22O AGRICULTURAL BACTERIOLOGY. 



keeping quality much increased. Experiment has shown that 

 milk thus treated will frequently remain good two days longer 

 than similar milk not pasteurized. Thus one purpose of the 

 heating is certainly accomplished, for, although the milk will 

 not keep indefinitely, it will remain sweet so much longer as 

 to render it easy to handle. 



Whether such temperatures destroy disease bacteria so as 

 to remove all danger of distribution of disease has been the 

 subject of much experiment and dispute. Of the diseases 

 which we have mentioned above as liable to distribution by 

 milk there is only one in regard to which there has been any 

 disagreement. It is admitted on all sides that typhoid and 

 diphtheria bacteria are killed by this heat (i 56 F.) ; the same 

 is probably true of scarlet fever and certainly true of cholera. 

 The tuberculosis bacillus, however, it has been claimed, will 

 stand this heat without injury, and hence, in order to be sure 

 of destroying these organisms, it is necessary to heat the milk 

 to a temperature of 185 F. At this temperature the cooked 

 taste and chemical changes begin to appear. Over this ques- 

 tion of the temperature necessary to destroy the tubercle 

 bacillus there has been much experimentation and dispute. 

 Into the details of this matter we cannot enter. The present 

 conclusion, which is the result of the most recent and most 

 careful experimenting, is happily a satisfactory one. It ap- 

 pears that if the milk is heated in such a manner as to avoid 

 the formation of a scum on its surface, a comparatively low 

 temperature is sufficient to destroy the virulence of the tuber- 

 culosis bacillus. A temperature no higher than 140 F., con- 

 tinued for twenty minutes, has been found sufficient to reduce 

 the virulence of the tubercle bacillus so much that milk which 

 originally contained the bacilli is rendered harmless. This 

 temperature is considerably below that at which the chemical 

 changes in the milk take place. Hence it follows that milk 

 may be thus deprived of its danger of distributing disease 



