378 MILK AND THE PUBLIC HEALTH CHAP. 



upon the diseases of wine and beer, and the temperature of 

 heating necessary to preserve them from abnormal fermentation. 

 Soxhlet in 1886 advocated the application of the same 

 principles to milk. The term, as practically applied to milk, 

 is taken to mean the heating of milk for a limited period, 

 to a temperature short of the boiling-point. The temperature 

 advocated has never been below 60 C. and has been as 

 high as 90 C., and the time for heating has varied from so 

 short a period as one to two minutes to as long as one hour. 



In view of this looseness of meaning it is well to define 

 exactly what is meant when the term " pasteurisation " is used. 



The best temperature and exposure time for pasteurisation 

 can only be arrived at when the exact objects of the process 

 are considered. Pasteurisation is, or should be, a makeshift 

 arrangement, to render milk which has been or which may 

 have been bacterially polluted, safe for consumption, while at 

 the same time not depriving it of any of its valuable properties. 

 Put scientifically it means that the temperature of the milk, 

 and the period during which it is maintained at that tempera- 

 ture, must be, on the one hand, sufficient to kill all harmful 

 organisms which may possibly be present, while, on the 

 other hand, the heating must not damage the milk as a food. 



Two types of pasteurisers are on the market. In the 

 one continuous -flow (or flasli) pasteurisers the rnilk is 

 heated to a certain temperature but only for a moment ; while 

 in the other retainer pasteurisers the milk is heated to a 

 lower temperature, but is kept at that temperature for a 

 definite and much longer period, for example 30 minutes. 



Of the two types of pasteurisers the " retainer " type would 

 seem to be the best from the hygienic point of view, while 

 the continuous-flow or flash pasteuriser is the more convenient 

 for trade purposes, since it allows a greater quantity of milk 

 to be treated in a given time than in the other kind. 



Ayers and Johnson from their work conclude : 



The " holder " process of pasteurisation is superior to the 

 " flash " process. With the " holder " process a high efficiency 

 may be obtained with a low temperature, while to obtain the same 

 efficiency with the " flash " process a high temperature would be 

 required. A temperature of 62*8 C. (145 F.) for thirty minutes 

 seems best adapted for pasteurisation. 



