Polyoses

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. 

For the total hydrolysis of polysaccharides, trifluoroacetic acid (TFA) has important advantages over sulfuric acid. The reaction time is short and there is no need for conventional neutralization, as TFA is volatile and can be removed by evaporation. Several methods have been developed, depending on the substance to be hydrolyzed. Soluble saccharides (e.g., polyoses) can be hydrolyzed with diluted TFA, while cellulose, pulp, and wood need treatments with concentrated TFA in homogeneous solution. The presence of lignin impedes the hydrolysis of polysaccharides thus, especially for wood samples, an intensive treatment with TFA is necessary, and correction values have to be considered. Several application examples show that the hydrolysis with TFA enables a rapid quantitative determination of the composition of polysaccharides, pulps, and woods. 

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. 

Table I. Hydrolysis of Polyoses A from Spruce Holocellulose with 72(%) Sulfuric Acid and 2N TFA6...

Figure I. Chromatograms of the hydrolysates of polyoses A from spruce-wood and beechwooa. Rha = rhamnose, Man = mannose, Ara = arahi-nose, Gal = galactose, Xyl = xylose, M-GluU = 4-O-methylglucuronic...

The main polyoses of softwoods are galactoglucomannan, with an average ratio of glucose to mannose of 1 3 and an average acetyl content of 1.3% (based on wood), and arabino-4-O-methylglucuronoxylan. 

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. 

Dorr also has commented on difficulties in viscose filtration caused by the presence of xylan, and has noted, further, a correlation between viscose-yarn strength and the total of xylan and mannan in pulps from which the yarns were made. In this case, the strongest yarns had the lowest, total-polyose content. It should be noted that, since the viscose process involves steeping in about 18% caustic solution, the xylan carried through to the final yarn will be chiefly resistant xylan (see page 308). 

The empirical nature of this fractionation may be appreciated by comparing the composition of a number of different preparations of alpha-cellulose. The alpha-cellulose from such coniferous woods as spruce, pine, and hemlock may contain appreciable amounts of mannan, although the xylan content is usually low. Similarly, the alpha-cellulose from hardwoods may contain appreciable quantities of non-cellulosic polyoses, although, in this case, the chief polyose is xylan, not mannan. 

On the other hand, certain types of wood pulp, prepared by se((uences involving both acidic and alkaline treatments, show a linear relationship between gamma-cellulose and non-cellulosic polyoses. This may be illus-strated from the studies of White, Steinman and Work on acetylation pulps. Acetylation-grade pulps are usually prepared from softwoods by sulfite cooking followed by hot-caustic refining. When the total polyose content of a number of such pulps was compared with their gamma-cellulose content, a linear relationship was obtained (see Fig. 3). In spite of this excellent correlation, it cannot be assumed that all of the non-cellulosic polyoses are contained in the gamma fraction of these pulps (see below). 

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.

Pentoses are distinguished by their action when boiled with phloroglucinol and hydrochloric acid arabinose and xylose produce a bright red color, which deepens in shade as the solution is boiled. Certain hexoses or polyoses give an orange color when heated with phloroglucinol and hydrochloric acid. 

Production The most important sources of crude C. are cotton and bast plants flax, ramie, jute, hemp (at present still very small amounts) for the textile industry and wood for the paper and pulp industry, in smaller amounts from bagasse. The isolation of C. from wood needs a series of processes in order to separate C. from lignin and polyoses. 

Lignin. A high-molecular-weight, aromatic substance filling the spaces between cell membranes in lignify-ing plants to form the wood (lignification). In this way a mixed unit of pressure-resistant L. and stretch-resistant cellulose is formed. L. is also bound to other polysaccharides (polyoses). The content of L. in dried plant material amounts to 27-33% in conifers and 22% in deciduous tress. 

Pentanedione see butter flavor. l-Penten-3-one see vegetable flavors (tomato). Pentosans see polyoses. 

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