174 CONTROL OF THE MILK-SUPPLY. 



Such moderate temperatures certainly do increase the keeping 

 quality of the milk. While a temperature of 156 F. does not 

 destroy spores, it does very largely destroy the active, non-spore- 

 bearing bacteria. Now the lactic acid bacteria, which are the cause 

 of the souring of milk, produce no spores, and consequently they are 

 largely killed by such moderate heat. Hence the total number of 

 bacteria in milk is immensely reduced, and the milk has its keeping 

 quality much increased. Milk thus treated will frequently remain 

 good two days longer than similar milk not pasteurized. 



Will such temperatures destroy disease bacteria? Of the 

 diseases mentioned above as liable to distribution by means of 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 the low heat (140 F. for one-half hour); the same 

 is probably true of scarlet fever. The tuberculosis bacillus, how- 

 ever, will withstand higher heat without injury, and hence, in order 

 to be sure of destroying these organisms, it has been thought neces- 

 sary to heat the milk to temperature of 185 F. At this temperature 

 the cooked taste and the chemical changes begin to appear. The 

 piesent conclusion, the result of the most recent and careful experi- 

 menting, is happily a satisfactory one. If milk is heated in such 

 a manner as to avoid the formation of a scum on its surface, at a 

 temperature no higher than 140 F., but continued for half 

 an hour, the virulence of the tubercle bacillus will be so much 

 reduced that milk containing these bacilli will be rendered harmless. 

 This temperature is considerably below that at which the chem- 

 ical changes in -the milk take place. Milk may thus be deprived 

 of its danger of distributing disease germs without having its 

 physical or chemical nature noticeably changed. Such milk, 

 when cooled, cannot be distinguished from fresh milk. 



The value of pasteurization is becoming rapidly recognized, and 

 this method of treatment is being widely adopted. The advantages 

 lie in the following facts: 



1. It produces milk which cannot be distinguished from fresh 

 milk and will be used as freely. 



2. It increases the keeping propeity of the milk, but not to the 



