118 THE CARBOHYDRATES. [cH. V. 



iodine. The grains are covered with a film of amylopectin, 

 which prevents the amylose from dissolving out in cold 

 water. 



" Starch paste " is obtained by pouring a suspension of 

 the grains in cold water into boiling water. It is to be 

 considered as a mixture of amylopectin and amylose, both 

 of the substances being colloids. It is opalescent, due to 

 the amylopectin. It is completely precipitated by half 

 saturation with ammonium sulphate, or by the addition 

 of an equal volume of strong alcohol. It has no reducing 

 properties. 



"Soluble starch " differs from starch paste in being clear 

 and limpid. It is produced by the action of amylases or 

 acids on starch paste. It is only slowly precipitated by 

 half saturation with ammonium sulphate, but is precipi- 

 tated immediately by full saturation. It has no reducing 

 properties. 



Dextrins are formed by the partial hydrolysis of starch 

 by amylases, acids or superheated steam. The name 

 arises from the fact that they are strongly dextrorotatory. 

 They differ considerably in complexity. There are two 

 main varieties : erythro-dextrins, giving a reddish colour 

 with iodine, and achroo-dextrins, which give no colour. By 

 fractionation with salt solutions Young has separated three 

 erythro-dextrins, I., II., and III. The first two are precipi- 

 tated by full saturation with ammonium sulphate, and give 

 a purple and a red colour respectively with iodine. Erythro- 

 dextrin III. is not precipitated by salts, and gives a red- 

 brown colour with iodine. The achroo-dextrins also are 

 not precipitated by salts. They are insoluble in strong 

 alcohol and in ether. They reduce Fehling's solution 

 slightly, but do not form osazones nor ferment with yeast. 



Stable dextrin is the dextrin obtained from starch by 

 the action of amylases continued until the hydrolysis shows 

 an apparent equilibrium. It is, as its name implies, very 

 resistant to the further action of the enzymes, but is 

 apparently broken down slowly to maltose and glucose. 



