Polyoses Hemicelluloses

In contrast to cellulose, polyoses are alkaline-soluble and partly water-soluble e.g. arabinogalactan of larch). 

I Cellulose Waste liquor for energy generation Xylose Acetic acid Furfural 

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For the study of soluble polysaccharides, a treatment with diluted TFA is sufficient and the reaction time can be kept short (7). Soluble polysaccharides of wood are separated from holocellulose by extraction with alkali. Wise et al. (10) term the extract with 5% KOH polyoses (hemicelluloses) A. Polyoses A can be hydrolyzed completely with 2N TFA within 1 hr. The chromatograms of the hydrolysates of polyoses A from spruce and beech holocelluloses recorded with a sugar analyzer (Biotronik ZA 5100) are shown in Figure 1. 

To the group of construction materials [54] belong, besides stone, those that have been used since humans began construction work. Wood is the oldest known construction material. Because of its imique characteristics, it is still in use today. The main components of wood are cellulose, polyoses (hemicelluloses), and lignin. The proportions are 40-50%, 15-35%, and 25-35%, respectively. Extractable compounds such as terpenes, waxes, or tannins may accoimt for 1-3% and mineral compounds for 0.1-0.5%. The tensile strength of wood is determined by the cellulose cell structure. The cellulose molecule contains from 10,000 up to 14,000 P-(l,4)-glycosidically bound glucose units and represents the valuable part of wood for further products such as paper, cardboard, foils, films, and fibers. 

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. 

Lignocellulosic materials have a common basic structure, but vary greatly in chemical composition and physical structure.4 Typically, these materials contain 30 percent to 60 percent cellulose, 10 percent to 30 percent hemicellulose (polyoses), and 10 percent to 20 percent ligmn. Cellulose provides strength and flexibility, while lignin supports and protects the cellulose from biological and chemical attack. Hemicellulose bonds lignin to cellulose. 

Hemicellulose (or polyose) is primarily composed of xylan, a branched polymer composed of five-carbon sugar, xylose. Typical polymerization degree of hemicellulose is 50 - 200, which is shorter than the cellulose molecules. The acid hydrolysis of hemicellulose, (C6H10O5)n, produces mainly xylose (C6H10O5), which can be converted to furfural, a chemical feedstock, or can be fermented to ethanol. 

Most of our knowledge of the chemistry of the polyoses in wood-cellulose preparations comes from studies made on fractions of the hemicelluloses obtained from alkaline extracts of wood, agricultural residues, or other plant material. In the following, some of these studies will be considered as a starting point, and then the effect of pulping procedures on the amount and nature of the associated polysaccharides will be discussed. It is not the purpose of this article to treat the chemistry of plant polysaccharides in detail, but rather to consider only those aspects which, in the opinion of the author, are pertinent to an understanding of the factors affecting the composition of wood-cellulose preparations. 

Hemicelluloses repr s ni a class of noncellulosic polysaccharides that is associated with cellulose in plant cell walls [56]. The term hemicelluloses was first used by Schulze in 1887 in the belief that (hemi- or half)-celluloses were perhaps components that were on the way of becoming cellulose a term meant to distinguish this group of noncellulosic polysaccharides from that which makes up the cell wall stmcture. It has long been recognized that the term is unfortunate and misleading, and that polyoses or heteropolysaccharides are better descriptors [57]. However, hemicelluloses is an often-used designation for the noncellulosic heteropolysaccharide components in plants. The latter term shall be used in this text. 

Hemicelluloses (also called polyoses) are the second most abundant polysaccharides in Nature after cellulose. Their structure (e.g., see Hg. 2) and chemical properties are presented in [6]. 

Figure 4.2.24 Typical structures of hemicelluloses or polyoses hardwood D-xylan and glucomannan and softwood galactan.

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