STUDIES ON THE MICROBIAL PRODUCTION OF XYLANASE BY ASPERGILLUS NIGER

Abstract

The present investigation was undertaken to isolate a persuasive microorganism having potential ability to secrete xylanase. An attempt to scale up and economize the process was made by employing the cheaper agro-residues. A thorough survey of various sites led to isolation of a potent xylanase producing fungal strain. The strain having notably higher levels of xylanase and identified to be Aspergillus niger from Indian Agricultural Research Institute, New Delhi was designated as NK-23. In order to improve the levels of enzymatic production, the isolated strain was subjected to both single step as well as mixed mutagenesis. UV mutagenesis followed by N-methyl-n'-nitro-N-nitrosoguanidine (NTG or MNNG) treatment resulted into amutant strain designated as NKUCN-3.40, which had remarkably higher expression ability of xylanase and P-xylosidase and interestingly had lower levels of cellulase expression. Mutagenesis of the wild type A. niger NK-23 had not only improved it's xylanase production ability but also resulted into the distinct morphological features of the mutagenized strain. A. niger NKUCN- 3.40 had dirty white coloured mycelia that appeared compact and thicker as compared to the yellow coloured and thinner mycelia of the wild type A. niger NK-23 strain. Xylanase production from the mutant NKUCN-3.40 strain was evaluated in surface, submerged and solid-state fermentation conditions; of these, the level of production observed was higher in solid-state fermentation. Among the ten different solid supports viz., wheat residue, wheat bran, coconut coir, cotton hull, bagasse, rice bran, loofa sponge, oil cake, polyurcthanc sponge and glass beads; cotton hull led to higher levels of xylanase production. The amount of substrate, moisture level, PH, temperature, inoculum and incubation period were also derived to achieve the maximum levels of enzymatic secretion. In order to further enhance the production level different additives were evaluated. Oil cake was found to be the best stimulator for xylanase production. These observations were utilized to scale up and develop a semi-continuous process for xylanase production. The designed bioreactor was used for recycling of the fungal biomass for multiple fermentation cycles. The cotton hull used for this process had yielded better results; since the fungi could anchor effectively and the presence of additional nutrients appeared supporting growth and enzymatic production. Applicability of the xylanase for pulp and paper processing was analyzed by using XCEHH and CEHH sequences. This had resulted into decrease in the chlorine consumption, which in turn reduces the pollution load on the environment. The enzyme pretreatment led to decrease in kappa number, BOD, COD and AOX. Additionally, it also had affected the increase in the mechanical and optical properties of the paper.