92 EXPERIMENTAL DAIRY BACTERIOLOGY 



color unchanged, it has been heated above 80 C. By the use 

 of this test 10 per cent of milk heated to 78 C. can be de- 

 tected when mixed with milk heated above 80 C. 



The test cannot be made with milk in which the acidity 

 has increased to any extent. In the testing of sour milk or 

 buttermilk the sample to be tested must first be neutralized. 

 The blue color does not develop except in the presence of 

 casein. In unheated whey a brown color develops, while in 

 whey heated above 80 C. no color is produced on adding 

 the reagents. 



Exercise. Heat a sample of milk to 70 C. and cool at once to 

 20 C. The heating should be done in such a manner that the sample 

 can be stirred during the process, so as to insure uniform application 

 of heat to all particles of the milk. Apply Storch's test to a portion 

 of the unheated milk and to the heated milk. 



Repeat the experiment at 75 and 80 C. Note comparative results. 



