8 LABORATORY WORK IN PHYSIOLOGICAL CHEMISTRY. 



ciently concentrated, gelatinize upon cooling. The most 

 characteristic reaction of the starches is their behavior toward 

 iodine, with which they form blue compounds whose color 

 disappears upon heating, but returns as the solution cools. 



(a) Examine under the microscope and sketch the follow- 

 ing starch granules: potato, corn, wheat, rice, and arrowroot. 



(6) Place some starch in a test-tube half full of water and 

 shake thoroughly until tho starch is finely divided. Heat 

 water to boiling in another test-tube and to this add enough 

 of the cold starch mixture to make a translucent solution 

 (about 2 per cent). What takes place upon pouring the 

 suspended starch into the hot water? The solution of starch 

 thus obtained is called a paste, and in making experiments 

 with this substance such a paste must always be prepared. 



(c) To 5 c.c. of starch paste add a drop of iodine solution. 

 Warm gradually and then allow to cool. Note changes. 



(d) Boil 10 c.c. of starch paste with 1 c.c. of concentrated 

 HC1 for 5 minutes. Observe the change which the solution 

 undergoes. Neutralize part of this cold solution with NaOH, 

 and apply Fehling's and Barfoed's tests. (If no reduction 

 appears, continue the boiling of the original acid solution for 

 some minutes longer and repeat the neutralization and tests.) 

 Determine the character of the sugar causing this reduction. 



For the DEXTRINS see Salivary Digestion; for GLYCOGEN 

 see Muscular Tissue. 



