Other Hardwood Hemicelluloses

Other Hardwood Hemicelluloses 1. Polysaccharides Extractable from Wood with Water 

Treatment of finely divided wood from angiosperms with water (hot or cold) removes small amounts of polysaccharides, which are probably largely of extracellular nature. A cold-water extract from aspen wood was found by QuickC to give galactose, glucose, mannose, arabinose, xylose, fucose, and rhamnose on hydrolysis, indicating the presence of a mixture of polysaccharides. Jones, Merler, and Wise, on exhaustive extraction of the same wood with cold water, obtained, in a yield of 2.1%, a product which could not be further resolved by precipitation with Fehling solution. 

The galactan of tension wood discovered by Meier differs not only from 

Brimacombe, and M. Stacey Department of Chemistry, University of Birmingham, Edgbaston, Birmingham, England 

Although, after 1880, various bacteria and vimses were known to cause pneumonia, the modem etiological classification of bacteria did not gain general acceptance until 1920. In this year, a Committee of the Society of 

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Hemicelluloses are quite difficult to extract from cell walls of softwoods (9,10) and are usually destroyed or depolymerized during the chemical pulping of these raw materials. However, other hemicelluloses, primarily xylans, can be extracted by cold, dilute sodium hydroxide from grasses and many hardwoods in very high yields (9,77). These xylans are deacetylated in an alkaline medium and are for the most part insoluble (hemicellulose A). A partially water soluble fraction (hemicellulose B) has also been... 

The hetero-l,4-linked xylans (or heteroxylans) constitute a well-characterized group of polysaccharides which form the major components of the hemicellulosic fractions of terrestrial plants (1-4). Softwoods are an exception, where the heteroxylans can be present as a minor component of the total hemicelluloses. They have been isolated from grasses, legumes, ferns, softwoods and hardwoods, and collectively may constitute up to 35% of the total dry weight of higher land plants (4). As such the heteroxylans rank second to cellulose in abundance as naturally occurring organic chemicals in the biosphere. The heteroxylans are closely associated with other... 

In the ALCELL process, conventional hardwood chips are cooked in batch extractors with an aqueous ethanol liquor at appropriate temperatures, pH, and time. In the process lignin, hemicellulose and other various components of wood are extracted from the chips into the aqueous ethanol forming a black liquor. 

Glucuronoxylan Even if hemicelluloses in various hardwood species differ from each other both quantitatively and qualitatively, the major component is an 0-acetyl-4-0-methylglucuronoi3-D-xylan, sometimes called glucuronoxylan. Often the xylose-based hemicelluloses in both softwoods and hardwoods are termed simply xylans. 

Closely associated with cellulose in the wood structure and paper products are other polysaccharides called hemicelluloses, which often have been labeled as the matrix material of wood. In hardwoods the primary hemi-cellulose is a xylan (polymer of xylose), whereas in softwoods the primary hemi-cellulose is a glucomannan, although both of these polysaccharides occur to some extent in both types of wood. The DP of the hemicelluloses is much less than that of cellulose, in the range of 100-200. 

Pectin and pectic materials that contain relatively small amounts of other materials can be readily converted to insoluble calcium pectate. When these materials have been isolated from a hardwood or similar material, they consist of pectic acid mixed with hemicelluloses, lignin and coloring matter. To remove these impurities, the material is treated with liquid bromine. The pectic acid and hemicelluloses are precipitated by alcohol and washed with this solvent until free of brominated lignin. The pectic acid is then separated from the hemicellulose by conversion to insoluble calcium pectate. 

Cellulose dominates the wood composition, although its proportion with respect to the other main components can vary appreciably from species to species. Conversely, polyphenols are the least abundant components and moreover, can exhibit quite different structures. As for lignins and hemicelluloses, their relative abundance and their detailed structures are essentially determined by the wood family softwoods are richer in lignins, wha-eas hardwoods are richer in hemicelluloses. These three basic polymeric components represent fundamental sources... 

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