hods of determining acidity in milk, much can be told
concerning the number of bacteria in the milk, and hence concerning
its keeping quality. Milk that has an acidity of over 0.2 per cent
is certain to contain many bacteria, and consequently will keep
poorly. Such milk is of low value for market milk, but may not be
objectionable for butter or cheese making. If the acidity is below
0.2 per cent, but little can be told as to the numbers of bacteria,
since any increase in acid is always preceded by an enormous
increase in the numbers of acid-forming bacteria.

A more important test than the acid test, from the standpoint of the
butter and cheese maker, and even the milk dealer, is the
fermentation test. In its simplest form, it consists in placing a
sample of the milk to be tested in a warm place and noting the time
required to curdle and the type of curd formed. In this country the
fermentation test has been largely supplanted by the Wisconsin curd
test which possesses the advantage of detecting the presence of
bacteria harmful in cheese making, especially the gas forming
bacteria.

The curd test is helpful in detecting the source of an abnormal
condition in a milk supply coming from diverse sources. The milk
furnished by each patron can be tested separately and the trouble
located, perhaps in an individual herd; the offending herd
determined, the test may then be used on the milk of individual
cows. The cheese maker and the milk dealer should be able not only
to detect which of the patrons furnish him poor milk, but he should
be able to give the patron definite instructions how to avoid the
sources of such trouble. This information can be given only when
the source is positively known.

[Illustration: Fig. 23.--Curd Test.

A good curd obtained from milk containing no harmful bacteria but
many desirable acid-forming organisms.]

The Wisconsin curd test is made as follows: Samples of the milk to
be tested are placed in sterile pint fruit jars. The milk is warmed
to 90 deg. F., ten drops of rennet are added to each sample, and as soon
as the curd is solid, it is cut into small pieces with a case knife
so as to facilitate the expulsion of the whey. As the curd settles
to the bottom of the vessel, the whey is poured off at intervals so
that a pat of firm curd is left. As the milk curdles the bacteria
are enmeshed and are carried with the curd. The jars are kept at a
temperature of 100 deg. to 105 deg. F., since this