Most of the other
bacteria in milk perish when lactic acid is formed. This is why stale
sweet milk is often harmful, when the same kind of milk allowed to sour
can be taken with impunity.
If the milk is kept in a cold place the bacteria multiply slowly. If it
is kept in a warm place they increase in numbers at a rate that is
marvelous, and consequently the milk sours much sooner. Even if the milk
is kept cold, bacterial growth will soon take place, but it will perhaps
not be lactic acid bacteria. It may be a form that causes the milk to
become ropy and slimy or one that gives it a bad odor.
Bacteria are like other forms of vegetation, such as grass, weeds,
flowers and trees, in that some flourish best under one condition and
others under dissimilar conditions, and they struggle one against the
other for subsistence and existence. Like flowers there are thousands of
different forms of bacteria and they vary according to their food and
environment.
Peculiar odors in milk generally come from certain kinds of food given
to the cows, such as turnips; from bacterial action; or from flavors
absorbed from other foods or from odors in the air.
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