necessarily true.

[Illustration: Fig. 40.--Good Milk.

A plate culture inoculated with 1/100 of a cubic centimeter of milk
containing 67 colonies, which equals 6,700 bacteria per cubic
centimeter of milk. Such milk will keep well.]

2. _Acidity test._ The acidity of the milk is also an indication of
its bacterial content. If the acidity has increased, above the
normal for fresh milk, the bacterial content is certain to be high,
and the keeping quality poor. An acidity above 0.2 per cent in
market milk is to be avoided, as an increase in acidity is always
preceded by a great increase of bacteria.

Whether the acidity is above or below this point can be rapidly and
easily determined at the receiving station by a modification of
the Farrington acid test. Dissolve one alkaline tablet in an ounce
of water. A unit volume of this solution added to a unit volume of
milk is equal to 0.1 per cent of acidity. If two measures are
provided,--one for the alkaline solution holding just twice as much
as that used for the milk, the approximate acidity can be quickly
determined by mixing a measure of each in a common white cup. If the
acidity is above 0.2 per cent the color will remain white; if a pink
color develops, it indicates an acidity less than this amount.
This test is also useful in the selection of milk or cream that is
to be used for special purposes, such as pasteurization.

[Illustration: Fig. 41.--Poor Milk.

A plate culture inoculated with 1/1000 of a cubic centimeter of
market milk containing 1,680 colonies, which equals 1,680,000
bacteria per cubic centimeter. Such milk has poor keeping
qualities.]

3. _Alcohol test._ A test giving similar information is made by
adding two parts of 70 per cent alcohol to one part of milk, and
noting whether curdling occurs.

4. _Curd test._ The curd test described on p. 100 gives no
indication of the number of bacteria present, only concerning the
types present. It has been proposed to combine the fermentation test
with the reduction test referred to below and thus gain some idea
of, not only the number, but the kinds of bacteria present.

5. _Reduction test._ The reduction test is made by adding to twenty
cubic centimeters of milk, one-half cubic centimeter of a solution
of methylene blue, a coal tar dye. A saturated solution of the dye
is made in alcohol, and 2.5 per cent of this solution added to
water. The time required for the reduction of the dye or the change
of the