SYNTHESIS OF STARCHES 



133 



[Chap. XV 



Starch grains. Starch grains in different kinds of plants may differ in 

 form, size, and internal structure (Fig. 60). This fact may be used as one 

 means of identifying certain commercially prepared drugs and foods, 

 and also certain impurities in them. 





-"i 



^Q^& 



B 



Oo^^ 



s^ ^ 



D E F 



Fig. 60. Diagrams of starch grains from several kinds of plants: A, bean; B, corn; 

 C, oats; D, potato; E, rice; F, wheat. Adapted in part from Leffmann and 

 Beam. 



Each grain is composed of at least two substances. The outer portion 

 is composed of amylopectin, which becomes gelatinous in boiling water, 

 but does not appear blue with iodine. The inner part of the grain is 

 largely amylose. It is colloidally dispersed in boiling water and becomes 

 blue with iodine. 



Dextrins. Dextrins are generally considered to be interaiediate prod- 

 ucts between sugar and starch. The probable steps in the formation and 

 digestion of starch may be represented in brief outline, in which diastase 

 is represented by three enzymes: 



Glucose ^^ Maltose ^=^ Dextrins ^=^ Starch 



(Mallase) 



(Dextrinases) 



(Amylase) 



In the young seeds of some plants (sweet corn, certain varieties of 



