Hemicellulose

This is unfortunately a misnomer, as the cell-wall hemicellulose fraction has no chemical relationship with cellulose. In the primary and secondary cellwalls, the cellulose microfibrils are embedded in an amorphous matrix consisting of pectins and hemicelluloses. Pectins are mainly polygalacturonic acids and predominate in the middle lamella between neighbouring cells; this is also the most highly lignified plant tissue (Coleman et al., 1999). Hemicelluloses are short chain partially soluble polysaccharides composed of a variety of sugars such as arabinose, galactose, glucose, mannose and xylose in combination with uronic type acids, such as glucuronic and galacturonic acid. Hemicelluloses that are basically xylose or arabinose in combination with glucuronic acid are termed xyloglucans or arabinoglucans, respectively. If arabinose and galactose are in approximately equal amounts, the hemicellulose is termed an arabinogalactan; arabinose and xylose combine to give arabinoxylan, which has few side chains. If mainly just glucose, galactose or xylose, they are called glucans, galactans or xylans respectively. Hemicellulose molecules, such as galactoarabinoxylan, are often branched and, like pectic compounds, very hydrophilic; they become highly hydrated and form gels. The hemicellulose in Poaceae (= Graminae, grasses, cereals) contains arabinoxylan, galactoarabinoxylan and glucan. Hemicellulose is abundant in primary walls but is also found in secondary walls.

The estimation of total hemicellulose, without characterization of individual sugars, may be achieved by subtracting the ADF value from the NDF value obtained from duplicate samples of the same weight. There are, however, four main possible interferences with this procedure.

1. Biogenic silica is largely dissolved by NDF method, but totally recovered with ADF.

2. Cell-wall protein is recovered in the NDF method, but mostly dissolves with ADF.

3. Pectin is dissolved in the NDF method, but is part precipitated with ADF.

4. Tannin is part dissolved by NDF, but precipitated as a protein complex with ADF.

It should be determined whether any of these factors is large enough to constitute a significant interference in the planned experiment. See also the comments on NDF below. An NDF method which includes pectic polysaccharides, P-glucans and fructans has been published by Hall et al. (1997).

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