Hemicellulose, definition

Hemicellulose [9034-32-6] is the least utilized component of the biomass triad comprising cellulose (qv), lignin (qv), and hemiceUulose. The term was origiaated by Schulze (1) and is used here to distinguish the nonceUulosic polysaccharides of plant cell walls from those that are not part of the wall stmcture. Confusion arises because other hemicellulose definitions based on solvent extraction are often used in the Hterature (2—4). The term polyose is used in Europe to describe these nonceUulosic polysaccharides from wood, whereas hemicellulose is used to describe the alkaline extracts from commercial pulps (4). The quantity of hemicellulose in different sources varies considerably as shown in Table 1. 

AG type II is most abundant in the heartwood of the genus Larix and occurs as minor, water-soluble components in softwoods. Certain tree parts of western larch (I. occidentalis) were reported to contain up to 35% AG [378]. The polysaccharide is located in the lumen of the tracheids and ray cells. Consequently, it is not a cell-wall component and, by definition, not a true hemicellulose. However, it is commonly classified as such in the field of wood and pulping research. This motivated us to include the larch AG in the review. 

Biomass is composed of various components such as cellulose, hemicellulose, lignin, extractives and mineral water. The composition of biomass plays a definitive role in altering the product distribution and their properties [2-3J. As is shown in earlier publications [4-S] different biomass, on pyrolysis, give different product yield with different product properties. In order to choose a biomass for a particular process (carbonisation, liquefaction, gasification or adsorbent char) knowledge on the product distribution and properties for various biomass are essential. 

The name lignin comes from the Latin word lignum which means wood. The discovery of lignin and its exploitation began over 150 years ago, when scientists considered wood to be a definite chemical substance based on three main components cellulose, lignin and hemicellulose. 

According to the classical definition, hemicelluloses are heterogeneous cell wall polysaccharides, which are normally extractable by a relatively concentrated aqueous alkaline solution. Hemicelluloses constitute 20-30% of the total bulk of annual and perennial plants and are consequently among the most abundant native polymers in the world. The hydrosoluble hemicellulose we are using occurs also in waste streams in the forest industry and can thus be obtained as a by-product from pulp and paper production. An additional benefit of using hemicelluloses is their biodegradability. 

Over the last 20 years many studies have shown that dietary fiber may have a hypocholesterolemic effeet[111,112] However, the largely held public belief that all fibers are hypolipidemic is not supported by the experimental data. Bran, for instance, which is mainly constituted by cellulose and hemicellulose, does not seem to have any effect on serum lipids[113], whereas it has been shown that pectin, guar-gum and alfa-alfa saponins have a definite hypocholesterolemic effect[114,115,116] ... 

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