ound. At ordinary air temperatures, the
acid-forming bacteria grow more rapidly in milk than do any other
forms, and the acid produced by them renders the milk an unfavorable
medium for the growth of other bacteria. This is the reason why milk
practically always undergoes the acid fermentation, although it is
contaminated with a host of other kinds of bacteria. If a mixture of
seeds is sown on low wet ground, certain kinds will grow best; if
the same mixture is sown on drier land, other types will find most
favorable conditions for growth, and the plants which appeared on
the low land will not appear. The same condition is found in milk
where the environment is most favorable for the acid-forming
bacteria.

=Amount of acid formed in milk.= In this country the acidity of milk
is expressed as so many per cent of lactic acid. A milk that shows
an acidity of one per cent should, theoretically, contain one pound
of lactic acid in each one hundred pounds of milk. The acid
determined does not actually represent lactic acid, as there are
other substances in milk which act as acids, with the reagents used
in the present methods of determining the acidity of milk. For
instance, perfectly fresh milk has an apparent acidity of 0.13 to
0.18 per cent, although no fermentation has occurred. Other acids
than lactic are formed in the acid fermentation, but the entire acid
content is referred to as lactic when speaking of the acidity of
milk. When the developing acidity of milk reaches 0.25 to 0.3 per
cent, a sour taste becomes evident and the milk will curdle on
heating. When the acidity increases to 0.6 to 0.7 per cent, the milk
curdles at ordinary temperatures. The acidity continues, however, to
increase until it reaches about 1 per cent, which is the maximum
amount that will be produced in milk by the ordinary acid-forming
bacteria. Milk contains about 4 per cent of milk sugar, all of which
is fermentable. If this were all decomposed by bacteria, the acidity
of the milk would actually exceed 4 per cent. It is thus evident
that the reason why more acid is not formed in milk is not because
of any lack of sugar. The bacteria, like all other kinds of living
things, are injured by their own by-products, unless these are
constantly removed in some way; in milk the bacteria cannot escape
the action of the acid which they themselves have formed,
consequently growth ceases. The amount of acid formed is dependent
on the kind of bacteria pres