AbstractsBiology & Animal Science

The term “Hemicellulose” is applied to the cell-wall constituents extracted from plant tissues by cold 4% sodium hydroxide, after preliminary extractions with cold water and 0.5% ammonium oxalate to remove pectic substances and water-soluble materials. The originator of the name was Schultze who, in 1891 and succeeding years, isolated there substances from a number of plant materials by extraction with dilute alkali and precipitation with acid. He observed that they were far more susceptible to dilute acid hydrolysis than cellulose, and believed them to be in some way related to cellulose, probably as intermediates in its formation. Although the eludication of the structure of cellulose and the analysis of hemicellulose have shown that there is little foundation for this belief, the name “hemicellulose” still persists. Hemicelluloses are polysaccharides which generally contain uronic acid. One of the few well-authenticated true hexosans that may be removed by alkali from the cell-wall of a higher plant is the mannan of the ivory nut. When dried by alcohol, hemicellulose preparations are obtained as fine, white or cream, amorphous powders; whereas, if dried from water, they form an extremely hard, horny mass, which will redissolve only with difficulty even if finely ground. The hemicelluloses are optically active, and usually more or less strongly laevo-rotatory. The majority of hemicellulose preparations give either no colour or a slightly greenish colour with iodine. The hemicellulose “A” of black-locust and of English oak sap-woods, precipitated by hydrochloric acid, was found to give a strong blue colouration with iodine. Anderson (1940) found that those hemicelluloses which are coloured by iodine solution usually give some d-glucose along with d-xylose in the products of hydrolysis. O’Dwyer suggests that in these hemicelluloses, anhydro-glucose units form a part of the molecule. It is more probable however, that they are derived from starch associated with it.