Impact of the extraction conditions and wood species on the chemical composition of hemicelluloses

摘要: Four different wood species (spruce, maritime pine, birch and eucalyptus) have been pre-extracted in different aqueous extraction liquors (acidic, near-neutral and alkaline) in order to isolated hemicelluloses prior cooking. Depending on the extraction conditions, hemicelluloses having completely different chemical composition in terms of the main sugars can be obtained. For softwoods, nearneutral or acidic extraction yields galactoglucomannans (GGM) rich fraction with low residual lignin content, while the alkaline extracted fractions are mixtures of GGM and arabinoxylans (AX). The hemicelluloses obtained from maritime pine in acidic conditions are essentially composed of galactans (up to 90% in some cases), while under the same extraction conditions, spruce GGM are rich in mannans. About hardwoods, in acidic or near-neutral conditions, mixtures of arabinoxylans and galactoglucomannans fractions are obtained. For the near-neutral, the mixture is composed of more or less equal amounts of the two types of hemicelluloses, but the addition of sulphuric acid shifts the selectivity towards galactoglucomannans. Contrarily, the fractions obtained in alkaline conditions are very rich in arabinoxylans (up to 80%). However, an important contamination of the all hemicelluloses with residual lignin and extractives (more important for eucalyptus than for birch) was observed, demanding the bleaching of the fractions for further use in papermaking. The extracted hemicelluloses are being tested as additives in different points of the papermaking chain.