Hemicelluloses of wood

Wood forms one of the world s most important chemical raw materials. It is the primary source of cellulose for the pulp and paper and cellulose industries. These industries are well up in the group of 10 major industries of the United Slates. For paper, rayon, films, lacquers, explosives and plastics, which comprise the greatest chemical uses of wood, it is the cellulose component (plus certain amounts of hemicellulose) of wood that is of value. The lignin forms a major industrial waste as a by-product of the paper and cellulose industries. Its major use is in its heat value in the recovery of alkaline pulping chemicals. A variety of minor uses for lignin have been developed, such as for the manufacture of vanillin, adhesives, plastics, oil-well drilling compounds and fillers for rubber. 

Analyses of the hemicelluloses of woods indicate the presence of a high proportion of D-xylose residues in association with those of a methyl ether of a hexuronic acid.64 The first evidence for the chemical structure of such wood xylans came from the isolation of xylobiose, xylotriose,65 and the aldobiouronic acid 0-(4-0-methyl-a-D-glucosyluronic)-(l —> 2)-D-xylose acid (IX)22 from partial hydrolyzates of aspen wood. The role of such oligosaccharide fragments in wood xylans became more apparent as the result of a study of a xylan from European beechwood,26 in which it was shown... 

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. 

Roudier A J, Eberhard L 1965 Hemicelluloses of wood of the maritime pine of Landes. IV. Polysaccharides extracted by boiling water. Structure of an arabinan present in the extract. Bull Soc Chim Fr 460 - 464... 

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. 

Wood is a composite material that is made, up basically of a mixture of three main constituents, cellulose, hemicellulose, and lignin (see Textbox 54), all of them biopolymers synthesized by the plants, which differ from one another in composition and structure (see Textbox 58). The physical properties of any type of wood are determined by the nature of the tree in which the wood grows, as well as on the environmental conditions in which the tree grows. Some of the properties, such as the density of wood from different types of trees, are extremely variable, as can be appreciated from the values listed in Table 71. No distinctions as to the nature of a wood, whether it is a hardwood or a softwood, for example, can be drawn from the value of its specific gravity. 

The conditions used for char preparation in the present chemisorption studies (i.e., progressive slow charring of wood) are intended to be relevant to "real life" smoldering combustion situations. Most previous studies of chemisorption have used chars from cellulose (i.e., avoiding hemicellulose and lignin... 

Aqueous alkaline extraction of wood was employed by Poumarede and Figuier48 in 1846 for the removal of a substance called wood gum. Similar very crude xylan or hemicellulose preparations were made by other workers of the early period. Extractions were made not only from wood sawdust, but from annual plant materials such as wheat straw, corn cob, etc.49-66... 

Many of the physical, chemical and biological properties of wood can be understood by referring to the polymeric chemical constituents. In many cases of wood modification, these polymeric components are modified to some extent. The three structural polymeric components of the wood cell wall are cellulose, hemicellulose and lignin. There are many excellent texts describing the structure and function of these components, and only a brief account is given here. 

Beall, F.C. (1969). Thermogravimetric analysis of wood hgnin and hemicelluloses. Wood arui Fiber, 1(3), 215-226. 

Biorefinery includes fractionation for separation of primary refinery products. The fractionation refers to the conversion of wood into its constituent components (cellulose, hemicelluloses and lignin). Processes include steam explosion, aqueous separation and hot water systems. Commercial products of biomass fractionation include levulinic acid, xylitol and alcohols. Figure 3.3 shows the fractionation of wood and chemicals from wood. 

Xyloisosaccharinic acid [2,4-dihydroxy-2-(hydroxymethyl)butanoic acid] is one of the major, alkaline-degradation products of wood xylan, in particular, that of birch. The disaccharide, 2-O-D-xylopyranosyl-L-arabinose, which was isolated as a hydrolysis product of corn-cob hemicellulose, is readily degraded at 100° in 15 mM Ca(OH)2 to acidic products, primarily saccharinic acids. Xylan oligosaccharides from corn-cob hemicellulose produced 2,4-dihydroxy-2-(hydroxymethyl)buta-noic acid when exposed to 0.02 M Ca(OH)2 at 25°. However, it was noted that the xylan, itself, was stable at 100° in Af NaOH. The major acidic component of the hemicellulose fraction of slash pine Pinus el-liotti) after acid hydrolysis was identified as 4-O-methyl-D-glucuronic... 

Solid wood material is built up of two major organic polymers (macro molecules) (1) polysaccharides and (2) polyphenylpropane [61,62], The polysaccharides consist of two groups - cellulose and hemicellulose, and make up around 65-75 % of the wood on dry basis. The polyphenylpropanes are more commonly termed lignins and constitute around 18-35 % of the wood on dry basis. In Table 9 we can see that wood fuels consist of extractives, minerals, and nitrogen as well. The chemical composition of wood of Sweden s most commonly wood species [63], the spruce, the pine and the birch are different, see Table 9. 

The FP cellulose per unit (ml) volume and enzyme yield per unit (g) cellulose or substrate obtained on wheat straw, wood, and CTMP in SSF were higher than those obtained in LSF on wheat straw and wood (Tables I, II, and III). And wheat straw proved to be a better substrate than wood for cellulose production in SSF. This could be attributed to the polysaccharides (cellulose and hemicelluloses) of wheat straw being more readily available for the organism s growth and cellulose synthesis than those of wood. The hemicelluloses and cellulose were presumably not as available in wood, because of its high lignin content and high cellulose crystallinity, as in wheat straw. 

---