44 PHYSIOLOGICAL CHEMISTRY. 



Starch is acted upon by amylases, e. g., salivary amylase (ptyalin) 

 and pancreatic amylase (amylopsin), with the formation of soluble 

 starch, erythro-dextrin, achroo-dextrins, maltose, iso-maltose and 

 dextrose (see Salivary Digestion, page 54). Maltose is the 

 principal end-product of this enzyme action. Upon boiling a starch 

 solution with a dilute mineral acid a series of similar bodies 

 is formed, but under these conditions dextrose is the principal 

 end-product. 



EXPERIMENTS ON STARCH. 



1. Preparation of Potato Starch. Pare a raw potato, com- 

 minute it upon a fine grater, mix with water, and " whip up " the 

 pulped material vigorously before straining it through cheese cloth 

 or gauze to remove the coarse particles. The starch rapidly settles 

 to the bottom and can be washed by repeated decantation. Allow 

 the compact mass of starch to drain thoroughly and spread it out 

 on a watch glass to dry in the air. If so desired this preparation 

 may be used in the experiments which follow. 



2. Microscopical Examination. Examine microscopically the 

 granules of the various starches submitted and compare them with 

 those shown in Figs. 7-17, page 45. The suspension of the granules 

 in a drop of water will facilitate the microscopical examination. 



3. Solubility. Try the solubility of one form of starch in each 

 of the ordinary solvents (see page 23). If uncertain regarding the 

 solubility in any reagent, filter and test the filtrate with iodine solu- 

 tion as given under 5 below. The production of a blue color. would 

 indicate that the starch had been dissolved by the solvent. 



4. Iodine Test. Place a few granules of starch in one of the 

 depressions of a porcelain test-tablet and treat with a drop of a 

 dilute solution of iodine in potassium iodide. The granules are 

 colored blue due to the formation of so-called iodide of starch. The 

 cellulose of the granule is not stained as may be seen by examining 

 microscopically. 



5. Iodine Test on Starch Paste. 1 Repeat the iodine test using 

 the starch paste. Place 2-3 c.c. of starch paste 2 in a test-tube, add 



1 Preparation of Starch Paste. Grind 2 grams of starch powder in a motar 

 with a small amount of cold water. Bring 200 c.c. of water to the boiling-point 

 and add the starch mixture from the mortar with continuous stirring. Bring 

 again to the boiling-point and allow it to cool. This makes an approximate T 

 per cent starch paste which is a very satisfactory strength for general use. 



2 For this particular test a starch paste of very satisfactory strength may be 

 made by mixing i c.c. of a i per cent starch paste with 100 c.c. of water. 



