THE CARBOHYDRATES 25 



When examined under the microscope it is seen to consist 

 of minute granules which vary in size, shape, and appearance. 

 The granules of potato starch are about -^ of an inch in 

 diameter, those of wheat and maize only about y~ of an inch. 

 The granules of rice starch are still smaller. The first are 

 kidney shaped and exhibit a peculiar concentric lamination. 

 The others are round, oval, or angular and differently marked. 



The granules are not entirely homogeneous. They contain 

 a skeleton part which is insoluble in saliva. This forms only 

 about 0*5 per cent, of the whole. It has been called starch 

 cellulose, but has apparently the same composition as the 

 bulk. 



When air dry, starch contains from 10 to 12 per cent, of 

 moisture. The larger parl of the water evaporates at iooC., 

 the remainder only on heating to about 1 10 C. Between 400 C. 

 and 5ooC. the starch is converted into dextrine; at higher 

 temperatures it chars, or burns and leaves a small proportion 

 of ash. 



Starch reacts with halogens in a manner that is not well 

 understood. Iodine dissolved in potassium iodide solution 

 produces an intense blue coloration. The colour vanishes 

 when the mixture is heated, but reappears on cooling. The 

 reaction is extremely delicate ; it is one of the best tests for 

 starch, and has many applications in volumetric analysis. 



Water, at ordinary temperatures, has no effect upon starch 

 (p. 23). Hot water causes the granules to swell up and form 

 a colloidal jelly commonly called starch paste. If the paste 

 is very thin, i.e. dilute, the starch passes through ordinary 

 filter paper, and the mixture is often called starch solution. 

 The starch, however, settles out on standing, and is probably 

 only suspended in the water. When mixed with water and 

 heated in sealed tubes to 190 C., starch forms a true solution. 

 This soluble form of starch is called amidulin. It reacts with 

 iodine, is insoluble in alcohol, and is, therefore, precipitated 

 as a gelatinous paste on addition of that reagent. It is also 

 precipitated by lime and other basic oxides. On continued 

 heating in sealed tubes at i9OC., or more rapidly at higher 

 temperatures, the amidulin undergoes hydrolysis producing 



