l8 A DAIRY LABORATORY GUIDE 



EXPLANATION OF BOILED MILK TEST 

 AND FORMALDEHYDE TEST 



There are two principal tests for boiled milk. 

 One test makes use of three chemicals hydrogen 

 peroxide, potassium iodide and starch. In the 

 second test two reagents are employed hydrogen 

 peroxide and paraphenylenediaminehydrochloride. 

 In both cases a blue color results if the milk has 

 not been boiled. If the milk has been boiled no 

 blue color will result. The last named test acts 

 more rapidly than the first one and also gives a 

 more intense color. Any double oxide may be sub- 

 stituted in place of the hydrogen peroxide. Such a 

 double oxide would be calcium peroxide. There is 

 in milk an enzyme galactase which is destroyed by 

 heat. When the milk has not been heated this 

 enzyme sets free the oxygen from the oxidizing 

 agent, and in case of the first test, the free oxygen 

 splits up the potassium iodide and liberates free 

 iodine. The starch in the presence of free iodine 

 turns blue. In case of the second test the oxygen 

 liberated by the galactase acts directly on the 

 paraphenylenediaminehydrochloride and turns the 

 solution blue. Hydrogen peroxide often contains 

 sulphuric acid. When this is the case, the reagent 

 is useless for the test with starch, as the free acid 

 would break up the potassium iodide. If this hap- 

 pened a blue color would result whether the milk 

 had been heated or not. 



The test for formaldehyde is a delicate one and 

 is easy to perform, inasmuch as it may be done in 

 connection with the Babcock test. The regular 

 amount of milk is measured with the 17.6 cc. pipette 

 into a Babcock test bottle and a few drops of ferric 

 chloride added. The regular amount of sulphuric 



