222 PHYSIOLOGICAL CHEMISTRY. 



and boil a few minutes. Note the formation of a film. Remove 

 the film and heat again. Does the film now form? Of what sub- 

 stance is this film composed ? The biuret test was positive, why do 

 we not get a coagulation here when we heat to boiling? 



6. Coagulation Test. Place about 5 c.c. of milk in a test-tube, 

 acidify slightly with dilute acetic acid and heat to boiling. Do you 

 get any coagulation? Why? 



7. Action of Hot Alkali. To a little milk in a test-tube add a 

 few drops of potassium hydroxide and heat. A yellow color de- 

 velops and gradually deepens into a brown. To what is the forma- 

 tion of this color due? 



8. Test for Chlorides. To about 5 c.c. of milk in a test-tube 

 add a few drops of very dilute nitric acid to form a precipitate. 

 Filter off this precipitate and test the filtrate for chlorides. Does 

 milk contain any chlorides? 



9. Guaiac Test. To about 5 c.c. of water in a test-tube add 3 

 drops of milk and enough alcoholic solution of guaiac (strength 

 about 1:60)* to cause a turbidity. Thoroughly mix the fluids 

 by shaking and observe any change which may gradually take place 

 in the color of the mixture. If no blue color appears in a short 

 time, heat the tube gently below 60 C. and observe whether the 

 color reaction is hastened. In case a blue color does not appear 

 in the course of a few minutes, add hydrogen peroxide or old 

 turpentine, drop by drop, until the color is observed. Fresh milk 

 will frequently give this blue color when treated with an alcoholic 

 solution of guaiac without the addition of hydrogen peroxide or old 

 turpentine. See discussion on page 192. 



10. Kastle's Peroxidase Reaction. The peroxidase reaction 

 of milk is founded upon the fact that small amounts of raw milk 

 will induce the oxidation of various leuco compounds by hydrogen 

 peroxide. This reaction has been used in a practical way as the 

 most convenient means of differentiating between raw milk and 

 boiled milk. Many substances have been employed for this purpose, 

 e. g.j guaiac, paraphenylenediamine, ortol, amidol, etc. Kastle has 

 found that a. dilute solution of " trikresol " 2 acts as a sensitizing 

 agent in the peroxidase reaction and offers the following test which 

 is based upon this fact : To 2-5 c.c. of raw milk in a test-tube add 



1 Buckmaster advises the use of an alcoholic solution of guaiaconic acid 

 instead of an alcoholic solution of guaiac resin. Guaiaconic acid is a constituent 

 of guaiac resin. 



a " Trikresol" is the trade name of an antiseptic which contains the three 

 cresols in approximately equal proportions. 



