THE CARBOHYDRATES 205 



THE POLYSACCHARIDES. 



The polysaccharides are substances of high molecular weight. The 

 size of their molecule is unknown, but it is composed of a large 

 number of monosaccharide units. Their empirical formula is usually 

 represented by (C 6 H 10 O 5 ) n , but many polysaccharides contain pentose 

 -units as well as hexose units and may consist entirely of pentose units 

 (C 5 H 8 O 4 ) n . They may be classified into the following groups : 

 Hexosans. Glucosans : Starch, dextrin, glycogen, cellulose. 

 Fructosans : Inulin. 

 Mannans. 

 Galactans. 



Pentosans. Gums, pectins. 

 Hexosan-Pentosans. Lignocellulose, hemicellulose. 



STARCH. 



Starch is present in various parts of plants and has been found in 

 green leaves, fruits, seeds, tubers, etc. The amount of starch present 

 in the seeds of cereals varies from 50-70 per cent, of the dry weight ; 

 potatoes contain from 15-30 per cent. It occurs in definite 

 granules starch grains which are made up of concentric layers 

 around a hilum. When examined under a microscope these granules 

 are seen to be of different forms. The source of starch grains can thus 

 be ascertained from their microscopic structure. 



Preparation. 



Starch is prepared from wheat, rice, maize, potatoes, etc., by 

 mechanical processes. The material is disintregrated by crushing, 

 washed with water and passed through sieves. The starch in suspen- 

 sion passes through and is allowed to settle. The water is drained off 

 and the starch grains are dried. 



Starch may be purified by making a I per cent, suspension in water, 

 freezing and allowing to melt. The starch is left as a residue whilst 

 the liquid contains the impurities. The operation is repeated four or 

 five times. 



Properties. 



Starch grains form a white powder which is insoluble in cold water. 

 If boiled with water the granules swell and burst forming an opale- 

 scent solution, termed starch paste. Such a solution is most con- 

 veniently made by rubbing starch grains into a cream with water and 

 pouring the cream into boiling water and boiling for some minutes. 

 The paste so formed varies in consistency with the amount of starch. 

 Dilute solutions from 1-4 per cent, are limpid, but stronger solutions 

 set into opaque white jellies. 



