PASTEURIZATION 131 



vessels has no effect. Indeed, if bottles are used, this is 

 to be forbidden as injurious to health. Very often the 

 milk is drunk directly from the bottle, which may be 

 infected by this means and, in spite of washing, fresh 

 milk may be infected as it is poured out. In this connec- 

 tion, what is to be understood by sterilization ? The aim 

 is, so far as possible, to kill all microbes. The task is 

 not difficult with metal vessels because, after thorough 

 mechanical cleansing, these can be subjected to very hot 

 steam or can be scalded. With glass bottles, the task is 

 more difficult, because strong heat causes much break- 

 age, and if the price of the milk is not such as to cover 

 this loss, another method must be used. Thorough me- 

 chanical cleansing, scouring the outside as well as the 

 inside, followed by long soaking, in a strong, warm solu- 

 tion of washing soda at 70 C. (158 F.) and rinsing in 

 pure, lukewarm water, is believed to kill with reasonable 

 certainty all pathogenic forms and also the majority of 

 other milk bacteria. 



A very large number of machines for the quick pas- 

 teurization of milk have been built, chiefly for use in 

 creameries. The principle of these machines is dif- 

 ferent. One of the first pasteurizers was that made by 

 Fjord; it consists, as the diagrams (Figs. 5 and 6, pp. 

 132 and 133) show, of a tinned copper tank provided with 

 an insulated steam jacket. The tank contains rotating 

 arms by which the milk is thrown against the heated 

 walls. The milk is admitted at the bottom through pipe 

 H and leaves the apparatus at the top. 



By regulating the quantity of milk admitted, the 

 rapidity of the revolving arms and the amount of steam, 

 the milk may be heated as desired. Many other pasteur- 

 izing machines are built according to the same principle. 



In other pasteurizers, the milk is heated while it 

 flows in thin layers over heated metal surfaces, or while 



