508 THE PASTEURIZATION OF MILK 



Starch's Test for Enzymes in Milk. This test is generally employed 

 in testing milk for the presence of the peroxydases. It is made as 

 follows : Take 10 c.c. of milk in a test-tube, add one or two drops of 

 dilute hydrogen peroxide (H 2 O 2 ), and mix well by shaking, then add 

 two drops of a 2 per cent, solution of paraphenylendiamine and 

 shake again. In raw milk an indigo-blue color is produced, depending 

 upon the presence of active oxydases. The test may also be made, 

 according to Rullman, as a contact ring test as follows: Take 10 c.c. 

 of milk in a test tube, add 10 drops of a 3 per cent, solution of hydrogen 

 peroxide, and shake well. Hold test-tube very obliquely and add 

 slowly from a pipette 1 c.c. of a 2 per cent, solution of paraphenylen- 

 diamine, allowing it to flow along the glass on the surface of the milk. 

 A grayish-blue ring is formed at the zone of contact between the milk 

 and the test fluid. According to Kastle, the reaction is delayed in 

 milk which has been heated to 70 C. for fifteen minutes, while it is 

 absent after heating to 70 C. for one hour. It is somewhat delayed 

 after heating to 60 C. for one hour, but not affected > after thirty 

 minutes' heating to 60 C. 



Home Pasteurization. Directions for the home pasteurization of 

 milk as given by Rogers in Circular No 152, of the Bureau of Animal 

 Industry, are as follows: 



" Milk is most conveniently pasteurized in the bottles in which it is 

 delivered. To do this use a small pail with a perforated false bottom. 

 An inverted pie tin with a few holes punched in it will answer the 

 purpose. This will raise the bottles from the bottom of the pail, 

 thus allowing a free circulation of water and preventing bumping of 

 the bottles. Punch a hole through the cap of one of the bottles and 

 insert a thermometer. The ordinary floating type of thermometer 

 is likely to be inaccurate and if possible a good thermometer with 

 the scale etched on the glass should be used. Set the bottles of milk 

 in the pail and fill the pail with water nearly to the level of the milk. 

 Put the pail on the stove or over a gas flame and heat it until the 

 thermometer in the milk shows not less than 150 F., nor more than 

 155 F. The bottles should then be removed from the water and 

 allowed to stand from twenty to thirty minutes. The temperature 

 will fall slowly, but may be held more uniformly by covering the bottles 

 with a towel. The punctured cap should be replaced by a new one, 

 or the bottle should be covered with an inverted cup. 



"After the milk has been held as directed it should be cooled as 

 quickly and as much as possible by setting in water. To avoid danger 

 of breaking the bottle by too sudden change of temperature this 

 water should be warm at first. Replace the warm water slowly with 

 cold water. After cooling, milk should in all cases be held at the 

 lowest available temperature. 



" This method may be employed to retard the souring of milk or 

 cream for ordinary use. It should be remembered, however, that 

 pasteurization does not destroy all bacteria in milk, and after pasteur- 



