METHODS OF EXAMINING MILK 295 



and sterilized by drawing boiling water into it several 

 times. 



The test is made as follows : The milk to be tested is 

 heated to a temperature of 45 to 48 C. (113 to 118 F.) ; 

 % c.c. of the methylene blue solution is placed in the 

 test tube and 20 c.c. of the milk is added. The test tube is 

 closed with the cotton plug, placed in a water bath or 

 incubator at 38 to 39 C. (100.4 to 102 F.), and ob- 

 served at frequent intervals to note the time of decolori- 

 zation. If the methylene blue solution is placed in the 

 tube first and the milk afterward, the two fluids will be 

 thoroughly mixed and there will be no opportunity for 

 a part of the methylene blue solution to be absorbed by 

 the cotton plug. It is not necessary to cover the mixture 

 with a layer of paraffine oil, as was formerly recom- 

 mended, because the difference in reduction time caused 

 by the exclusion of oxygen is not sufficient to be taken 

 into consideration in routine milk testing. 



Several investigators have compared the reduction 

 time of market milk with the number of bacteria as de- 

 termined by the plate method. O. Jensen, using a test 

 solution prepared from a saturated alcoholic solution of 

 methylene blue, found the relations to be as follows : 



1. Decolorization in 7 hours or over, 100,000 bacteria 

 per c.c. 



2. Decolorization in from 2 to 7 hours, 100,000 to 

 300,000 bacteria per c.c. 



3. Decolorization in from % to 2 hours, 300,000 to 

 20,000,000 bacteria per c.c. 



4. Decolorization in less than % hour, 49,000,000 to 

 264,000,000 bacteria per c.c. 



In testing market milk with methylene blue solution 

 prepared from tablets, O. Jensen and Barthel found the 



