Modification of hemicelluloses

The wake-up call for this enormous renewable resource has gone through Scandinavia and Europe to North America. The editors organized a l International Symposium on Xylans, Mannans and Other Hemicelluloses as a Symposium of the American Chemical Society Division of Cellulose and Renewable Material in Orlando, Florida in April 2001. The meeting was very successful and resulted in fiiiitful interaction between scientists and resulted in this book, which covers topics dealing with the isolation processes of the hemicelluloses and the characterization of their structure. The development of new analytical tools for determining the molecular architecture of hemicelluloses is described. The assembly characteristics and the interactions of hemicelluloses with cellulose are recurrent themes in this book, and the enzymatic and chemical modifications of hemicelluloses along with new applications for materials based on hemicelluloses are also covered. We hope that this book will play an important role in the process of utilization of hemicelluloses. 

Due to the complex structures of hemicelluloses, several different enzymes are involved in their enzymatic degradation and modification (Table II) (7-70). Enzymes are nature s own catalysts, which act in mild conditions. Consequently, the use of enzymes offers an excellent alternative to engineer the properties of hemicelluloses in a controlled way. This paper discusses properties of hemicellulases and the possibilities for enzyme-aided modifications of hemicelluloses. 

The utilization of various hydrolases for the modification of hemicelluloses in large scale is, however, still restricted due to the limited industrial availability of the enzymes discussed above. Endoxylanases, endomannases and endoglucanases can be obtained in substantial quantities from the enzyme produces. However, other backbone-hydrolyzing enzymes are not available without side-activities and thus cannot yet be used for selective modifications. The only accessory enzyme currently on the market is a-galactosidase. New enzyme products containing other hemicellulases are still needed before the enzymatic tailoring of hemicellulases can be performed in industrial scale. 

The enzymes forming new glycosidic linkages are involved in the synthesis of hemicelluloses and exist in all plants. They are, however, not yet available for in vitro modifications of hemicelluloses. Plant polysaccharide synthetases are still poorly characterized and only a few of the enzymes participating in the synthesis of hemicelluloses have been isolated and characterized (41). Furthermore, even fewer of them have been cloned and produced in another host organisim in significant amounts. 

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 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. 

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). 

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. 

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. 

Shiraishi and coworkers have had a 10-year program on wood molding at Kyoto University in Japan [26]. Their approach renders the entire wood structure thermoplastic through chemical modification of wood meal, which means that lignin and hemicelluloses are modified and the cellulose is decry-... 

Light toast indicates a toasting time of approximately 5 minutes, with a surface temperature between 120 and 180°C. The inside of the barrel has a spongy appearance, due to modification of the lignins and hemicelluloses, while the cellulose structure remains intact. 

There have been many research projects over the years studying ways to thermoform ligno-cellulosics. Most of the efforts have concentrated on film formation and thermoplastic composites. The approach most often used involves the ehemieal modification of cellulose, lignin, and the hemicelluloses to decrystallize and modify the eellulose and to thermoplastieize the lignin and hemicellulose matrix to mold the entire lignoeellulosic resource into films or thermoplastic composites [92 97]. 

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