Hemicelluloses in softwood

The other principal hemicellulose in softwoods is arabino-4-O-methylglucurono-xylan (degree of polymerization of 70-180). The backbone is composed of about 150 P D 1,4 xylopyranose units which are partially substituted at the C2 position by 4-0-methyl-a-D-glucuronic acid groups (approximately one group for every 5-6 xylose units). Also an a-L-arabinofuranose unit is linked by a-1,3 bond on approximately every 6 to 10 xylose units. Arabinofuranose is so called because it is a furanoside having a five-membered ring. 

Hemicellulose compositions vary widely among tree species and wood structures. The different major wood hemicelluloses listed in Table I are named for the simple sugars from which they are formed. The major hemicellulose in softwoods is a glucomannan that contributes 10-15% of the wood mass. This polymer consists of an unbranched chain containing about one... 

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. 

The second important group of hemicelluloses found in softwoods are the arabino-(4-0-methylglucurono)xylans, which may make up from 5 to 10% of the dry weight of the wood. These consist of /3-1,4-linked D-xylopyranose units, partially substituted by L-arabino-furanose (at the 3 position) and by 4-O-methyl D-glucuronic acid (at the 2 position). The frequency of these substituent groups is around 1 to 2 residues per 10 xylose units. 

Smaller amounts of other hemicellulose polysaccharides are also found in softwoods. In particular, larch contains an unusually large amount of arabinogalactan, which is usually only a minor components of other wood species. 

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. 

The pyrolysis oil collected between 200 to 275 C in the second step was 18.14 % by wt. on an anhydrous wood basis and corresponded to the hemicelluloses degradation . In general, hemicelluloses decompose to yield furans and its derivatives as well as a series of aliphatic carboxylic acids. Hardwoods are composed of pentosan-based hemicelluloses, while hexosan-based hemicelluloses are found in softwoods. Hardwood produces more low molecular weight carboxylic acids and furans than softwood upon decomposition . In contrast, hardwood contains a lower quantity of fatty acids than softwood. Cl - C6 carboxylic acids, fatty acids (e.g. Cia), fiiran and derivatives were identified in this fraction. In the second pyrolysis step, a little rearrangement of the lignin polymer occurred which was based on the formation of small amount of phenolic compounds 0,395 % by wt., anhydrous wood basis). The main components of this fraction are listed in Table 2. Betulin and lupeol were abundant in this fraction. Furthermore, levoglucosan which has been reported earlier as a major compound in the cellulose-derived pyrolysis oils, was identified in the second... 

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. 

As mentioned previously (see Part I, p. 255), only minor quantities of hemicelluloses can be removed from softwood by direct extraction with alkali. Very likely, the hemicelluloses are protected by the lignin, especially by that portion which surrounds the polysaccharides in the secondary wall. Isolation of softwood xylans by direct, alkaline extraction of tbe wood seems to have been used on only two occasions. Saarnio, using wood shavings of Norway spruce Picea abies), was able to obtain a relatively pure arabino-(4-0-methylglucurono)xylan in a yield of 3%, simply by extracting with 5% aqueous potassium hydroxide, a remarkable result. With wood of loblolly pine, Jones and Painter obtained a mixture of hemicelluloses, in a yield of about 1%, on direct extraction with 25% aqueous potassium hydroxide solution containing 4% of borate. 

As will be discussed later (see p. 473), the alkali-soluble glucomannans in softwoods have now been shown to be partially 0-acetylated in their native state. If the 0-acetyl groups in softwoods are associated only with the D-mannose-containing hemicelluloses, experimental data indicate that the galactoglucomannans must also carry 0-acetyl groups. This conclusion is strengthened by the fact that the extracellular galactoglucomannans are partly 0-acetylated. 

All of the major polysaccharides in softwoods are now known these are cellulose, galactoglucomannans, arabino-(4-0-methylglucurono)xylan, ara-binogalactan, galactan, pectic material, and starch. It is clear, however, that other minor polysaccharide constituents, still unknown, must also be present. These polysaccharides have not yet been isolated, but their presence may be inferred from the formation of di- and tri-saccharides of unknown origin on partial hydrolysis of wood or crude hemicellulose preparations. 

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). 

Another major hemicellulose component in softwood is arabino-(4-(7-methylglucurono)-xyIan (Fig. 1), which represents about 7-15% of the wood [3]. Arabinoglucuronoxylans are composed of approximately 200 (1 4)-Iinked (1-D-xylopyranosyl units that are partially substimted by 4-0-methyl-a-D-glucuronic acid moieties at the C-2 position, with an average of (Mie glucuronic acid unit per 5-6 xylopyranosyl units [12]. In additimi, a-D-arabinofuianosyl units are bonded at the C-3 position of the main xylan chain, with an average of 1.3 arabinofuranosyl units per 10 xylopyranosyl units. 

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