Linkages between residual lignin and carbohydrates in bisulphite (Magnefite) pulps

The presence of bonds between residual lignin and carbohydrates in bisulphi te (Magnefite)pulps from both hardwood and softwood has been studied using a combination of carbohydrate-degrading enzymes and an analysis of the wood polymers with size-exclusion chromatography (SEC) in lithium chloride-dime thylacetamide (LiCl-DMAC). From the experiments we suggest that a fraction of the residual lignin is bonded to cellulose in bisulphite pulps of softwo od. The conclusion is based on the fact that extensive treatment of soft wo od bisulphite pulp with a commercial cellulase preparation results in simul taneous changes in the molecular weight distribution pattern for cellulose and lignin (MW 100-2000 kDa). Results from alkali extraction of the pulp su pport this finding. Treatments of the pulp with both xylanase and extractio n experiments indicate that alkali-stable lignin-xylan bonds in softwood bi sulphite pulps might exist to a certain extent. In hardwood bisulphite pulp s the extent of such bonds seems to be low. The experiments suggest that co nsiderably less lignin is bound to the high-molecular-weight cellulose in t he softwood bisulphite pulp than in the kraft pulp.