Hemicelluloses: Major Sources,

Properties and Applications

Iuliana Spiridon and Valentin I. Popa

ABSTRACT

This chapter deals with the major representatives of hemicelluloses, their properties and application. Hemicelluloses are plant cell wall polysaccharides that are not soluble in water, but they can be separated by aqueous alkali, or hydrolyzed by diluted acids. The main hemicelluloses include the following polysaccha r-ides: xylan, glucuronoxylan, arabinoxylan, mannan, glucomannan and galactoglucomannan, and their rep-resentatives are different as a function of the plant species. The general structure of hemicelluloses is based on a sugar backbone substituted with side chains. The nature of the monosaccharide, as well as the linkages between structural units, determines some properties, such as solubility and the three-dimensional conform-ation of hemicelluloses. At present, there is an increasing interest to develop new applications of hemicellu-loses as raw materials for chemical industry and also in the fi elds of food and pharmaceutical industries.

Keywords

Hemicelluloses, Polysaccharides, Xylan, Glucuronoxylan, Arabinoxylan, Mannan, Glucomannan and galactoglucomannan, Polymer, Biosynthesis, Biodegradation, Properties, Applications

13.1 STRUCTURE, SOURCES AND PROPERTIES

Hemicelluloses are the second most abundant polysaccharides in nature after cellulose. They occur in close associ-ation with cellulose and lignin and contribute to the rigidity of plant cell walls in lignifi ed tissues. Hemicelluloses constitute about 20–30 per cent of the total mass of annual and perennial plants and have a heterogeneous com-position of various sugar units, depending on the type of plant and extraction process, being classifi ed as xylans ( � -1,4-linked d -xylose units), mannans ( � -1,4-linked d -mannose units), arabinans ( � -1,5-linked l -arabinose units) and galactans ( � -1,3-linked d -galactose units).

In Table 13.1 gives the main hemicelluloses of hardwood and softwood. Xylan [1] is one of the major constituents (25–35 per cent) of lignocellulosic materials. Its structure has a linear

backbone consisting of � -1,4-linked d -xylopyranose residues. These may be substituted with branches containing acetyl, arabinosyl and glucuronosyl residues, depending on the botanic source and method of extraction.

Xylans are the main hemicelluloses in hardwood and they also predominate in annual plants and cereals making up to 30 per cent of the cell wall material. Hardwood xylan ( O -acetyl-4 methyl-glucuronoxylan) is substituted at irregular intervals ( Fig. 13.1 ) with 4- O -methyl- � - d -glucuronic acid groups joined to xylose by � -1,2-glycosidic link-ages. On an average, every tenth xylose unit has an uronic acid group attached at C2 or C3 of the xylopyranose.

Barley arabinoxylans are composed of d -xylopyranosyl (Xyl) units linked by � -(1,4) bonds with single l -arab-inofuranosyl residues connected to the backbone of one or both of the � -(1,2) or � -(1,3) bonds [2, 3] Another unique

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