Agro-Residue Valorization for Thermostable Xylanase Production by Aspergillus caespitosus and Its Eco-Friendly Application in Pulp Biobleaching

The fungus Aspergillus caespitosus was cultivated under solid-state fermentation (SSF) using wheat bran (WB) and sugarcane bagasse (SCB) as agro-industrial substrates to produce xylanase. WB supported the highest enzymatic activity (approximately 1100 U g⁻¹ dry substrate), while pretreatment of SCB with NaOH (AT-SCB) enhanced productivity to about 1500 U g⁻¹, confirming the positive effect of lignocellulosic modification. The optimal moisture ratio (1:3 water: solid) yielded approximately 2400 U g⁻¹, and supplementation with 1% NH₄NO₃ and trace salts further increased xylanase synthesis by approximately 40%. The crude extract retained more than 80% of its activity after 72 h at 50 °C, indicating good thermal stability. In kraft pulp biobleaching, treatment with WB-derived xylanase (10 U g⁻¹ pulp, 50 °C, 3 h) resulted in a 15% reduction in the kappa number and a 2.5 ISO-point increase in brightness, with no detectable cellulose degradation. These findings demonstrate that A. caespitosus efficiently produced a thermostable and selective xylanase under SSF, highlighting the potential of agro-residue valorization for developing environmentally friendly processes in the pulp and paper industry.