2l6 AGRICULTURAL BACTERIOLOGY. 



undergoing any other deleterious change. This appeals to the 

 producer, the distributer and the consumer. 



2. It is desired to destroy the pathogenic bacteria which 

 may be present so as to remove all danger to health in drink- 

 ing the milk. 



3. It is sometimes desired to remove the bacteria already 

 present so that others may be added to produce certain desired 

 effects, as in the ripening of cream (see page 231). 



There is no difficulty in accomplishing all of these purposes 

 by the use of heat, for a sufficiently high temperature will 

 destroy all bacteria and attain all the ends at once. The prac- 

 tical application of heat to milk, however, presents certain im- 

 portant problems not easily solved. 



Sterilization. The most obvious suggestion would be to 

 heat the milk to a temperature which would destroy all bac- 

 teria at once. This is perfectly possible, but, owing to the 

 fact that milk always contains spore-bearing bacteria, it re- 

 quires a high temperature for the purpose. A temperature of 

 boiling water will not destroy the spores, and for this purpose 

 it is necessary to heat the milk to several degrees above boil- 

 ing. This involves the use of special apparatus, in which 

 bottles of milk can be inclosed in special vessels, subjected to 

 steam under pressure, and subsequently hermetically sealed 

 while still within the closed vessels. Such apparatus can be 

 used only where there is a steam supply on hand and it in- 

 evitably makes the milk rather expensive. The milk so pre- 

 pared is, however, supposed to be germ-free and consequently, 

 to keep indefinitely. Unfortunately, it has been shown that 

 even these temperatures of superheated steam do not always 

 destroy all the spores, and some of the samples of milk thus 

 treated will subsequently undergo fermentative changes due to 

 the spores that are left. Further it has appeared that these 

 later changes, due to the resisting spores, are frequently such 

 as do not change the appearance of the milk to the eye, so 



