Xylans Subject

Pure xylan is not employed in industry. but crude xylan or pentosans are of industrial importance. Xylan has been proposed as a textile size but is not employed as yet for this purpose.130 Perhaps the largest use of pentosans is in their conversion to furfural, which has many applications and serves as the source of other furan derivatives. At the present time, large quantities of furfural are used in the extractive purification of petroleum products, and recently a large plant has been constructed to convert furfural by a series of reactions to adipic acid and hexamethylene-diamine, basic ingredients in the synthesis of nylon. In commercial furfural manufacture, rough ground corn cobs are subjected to steam distillation in the presence of hydrochloric acid. As mentioned above, direct preferential hydrolysis of the pentosan in cobs or other pentosan-bearing products could be used for the commercial manufacture of D-xylose. 

Other workers140 have also examined the products derived by hydrolysis of periodate-oxidized xylan. Both wheat straw and com cob xylan, after oxidation and hydrolysis, yield small amounts of L-arabinose and D-xylose. These sugars are obtained even after the xylans have been subjected to extended periods of oxidation. It is concluded that the D-xylose constituted branch points in the xylan. Likewise, the L-arabi-nose molecules must not have been terminal units in a xylan chain but must have been either interior units in the xylan molecule or have constituted an araban-like polysaccharide which is in combination or admixture with the xylan polysaccharide. 

Selective Production of Xylanases by Cellulolytic Microorganisms. Until recently there was little information on common or separate genetic control of cellulase and xylanase synthesis in microorganisms (60). Studies on this subject were complicated by the fact that numerous microbial ceUulases and xylanases are non-specific with respect to cellulose and xylan as substrates. As could be expected from a comparison of both polysaccharide structures, non-specificity is more frequently observed with cel-lulases, because their substrate binding sites can easily accommodate substrate using an unsubstituted p-(l 4)-linked chain of D-xylopyranosyl units. 

In these early years, Heuser wrote chapters for various texts and monographs, on such varied subjects as paper pulp and artificial silk, the coloring of paper on paper-machines, celluloid and various cellulose films, methods of determining furfural, and xylan. 

A comprehensive review on D-xylanases and their degradation of D-xylans was written by Sorensen5 in 1957, and since then, brief reviews of the same subject have been written in English by Timell6 and in Japanese by Fukui7 (1961) and Sasaki8 (1971). Work on D-xylanases from 1957 onwards will, therefore, be discussed in detail in this article. 

In an attempt to improve the yield of hemicelluloses. Sands and Nutter subjected mesquite (Prosopis julifiora) wood to alternate extraction with alkali and delignification with chlorine and aqueous ammonia. The resulting fractions all consisted of a xylan whose average composition corresponded to one methoxyl group, one uronic acid residue, and ten xylose residues, or exactly the same composition as had been noted by O Dwyer for her oak xylan." ... 

Dutton and Unrau have subjected (4-0-methylglucurono) xylans from apple and cherry wood to a Smith degradation. The acid portion of the... 

On extraction of wood of white spruce with water, Adams obtained two polysaccharides, one of which, a galactoglucomannan, will be discussed later (see p. 458). The other, an acidic arabinogalactan still containing some xylan, was subjected to a structural investigation. The methylation data (see Table XI) were very similar to those observed for the polysaccharide... 

As the chlorine-2-aminoethanol method offers no obvious advantages in the case of softwoods, most investigators have used chlorite holocelluloses for isolating the hemicelluloses. The most common procedure has been to subject the holocellulose to a preliminary extraction with aqueous potassium hydroxide or sodium hydroxide solution. Low concentrations of alkali favor removal of the acidic xylan, as does the use of potassium... 

Acid hydrolysis of hitman fecal samples collected after Ingestion of different dietary fibers produced monosaccharides that were related to the fibers Ingested. Dietary fibers studied In five normal male subjects were cellulose, xylan, corn bran and pectin. Increases In monosaccharides In recovered hydrolyzed feces from fiber diets compared to those from the fiber-free diet were galactose for the pectin diet xylose and glucose for the cellulose diet xylose and mannose for the xylan diet and arablnose, xylose, galactose and glucose for the corn bran diet. Comparison of the fecal results to those for the Intact food fibers showed that fermentation of fiber components In the colon was different for each fiber. 

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