194 BACTERIOLOGY. 
be used as an article of food, especially for infants. 
Milk as it reaches the city contains enormous num- 
bers of germs, and these will produce fermentation, 
even though the milk is kept on ice. Unclean vessels 
hasten this process. No matter how good milk may 
be in the morning, when comparatively fresh, toward 
evening, unless it has been partly or completely steril- 
ized, it may be dangerous to an infant, and may, espe- 
cially in summer, cause fatal illness, even though it 
still tastes sweet. 
Complete sterilization destroys all the germs in milk, 
and so prevents permanently fermentative changes. By 
partial sterilization most of the germs which are not in 
the spore form may be destroyed, so that the milk will 
remain wholesome for at least twenty-four hours in the 
warmest weather. 
Milk is best sterilized by steam, for nearly all chem- 
icals, such as boric acid, salicylic acid, and formalin, 
make the milk less digestible, and, as a rule, unfit for 
food. It may be sterilized at a high or low tempera- 
ture—that is, at the boiling temperature (212° F.)—or 
at a lower degree of heat, obtained by modifying the 
steaming process. 
It has been found that milk sterilized at a high tem- 
perature (212° F.) is not desirable for prolonged use, as 
the high temperature causes certain changes in the milk, 
which make it less suitable as a food for infants. These 
changes are almost altogether avoided if a temperature 
below 80° C. is used. It is recommended, therefore, 
that the lowest temperature be used for partial sterili- 
zation, which will keep the milk wholesome for twenty- 
four hours in the warmest weather and kill the tubercle, 
typhoid, and other non-spore-bearing bacilli. Raising 
