38 



The percentage of acidity is calculated by multiplying 

 the number of cubic centimeters of alkali solution used 

 by 0.009 and dividing by the number of cubic centimeters 

 of milk or cream taken, the quotient being multiplied by 

 100. Thus: 



c. c. alkali X. 009 . 

 Percentage of acidity te8ted X 100. 



If 50 cubic centimeters of the sample required 10 cubic 

 centimeters of the alkali to neutralize, the percentage of 

 acidity would be 



10 * Q 09 X100, or 0.18 per cent. 



DETECTION OF PRESERVATIVES. 



The preservatives usually met with are formaldehyde, 

 borax, and boric acid, and these are not difficult to detect 

 if care is used in conducting the tests. Until one is thor- 

 oughly familiar with the tests it is a good plan to run three 

 samples together, one being the suspected sample, one 

 which is known to contain the preservative looked for, and 

 one known to be free from that preservative. 



Formaldehyde. There are two well-known tests for 

 detecting formaldehyde, one known as the Hehner test 

 and the other as the Leach test. 



In the Hehner test, about 5 cubic centimeters of the 

 milk is placed in a 6 by J inch test tube, and then about 

 the same quantity of concentrated sulphuric acid to which 

 a trace of ferric chlorid has been added. The acid is 

 allowed to run down the side of the test tube so as not to 

 mix with the milk. In a few minutes the presence of 

 formaldehyde will be indicated by a violet coloration at 

 the juncture of the milk and the acid. This must not be 

 confused with the charring of the milk by the acid. A 

 modification which avoids this charring is in use in the 

 dairy laboratory of the Bureau of Chemistry, United 

 States Department . of Agriculture, the only difference 

 being that the sulphuric acid used is diluted with water 

 until it has a specific gravity of 1.8. 



