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 well as mono-saccharides, increased as the temperature rose to 190°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 lignin 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 180°C using the flow-through vessel.
- Acetyl groups
- Pressurised hot water extraction