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 
