Pentosan fraction, xylan

While there are many chemicals possessing the furan nucleus, furfural and furfuryl alcohol are of major industrial significance. The chief source of furfural in plant materials is the pentosan fraction, predominantly xylan (Scheme 1). 

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. 

That not all of the xylans of wood are homogeneous polymers of anhydro-D-xylose units has been shown by the classical studies of O Dwyer (1923 to 1940) on hemicelluloses of American white oak. O Dwyer prepared a hemi-cellulose fraction from water-extracted, oakwood sawdust by extraction for two days with 4% aqueous sodium hydroxide solution. The polysaccharide material was obtained, after acidification, by the addition of ethanol. The product ([a]n —75° in 1 % sodium hydroxide), contained 70% of pentosan, and yielded n-xylose, n-maiinose, n-galactose, and L-arabinose on hydrolysis. 

A summary of the yields obtained, and some of the properties of the softwood xylans thus far isolated, is presented in Table XII. The pentosan content of most gymnosperm species lies between 5 and 12% (by weight) of the wood. Many of the yields reported in the literature are extremely low, and it is not certain whether the small fraction investigated was really representative of the entire xylan portion of the wood. It is notable... 

The encrusting of cellulose fibrils by lignin-carbohydrate complexes within the cell wall is still a possible mechanism for lowered cellulose availability. The nutritive availability of the different fractions of hemicellulose and cellulose is quite different as shown by the data of Gaillard (26) in Table IV. Xylan is generally less digestible than other fractions (26, 30), and Lyford et ah (36) have shown that the micellular pentosans have a low digestibility. 

Pentosans present in wheat and rye diets are often poorly metabolized. Especially arabinoxylans negatively influence the digestion and absorption of nutrients in the foregut of animals. When a xylanase (EC 3.2.1.8) treated arabinoxylan fraction was used, the nutritional parameters were similar to those when an arabinose and xylan monomeric mixture was used, indicating that xylanases are a valuable feed additive. Especially sought are enzymes with endo-l,4-P-xylanase activity that are stable in the digestive tract of poultry (Mondou et al. 1986). 

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