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, (118 to 118° F.) ; 
Ye 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 14 to 2 hours, 300,000 to 
20,000,000 bacteria per c.c. 
4. Decolorization in less than 1% 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 
