Hemicelluloses solubility

Prior to xylan removal it is possible to extract from the plant material those hemicelluloses soluble in dilute alkali. Soluble hemicelluloses of this type are the principal impurities extracted with xylan and consist for the most part of low molecular weight polysaccharides and polyuronides. Corn cob holocellulose may be freed of these compounds (called hemicellulose-B fraction, see Figure 2) by extraction with potas-... 

Some hemicelluloses are partiaUy extractable with water, but they are usuaUy extracted with alkaline solutions foUowing removal of Upids and lignin. DeUgnifted plant material is termed, holoceUulose. Neutralization of the alkaline extract effects precipitation of the more linear and less acidic hemicelluloses, termed the hemiceUulose A [63100-39-0] fraction. The more acidic and more branched material, termed hemiceUulose B [63100-40-3], is precipitated with ethanol (70%). HemiceUulose B types type are usuaUy water-soluble after extraction. 

Recently, the alkah-soluble hemicelluloses of hardwood dissolving pulps have been investigated [28]. Their composition and molecular properties depended on the pulp origin and steeping conditions. The MGX of the j8-fraction from press lye had a low uronic acid content (ratio of MeGlcA to Xyl is about 1 20). The molecular weight of the hemicellulose fractions varied between 5000 and lOOOOg/mol. 

From recent literature it is known that the disintegration of lignified cell walls can be achieved by steam explosion treatments resulting in solubilization of partially depolymerized hemicelluloses [91,92]. The application of this method on wheat bran yielded feruloylated GAX with different feruUc acid content [93]. Partly depolymerized water-soluble, acetylated AGX was obtained from spruce wood by employing microwave treatment [94]. 

AG type II is most abundant in the heartwood of the genus Larix and occurs as minor, water-soluble components in softwoods. Certain tree parts of western larch (I. occidentalis) were reported to contain up to 35% AG [378]. The polysaccharide is located in the lumen of the tracheids and ray cells. Consequently, it is not a cell-wall component and, by definition, not a true hemicellulose. However, it is commonly classified as such in the field of wood and pulping research. This motivated us to include the larch AG in the review. 

The carbohydrates in rice bran are made up of the cell wall components such as polysaccharides, oligosaccharides, hemicelluloses, starch and some sugars. The health benefits of rice bran polysaccharides and hemicelluloses are discussed under phytochemicals of rice bran below. It is lactose-free and gluten-free. The water-soluble non-starchy polysaccharides, oligosaccharides and hemicelluloses are concentrated in the water-soluble fraction of rice... 

NORMAND F L, ORY R L, MOD R R (1987) Binding of bile acids and trace minerals by soluble hemicelluloses of rice The ability of rice fiber components to bind bile acids may play a role in lowering serum cholesterol. Food Technology, 41(2) 86-90. 

In the absence of suitable cell wall mutants, DCB-adapted tomato cells provide an opportunity to characterise the pectin network of the plant cell wall. It should be noted that synthesis and secretion of hemicellulose is not inhibited but, in the absence of a cellulose framework for it to stick to, most of the xyloglucan secreted remains in soluble form in the cells culture medium (9, 10) while other non-cellulosic polysaccharides and other uronic-acid-rich polymers predominate in the wall. 

The AIS of fresh and canned carrots were sequentially fractionated into water-soluble pectin (WSP), oxalate-soluble pectin (OXSP), acid-soluble pectin (HSP), alkah-soluble pectin (OHSP), hemicellulose and cellulose. The distribution of uronic acid among various fractions is presented in Table 1. WSP of fresh carrots accounted for 19.0% of the total pectin, while OXSP constituted 29.6%. HSP represented the lowest (12.0%) pectin fraction, whereas OHSP was the highest (35.6%) In the hemicellulose and cellulose fractions significant amounts of uronic acid were found. Heat treatments during canning altered the proportion... 

Xylan has the general properties of insolubility in water, solubility in alkaline solutions, ease of acid hydrolysis, high negative optical rotation, and non-reducing action toward Fehling s solution. It can be placed in three general polysaccharide classes (1) pentosan, (2) glycan, and (3) hemicellulose. It is classed as a pentosan because it is principally a polymer of a pentose. It is by far the most abundant pentosan. 

Usually it is considered that xylan has been effectively removed if the extracted residue compares in composition to a-cellulose. Yet, even a-cellulose contains small amounts of xylan as well as other hemicelluloses such as mannan.67 69 Beta and 7-cellulose are mixtures which contain pentosans along with other alkali-soluble extractives.1M0... 

The hemicelluloses are soluble in alkali, and can therefore be readily separated from the cellulose component by alkali extraction. However, this can only be done when the wood has first been delignified. This is because they are probably linked to lignin via covalent ester linkages (see Chapter 3) which need to be cleaved... 

Unlike cellulose, hemicelluloses consist of different monosaccharide units. In addition, the polymer chains of hemicelluloses have short branches and are amorphous. Because of the amorphous morphology, hemicelluloses are partially soluble... 

Other abundant carbohydrates, such as hemicelluloses and pectin, are usually highly branched and thus not very suitable for fiber and film production. Hemicelluloses and some pectins are also acetylated in the native state, which makes them more resistant to enzymatic hydrolysis (20,21) and changes their solubility properties (9-77,75). Branching does not, however, preclude their utilization in such potentially large markets as thickeners and adhesives. Xylans, for example, show such a strong adhesion to cellulose fibers that they are very difficult to remove completely by both acidic and alkaline pulping processes (22). 

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... 

Unfortunately, pure xylan is an expensive carbon source for commercial-scale xylanase production. Therefore, several groups have tried to develop xylanase production on cheaper xylan-rich materials. The best candidates for the purpose appear to be water-soluble hemicellulose from steam-treated wood (63,69) and residues of annual plants like wheat bran (70). 

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