hemicellulose

n.
Any of several polysaccharides that are more complex than a sugar and less complex than cellulose, found in plant cell walls and produced commercially from corn grain hulls.

A term designating plant cell components which are made soluble by dilute alkali or which go into solution quite readily in hot dilute mineral acids with the formation of simple sugars. Hemicelluloses constitute about one-fourth of perennial plants and about one-third of annual plants. The term is usually applied to those polysaccharides in the cell wall of land plants which are extractable by dilute alkaline solutions. The term has also been used to include all the polysaccharide components of the cell wall other than cellulose. See also Cell walls (plant); Cellulose.

Hemicelluloses extracted from different plant sources are rarely identical. In fact, many different hemicelluloses usually occur intermixed with each molecular type representing different degrees of polymerization. Because of this heterogeneity, few hemicelluloses have been isolated in a homogeneous state. Therefore, relatively little is known of the structure of these compounds that compose almost one-third of the carbohydrates in woody tissue.

D-Xylose is the dominant building unit of the hemicelluloses of most woods and annual plants. D-Mannose is also very abundant in hemicelluloses; the mannose content of softwoods is usually higher than that of hardwood. Often it occurs as a polymer, mannan, or in combination with D-glucose or D-galactose as a glucomannan, galactomannan, or galactoglucomannan.

Hemicelluloses are important to the paper industry. In chemical wood pulps, hemicellulose is needed for satisfactory pulp quality. Its presence aids the swelling of the pulp, the bonding of the fibers, the bursting strength, tensile strength, tear resistance, folding endurance, opacity, and specific surface of the pulp sheet.

Hemicelluloses also serve as nutrients for yeasts, and they can be used for raw material in the production of furfural and ethyl alcohol.

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Complex carbohydrates included as dietary fibre, composed of polyuronic acids combined with xylose, glucose, mannose, and arabinose. Found together with cellulose and lignin in plant cell walls; most gums and mucilages are hemicelluloses.

Structural polysaccharide of plants. Consists of β1,4-linked pentose sugars, mainly xylose.

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