294 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
separated if sodium bicarbonate has been used as a pre- 
servative. The catalase test is not as suitable for the ex- 
amination of market milk as some of the other methods, 
but it is one of the most delicate tests for detecting path- 
ological conditions when individual milk is examined. 
REDUCTASE TEST 
In this test, methylene blue solution and milk are 
mixed together and the amount of reductase present, or 
the reduction power, is indicated by the length of time 
required for the blue color to disappear. Different prep- 
arations of methylene blue vary in composition and it is 
therefore recommended that Merck’s B extra methylene 
blue be always used. The solution is prepared as fol- 
lows: methylene blue is added to absolute alcohol to sat- 
uration; 5 c.c. of this saturated alcoholic solution is then 
added to 195 c.e. of sterile distilled water; this is the so- 
lution used in making the test. The saturated alcoholic 
solution is rather unstable, and Barthel and O. Jensen 
therefore recommend that the test solution be made with 
tablets of Merck’s B extra methylene blue prepared by 
Blauenfeld and Tvede, of Copenhagen, Denmark. One 
tablet is dissolved in 200 c.c. of sterile distilled water. 
The solution will keep two weeks. When milk is tested 
with the tablet solution, the reduction time will be shorter 
than when it is tested with the solution made from the 
saturated alcoholic solution, because the tablet solution 
contains less methylene blue. 
Ordinary test tubes are used in making the test. 
These should be cleaned, plugged with cotton and steril- 
ized. The pipettes used in measuring the milk and the 
methlyene blue solution should also be cleaned and steril- 
ized. After each sample of milk is measured, the pipette 
used for this purpose should be rinsed with cold water 
