilization of Corn Cob Waste for Cellulase-Free Xylanase Production by Aspergillus niger DX-23: Medium Optimization and Strain Improvement

Aspergillus niger DX-23 produces a cellulase-free xylanase which showed efficient deinking of old newspaper pulp for recycled paper production. Towards economical production of the above xylanase when various agro-waste biomass were evaluated as substrates, under shake flask conditions, A. niger DX-23 produced highest amount of xylanase (59.5 ± 5.0 U/mL) using corn cob powder as substrate. A central composite design was used to optimize concentration of corn cob powder, NaNO3 and KH2PO4 in the medium for maximum xylanase production. The optimum concentration of the above nutrients were determined to be 37.0 g/L corn cob powder, 2.5 g/L of NaNO3 and 1.0 g/L of KH2PO4 at which level xylanase yield of 110.4 U/mL was obtained, which was 82.9 % more than the yield obtained in unoptimized medium. Moreover, under shake flask conditions, 5.0 % (v/v) of inoculum, pH of 5.0 and incubation time of 84 h was found to be suitable for maximum xylanase production. At laboratory fermentor level, A. niger DX-23 produced 79.4 U/mL (after 96 h) and 117.9 U/mL (after 72 h) xylanase using untreated corncob powder and alkali treated corn cob powder, respectively. In order to obtain higher xylanase procuring strains, A. niger DX-23 was mutagenized using UV rays. In optimized medium mutant strain of A. niger produced 150.9 ± 3.4 U/mL of xylanase which was 118.0 % higher than the xylanase yield obtained for parent type in unoptimized medium (60.2 ± 2.6 U/mL). The above results suggested effectiveness of the combined strategy of medium optimization and mutation towards enhanced production of xylanase by A. niger.