150 



THE CARBOHYDRATES 



while potato and arrowroot starch contain hardly any (cf. 

 Amylohemicellulose, p. 153). 



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 CgHioOg, known as British gum or dextrin 



(q.v.). 



Starch is insoluble in cold water, but if dry starch is finely 

 ground for some time in an agate mortar and then stirred up 

 with cold water and filtered through a gravimetric filter paper, 

 such as will retain the finest suspended solids, the filtrate 

 may be shown to have taken up some of the starch in colloidal 

 solution since on addition of a solution of iodine a deep blue 

 coloration results. If a suspension of starch in water is heated, 

 the particles gradually swell and finally burst, forming an 

 opalescent solution, known as starch paste, which is more or 

 less mucilaginous according to the amount of starch employed ; 

 the optimum temperature for bringing about this change varies 

 with the starch as may be seen from the following figures : — 



Too high a temperature tends to the formation of lumps, 

 and it is generally best not to boil the solution but to add a 

 fine suspension of starch in cold water to the requisite amount 

 of warm water heated over a boiling water bath. 



A solution of starch so prepared is not to be regarded as 

 a true molecular disperse solution, but as a colloidal solution 

 of soluble amylose thickened by a suspension of the insoluble 

 gelatinizing material amylopectin. 



A solution of starch paste undergoes a change on keeping, 

 known as retrogradation, and deposits a white fiocculent pre- 

 cipitate which, microscopically, resembles starch. For this 



