LOW -TEMPER ATURE PASTEURIZATION. 117 



cidal efficiency of a pasteurizing machine at different times, 

 even if run under constant conditions. 



Bactericidal efficiency. Russell (15) has shown that an 

 exposure to 140F. (60C.) for twenty minutes results in the 

 destruction of as great a number of bacteria as exposure to 

 higher temperatures for the same time. The bacteria remain- 

 ing alive after exposure to 160F. were in spore form, and were 

 uninjured by the higher ranges of temperature. Milk which 

 would ordinarily sour in two days would keep fresh for six 

 days when pasteurized at 140F. for fifteen to thirty minutes 

 (2). Over 99% of the bacteria were killed. 



Low-temperature pasteurization* The various limitations to 

 the application of heat for the purpose of killing bacteria per- 

 mit the use of 140F. (60C.) for twenty minutes. All the 

 physical, chemical and bacteriological considerations involved 

 favor the selection of this combination of time and temper- 

 ature, for it presents the widest margin of safety from violation 

 of the limiting conditions. 



The earliest machines for accomplishing low temperature 

 pasteurization embodied the idea of heating a definite quantity 

 of milk, -with proper agitation, to the required temperature, 

 after which the milk was rapidly chilled. The principle of a 

 reservoir, alternately filled and emptied, is common to many. 

 In some, heating and chilling the milk are accomplished in the 

 same chamber. The process of intermittent pasteurization in 

 the reservoir type of machine necessitates considerable expense 

 for labor and is slow. These considerations have largely pre- 

 vented the general employment of the type of machine in ques- 

 tion. There are no insurmountable difficulties in the way of 

 the development of machines for accomplishing low temper- 

 ature pasteurization at a rate of speed and low cost appropriate 

 to the demands of milk distributers. There are now on the 

 market machines designed to hold the milk at a given temper- 

 ature for twenty or thirty minutes, and at the same time 

 permit continuous operation. The milk is first heated and 

 then passed through a ' ' holding device ' ' by which the desired 

 temperature is maintained for the proper time. 



The pasteurizing process may be carried out in the house- 



