Hemicellulose materials

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. 

Wood chips can also be utilized as such to produce bioethanol. The cellulose and hemicellulose material is hydrolyzed in the presence of acids (H2SO4, HCl, or HCOOH) or enzymes to yield glucose and other monosaccharides [16]. Lignin is separated by filtration as a solid residue and the monosaccharides are fermented to ethanol, which, in turn, is separated from water and catalyst by distillation. Ethanol can be used not only as energy source but also as a platform component to make various chemicals, such as ethene and polyethene. Today green acetaldehyde and acetic acid from wood-derived bioethanol is manufactured by SEKAB Ab, at the Ornskoldsvik Biorefinery of the Future industrial park. 

Such activation energies of abiotic heat release for lignocellu-losic material have recently been summarized and and compared in detail by Kubler (14). These activation energies in the range from 120°C to 220°C, when air is present, usually are between 60 and 115 kJ/mole, i.e. 15 to 30 kcal/mole (14). Activation energies for the delignified cellulose-hemicellulose material presented here are at the lower end of this range. 

The major soluble components of acid hydrolysates are sugars, such as xylose, glucose, and cellobiose furfurals, such as furfuraldehyde and hydroxymethyl furfural and organic acids, such as levulinic acid, formic acid, and acetic acid (13). When natural sources of cellulose are acid-hydrolized, numerous products can result, largely because of the hemicellulose materials. These make it difficult to produce a relatively pure sugar product and limit the utility of the acid hydrolysis process. 

A very important step in the process is the simultaneous saccharification and fermentation. This requires enzymes that can effectively break the cellulosic and hemicellulosic material into sugar components. Additionally, micro-organisms that can use a wide range of sugars are desired. These are challenges to be solved by future research in the field of biotechnology. 

Farone, W. A. and Cuzens, J. E., Methods of Producing Sugars Using Strong Acid Hydrolysis of Cellulosic and Hemicellulosic Materials. U S. Patent No. 5,562,777, October 8, 1996. 

Non-endospermic tissues of grasses are often subjected not only to delignification but to other treatments before the intentional extraction of hemicellulosic materials. It is not always clear that a useful purpose is served by some of these treatments, or that they do, or only do, what is wanted. The fact that a procedure has often been used does not alone justify its routine adoption or retention. As mentioned, enzymes may be inactivated by hot, aqueous alcohol78,79 in which oligo-... 

The hemicellulosic materials extracted by alkali are, in part, precipitated on neutralization or mild acidification, and, in further part, by the subsequent addition of an excess of acetone or ethanol.29 Under these conditions, part of the hemicellulosic material remains in solution, and is commonly not recovered. An oat-leaf holocellulose was treated with aqueous acid, and the precipitated hemicelluloses were recovered after the addition of an excess of acetone. The solvents were removed from the remaining solution and, after dissolving the... 

Some of the structural features account for galactose tanits139-141 143 145 in the parent hemicellulosic materials. Both the D and the L enantiomer are present,139-145 but it is often assumed that the galactose in land plants is the D enantiomer. 

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. 

Although non-endospermic, hemicellulosic materials have often been reported to yield glucose on hydrolysis, there is little evidence of the presence of glucosyl residues in xylans. Non-cellulosic glucose occurs mainly, or exclusively, in /J-D-glucans (described later in this Section). 2,6-Di-O-methylglucose was obtained on methylation and hydrolysis of the xylans of wheat straw and com cob,132 and its presence is indicative of doubly linked glucose residues attached in some way to these xylans.132... 

Glycosyluronic acid groups are often noted in non-endospermic xylans157 and, more rarely, in those from the endosperm. Endospermic material may be contaminated by husk or bran containing hemicelluloses bearing uronic acid units.67,153,158,159 There is much information on uronic acids in hemicellulosic materials, but it is not easily inter-... 

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... 

Using the advanced bioethanol technology available, it is possible to produce ethanol from any cellulose/hemicellulose material, which means any plant or plant-derived material. Many of these materials are not just underutilized and inexpensive, but also create disposal problems. For example, rice straw and wheat straw are often burned in the field, a practice that is becoming limited by air pollution concerns. Also, much of the material now going into landfills is cellulose/hemicellulose material and could be used... 

Probably, the principal reason why it is so difficult to find covalent ester bonds between coupling agents and wood fiber in WPC, employing spectroscopic observations, is that wood fiber itself contains plenty of hemicellulosic materials (besides cellulose and lignin), which in turn contain plenty of ether and ester bonds, creating a heavy background for spectroscopic measurements. It would have been more reasonable to use, for such studies, bleached cellulose or pure cellulose, such as of cotton fiber. 

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