Hardwood hemicellulose content

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. 

Attempts to remove hemicellulose for production of dissolving pulps with very low hemicellulose contents have shown that complete enzymatic hydrolysis of hemicellulose within the pulp is difficult to achieve. The xylan content in delignified mechanical aspen pulp was reduced from approximately 20 to 10%, whereas in bleached hardwood sulphite pulp the xylan content was decreased from 4 to only 3.5% even at very high enzyme dosages (50). The complete removal of residual hemicellulose seems thus unattainable, apparently due to modification of the substrate or to structural barriers. 

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. 

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. ... 

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,... 

In the cell wall, hemicelluloses are thought to surround and sheath microfibrils, forming a chemically bonded cellulose-hemicellulose network. Whereas the composition of cellulose is consistent for softwoods and hardwoods, the chemistry and structure of softwood and hardwood hemicelluloses differ. Even within a single tree, different parts will contain varying amounts of certain hemicellulose species (9). Given this variability, hemicellulose contents for hardwoods and softwoods are generally reported as ratios, rather than as exact quantitative measurements. 

When hardwoods are to be used, it is necessary to remove a part of the hemicellulose first by prehydrolysis. This has been done on a large scale with straw, a substance chemically similar to hardwood, by heating it in an 8-to-l liquid-to-solid ratio, with 0.5 percent sulfuric acid for 2 to 3 hours at 130°C (266°F). Without prehydrolysis, hardwoods and straws form slimy materials, probably because of their high hemicellulose content, which prevent the flow of the hydrolyzing acid. 

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. 

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). 

Tritiated Water. Exchange with tritiated water and subsequent scintillation analysis of the tritium content is a very useful method for accessibility measurements when the samples are not transparent to IR and visible light. Thin shives of sapwood from black spruce (a common softwood), and white birch (a common hardwood) where studied before and after delignification by treatment with peracetic acid (17). Also wood cambium from the same softwood was studied. The cambium is the recently formed fiber layer, located close to the bark and not yet lignified. Spruce and birch wood contain about the same amounts of cellulose, 42 and 44%, respectively. Spruce wood has more lignin (28 vs. 18% ), while birch has more hemicellulose (35 vs. 2S% ), in particular more pentosans than spruce (24 vs. 14%). 

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. 

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). 

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]. 

Wood is a naturally formed organic material composed of cells arranged in a parallel manner. The chemical composition of the woody cell walls is approximately 40-50% cellulose, 15-30% lignin, less than 1% mineral, 25-35% hemicellulose, and the remainder extractable matter of various kinds. Softwoods and hardwoods both contain approximately the same cellulose content. 

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