Enzymatic modifications hemicelluloses

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. 

Complete utilization of cellulose and hemicellulose requires selection or genetic modification of an organism that is able to ferment pentoses. In order to obtain monosaccharides from the raw material, several pretreatments and/or separations are required. First, the lignocellulosic material is mechanically treated and then delignified (pulped) by strong alkali or acid treatment. The (hemi)cellulose part becomes more accessible for enzymes at the same time. Subsequent enzymatic treatment mainly yields glucose and xylose and some arabinose. The enzymatic treatment and subsequent fermentation can be done in separate reactors or in one fermenter, in an SSF concept similar to starch SSF [57]. 

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. 

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