Hemicellulose of hardwoods

Recently, the alkah-soluble hemicelluloses of hardwood dissolving pulps have been investigated [28]. Their composition and molecular properties depended on the pulp origin and steeping conditions. The MGX of the j8-fraction from press lye had a low uronic acid content (ratio of MeGlcA to Xyl is about 1 20). The molecular weight of the hemicellulose fractions varied between 5000 and lOOOOg/mol. 

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. 

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. ... 

Figure 4.46 Xylan from grasses. The major hemicellulose of hardwoods lacks the arabinofuranosyl groups and the 4-O-methyl-a-glucuronosyl residues are attached only to 02 of xylose. The minor hemicellulose of softwoods lacks the acetyl groups, but again the 4-O-methyl-a-glucuronosyl residues are attached only to 02 of xylose.

Glucuronoxylan (15-30%) is the major hemicellulose of hardwoods such as birch, eucalyptus and poplar/aspen. It is similar to grass xylan, but lacks the arabinofuranosyl decoration (Figure 4.44). 

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

Characterization of Alkali-Soluble Hemicelluloses of Hardwood Dissolving Pulps... 

Glucomannans (GM) and galactoglucomannans (GGM), common constituents of plant cell walls, are the major hemicellulosic components of the secondary cell walls of softwoods, whereas in the secondary cell walls of hardwoods they occur in minor amounts. They are suggested to be present together with xylan and fucogalactoxyloglucan in the primary cell walls of higher plants [192]. These polysaccharides were extensively studied in the 1960s [6,193]. 

Maloney, M.T., Chapman, T.W., and Baker, A.J. (1986) An engineering analysis of the production of xylose by dilute acid hydrolysis of hardwood hemicellulose. Biotechnol Progr., 2, 193. 

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. 

Glucomannan is a minor fraction of hardwood hemicelluloses (about 3-5%), and is a P-(l-4)-linked linear copolymer of glucose and mannose at the ratio of 1 1-2. 

The hemicelluloses of wood contain uronic acid residues which frequently bear methyl ether groups. Hydrolysis of the hemicelluloses yields aldobioLironic and aldotriouronic acids. In researches referred to previously (page 300), O Dwyer " investigated the hemicelluloses from oakwood. She isolated an aldobiouronic acid as a barium salt after combined enzymic and acidic hydrolysis. Analyses of this compound were in agreement with the empirical formula for a barium salt of an 0-methyl-xylo-aldobiouronic acid. This compound was isolated from a polysaccharide material which contained xylose (6 parts), uronic acid (1 part), and methoxyl (1 part). Studies by Anderson on the hemicelluloses of a number of hardwoods also showed that 0-methylhexuronic acid residues are present in glycosidic union with xylose, in ratios extending from 8 to 19 xylose units per uronic acid unit. 

Newman RH (2005) Solid-state NMR as a tool for studying dancing molecules. In Entwistle KM and Walker JCF (eds). The Hemicelluloses Workshop 2005, Wood Technology Research Centre, University of Canterhury, Christchurch, New Zealand, 77-86 NHLA (1991) An introduction to grading of hardwood lumber. National Hardwood Lumber Association, Memphis, Tennessee... 

Infestation is usually promoted by a warm climate and an adequate supply of high-starch-content sapwood of hardwood. Larvae cannot digest lignin, hemicellulose, or cellulose. Females tend to oviposit in wood with a starch content of at least 3%. Larvae can survive in moisture contents (MC) between 8% and 32%. Dehydration is a primary threat to the beetle. 

Acetic acid, formic acid and ievulinic acid are the most common carboxylic acids found in the hydrolyzates. Acetic acid is not only a by-product of hydrolysis (45) but is also a well-known by-product in fermentation (46). Acetic acid is mainly formed from acetylated sugars in the hemicellulose, which are cleaved off already at mild hydrolysis conditions. Therefore, die acetic acid yield in the hydrolysis does not significantly depend on the severity of the hydrolysis process (2). Hydrolysis of hardwoods (alder, aspen and birch) at 198-234°C, 0.5 g/1 H2SO4, and 33% wood dry materials for 7 min resulted in approximately 10 g/l acetic acid, whereas the hydrolysis of softwoods (pine and spruce) produced 3 g/1 acetic acid at similar conditions (2). 

Xylan is the main component of hardwood (HW) hemicelluloses (10-35% of the wood). In contrast to softwood Xylans, hardwood xylans do not contain a-D-arabinofuranosyl units and, therefore, they are glucuronoxylans. The content of 4-0-methyl-a-D-glucuronic acid moieties in HW xylans is lower than in the softwood ones, about 1 unit per 10 xylopyranosyl units [1-3]. In addition, xylopyranosyl units in HWs are partially acetylated at the C-2 and/or C-3 position with total amounts of 3.5-7 acetyl groups per 10 xylopyranosyl units [13]. 

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