TRANSPORTATION PROBLEMS 187 
milk at all, while higher temperatures-will bring them about. For 
this reason the lower temperatures are better. But will these 
moderate temperatures accomplish the desired ends? 
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, however, will withstand higher heat without injury, and 
hence, in order to be sure of destroying these organisms, it has 
been thought necessary to heat the milk to temperature of 185°F. 
At this temperature the cooked taste and the chemical changes 
begin to appear. The present conclusion, the result of the most 
recent and careful experimenting, 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 chemical 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. 
