Isolation and structural characterization of polysaccharides dissolved in Eucalyptus globulus kraft black liquors

Polysaccharides dissolved in black liquors from Eucalyptus globulus kraft pulping were isolated by acid precipitation, analysed for monosaccharide composition and structurally characterized by methylation analysis and NMR spectroscopy. The precipitated oligo- and polysaccharides, representing 20% of the dissolved and/or degraded wood polysaccharides are essentially composed of xylan and amylopectin. The xylan dissolved in the alkaline black liquor showed a Mw of about 17-19 kDa as revealed by GPC analysis. The major part of the 4-O-methylglucuronic residues in the xylan from black liquor were degraded to hexenuronic (4-deoxy-β-l-threo-hex-4-enopyranosyluronic acid) units, which was not found to be so pronounced in the case for xylan in the corresponding kraft pulps. The overall structural and molecular weight features of both black liquor and pulp xylans during the kraft pulping are discussed.