270 ORGANIC AGRICULTURAL CHEMISTRY 



facture of glucose, which is really a food use, and as laundry 

 starch, or as sizing for cloth or paper, it is generally obtained 

 from either corn or potatoes, which are the two most common 

 and abundant sources. 



Starch Content of Crops. — The preceding table gives the 

 percentage amounts of starch (in some cases including small 

 amounts of sugars) in some of the more important crops. The 

 figures are those for " nitrogen free extract," which ordinarily 

 includes with the starch the small amounts of sugars and pen- 

 tosans if these have not been separately determined. 



EXPERIMENT STUDY XXXIV 



Starch 



(i) Cut a wheat grain and moisten the cut surface with iodine solu- 

 tion. Note test for starch. Repeat with other cereal grains. 



(2) Cut a slice of potato and moisten similarly with iodine solution. 



(3) Cut a slice of a green apple and similarly test for starch by 

 moistening the entire surface. Repeat the test with a slice of a ripe 

 apple. Note difiference in amount and location of starch. 



(4) Wash out the starch from some wheat flour made into a dough 

 and kneaded in the hand under a small stream of running water. 

 Keep the dough in a compact ball and continue to knead until prac- 

 tically all starch is washed out and an elastic gummy ball is left. 

 This is gluten. Test for protein (Experiment XVIII). Collect the 

 starchy washings and allow to settle. Examine the starch residue. 



(5) Peel a potato and grate it into a large dish of water. Allow 

 the starch to settle and examine it. This is the principle of the prepa- 

 ration of commercial potato starch. 



Dextrin, Inulin, Mannan, Galactan 



While there is only one form of starch chemically considered, 

 there are several other substances occurring in plants which 

 are isomeric with it and which are often included with it in a 

 general group sometimes termed the starches. They all have 

 the same composition formula, viz. (CeHioOs)^. They differ 

 probably in complexity of the molecule, but their chief difier- 



