Lignin-carbohydrate network in wood and pulps: A determinant for reactivity

Pretreatment of wood or kraft pulp with endoglucanase followed by swelling in urea leaves a non-crystalline residue that can be dissolved in strong aqueous sodium hydroxide-sodium borate solution. A stepwise precipitation process employing acid and barium ions can separate lignin-carbohydrate complexes enriched in individual polysaccharides. This procedure has been applied to eucalypt and birch wood and to the corresponding kraft pulps. Thioacidolysis of the various lignin-carbohydrate complexes was used as the major analytical technique to obtain information about the structure and structural changes in lignin. A combination of thioacidolysis and size exclusion chromatography was used to obtain knowledge on the degree of depolymerisation and repolymerisation of lignin when going from wood to chemical pulp. In contrast to spruce wood and kraft pulp, complete recovery of the lignin-carbohydrate complexes could not be obtained from hardwood samples.