202 PASTEURIZATION 



into a test-tube and one or two drops of a 0.2 per cent solution of 

 hydrogen peroxide is added from a dropping bottle, also two 

 drops of a 2 per cent solution of paraphenylene diamine, from a 

 dropping bottle. Brown dropping bottles should be used to 

 prevent the light from weakening the reagents. The test-tube 

 is then well shaken, and if the milk has not been heated above 

 78 C., or 172 F., or if not heated at all, an intense blue coloration 

 is produced. If at once or after half a minute the milk becomes 

 bluish-grey, it indicates that it has been heated to a temperature 

 of 78 C. to 80 C., or 172 F. to 176 F. When the color of the 

 milk is unchanged after addition of the reagents, it may be 

 concluded that the heating has exceeded 80 C. The blue color 

 that develops on standing has no significance. 



Storch's test has shown itself to be the most reliable of all the 

 methods proposed for distinguishing heated from unheated 

 milk. All the so-called improvements which have been advo- 

 cated by other chemists have proven to be of no benefit, often 

 indeed the opposite. 



If during the pasteurization of the milk the temperature falls 

 below 80 C. for a time, the whole of the milk after being mixed 

 reacts to Storch's test. The sensibility of the test is so great 

 that the admixture of 10 per cent of milk which has only been 

 heated to 78 C., suffices to make the whole volume of milk react 

 to the test. 



Pasteurization Temperatures. In pasteurizing or heating 

 milk for city trade or immediate consumption, low temperatures 

 are used. This is done for the purpose of avoiding a cooked or 

 heated taste. The method found most satisfactory for milk is 

 to heat to 145 F., and hold at this temperature for twenty 

 minutes. This is known to the trade as the holding method. 



Where the flash or instantaneous method is used, the milk or 

 cream is heated to a temperature of 175 to 190 F. The tem- 

 peratures most commonly used in butter-making in creameries 

 are 1 80 to 185 F. Of late years the higher temperatures have 

 been used for butter-making, even in the holding method. 



Marker, in the Canadian Northwest, recommends heating to 

 170 or 175 F., and holding for fifteen or twenty minutes. This 



