122 ORGANIC AGRICULTURAL CHEMISTRY 



is composed of small pieces or grains. These grains possess a 

 definite microscopical structure which is different for the starch 

 obtained from different plants. This makes possible the iden- 

 tification of starches as to their source, and is used in detecting 

 adulteration of starches or starch-containing materials. When 

 placed in cold water these starch grains remain unbroken and 

 the starch does not mix with the water, but settles out on 

 standing. If, however, the starch is boiled in water the grains 

 are ruptured and the contents then mix with the water as an 

 emulsion or colloidal suspension known as starch paste. In 

 this form starch is adhesive, and is used as a substitute for 

 gums and in sizing cloth and paper. When starch is brought 

 in contact with a solution of iodine, a blue compound is formed. 

 This blue color of iodine and starch is used in a number of ways 

 as a test for starch or for free iodine. Starch grains treated 

 with iodine take on a beautiful blue color under the micro- 

 scope. The structure of the grains is shown very clearly so that 

 starch grains and other similar granular substances are readily 

 distinguished. Paper moistened with a starch paste to which 

 potassium iodide has been added turns blue as soon as any io- 

 dine is liberated £rom the potassium iodide. This paper is thus 

 a test for ozone, hydrogen peroxide, nitrous oxide or any other 

 oxidizing substance which will set iodine free from its iodide 

 salts. When starch is hydrolyzed by enzymes or by boiling 

 with acids, a test with iodine solution readily indicates when 

 all starch has been hydrolyzed. Starch does not reduce 

 Fehling^s solution^ is non-fermentable by zymase and is inactive 

 optically. 



Hydrolysis. — When hydrolyzed by acids or enzymes, starch 

 yields glucose only. In the enzymatic hydrolysis, two enzymes 

 are chiefly concerned. In plants, both in green leaves and in 

 seeds, the enzyme diastase converts starch into maltose sugar. 

 In the animal body the saliva contains the enzyme ptyalin and 

 the pancreatic juice contains the enzyme amylopsin both of 

 which similarly hydrolyze starch into maltose. Associated 

 with diastase in plants and with ptyahn in saHva and also pres- 



