Investigation of lignin-carbohydrate complexes in kraft pulps by selectiveenzymatic treatments

The occurrence of covalent bonds between residual lignin and polysaccharide s in birch and pine kraft pulps was investigated by specific enzymatic trea tments. Pure enzymes degrading cellulose, xylan and mannan were used both s eparately and in combination. Comparison of the molar masses of polysacchar ides and lignin in the original pulps and in the residual pulps after enzym atic treatments showed that residual lignin in birch kraft pulp is linked a t least to xylan. A minor portion may also be linked to cellulose. In pine kraft pulp some of the residual lignin appears to be linked to cellulose, g lucomannan and xylan. The linkages between lignin and cellulose and hemicel luloses may be either native or formed during pulp processing. The results also provided new information on the synergistic action of cellulose- and h emicellulose-degrading enzymes on pulp fibres. The synergism appears to be mainly due to the structure of the pulp fibres, with different layers of ce llulose sheets, hemicelluloses and lignin. On the other hand the results al so provided information about fibre structure. The degradation of xylan cle arly enhanced the action of enzymes on cellulose, suggesting that xylan par tially covers the cellulose. A similar phenomenon was not observed in the s imultaneous hydrolysis of glucomannan and cellulose. However, the results s uggest that glucomannan does interact with cellulose, possibly by non-coval ent linkages.