Flax pulp bleaching and residual lignin modification by laccase-mediator systems

Enzymatic bleaching of flax alkaline pulp is intended here by using laccase-mediator systems (LMS) based on Pleurotus eryngii, Trametes versicolor and Pycnoporus cinnabarinus laccases and two mediators. The efficiency of the different LMS treatments was compared in terms of brightness, kappa number and viscosity modifications. Furthermore, changes in the molecular structure of residual lignin and lignin/carbohydrate ratio were analyzed by pyrolysis-gas chromatography-mass spectrometry of the treated pulps. Pycnoporus cinnabarinus and T. versicolor laccases in the presence of 1-hydroxybenzotriazole (HBT), were the most effective for flax pulp delignification. Up to 20% ISO brightness increase with respect to the initial pulp and a decrease of kappa number from 9 to 3 were attained after alkaline extraction of the pulps treated with those LMS. Pycnoporus cinnabarinus laccase plus HBT gave also the best selectivity in lignin removal. Analytical pyrolysis of the LMS-treated pulps showed a selective removal of lignin against cellulose and a strong alteration of the residual lignin with preferential removal of the syringyl units. Pycnoporus cinnabrinus LMSHBT gave the best results in terms of decrease of lignin/cellulose and syringyl/guaiacyl ratios, in agreement with data from pulp parameters. For the above reasons laccase from P. cinnabarinus and HBT were selected for further assays including a peroxide step. Up to 89% relative selignification rate (attaining 1.3 kappa number) and an incease of 44% ISO brightness were atained after LMSHBT-peroxide bleaching. Ongoing studies focus on improvement of a LMSHBT-based totally chlorine free sequence for flax pulp bleaching.

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