Pentosans hemicellulose polymer

Xylan has the general properties of insolubility in water, solubility in alkaline solutions, ease of acid hydrolysis, high negative optical rotation, and non-reducing action toward Fehling s solution. It can be placed in three general polysaccharide classes (1) pentosan, (2) glycan, and (3) hemicellulose. It is classed as a pentosan because it is principally a polymer of a pentose. It is by far the most abundant pentosan. 

The hemicelluloses also include the polyuronides, or polyuronic acids, for instance a polymer of a hexuronic acid such as galacturonic acid. There is a possible generic link between the polyuronides and the pentosans since the latter might be produced as the result of the decarboxylation of hexuronic acids. The possibility of transforming hexuronic acids into pentosans by the removal of carbon dioxide... 

That not all of the xylans of wood are homogeneous polymers of anhydro-D-xylose units has been shown by the classical studies of O Dwyer (1923 to 1940) on hemicelluloses of American white oak. O Dwyer prepared a hemi-cellulose fraction from water-extracted, oakwood sawdust by extraction for two days with 4% aqueous sodium hydroxide solution. The polysaccharide material was obtained, after acidification, by the addition of ethanol. The product ([a]n —75° in 1 % sodium hydroxide), contained 70% of pentosan, and yielded n-xylose, n-maiinose, n-galactose, and L-arabinose on hydrolysis. 

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... 

The main direction of research now appears to be toward the utilization of wood by-products. Wood generally contains from 50-60% cellulose, 20-30% lignin, 10-20% pentosans, 0-7% mannans and some acetyl linkages (usually 0.5-2% acetic acid) (48). This wide assortment of carbohydrates is of interest to ruminant nutritionists for at least three reasons. Ruminants need a source of readily available energy, usually obtained as hexoses derived from soluble carbohydrates. Most forages contain pentosans or hemicelluloses, but no reliable concentrated pure source of these materials is available and until recently it was not possible to obtain enough to evaluate the utility of these products in defined rations. Finally, the possibility of producing polymers from feed material is attractive. 

Hemicelluloses have relatively low molar mass compared with other wood polymers. The degree of polymerization of hemicelluloses is around 80-200. The general units of hemicelluloses are pentosans and hexosans. Their general formulas are (CsHgOJn and [CgHioOsin, respectively. Hemicellulose does not have a repeated structure. There are many variations in the structure that are not well known. The detailed structure of the hemicelluloses and their abundance vary. 

Hydrolyses of hemicelluloses yield mixtures of glucose, glucuronic acid, xylose, arabinose, galactose, galacturonic acid, mannose, and rhamnose. Some common polymers of pentoses, also known as pentosans, are xylan, galactan, araban, and others. Pentosans are found in large amounts (20-40%) in cereal straws and in brans. Large-scale industrial preparations of furfural, for instance, are based on these materials. 

For several years now, dietary fibre has received considerable attention because of possible benefits to human health. Fibre is the collective name for the insoluble indigestible cell wall constituents of plant materials it consists of cellulose (a glucose polymer), hemicellulose (a heterogeneous group with pentosans predomi-nant-hexosans, and poly uronides), lignin (an aromatic compound), protopectin and some nitrogen-containing materials (Van Soest 1963 Worth 1967 Norman 1973 Hellendoorn et al. 1975). 

Wood (xylem) is a vascular tissue which occurs universally in gymnosperms (soft wood trees) and dicotyledonous angiosperms (hard wood trees). The wood consists of lignin and holocellulose. The former is a polymer consisting of repeat phenolpropane units and the latter consists of cellulose and hemicellulose. Hemicellulose is a mixture of pentosans, hexosans and polyuronides. 

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