Xylan resistant

Schoettler has studied the alkali resistance of the xylan in pulps made from aspen wood. Verifying and extending previous work by March, he" observed a higher content of xylan resistant to 17.5% sodium hydroxide in aspen alkaline pulps (sulfate or soda process) than in aspen, chlorite holo-cellulose. Further chlorite treatment of an alkaline pulp did not reduce its xylan or resistant-xylan content. Alkaline (sulfate) cooking of the holo-cellulose did not increase the resistant-xylan content. These results are summarized in Table X. 

The end group which is produced contains a carboxylic acid functionality which has an influence on the anionicity of pulp fibres (Chapter 6) but, in this form, it is resistant to further alkaline degradation. The hemicelluloses are also able to undergo the same type of peeling reaction but at different rates from each other and from cellulose. The /3-1,4-xylans, for example, are more stable to alkaline degradation than the glucomannans. 

Other abundant carbohydrates, such as hemicelluloses and pectin, are usually highly branched and thus not very suitable for fiber and film production. Hemicelluloses and some pectins are also acetylated in the native state, which makes them more resistant to enzymatic hydrolysis (20,21) and changes their solubility properties (9-77,75). Branching does not, however, preclude their utilization in such potentially large markets as thickeners and adhesives. Xylans, for example, show such a strong adhesion to cellulose fibers that they are very difficult to remove completely by both acidic and alkaline pulping processes (22). 

The removal of xylan by the xylanase appeared more pronounced in T than in Si. This is in disagreement with microdensitometrical evaluations of substance losses in cell walls of spruce- and pinewood holocellulose reported by Boutelje and co-workers (5,6). They found that the hydrolysis of xylan by xylanase treatment occurred mainly in the S2 layer the Si and in some cases also the S3 (T) layer appeared not to be affected. In the same way, they showed that the Si layer is more resistant to cellulolytic attack than the other cell wall layers. 

Many polysaccharides of the xylan group contain residues of D-glucuronic acid or 4-0-methyl-D-glucuronic acid. Since glycosiduronic acids are particularly resistant to acid hydrolysis, aldobiouronic acids may be isolated... 

Alkali-induced deacetylation and hydrolysis of the uronic acid group of xylans proceed readily under alkaline pulping conditions and contribute significantly to xylan redeposition onto the fibers. The galactose side chain in galactoglucomannans is fairly resistant to alkaline hydrolysis (Table 2). Wood xylans are much more stable than glucomannans under alkaline conditions [34]. 

Bleached, soda pulp contained about the same amount of resistant xylan as bleached, sulfate pulp, and a bleached, sulfite pulp contained only a small amount (Table X). In this same study," it was observed that the resistant-xylan content was greater in an air-dried or oven-dried pulp than in the same pulp before drying (see Table XI). In agreement with the work of Meller," a reduction in resistant xylan resulted from mechanical beating or grinding. 

Pulp Xylan, % Xylan in alpha-cellulose Resistant xylan), %... 

Effect of Drying on Resistant-xylan Content of Aspen Bleached Kraft Pulp ... 

Most of the above results are in accord with the idea that xylan and cellulose co-exist in a sort of mixed crystallization" held together by secondary valence forces (such as hydrogen bonds). Mechanical treatment w ould be expected to disrupt such bonds, thus decreasing the resistant xylan, and the removal of water from a wet pulp would allow neighboring chains to come into contact, resulting in augmentation of the number of secondary bonds and thus increasing the resistant xylan. 

Dorr also has commented on difficulties in viscose filtration caused by the presence of xylan, and has noted, further, a correlation between viscose-yarn strength and the total of xylan and mannan in pulps from which the yarns were made. In this case, the strongest yarns had the lowest, total-polyose content. It should be noted that, since the viscose process involves steeping in about 18% caustic solution, the xylan carried through to the final yarn will be chiefly resistant xylan (see page 308). 

Bergius-Willstatter saccharification process. Process for industrial production of fermentable sugar from wood by hydrolysis of tannin and xylan-free cellulose with 40-45% hydrochloric acid. The use of concentrated acid requires acid-resistant equipment and recovery of acid. The sugar produced must be rehydrolyzed prior to fermentation. 

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