Potential of solid-state fermentation enzymes of Aspergillus oryzae in biobleaching of paper pulp

Abstract

Production of xylan-degrading enzymes, endo-β-xylanase (EC 3.2.1.8), β-xylosidase (EC 3.2.1.37), α- l-arabinofuranosidase (EC 3.2.1.55 and α-galactosidase (EC 3.2.1.22) by Aspergillus oryzae NRRL 1808 was investigated in solid-state fermentation using eucalyptus soda-anthraquinone and bagasse soda pulps as carbon feedstock. The impact of different nitrogen sources and initial fermentation pH on enzyme production was studied in medium optimisation experiments using fractional factorial design. The use of ammonium nitrate and corn steep liquor favoured xylanase production on eucalyptus pulp, whereas potassium nitrate and corn steep liquor increased xylanase yields on bagasse pulp. Xylanase production on eucalyptus and bagasse pulps under optimised conditions peaked on day 4 of cultivation (3200 and 2675 IU g −1 dry matter, respectively) which represents an increase in xylanase activity of 60% and 37%, respectively, over the non-opimised conditions. Biobleaching of eucalyptus and bagasse pulps was carried out using the whole fermented material (residual pulp, fungal biomass and in situ enzymes) from the cultivation of A. oryzae without a prior down streaming of the in situ enzymes from these pulps. Overall, the bagasse pulp was more susceptible to enzyme bleaching than eucalyptus pulp. In all instances, a greater brightness gain was induced with the in situ solid-state enzymes (0.9–3.0 points brightness increase) than a commercial enzyme control (0.8–2.5 points). At the same bleaching costs (US$ 1 or 3 t −1 pulp), the solid-state enzymes produced under optimised conditions were 20–36% more efficient in improving the brightness of paper pulp than the commercial enzyme. However, the presence of increased levels of side chain xylan degrading enzymes in the optimised medium did not improve the bleaching abilities of xylanase. It was demonstrated that bagasse pulp could successfully be used as carrier of A. oryzae enzymes in biobleaching of eucalyptus pulp without sacrificing the bleaching efficiency of xylanase.