20 



FOOD MATERIALS AND HOW TO DETECT THEM 



Method. (a) Physical tests. Put a small quantity of 

 the egg in a test tube, add a little cold water, and shake the 

 mixture vigorously. What happens? Stand the tube in 



a test tube rack for some time. 

 Compare this tube with a tube 

 containing clear water. 



Heat a small portion of 

 white of egg in water until it 

 boils. What change does it 

 undergo ? 



Burn a small piece of white 

 of egg or lean meat and note 

 the odor of the smoke. This 

 peculiar smell is characteristic 

 of nitrogenous substances. 

 Name some other things which 

 emit this odor when burning. 

 (6) Chemical tests. The yellow test (xanthoproteic) . To 

 a small portion of white of egg add a few drops of dilute 

 nitric acid and heat the mixture. How does the nitric acid 

 affect the raw albumin ? What change of color follows heat- 

 ing ? Now neutralize with ammonia. What is the final color ? 

 The red test (Millon's). To another portion of egg 

 albumin add a few drops of Millon's reagent and warm 

 gently over a lamp. What change of color occurs ? 



Note. Heat gently at first, or this test is not likely to work. 



The violet test (biuret). To a small amount of soluble 

 protein, for example, albumin or milk in water, add a little 

 of the biuret reagent. What change occurs ? Repeat this 

 test, using only a drop of the egg solution in a very large 

 quantity of water. This will show how delicate this test is. 



Biuret. Xanthoproteic. Millon's. 

 FIG. 4. 



