the stove. The
pail should be filled with water so that its level is about the same
as that of the milk. The water is then heated to the desired
temperature, maintained for the requisite period of time, and is
then cooled as rapidly as possible. During the heating, the mouth of
the bottle should be covered, either with an inverted glass tumbler,
or the paper cap may be left in place, simply punching a small hole
through it so as to permit of the insertion of a thermometer.

[Illustration: Fig. 30.--A Pasteurizer for Use in the Home.

A milk bottle with a tumbler for a cover. The cover prevents the
formation of the "scalded layer" on the milk during the heating and
also protects the mouth of the bottle from dust.]

=Efficiency of pasteurizing.= It is easy to destroy over 99 per cent
of the bacteria present by the use of any of the modern types of
machines. The number remaining after treatment will be largely
dependent, other things being equal, upon the number of bacteria
before pasteurization. The pasteurizing process is not one by which
poor milk can be changed into good milk, nor is it legitimate to use
the process in place of cleanliness, as is sometimes done. There is
a legitimate field for the process in the handling of market milk,
as well as in the creamery; but it should be used to improve the
keeping quality, and to insure the freedom of the milk from
pathogenic bacteria, when other protective measures have been
carried as far as possible under the prevailing conditions.

=Details of process.= If the process is to be successful, due
attention must be given to certain details. In the treatment of
market milk, care should be taken to use only that in which the
acidity has not materially increased. A fair standard is about 0.2
per cent. High acid milk usually means old milk or dirty milk,
either of which is very likely to contain many more spore-bearing
bacteria than clean, fresh milk. The greater the number of spores,
the more rapidly will the pasteurized milk spoil. If it is possible
to exercise any selection of milk prior to pasteurization, the rapid
test for determination of acidity will prove of great advantage.

Care should be taken to prevent fluctuations in the temperature to
which the milk is heated. With varying steam pressure and variations
in the rate of flow of milk, these fluctuations may be very
considerable. Regulators are now made that will control the
temperature within narrow limits.

I