154 PRACTICAL DAIRY BACTERIOLOGY 



The efficiency of pasteurization differs much with the method 

 adopted. When well done it may reduce the bacteria very 

 greatly, for example, from 3,000,000 per c.c. to 10,000 per c.c., 

 or even less. If less efficient the number at the close of the 

 process would be perhaps some hundreds of thousands. The 

 efficiency depends upon several factors. Dirty milk jars, or 

 jars with dirty rubber rings, produce trouble, and other careless 

 methods of handling decrease the value of pasteurization. 1 The 

 most considerable factor affecting it, however, is the temper- 

 ature, together with the length of exposure of the milk to the 

 temperature. In general it is best to . use lower temperatures 

 for a longer time than higher ones for a shorter period, and it 

 is doubtless best to pasteurize at a temperature no higher than 

 140, if the process is maintained for a long enough period, 

 e.g.., two hours ; for this destroys all disease germs and produces 

 the least change in the milk. 2 It is also important to do the 

 heating in such a way as to avoid the formation of a scum on 

 the surface of the milk, for such a scum makes the process less 

 efficient in destroying disease bacteria. This can be done by 

 some device to produce a constant agitation. 



Pasteurization is sometimes adopted .in a household on a small 

 scale and sometimes in a central station for large quantities of 

 milk. When adopted in the household it can be done by a very 

 simple, easily devised apparatus. Figure 56 shows an apparatus 

 which makes it possible to pasteurize the milk without the 

 necessity of using a thermometer. The milk is placed in bottles, 

 which are placed in a receptacle that has been partly filled with 

 boiling water. The whole apparatus is then set aside to cool. 

 The milk lowers the temperature of the water, and the hot 

 water raises the temperature of the milk until they each reach 

 the same temperature, after which the whole apparatus cools 

 down. By proper adjustment of the size of the bottles and the 



1 Swellengrebel. Cent. f. Bact., xii., p. 440, 1904. 



2 Jensen and Plattner. Rev. Gen. d'Lait, iv., p. 419, 1905. 



