Effects of lignin-modifying enzymes on pine kraft pulp

Oxygen delignified pine kraft pulps were treated with lignin-modifying enzymes: laccase (EC 1.10.3.2), Mn-dependent peroxidase (EC 1.11.1.13), lignin peroxidase (EC 1.11.1.14), and xylanase (EC 3.2.1.8), in order to improve bleachability. Pulps were treated either with a purified single enzyme or alternatively enzymes were successively added in different combinations. The solubilized components were analysed using HPLC, GPC and IEF. The residual pulps were delignified with alkaline hydrogen peroxide and analysed for Kappa number and ISO-brightness. Lignin-modifying enzymes did not improve bleachability when acting alone. Xylanase treatment increased the brightness by 1–2.5 ISO-units, and xylanase combined with lignin-modifying enzymes increased the brightness by a further one unit. By extending the alkaline peroxide step the final brightness increased in all samples, whereas the relative differences between reference and enzyme-treated samples remained constant