Biobleaching Efficiency of Aspergillus oryzae Xylanases Produced by Solid Substrate Fermentation

Abstract

The direct use of substrate specific enzymes produced by solid substrate fermentation (SSF) without a downstream processing represents an innovative method for modification of pulp and paper properties and reduction of chemical consumption during bleaching that can bring about positive economical and environmental benefits to the industry. Screening of a number of Aspergillus oryzae isolates on oxygen delignified soda-aq pulp of Eucalyptus grandis as carbon source and enzyme inducer yielded xylanase activities for the best six strains in the range of 500 - 4300 IU/g dry material (DM) with low levels (<1.0 FPU/g DM) of simultaneously produced cellulase. The in situ produced enzymes (whole SSF culture) were used in pulp bleaching without a prior downstream processing of xylanase. The brightness gain over control was enzyme dose dependent and varied between 0.7 and 1.4 points. The optimum moisture content (83%), initial pH (8.3) and time course (4 d) of SSF were determined for the best-performing strain in the bleaching studies (A. oryzae NRRL 3485). Addition of corn steep liquor, an inexpensive nitrogen source, enhanced the xylanase yield considerably. The pH and temperature optima of the xylanase complex of A. oryzae NRRL 3485 was found to be 6.5 and 65°C, respectively. An improved thermal stability was observed at 50°C and pH 6.5 in the presence of eucalyptus pulp substrate. When used at equal enzyme costs and charges in biobleaching of eucalyptus soda-aq pulp, the A. oryzae enzyme outperformed a commercial xylanase control in its efficiency to improve pulp brightness.