124 Dairy Bacteriology. 



be present in the milk. The lactic acid bacteria are non- 

 spore-bearing and are not resistant to heat, Most of 

 them are destroyed when the milk is heated to 140 F. 

 for fifteen minutes or to 160 F. for a moment. To in- 

 sure proper keeping quality, somewhat higher tempera- 

 tures must be employed, such as 145 to 150' F. for 

 fifteen to twenty minutes. 



Milk pasteurized at these temperatures will, as a rule, 

 undergo an acid fermentation in much the same manner 

 as will raw milk. The rate with which the acid develops 

 is of course much slower than in the raw milk, due to 

 the destruction of 95 to 99 per cent of the acid-forming 

 bacteria. If the milk has been pasteurized at higher 

 temperatures, the acid fermentation may not appear. 

 The spores of the spore-bearing organisms will be left; 

 these may germinate and cause their characteristic 

 change in milk, which, as previously noted, is usually a 

 sweet-curdling or a digesting fermentation. Since the 

 changes they produce in the milk are not evident at first, 

 it might be used as food even though it was so far ad- 

 vanced in decomposition as to be undesirable or even 

 harmful as food. Indeed one of the objections urged 

 against pasteurization is that it destroys the natural 

 safe guard, the acid-forming bacteria. Many people are 

 so accustomed to use this as the indication of spoiled 

 milk that they will use milk long after it should be used 

 if -it does not show an acid fermentation. 



The butyric acid organisms are spore forming and 

 may at times produce their characteristic fermentation 

 in pasteurized milk. The milk shows gas formation and 

 develops an objectionable odor. 



The pathogenic bacteria most likely to be present in 

 the milk are the typhoid and the tubercle organisms. 



