The modification of fibre surface by an enzymatic pretreatment was stu
died by analyzing the hydrolysis products released from the pulps and
the molar mass distribution of the alkali-extracted residual lignin. I
n kraft pulp, the reprecipitated or readsorbed alkali-resistant xylan
appears to form a physical barrier to the extraction of residual ligni
n from the fibres. The treatment of kraft pulp with hemicellulases rem
oves some of this xylan and renders the fibre structure more- permeabl
e. The increased permeability allows the passage of lignin or lignin-c
arbohydrate molecules in higher amounts and of higher molecular masses
in the subsequent chemical extraction. The improved extractability of
lignin leads to higher brightness of pulp, or can alternatively be ex
ploited to reduce the consumption of bleaching chemicals.