Hemicellulose

Hemicellulose is a collective term for plant biomass occurring in mixtures of polysaccharides ( multiple sugars ) in variable composition. The most common monomers ( monosaccharides = simple sugars ) are pentoses such as D- xylose and L- arabinose.

The name of hemicellulose is based on the original, incorrect assumption that they ( hemi = Greek for semi-) is a precursor of cellulose. However, it is a homopolymer of the hexose glucose.

Definition

Hemicelluloses are a component of plant cell walls, whose matrix consists of fibrillar, partly crystalline cellulose. Lignification in this matrix is also penetrated by the lignin macromolecule, thus forming the lignocellulose. The hemicelluloses thus form part of the supporting and structural substance of cell walls and accounts for 1 /4 to 1 /3 of the plant mass from. It is amorphous and does not form higher structures.

The variety of carbohydrates or monosaccharides in nature is very large. In addition, they can be linked in different ways ( by glycosidic bonds) to polymer ( polysaccharides ) which are still also can differ in the number of the monomers. Even with hemicelluloses there is this great diversity, so that a general definition based on the chemical structure is only vaguely possible.

Physics

The hemicelluloses are soluble in alkaline or slightly acidic environment. The physical properties of the dry matter is determined by the mixture. Hemicelluloses are dependent in their mechanical and other physical properties, especially on the chain length of the sugar molecules.

Hemicelluloses are the component of the cell wall, which can be solved by alkali treatment or short-term extraction with dilute hot acid. Other components, such as lignin and difficult to access due to their highly ordered structure of cellulose, not go into solution. These methods are used, for example, in the feed analysis to determine the levels of certain fractions ( cellulose, other carbohydrates ).

Chemistry

Monosaccharides and derivatives

The polysaccharides of the hemicelluloses consist of different monosaccharides. Frequently represented are the pentoses (sugars with five carbon atoms), xylose and arabinose. Also hexoses (sugars with 6 carbon atoms), such as glucose, mannose, and galactose, are found. With sugars related compounds, such as uronic acid (certain sugar acids ), for example from the group of hexuronic acids ( glucuronic acid, methylglucuronic acid, galacturonic acid ) and special sugars, such as deoxyhexoses ( hexoses wherein an alcohol group (- OH) by a hydrogen atom ( is substituted -H) ), for example ( rhamnose ) occur.

Polymers

The monosaccharides are with polymers (more precisely, polysaccharides ) linked. In homopolymers only a monosaccharide in the respective polymer occurs in heteropolymers ( heteroglycans ) there are two or more different monosaccharides. In part, this is made clear on behalf of the polymer which bears the name of one or of a common monosaccharide, provided with the ending ane. Examples are xylans, mannans and galactans, or the parent names, such as pentosans and hexosans.

Mostly, the main chain of a homopolymer with Xylan for example, xylose, or a hetero- polymers of two or more blocks dar. to this are branching more sugar bound to form an irregular macromolecule.

Occurrence

Wheat and barley contain predominantly hexosans, rye and oats pentosans. The swellable, slimy pentosans in Rogge flour prevent the formation of a gluten framework as with the wheat dough during dough preparation. This polysaccharide is mainly to arabinoxylan, the main chain is supplemented from xylose by branches in the form of arabinose. Bran contains up to 40% hemicellulose.

Needle wood contains predominantly mannans, during itself find in deciduous wood above all xylans.

Use and meaning

May be hemicelluloses an important part of chemical pulp for papermaking and is relevant to properties of the paper such as tensile strength and tensile strength, opacity, etc..

With the increasing importance of renewable resources is also increasingly the opening up of of hemicelluloses, eg in biorefineries, discussed. Wood and cereals could be the source of raw materials, the compounds such as furfural from pentoses, eg for the preparation of nylon delivers, . Of furfural and furan is derived which is used for example for the production of solvents.

A number of other platform chemicals can be derived from hemicelluloses and certain monomers.

With special yeasts that can utilize pentoses, the future could be a production of bio-ethanol from hemicelluloses, eg, for use as biofuel or basic chemical, be economical.

In the human food are hemicelluloses, in addition to the polysaccharides cellulose and pectin, a major physiologically acceptable dietary fiber, since they are neither digested in the stomach or in the intestines.