116 Dairy Bacteriology. 



they make the milk appear cleaner and better than it 

 really is, the harm having been done when the dirt with 

 the adherent bacteria found its way into the milk. The 

 removal of the foreign matter that has been introduced 

 into the milk will have but little effect in reducing the 

 number of bacteria, since a large part of the organisms 

 will have been washed off the insoluble material. All of 

 these processes improve the appearance of the milk but 

 have little or no influence in increasing its keeping qual- 

 ity or its healthfulness. 



Preservation by cold. The only legitimate way of 

 preventing the growth of bacteria in milk is by holding 

 it at temperatures at which the ordinary forms of bac- 

 teria cannot thrive. Bacterial growth is greatly checked 

 at temperatures approximating 50 F., or below, al- 

 though certain types multiply at the freezing point or 

 slightly above. If food products are actually congealed, 

 no germ growth occurs, and they may be kept -quite in- 

 definitely, but this process cannot be successfully applied 

 to milk, as the fat and casein are physically changed, so 

 that a normal emulsion can not again be made when the 

 frozen milk is melted. The fat separates in visible 

 masses as though the milk had been partially churned. 

 On account of this fact milk must be stored at tempera- 

 tures above the freezing point. In Denmark efforts have 

 been made to preserve milk, that is to be shipped long 

 distances, by freezing a portion of the milk, and placing 

 a block of the frozen milk in each can after cooling the 

 main mass of mi| nearly to the freezing point. Even 

 this method has not proven practical, and at present 

 reliance is placed on thorough chilling of the milk. At 

 32 F., the lactic bacteria cannot grow, but other types, 



