286 BACTERIOLOGICAL ANALYSIS OF MILK. 



of tuberculosis bacillus inasmuch as there are other bacilli 

 liable to be found in milk, which have similar staining proper- 

 ties. On the other hand, the failure to find such bacilli does 

 not indicate the absence of the tuberculosis organism, since 

 it is so difficult to detect its presence that negative evidence 

 has little meaning. The microscopic study of the milk for 

 detecting the presence of the tuberculosis bacillus, is there- 

 fore, unsatisfactory and of little value. 



TO DETERMINE THE ACIDITY OF MILK. 



Although not strictly bacteriological, the test for the 

 amount of acidity in milk is occasionally necessary in connec- 

 tion with bacteriological work, and is extremely useful in 

 many respects. For this reason the method of making this 

 test is here briefly described. 



In testing fresh milk it is necessary, if one wishes to be 

 accurate, to boil the sample of milk tested in order to drive 

 off the CO 2 . If the milk is diluted with ten times its bulk 

 of water and boiled for a few minutes before testing, the 

 errors introduced by the CO 2 are avoided. The readings will 

 be a little less than without such previous treatment. If the 

 milk is old, such boiling will curdle it and interfere with the 

 ease of the test. 



For an accurate test it is necessary to prepare a one tenth 

 normal NaOH solution. A good-sized burette should be 

 filled with this solution. 



A carefully measured quantity of milk (50 c.c. is a con- 

 venient amount) is placed in a porcelain dish and I c.c. of 

 phenolphthalein is added. 



The alkaline solution is now to be drawn from the burette 

 into the milk by small additions, the milk being stirred 

 between each addition, and this is continued until the milk 

 retains a permanent faint pink color. This indicates that the 

 acid has been neutralized, and the number of c.c. of the 

 alkaline solution used is read from the burette. 



