Investigation of the digestion of cell-wall polysaccharides of spear grass and of cotton cellulose by viscometry and by x-ray diffraction

Abstract

The lag of ≈ 10 hours in the onset of digestion of cotton cellulose in the rumen, observed by previous workers, has been confirmed. The molecular weight of the remaining cotton decreases only slowly during digestion, and the polysaccharide retains its fibrous form. The crystallinity decreases slightly at the same time, and it is concluded that the amorphous and crystalline regions of cellulose are attacked at approximately the same rate. The hemicelluloses of grass partly digested in the rumen and of faeces fibre have been isolated and found by viscometry to have molecular weights similar to those of the material isolated from the original grass. This finding confirms earlier conclusions that the digestion-resistant hemicelluloses are chemically identical with the digestible hemicelluloses and that the resistance is due to protection by lignin. The holocellulose prepared from faeces fibre by removal of lignin showed slightly less X-ray crystallinity than that from the original grass, but this effect is probably due to a decrease in cellulose-hemicellulose ratio during passage through the animal, rather than to preferential digestion of crystalline cellulose. A comparison of the chemical composition of the polysaccharides of grass and faeces fibre confirms that cellulose is digested more rapidly and completely than hemicelluloses, presumably because it is less effectectively protected by lignin. In the corresponding holocclluloses, however, where the lignin has been removed, the cellulose and hemicelluloses are digested at about the same rate.