Hemicellulosic materials isolation

Within the scope of this review, the contributions of the last decade concerning cell-wall polysaccharides isolated from woody and other plant tissues will be reviewed according to the above-proposed classification of hemicelluloses including larch arabinogalactans. The present review article updates and extends previous reviews [3-5] and will focus in particular on new investigated plant sources, isolation methods, structural features, physicochemical and various functional properties of hemicelluloses. Attention will also be paid to the modification of isolated hemicelluloses or hemicellulosic materials and the appHcation possibiUties of hemicelluloses and their derivatives, including their use for the production of composite materials and other biomaterials. 

No complete picture of biological materials can be given without reference to their original condition in their natural environment. Brief consideration will, therefore, be given to the biosynthesis of the hemicelluloses and to their relationship, in the plant, to cellulose and lignin. Since much of our present knowledge of the chemistry of the hemicelluloses is based upon examination of materials isolated by alkaline extraction from plant tissues which, in many cases, have been previously treated to remove lig-... 

Non-endospermic xylans from grasses are frequently stated to contain low proportions of non-xylosidic sugar units, whereas endosper-mic xylans, which are commonly isolated without deliberate fractionation of the readily extracted hemicellulosic material, are stated to be highly substituted by L-arabinofuranosyl groups. It is beyond dispute that xylans having low proportions of non-xylosidic units are present in, and commonly isolated from, the molecular populations of non-endospermic, hemicellulosic materials. 

With discretion, the structural information on features in xylans from other, higher land-plants can be used to assist, as in the past, in the search for similar features in the hemicellulose of plantstuffs under examination—no matter how often they have been previously examined. If new facts suggest new interpretations, earlier studies should be re-examined to determine whether the newer interpretation accords with facts established earlier. It is a misfortune of hemicellulose chemistry that it is rarely possible to disprove a reported fact, as, unless total hemicelluloses30,31 are isolated from plants of the same variety or cultivar grown under very similar conditions to the plants studied earlier, the hemicellulosic material under examination may not contain molecules having detectable amounts of the structural features noted earlier. 

A pentasaccharide and a trisaccharide have been isolated from an enzymic hydrolyzate of hemicellulosic material from oat coleoptiles.224 Methylation analysis of their structures indicated that these compounds may derive from j8-D-(l — 4)-linked D-glucans having terminal D-xylopyranosyl groups on some 0-6 atoms. Xylo-glucans having these structural features are present in the primary cell-walls of dicotyledons.22411... 

One complication that has been reported in connection with the SEC analysis of cellulose diacetate is the presence of a small, separate peak ( prehump ) or shoulder on the high-molecular-weight end of the chromatogram (10,94) obtained from wood pulps. Material isolated from this peak is enriched in mannose and xylose, and the size of the peak can be directly correlated with the amount of hemicellulose in the pulp. The hemicellulosic material causing the prehump can be removed as a small amount of poorly soluble material by fractional precipitation. 

Endo-xylanases (1,4-p-D-xylan xylanohydrolase, EC 3.2.1.8), which hydrolyze mainly interior p-(l- 4)-D-xylosyl bonds in the plant hemicellulose xylan, have been isolated from many fungi and bacteria and occasionally from plants and invertebrates. Because xylan is one of the most common of all natural materials, and its removal or conversion is often desirable, the endo-xylanase family has high commercial as well as scientific interest. 

The fact that EMC reduction as a function of acetyl content is the same for many different llgnocelluloslc materials Indicates that reducing moisture sorption and, therefore, achieving cell wall stability are controlled by a common factor. The lignin, hemlcellulose, and cellulose contents of all the materials plotted in Figure 2 are different (Table II). Earlier results showed that the bonded acetate was mainly in the lignin and hemicelluloses (33) and that Isolated wood cellulose does not react with uncatalyzed acetic anhydride ( 4) ... 

Holocellulose Cellulosic material obtainable from wood after removal of lignin. The term therefore means total carbohydrates (cellulose and hemicellulose) present in the wood (Refs 1 2) Ritter (Ref 3) succeded in isolating holocellulose from wood pulp by repeated chlori-nations, followed by extraction with alcohol containing 3% of monoethanolamine. The resulting product was white but changed color on standing... 

In a typical hemicellulose isolation process, finely ground plant material (200 g) is treated with 5-10% sodium hydroxide in oxygen-free water at 25°C for 18-24 h (Whistler and Feather, 1965). The water-insoluble fraction, containing two subfractions differentiated by density, is extractable with higher rather than lower NaOH concentrations progressively higher concentrations of alkali extract progressively more of the dense saccharides. 

The majority of the material in sulfite spent liquors originates from lignin (lignosulfonates) and hemicelluloses (Tables 7-5 and 10-1). A variety of useful products can be isolated and prepared from these liquors (Table 10.-2)... 

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