ii4 THE CARBOHYDRATES 



ture to stand until the gluten contained in the flour undergoes 

 fermentation, when it dissolves and may be removed by wash- 

 ing. On a small scale the separation is most conveniently 

 effected by kneading some flour in a muslin bag which is held 

 under a stream of water. The starch granules are hereby 

 washed through the muslin, while the gluten remains behind 

 in the bag as a sticky grey mass. 



Starch may also be obtained from potatoes by macerating 

 them with water and separating the non-starchy material from 

 the starch by filtration. The starch is then allowed to settle at 

 the bottom of the water, when it is collected and dried 



Purification. 



Malfitano and Moschkoff * give the following method for 

 the purification of starch : A one per cent colloidal solution of 

 starch is frozen and then allowed to melt. When melted, most 

 of the starch is deposited in a floccular precipitate, whilst the 

 clear liquid contains some starch and the greater part of the 

 mineral impurities. On repeating the operation four or five 

 times, the purified product yields less than -02 per cent of ash. 



Properties. 



Air-dried starch contains a considerable quantity of water, 

 as much as 20 per cent being not uncommon ; it can -be made 

 to part with this water by carefully heating to 100. If heated 

 to about 200 it is converted into a sticky soluble substance, 

 which is probably a mixture of isomeric substances of the 

 empirical formula C 6 H 10 O 5 , known as British gum or dextrin 

 (q.v.)- 



Starch is quite insoluble in cold water, but if heated with 

 water the granules swell and burst, a slimy opalescent mass 

 known as starch paste being formed. The consistency of this 

 paste varies of course with the concentration, and also with the 

 particular kind of starch employed ; this may be accounted for 

 by assuming that some starches are richer than others in the 

 constituent which produces the viscosity (p. 115). If a dilute 

 starch paste be filtered, a gelatinous residue remains on the 

 filter paper ; the filtrate contains some starch, since it gives a 



* Malfitano and Moschkoff: Compt. rend.," 1910, 151, 817. 





