Preservation of Milk. 125 



The typhoid bacillus is no more resistant to heat than 

 the ordinary acid-forming bacteria, and all milk that has 

 been heated, so as to impart to it satisfactory keeping 

 properties, will certainly be free from typhoid bacilli. 

 It has sometimes been asserted that the tubercle bacillus 

 is very resistant to heat ; some claiming that it is neces- 

 sary to heat milk to 200 F. in order to destroy it. 

 Other experimenters have asserted that lower tempera- 

 tures would suffice, but the temperatures were still above 

 those at which the milk is physically and chemically 

 changed by the heating process. More recent work has 

 shown that not all sources of error were avoided in the 

 earlier attempts to determine the thermal death point of 

 the tubercle bacillus, as, for example, it has been shown 

 by the authors that the "scalded film" that forms on 

 the surface of milk when heated in an open vessel will 

 protect the bacteria imbedded in it. It has also been 

 shown by the authors that a temperature of 140 F., for 

 twenty minutes or 160 F. for one minute will destroy 

 the tubercle bacilli in milk, in case the heating is done 

 with sufficient thoroughness to insure all particles of the 

 milk being heated to the same temperature for these 

 periods of time. 



The pasteurization of milk can be done in such a man- 

 ner as to impart to it good keeping qualities and to in- 

 sure its freedom from pathogenic bacteria, and yet not 

 impair its physical and chemical properties, but much 

 of the so-called pasteurized milk placed on the market is 

 not treated in accordance with proper hygienic methods. 



Methods of pasteurization. In order to destroy the 

 bacteria in milk, it is necessary that the milk be heated 

 for a varying time dependent upon the temperature em- 

 ployed. A lower temperature for a considerable period 



