Hemicelluloses in hardwood

Molecular structures of the predominant forms of hemicellulose in hardwood and softwood tree species. 

Hardwoods are less thermally stable than softwoods and this is attributable to differences in the hemicellulosic content and composition. Pentosans (which are found in higher proportions in hardwood hemicelluloses) are more susceptible to thermal degradation than hexosans (Fengel and Wegener, 1989). Additionally, hardwoods, in general, have a higher proportion of hemicellulose, and the hemicelluloses of hardwoods also have a higher acetyl content compared to softwoods. 

The amount of hemicelluloses of the dry weight of wood is usually between 20 and 30% (cf. Appendix). The composition and structure of the hemicelluloses in the softwoods differ in a characteristic way from those in the hardwoods. Considerable differences also exist in the hemicellulose content and composition between the stem, branches, roots, and bark. 

Glucuronoxylan Even if hemicelluloses in various hardwood species differ from each other both quantitatively and qualitatively, the major component is an 0-acetyl-4-0-methylglucuronoi3-D-xylan, sometimes called glucuronoxylan. Often the xylose-based hemicelluloses in both softwoods and hardwoods are termed simply xylans. 

Closely associated with cellulose in the wood structure and paper products are other polysaccharides called hemicelluloses, which often have been labeled as the matrix material of wood. In hardwoods the primary hemi-cellulose is a xylan (polymer of xylose), whereas in softwoods the primary hemi-cellulose is a glucomannan, although both of these polysaccharides occur to some extent in both types of wood. The DP of the hemicelluloses is much less than that of cellulose, in the range of 100-200. 

Further studies on hemicelluloses of hardwoods have shown that these may exhibit a considerable range in the content of xylan and 0-methylhexuronic anhydride. In a recent review of hemicelluloses, Wise suggests that hardwood hemicelluloses may have a rather simple, architectural pattern, consisting predominantly of n-xylose units and smaller amounts of 0-methyluronic acid units. Softwood hemicelluloses also contain this fundamental structural unit consisting of a mono-O-methyluronic acid in combination with n-xylose units. In the case of pine, this fundamental unit may be associated with a part of the maiuian. ... 

In the isolation of hemicelluloses from plant materials containing relatively large amounts of hemicelluloses and small amounts of insoluble pectic materials, as in hardwoods, the powdered material is first extracted with organic solvents and hot water. The hemicelluloses are then removed by two extractions with a cold 4% solution of sodium hydroxide. The material is made neutral with dilute hydrochloric acid and washed with water. It is then extracted successively with a hot. 05 N solution of hydrochloric acid and a cold 5% solution of ammonium hydroxide. This procedure removes only a part of the hemicelluloses and pectic materials from a wood since lignin protects the underlying hemicelluloses from the alkaline solution or is combined with them. - To overcome this difficulty the wood is suspended in water and treated with chlorine gas. - - The subsequent addition of a large volume of... 

Pentosan. Pentosan analysis measures the amount of five-carbon sugars present in wood (xylose and arabinose residues). Although the hemicelluloses consist of a mixture of five- and six-carbon sugars see discussion of hemicelluloses), the pentosan analysis reports the xylan and arabinan content as if the five-carbon sugars were present as pure pentans. Pentoses are more abundant in hardwoods than softwoods the difference is due to a higher xylose content in hardwoods see Table XIII for examples). 

In hardwoods xylose is by far the most important hemicellulose monomer followed by mannose, glucose and galactose, with smaller amounts of arabinose and rhamnose. The xylose occurs predominantly as O-acetyl-4-O-methylglucuronoxylan (degree of polymerization of 100-200). The basic skeleton of all xylans is a linear... 

Lignin is present in plants for which water conduction is important. Of greatest interest is its presence in trees. The lignin content depends on the type of tree about 28% for softwoods and 20% for hardwoods. The cellulose content is approximately 45% in the wood of both types, while the hemicellulose content is roughly 17% in softwoods and 25% in hardwoods. Lignin structure can vary within the same plant,... 

The strainTFfects due to dehydration are nowhere better illustrated than in the precautions that must be taken to dry and season wood before its use. This problem relates to water removal from capillaries but certainly the dehydration of the hemicelluloses which are in a paracrystal1ine order at the surface of the microfibrils ( ) must play a role. Xylan (4) in hardwoods and galactoglucomannans (IJ ) in softwoods have the characteristics of columnar and sheet hydration, respectively. 

Hemicellulose is one of the major components of lignocellulose. Depending on the nature of the raw material, the hemicellulose fraction contains varying levels of xylose-based hemicelluloses, xylans (Table 1). The xylan content is generally high in hardwood (wood from deciduous trees) and in agricultural residues, and somewhat lower in softwood (wood from coniferous trees). 

In Table 13.1 gives the main hemicelluloses of hardwood and softwood. 

The glucuronoxylans are composed of linear chains of p-(l->4) xylopyranosyl residues branched with a-(l->2)-linked 4-0-methylglucuronopyranosyl residues (4-0-MeGlcA). The 4-0-MeGlcA-to-xylose ratio is -1 6 for softwood and -1 10 for hardwood xylans. (7-acetyl-(4-D-methylglucurono)-xylan is the major hemicellulose present in hardwood, whereas arabino-(4-6>-methylglucurono)-xylan is present in softwood. 

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