Extraction of galactoglucomannan from spruce wood with pressurised hot water

Sapwood chips and ground wood of spruce have been extracted with pressurised pure water in an accelerated solvent extractor at temperatures from 100 to 180°C. The water extracts were freeze-dried, weighed, and analysed for carbohydrates by acid methanolysis followed by GC determination of monomeric sugars. Lignin was determined by measuring UV absorption and acetyl groups by HPLC after alkaline hydrolysis. Average molar masses of hemicelluloses in water extracts were determined by HPSEC-MALLS after filtration. Typically, approximately 70% of the total weighed extracts were composed of carbohydrates derived from hemicelluloses. Approximately 75% of the extracted carbohydrates were from galactoglucomannan (GGM). Other extracted substances were xylans, arabinogalactans, lignin and acetic acid. As much as 80–90% of the GGM in the wood, i.e., approximately 15% based on the original wood, was extracted from ground wood at 170–180°C for an extraction time of 1 h. The yields from chips were lower: approximately 60% from that of ground wood. The hemicelluloses were partly hydrolysed during the extractions at 160–180°C. Xylans and arabinogalactans were hydrolysed to a higher extent than GGM. Acetyl groups in GGM were also partly hydrolysed, which resulted in lower GGM solubility. A key factor for achieving a high yield of high-molar mass GGM is an optimised pH profile in order to minimise hydrolysis of acetyl groups and hydrolytic cleavage of GGM chains.