The mechanism of xylanase prebleaching of kraft pulp: An examination using model pulps prepared by depositing lignin and xylan on cellulose fibers

Model pulps, made by depositing lignin and/or xylan on cellulose fibers under alkaline conditions, were subjected to a xylanase-chelation-peroxide bleaching sequence. On average, 14 % (0.7g) of the added xylan had been redeposited on 20g cellulose. However, the kappa numbers of the pulps were smaller than 1, indicating that only a small amount of oxidizable compounds, e.g. lignin and sugar chromophores, had precipitated. A brightness increase occurred immediately after the xylanase stage. After peroxide bleaching a further brightness increase was observed with model pulps containing both lignin and xylan. Analysis of the filtrate obtained from the enzyme stage showed that only model pulps containing xylan released sugar. The molecular mass distribution of solubilized material was analyzed with pulsed amperometric and UV detectors. The results suggested that lignin-carbohydrate bonds are formed during the redeposition of lignin and xylan on the cellulose fibers and that xylanases can partly hydrolyze the lignin-carbohydrate complexes. This mechanism is proposed to be a major contribution to the bleach boosting effect of these enzymes. Although the formation of chromophoric xylan was also observed during alkaline cooking, the results indicate that the hydrolysis of this class of compounds has a limited role in xylanase prebleaching.