Glucomannans degradation

Glucomannans, in contrast to xylans, undergo extensive endwise depolymerization [268,269], partly because of the lack of substitutents at the C2 or C3 position. The extent of glucomannan degradation at 1(X)°C [269] was higher for pine galactoglucomannans (57%) than for spruce glucomannans (47%). 

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. 

The most characterised D-mannan degrading enzymes are the endo- -mannanases (5). These enzymes act on a range of 1,4-p-D-mannan-type polysaccharides including D-mannan, glucomannans (acetylated and deacetylated), galactomannans and galacto-glucomannans. The extent and... 

Figure 7-32 illustrates the glucomannan and xylan losses during kraft pulping of pine wood. As can be seen, an appreciable portion of the lost xylan is actually not degraded but dissolves in the cooking liquor as a polysaccharide. The amount of dissolved xylan reaches a maximum around the midpoint of the delignification process. 

Enzymes degrading konjac glucomannan have also been found (see Ref. 88) to be produced by the following invertebrates crab, earshell, earthworm, lobster, sea-urchin, snail, and turbo. 

Pectin, guar and glucomannans are examples of gel-forming fibres, whereas cellulose, lignins and seed coats are examples of nonsoluble fibres. The non-soluble fibres bind water and swell and keep water in the intestine, while the gel-forming fibres are degraded, which make them unsuitable as laxatives. 

The original observation of Yundt, that a degraded glucomannan from slash pine is crystalline, has lately been confirmed by others. The rod-shaped crystals, 2.5A. long, gave, on hydrolysis, n-glucose and o-man-nose in the ratio of 1 3.3. After purification, the x-ray diffraction patterns showed relatively sharp peaks. 

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