STUDIES ON THE MICROBIAL PRODUCTION OF LACCASE BY ASPERGILLUS FUMIGATUS

Abstract

The present investigation was undertaken to isolate a potent microorganism with laccase production ability and to develop and economize the process by employing agro-horticultural residual resources. The persuasive fungal strain secreting laccase was isolated by scrupulous survey of various sites. The strain VkJ was observed to have remarkable levels of laccase production and identified as Aspergillusfumigatus from Indian Agricultural Research Institute, New Delhi. To achieve an improvement in the enzyme production ability of the selected strain, physical, chemical and mixed mutagenesis of the strain was attempted. Double stage mutagenesis by UV irradiation led to the development of strain VkJ2.4.5 having notably higher levels of laccase production. A detectable level of manganese peroxidase production was also observed. Mutagenized strain A. fumigatus VkJ2.4.5 had discrete morphological features. The mycelia of the mutant VkJ2.4.5 were loose and dirty white as compared to the wild type VkJ strain which had compact and white mycelia. Surface, submerged and solid-state fermentation systems had been evaluated for laccase production from the mutant VkJ2.4.5 strain that demonstrated that level of enzyme production was higher in solid-state fermentation. Among the six different solid supports viz., wheat straw, wheat bran; banana peel, bagasse, rice bran and poplar leaves; banana peel led to higher levels of enzyme production. In order to further enhance the enzyme production levels; amount of substrate, moisture level, incubation period, aeration volume and additives were evaluated. An attempt to scale up the production was made by designing a column-tray bioreactor using the derived conditions. In successive batches of fermentation same fungal mycelia and additional nutrients were utilized to accomplish maximal production levels in an economic manner. The banana peel employed, was observed to be the best substrate that led to remarkable productivity; since fungi could anchor efficiently and additional nutrients appeared supporting growth and enzyme production. Biobleaching efficiency of the developed enzyme preparation was evaluated for mixed wood pulp in XCEHH and XODED sequences. Analysis of FTIR spectra of residual lignin revealed characteristic modifications following enzymatic bleaching. Variations in morphology and crystallinity of pulp were evaluated by scanning electron microscopy and X-ray diffraction analysis. The enzyme pretreatment led to decrease in kappa number, yellowness, AOX and COD. Additionally, it improved the mechanical and optical properties of the paper along with reduction in the chlorine consumption which in turn diminish the pollution denoting it therefore to be an ecofriendly and benign bleaching technology.