Structures of Hemicelluloses and Pectins in Wood and Pulp

This work aims to fill gaps in the present knowledge of the structures of wood polysaccharides of potential importance in mechanical and chemical pulping. The detailed structures of the polysaccharide components from wood that dissolve during mechanical pulping were determined. The structures of polysaccharide components linked to residual lignin in chemical pulp were also determined. Methylation analysis as linkage analysis was further developed and verified for this purpose using the procedure methylation, methanolysis, silylation and GC/MS analysis. Water-soluble arabinogalactans obtained from spruce and pine heartwood were analyzed using methylation analysis, NMR spectroscopy and carbohydrate composition analysis. The arabinogalactans isolated were acidic and had a backbone of β-(1 3)-linked Dgalactopyranose units, highly branched at C6. Water-soluble galactoglucomannans were extracted from spruce wood and TMP. Based on both methylation analysis and NMR spectroscopic analysis, the main C6-branched unit was D-mannopyranose. Some C6branched D-glucopyranose units were also detected by methylation analysis. The polysaccharides that dissolved during oxygen delignification of pine kraft pulp comprised mainly xylan, together with galactans, 1,3-linked glucan, arabinan and glucomannan. This indicated that the glucose units present in the dissolved polysaccharides did not originate from cellulose but from a 1,3-linked glucan. This was a new and unexpected finding indicating that the content of dissolved glucose is not related to the degree of cellulose degradation. Polysaccharides in the corresponding filtrates from birch kraft pulp also contained mainly xylan. An indication was found for the presence of bonds between lignin and position C3 of the backbone units in birch xylan. Residual lignin-carbohydrate complexes (RLCCs) isolated from spruce and pine kraft pulps contained oligosaccharides representing the main polysaccharides in softwood. In addition, 1,4-linked and 1,3/6-linked galactans were present in considerable amounts, together with smaller amounts of 1,3-linked glucan and 1,5-linked arabinan, and possibly also xyloglucan. 1,4-linked galactan was enriched in the RLCC of spruce fiber surface material. Towards the inner part of the fiber, 1,3/6-linked galactan became the major lignin-bound galactan structure. Some differences related to the alkaline cooking method used were detected in the carbohydrate component of pine pulp RLCCs. Oxygen delignification removed 1,3/6-linked galactan from the inner parts of fiber in preference to 1,4-linked galactan.