CARBOHYDRATES 61 



'These transformations can be traced by the iodine color reaction, 

 as starch will show its characteristic blue, dextrins purple or rose- 

 red, and maltose and glucose no color with iodine. 



Dextrins may occur in plants as transition products in the 

 transformation of starch into sugars, or vice versa. Most com- 

 monly, however, they are artificial products resulting from the 

 partial hydrolysis of starch hi the laboratory or factory. They 

 are amorphous substances, which are readily soluble in water, 

 forming sticky solutions which are often used as adhesives ("library 

 paste " is a common example of a very concentrated preparation 

 of this kind). They are precipitated from solution by alcohol, 

 but not by lead subacetate (distinction from starch). They are 

 strongly dextrorotatory (specific rotatory power 4-192 to +196); 

 are not fermented by yeast alone, but readily undergo hydrolysis 

 to glucose which does ferment. There are several different mod- 

 ifications, or forms, of dextrins, depending upon the extent to 

 which the simplification of the starch molecule by hydrolysis is 

 carried. Three fairly definite forms are generally recognized, as 

 follows: amylo-dextrin, or soluble starch, slightly soluble in cold 

 water, readily so in hot water, giving a blue color with iodine; 

 erythro-dextrin easily soluble in water, neutral taste, red color with 

 iodine; and achroo-dextrin, easily soluble hi water, sweetish taste, 

 no color with iodine. 



Commercial dextrin, which is much used in the preparation 

 of mucilages and adhesive pastes, is prepared by heating dry 

 starch to about 250 C. It is composed chiefly of achroo-dextrin, 

 mixed with varying quantities of erythro-dextrin and glucose. 



Glycogen, or " animal starch/' is one of the most widely dis- 

 tributed reserve foods of the animal body; in fact, it is the only 

 known form of carbohydrate-reserve in animal tissues. But it is 

 present only rarely in plants. It occurs in certain fungi, par- 

 ticularly in yeasts. In the animal body, glycogen is found in all 

 growing cells; also in the muscles and blood; but most largely in 

 the liver, where it is stored in large quantities. The glycogen 

 found in yeasts is identical with that found in animal tissues. The 

 quantity of glycogen in a yeast cell increases rapidly as the yeast 

 grows during the fermentation process. 



Glycogen is a white, amorphous compound, readily soluble in 

 hot water, forming an opalescent solution similar in appearance 

 to the solutions of soluble starch. It is strongly dextrorotatory 



