Hemicellulose modification

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. 

The GGM-rich hemicelluloses, isolated from water-impregnated spruce chips by heat-fractionation [218], has been used as pre-polymers after modification with methacrylic functions [439]. Radical polymerization of the modified hemicelullose with 2-hydroxyethyl methacrylate in water yielded elastic, soft, transparent, and easily swollen hydrogels. 

The structural varieties of hemicelluloses offer a number of possibilities for specific chemical, physical, and enzymic modifications. Future advancements will be based on the synthesis of hemicellulose-based polymers with new functionalities and with a well-defined and preset primary structure both on the level of the repeating imit and the polymer chain. Hemicelluloses have also started to be attractive to synthetic polymer chemists as... 

Modifications of the cell wall composition of released cells occured during the maceration (Fig. 5). In pectic polysaccharide, galactose increased whereas galacturonic acid and arabinose decreased. Cellulose and hemicellulose compositions were not modified. 

Many of the physical, chemical and biological properties of wood can be understood by referring to the polymeric chemical constituents. In many cases of wood modification, these polymeric components are modified to some extent. The three structural polymeric components of the wood cell wall are cellulose, hemicellulose and lignin. There are many excellent texts describing the structure and function of these components, and only a brief account is given here. 

Attempts to remove hemicellulose for production of dissolving pulps with very low hemicellulose contents have shown that complete enzymatic hydrolysis of hemicellulose within the pulp is difficult to achieve. The xylan content in delignified mechanical aspen pulp was reduced from approximately 20 to 10%, whereas in bleached hardwood sulphite pulp the xylan content was decreased from 4 to only 3.5% even at very high enzyme dosages (50). The complete removal of residual hemicellulose seems thus unattainable, apparently due to modification of the substrate or to structural barriers. 

The major polymers that make up the wall are polysaccharides and lignin. These occur together with more minor but very important constituents such as protein and lipid. Water constitutes a major and very important material of young, primary walls (2). The lignin is transported in the form of its building units (these may be present as glucosides) and is polymerized within the wall. Those polysaccharides which make up the matrix of the wall (hemicelluloses and pectin material) are polymerized in the endomembrane system and are secreted in a preformed condition to the outside of the cell. Further modifications of the polysaccharides (such as acetylation) may occur within the wall after deposition. Cellulose is polymerized at the cell surface by a complex enzyme system transported to the plasma membrane (3). 

This is based on the method by Van Soest and Wine (1967) which has been modified according to subsequent recommendations. It is the only fibre determination suitable for non-ruminants. The residue consists of the plant cell-wall constituents cellulose, hemicellulose, lignin, cutin, NDF-insoluble tannin and ash. See the article by Cherney (2000) for current modifications these include the use of amylase to aid in the removal of starch from forages containing grain (Van Soest et al., 1991), which has been adopted by MAFF... 

H. Chemical Modification. The chemical modification of wood involves a chemical reaction between some reactive part of a wood component and a simple single chemical reagent, with or without catalyst, to form a covalent bond between the two. The wood component may be cellulose, hemicellulose, or lignin. The objective of the reaction is to render the wood decay resistant. The mechanism of the effectiveness is not known, but some possible explanations were given earlier. 

Apart from hemicellulose and cellulose, lignin is also a promising raw material for various applications. Huttermann et al. (2001) described various potential technologies for lignin modification for the production of new compounded materials. Pan et al. (2005) demonstrated that lignin extracted during ethanol organosolv... 

Table VII shows that the activation energies of artificially aged pulps BBC and BPC are consistently lower than for the control samples. The lower values indicate that the rate-determining reaction can proceed more readily after artificially aging. This may be a result of the introduction of carbonyl groups or of the introduction of chain ends as a result of hydrolysis of the hemicellulose and cellulose. Such modifications of the original pulp may act as weak links or sites at which further degradation is facilitated.

Meshitsuka, G, and Isogai, A. 1996. Chemical structure of cellulose, hemicellulose, and lignin. In Hon, D. N.-S. (Ed.), Chemical modification of lignocellulosic materials (pp. 11-34). New York Marcel Dekker. 

Extraction of lignocellulosic materials with dioxane has been used for quite some time as a method for lignin isolation at atmospheric pressure (1) or, as recently reported, at high pressure and with supercritical C02 mixtures (2-5). In this case, it was possible to extract from wood lignin oligomers with a low degree of chemical modification, hemicelluloses were also depolymerized and extracted, but cellulose remained without significant mass losses. 

This chapter discusses major factors affecting the reactivity of cellulose, hemicelluloses, and lignin under both acidic and alkaline modifications. 

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