ed by determining the acidity of the whey as it comes from the
curd at different stages in the making. This test, which is often
used in place of the "hot iron" test is carried out in the same
manner, as in determining the acidity of milk or cream. The quality
of the cheese, both as to texture and flavor, is dependent to a
great degree upon the amount of acid that is formed during the
various stages in making; hence, the successful maker must follow
closely by some means the acid formation in the curd until it is put
to press.

It is very necessary that the milk shall contain a sufficient number
of acid-forming bacteria to produce the required amount of acid. If
a sufficient number of bacteria are not present in the milk as it is
received, as is the case with very sweet milk, they must be added
by the maker in the form of a starter, or the process of making will
be much prolonged.

[Illustration: Fig. 35.--Bacteria in Cheese.

A photomicrograph of curd just after curdling has taken place. Note
the few lactic acid bacteria embedded in the curd.]

=Starters in cheese making.= The starters used in cheese making, are
identical with those employed in butter making and the same
precautions should be observed in their propagation. It is important
that the starters should not be such as to form a hard curd that
cannot be mixed uniformly with the milk, since the curd particles
would appear as white specks in the cheese. The starter should be
added to the milk through a hair sieve, and well mixed with the
milk, so as to distribute the bacteria uniformly. Amounts varying
from 0.5 to 2 per cent are used. In butter making, it is essential
that the bacteria of the starter be able to form not only acid, but
sufficient flavor-forming substances to impart to the butter a
desirable flavor. In cheese making it is not probable that this
latter characteristic is of any particular importance.

[Illustration: Fig. 36.--Bacteria in Cheese.

A photomicrograph of curd at the time the salt is added. The lactic
acid bacteria have increased materially in numbers.]

It is desirable that the process of cheese making shall conform as
closely as possible to that which experience has shown to give the
best results. The rate at which acid is developed in the curd and
the rapidity with which the whey is expelled therefrom should bear a
certain ratio to each other. If the milk has too high a degree of
acidity, _i.e._, is overripe, the acidity deve