Xylan, fractionation

Xylanases from A. niger with unusual hydrolysis specificities were recently reported (54), and five xylanases were isolated and purified. Xylanase 1 attacked soluble larchwood arabinoglucuronoxylan and xylose oligosaccharides of DP > 3, to mainly Xyl2 and xylose, but was inactive towards an insoluble xylan fraction (larchwood) prepared by dissolving the aforementioned xylan in water and separating the undissolved fraction... 

Variations on the filter paper assay have been developed. Irwin and coworkers have established a method for differentially measuring soluble and insoluble reducing sugars released from filter paper.192 Ghose and the IUPAC committee also proposed a similar approach for measuring hemicellulases in 1987.193 This method relies on meeting a standard level of conversion of the xylan fraction in oat spelt xylan to xylose in a specified... 

Muller M (1980) Beitrag zum Einsatz der Curie Punkt-Pyrolyse Massenspektrometrie in der Analytik von Naturstoffen Dissertation ETH 6743, Zurich Obst JR (1983) Analytical pyrolysis of hardwood and softwood lignins and its use in lignin type determination of hardwood vessel elements J Wood Sci Technol 3 377-397 Pouwels AD, Boon JJ (1987) Analysis of lignin and chlorohgntn residues in a beech xylan fraction by pyrolysis gas chromatography mass spectrometry J Wood Sci Technol 7 197— 213... 

Table 7.7.2. Chemical constituents of the pyroly sate of holocellulose (A), a xylan fraction (B), and technical xylan (C) from beech wood [64], (The origin from lignin of certain compounds is also indicated in the table.)...

Kim and Lee [14] have reviewed the kinetics of acid-catalyzed hydrolysis of hemicellulose and provide a useful tabulation of previously published kinetic rate constants. There is wide variation in reported rate constant values, reflecting again the differences in the substrate and pretreatment conditions, the model employed and the method by which the effective acid concentration (or weight percent) was determined [16]. Unfortunately, only a few researchers have performed detailed kinetic studies in which both hydrolysis and decomposition rates are determined. Kim and Lee [14] have carried out the only study including both the fast and slow xylan fractions and xylose decomposition. 

Evidently, the indiAddual xylan molecules do not contain the same relative number of acid side-chains, and it is to be expected that these groups will have a more decisive influence on the solubility of the polysaccharide than will its molecular weight. Similar variations in uronic acid content of birch-xylan fractions have been reported by other investi-gators. ... 

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. 

A wood xylan was prepared from birch fibers that had been subjected to acetone extraction and chlorite delignification. The xylan fraction was extracted with a solution of 6.5% KOH in a nitrogen atmosphere at 20 C for 3.7 hours. The extract was acidified and the xylan was precipitated with ethanol. 

The neutral xylan fraction from Palmaria decipiens was treated with aqueous bromine solution (25). This xylan contains l->4- as well as l->3-linked sugar units. Carbonyl groups are formed which was revealed by means of FTIR (vco at 1741 cm ) and NMR spectroscopy. The peaks found in the C-NMR spectrum were assigned (Table 5). The presence of C=0 was also revealed... 

Xylan Fractional precipitation Dimethyl suUbxide/ethanol 1887... 

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