96 THE CARBOHYDRATES 



mineral impurities. On repeatincj 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,.Hi^Or,, 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. 97). If a dilute 

 starch paste be filtered, a gelatinous residue remains on the 

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

 blue colour with iodine, but it is doubtful whether the liquid is 

 a true solution ; it is more likely a colloidal solution in which 

 the particles are sufficiently small to pass through the pores of 

 the filter paper. 



With regard to the chemical nature of starch granules there 

 are considerable differences of opinion. The researches of Nageli 

 have shown that when starch is treated with dilute hydrochloric 

 acid, malt extract, or saliva, a considerable portion goes into 

 solution, leaving a transparent skeleton undissolved. The 

 soluble portion, which gives a blue colour with iodine, Nageli 

 regarded as the true starch constituent of the granule, and 

 named it granulose ; on the other hand, the undissolved skele- 

 ton, which is not turned blue by iodine, he considered to be 

 of a cellulose nature, and called it starch cellulose or amylo- 

 cellulose. 



On the other hand, Meyer * was of opinion that starch 

 granules consisted essentially of two substances known respec- 

 tively as a and ^ amylose. The former, which was insoluble, 



* Meyer: " Unters. u, d. Starkekorner," Jena, 1895. 



