scalded film"
that forms on the surface of milk when heated in an open vessel will
protect the bacteria imbedded in it. It has also been shown by the
authors that a temperature of 140 deg. F., for twenty minutes or 160
deg. F. for one minute will destroy the tubercle bacilli in milk, in
case the heating is done with sufficient thoroughness to insure all
particles of the milk being heated to the same temperature for these
periods of time.

The pasteurization of milk can be done in such a manner as to impart
to it good keeping qualities and to insure its freedom from
pathogenic bacteria, and yet not impair its physical and chemical
properties, but much of the so-called pasteurized milk placed on the
market is not treated in accordance with proper hygienic methods.

[Illustration: Fig. 28.--The Pott's Discontinuous Pasteurizer.

The milk is placed in the inner compartment. For heating and
cooling, hot or cold water is passed between the jackets.]

=Methods of pasteurization.= In order to destroy the bacteria in milk,
it is necessary that the milk be heated for a varying time dependent
upon the temperature employed. A lower temperature for a
considerable period may exert the same effect on the bacteria as a
higher temperature for a shorter time. In practice, two types of
pasteurizing machines are employed, depending on the temperature at
which the milk is to be treated. The discontinuous machines or
intermittently operated pasteurizers are those in which the milk is
heated for any desired time at any temperature. Such machines
consist of jacketed containers the inner receptacle being filled
with milk, while the outer space between the walls is filled with
circulating hot water or steam. The milk is kept agitated by the
rotation of the machine. After it is heated, it is cooled in the
same container by replacing the hot water first with cold water,
then ice water. The disadvantage of this process is that the
capacity of the machine is limited which precludes its use in places
where large quantities of milk or cream are handled; for the
pasteurization of limited quantities, it is very successful, as
every particle of milk or cream is under the direct control of
the operator and may be thoroughly and efficiently treated.

As pasteurization was introduced for the treatment of market milk,
and for the preparation of cream for butter, machines have been
devised which permit large quantities, as thousands of pounds, to be
ha