No abstract

The two major xylanases of Trichoderma reesei with different pI values and pH optima were compared for increasing the bleachability of pine kraft pulp. The efficiencies of the two enzymes acting on pulp substrate were very similar in hydrolysis yield, extraction kappa number or final brightness value. Only slight synergism between the two enzymes was observed in both hydrolysis and bleaching tests. The pH optimum of the pI 5.5 xylanase was similar in pulp treatment and in the hydrolysis of isolated substrates, and the bleaching result also correlated well with the hydrolysis of pulp xylan. By contrast, the pI 9.0 xylanase acted differently on pulp than on isolated xylans at different pH values and the pH optimum on pulp was increased. The bleachability of pulp by the pI 9.0 xylanase was improved more than expected at pH 7.0, although the hydrolysis of pulp xylan was substantially decreased. A similar phenomenon was also observed when the hydrolysis was performed in water instead of buffer. It thus appears that the degree of hydrolysis needed to obtain improved bleachability with pI 9.0 xylanase can be minimized by proper adjustment of the hydrolysis conditions.

Xylanase treatment improved the bleachability of softwood kraft pulp both in conventional chlorine bleaching and in peroxide delignification. The mannanase isolated from Trichoderma reesei acted differently and improved the bleachability in the peroxide delignification significantly more than in the chlorine bleaching. When mannanase and xylanase were dosed together a positive effect was detected in both bleaching sequences.The reason for the different efficiencies of these enzymatic treatments in the bleaching may be the selectivities of the bleaching chemicals. The mannanase could hydrolyze amounts of glucomannan comparable to the xylan hydrolyzed by xylanase, suggesting that at least part of both hemicelluloses are located on the surface of the fibres.

No abstract

No abstract