heat.
Most of them are destroyed when the milk is heated to 140 deg. F. for
fifteen minutes or to 160 deg. F. for a moment. To insure proper keeping
quality, somewhat higher temperatures must be employed, such as 145 deg.
to 150 deg. F. for fifteen to twenty minutes.

Milk pasteurized at these temperatures will, as a rule, undergo an
acid fermentation in much the same manner as will raw milk. The rate
with which the acid develops is of course much slower than in the
raw milk, due to the destruction of 95 to 99 per cent of the
acid-forming bacteria. If the milk has been pasteurized at higher
temperatures, the acid fermentation may not appear. The spores of
the spore-bearing organisms will be left; these may germinate and
cause their characteristic change in milk, which, as previously
noted, is usually a sweet-curdling or a digesting fermentation.
Since the changes they produce in the milk are not evident at first,
it might be used as food even though it was so far advanced in
decomposition as to be undesirable or even harmful as food. Indeed
one of the objections urged against pasteurization is that it
destroys the natural safe guard, the acid-forming bacteria. Many
people are so accustomed to use this as the indication of spoiled
milk that they will use milk long after it should be used if it does
not show an acid fermentation.

The butyric acid organisms are spore forming and may at times
produce their characteristic fermentation in pasteurized milk. The
milk shows gas formation and develops an objectionable odor.

The pathogenic bacteria most likely to be present in the milk are
the typhoid and the tubercle organisms. The typhoid bacillus is no
more resistant to heat than the ordinary acid-forming bacteria, and
all milk that has been heated, so as to impart to it satisfactory
keeping properties, will certainly be free from typhoid bacilli. It
has sometimes been asserted that the tubercle bacillus is very
resistant to heat; some claiming that it is necessary to heat milk
to 200 deg. F. in order to destroy it. Other experimenters have asserted
that lower temperatures would suffice, but the temperatures were
still above those at which the milk is physically and chemically
changed by the heating process. More recent work has shown that not
all sources of error were avoided in the earlier attempts to
determine the thermal death point of the tubercle bacillus, as, for
example, it has been shown by the authors that the "