Hemicellulose xylan derivatives

Derivatives of hemicellulose components have properties similar to the ceUulosic equivalents but modified by the effects of thek lower molecular weight, more extensive branching, labile constituents, and more heterogeneous nature. Acetates, ethers, carboxymethylxylan (184), and xylan—poly(sodium acrylate) (185) have been prepared. 

During the past decade, MALDI-TOF MS has proven to be an effective tool for the analysis of oligo- and polymeric mannoglucans (for extensive reviews see [222,223]). SEC/MALDI mass spectrometry was employed in the analysis of hemicelluloses isolated by microwave heat-fractionation from spruce and aspen wood [94]. These methods allowed the separation and characterization of the oligo- and polysaccharide fractions derived from the xylan and mannan components of both woods [224]. 

From this type of schematic representation it would be expected that a more highly branched xylan chain would give more potential for reaction of the arabinose residues, either through crosslink formation or by Maillard-type reactions. Indirect evidence supporting this concept can be derived from pentosan analysis of the hemicellulose fraction of grasses and legumes. 

Although most polysaccharides may be acylated without degradation, acyl derivatives of xylans29 are often unsatisfactory for molecular-weight determinations owing to their poor solubility characteristics. Mixed esters of xylans143 are often superior in this respect, but there are obvious difficulties in the differential analysis of acyl groups. Hemicellulose nitrates have been used for measurements of molecular size,168 but xylan nitrates,29 like xylan acetates, are reported to have poor solubility characteristics. 

Non-endospermic xylans have substituent groups of D-glucopyrano-syluronic acid, or its 4-methyl ether, or both. Individual molecules may lack such residues, but, almost certainly, other xylan molecules in the total hemicelluloses from each grass have both groups.137,157 Uronic acid groups may escape detection or identification in hydrolyzates of xylans studied by paper chromatography, and in hydrolyzates and methanolyzates of methylated xylans studied by g. l.c. after conversion of the products into volatile derivatives. 

Xylitol, a sugar alcohol, has potential use as a natural food sweetener, a dental caries reducer and a sugar substitute for diabetics. It is produced by chemical reduction in alkaline conditions of the xylose derived mainly from wood hydrolyzate (169). The recovery of xylitol from the xylan fraction is about 50-60% or 8-15% of the raw material employed. Drawbacks of the chemical process are the requirements of high pressure (up to 50 atm) and tenq>erature (80-140°C), use of an expensive catalyst (Raney-Nickel) and use of extensive separation and purification steps to remove the by-products that are mainly derived from the hemicellulose hydrolyzate (770). The bulk of xylitol produced is consumed in various food products such as chewing gum, candy, soft drinks and ice cream. It gives a pleasant cool and fresh sensation due to its high negative heat of solution. 

Among the several cellulose derivatives are xylan-type hemicelluloses, which are the most abundant heteropolysaccharides derived as by-products from agricultural processes. Over the past few years their use in food packaging and other applications has increased considerably [121]. Hemicelluloses such arabinoxylan are hydrophilic polysaccharides and their films tend to show high water uptake, becoming a drawback in the production of materials for the food sector. In order to improve both mechanical and barrier properties, Sarossy et al. [122] adopted the... 

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