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.c. 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 

 Blauenf eld 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 



