10 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 



then immersing them in hot water until heated to 145 F. and hold- 

 ing them at that temperature for from 20 to 30 minutes. The cool- 

 ing was accomplished by gradually lowering the temperature of the 

 water until that of the milk reached 50 F. 



The advantage of this process is in the fact that the milk after 

 heating is not exposed until it reaches the consumer, thereby elimi- 

 nating any danger of reinfection with disease-producing organisms 

 through handling. For this process to be successful, however, it is 

 necessary that the temperature of the milk in bottles be measured at 

 the bottom of the bottle, and that the holding period of 30 minutes 

 begin when the temperature at the bottom has reached 145 F. This 

 is essential, because the milk in the top heats faster than that in 

 the bottom of the bottle. 



The matter of seals is also important. They should be absolutely 

 water-tight, as the bottles are submerged in water, and during cool- 

 ing a defective cap might allow infection by polluted cooling water. 

 The disadvantage of this process is in the increased cost of pasteuri- 

 zation, caused by the cost of the seal caps. It is claimed, however, 

 that the saving in milk losses by pasteurization in bottles makes up 

 for the added expense of caps. It is now possible to pasteurize milk 

 in this manner without using water-tight caps. This is accomplished 

 by the aid of devices which fit over the tops and necks of the bottles, 

 thereby protecting the ordinary paper caps from the water which is 

 sprayed on the bottles for the purpose of heating or cooling. This 

 method of protecting the tops permits the use of the ordinary caps 

 and seems to remove the possible danger of polluted water infecting 

 the milk. 



ADVANTAGES OF LOW-TEMPERATURE PASTEURIZATION. 



In general, the trend of pasteurization has been toward the holder 

 process, and with this tendency the use of lower temperatures has 

 become more common. As a general rule, when the holder process 

 is used milk is heated to about 145 F. for from 20 to 30 minutes and 

 to at least 160 F. for 1 minute when the flash process is used. From 

 bacteriological, chemical, and economical standpoints it is highly 

 desirable that milk be pasteurized at the lower temperature. 



From a bacteriological standpoint, pasteurization at 145 F. for 

 30 minutes gives assurance, so far as we know, of a complete de- 

 struction of nonspore-forming disease-producing bacteria and at 

 the same time leaves in the pasteurized milk the maximum percent- 

 age of the bacteria that cause milk to sour (lactic-acid bacteria) 

 and only a small percentage of those that cause it to decompose 

 (peptonizers). When higher temperatures are used, while the total 

 number of all kinds of bacteria is reduced, the percentage of lactic- 



