MILK 337 



peroxidases, and reductases have been identified in milk, but not all of 

 them in milk of the same species of animal. 



Among the principal preservatives used in connection with milk are 

 formaldehyde, hydrogen peroxide, boric acid, borates, salicylic acid, 

 and salicylates. The use of milk preservatives is illegal in most states. 



EXPERIMENTS ON MILK 



1. Reaction. Test the reaction of fresh cow's milk to litmus, phenolphthalein 

 and Congo red. 



2. Biuret Test. Make the biuret test according to directions given on 

 page 100. 



3. Microscopical Examination. Examine fresh whole milk, skimmed or 

 centrifugated milk, and colostrum under the microscope. Compare the micro- 

 scopical appearance with Fig. 104, page 331. 



4. Specific Gravity. Determine the specific gravity of both whole and 

 skimmed milk (see page 342). Which possesses the higher specific gravity? 

 Explain why this is so. 



5. Film Formation. Place 10 c.c. of milk in a small beaker and boil a few 

 minutes. Note the formation of a film. Remove the film and heat again. Does 

 the film now form? Of what substance 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 hi 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 develops and gradually deepens 

 into a brown. To what is the formation of this color due? (See Moore's Test, 

 Chapter II.) 



8. Test for Chlorides. To about 5 c.c. of milk hi 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 hi a test-tube add 3 drops of 

 milk and enough alcoholic solution of guaiac (strength about i : 60) l to cause 

 turbidity. Thoroughly mi* the fluids by shaking and observe any change which 

 may gradually take place hi the color of the mixture. If no blue color appears 

 in a short time, heat the tube gently below 6oC. and observe whether the 

 color reaction is hastened. In case a blue color does not appear hi 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. Those milks which respond positively, fail to do so 

 after boiling 15-20 seconds. What substances beside milk respond to 

 this test? See discussion on page 261. 



10. Differentiation of Human and Cow's Milk (Modification of Bauer's Test). 1 

 Introduce 2 c.c. of fresh human milk into a 50 c.c. test-tube and 2 c.c. of fresh 



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. 

 * Bauer: M onatssch. f. KinderheiL, n, 474, 1912-13. 

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