e tests are most frequently used at the factory, to
enable the maker to detect the presence of milk that is likely to prove
unfit for use, especially in cheese making. They are based upon the
principle that if milk is held at a moderately high temperature, the
bacteria will develop rapidly. A number of different methods have been
devised for this purpose. In Walther's lacto-fermentator samples of milk
are simply allowed to stand in bottles or glass jars until they sour.
They are examined at intervals of several hours. If the curdled milk is
homogeneous and has a pure acid smell, the milk is regarded as all
right. If it floats in a turbid serum, is full of gas or ragged holes,
it is abnormal. As generally carried out, no attempt is made to have
these vessels sterile. Gerber's test is a similar test that has been
extensively employed in Switzerland. Sometimes a few drops of rennet are
added to the milk so as to curdle the same, and thus permit of the more
ready detection of the gas that is evolved.

~Wisconsin curd test.~ The method of testing milk described below was
devised at the Wisconsin Experiment Station in 1895 by Babcock, Russell
and Decker.[77] It was used first in connection with experimental work
on the influence of gas-generating bacteria in cheese making, but its
applicability to the detection of all taints in milk produced by
bacteria makes it a valuable test for abnormal fermentations in general.

In the curd test a small pat of curd is made in a glass jar from each
sample of milk. These tests may be made in any receptacle that has been
cleaned in boiling water, and to keep the temperature more nearly
uniform these jars should be immersed in warm water, as in a wash tub or
some other receptacle. When the milk is about 95 deg. F., about ten drops
of rennet extract are added to each sample and mixed thoroughly with the
milk. The jars should then remain undisturbed until the milk is
completely curdled; then the curd is cut into small pieces with a case
knife and stirred to expel the whey. The whey should be poured off at
frequent intervals until the curd mats. If the sample be kept at blood
heat (98 deg. F.) for six to eight hours, it will be ready to examine.

[Illustration: FIG. 18. Improved bottles for making curd test. _A_, test
bottle complete; _B_, bottle showing construction of cover; _S_, sieve
to hold back the curd when bottle is inverted; _C_, outer cover with _(D
H)_ drain holes to permit of remov