BIOREFINERY. Pressurised hot water extraction of acetylated xylan from birch sawdust

Abstract Birch sawdust was extracted using pressurized hot water (PHW), with the ultimate aim of maximizing the yield of large and intact non-cellulosic heteropolysaccharides, i.e. hemicellulose molecules. The extractions were performed using a flow-through vessel. Effects of different extraction temperatures between 140 and 200°C were evaluated. The amount of extracted xylans, as weil as monosaccharides, increased as the temperature rose to 1 90°C, accordingly the degree of polymerization of the xylans decreased. The extracted xylans retained most of the native acetyl groups and were thereby water-soluble after the extraction. The PHW extract contained not only xylan and other hemicellulose-derived oligomers, but fragmented Iignin molecules and some extractives as well. Only trace amounts of furfurals were found in the extract at lower extraction temperatures. An optimal yield of water-soluble, polymeric xylans with the smallest amount of unwanted impurities and degradation products was thus achieved at an extraction temperature of 1 80°C using the flow-through vessel