oyed at 140 deg. F. in 10-15 minutes, while an exposure at
160 deg. F. requires only about one minute.[140] If the conditions of
heating are such that the surface of the milk is exposed to the air, the
resistance of bacteria is greatly increased. When heated in open vessels
Smith found that the tubercle organism was not killed in some cases where
the exposure was made for at least an hour. Russell and Hastings[141]
have shown an instance where the thermal death-point of a micrococcus
isolated from pasteurized milk was increased 12.5 deg. F., by heating it
under conditions that permitted of the formation of the scalded layer.
It is therefore apparent that apparatus used for pasteurization should
be constructed so as to avoid this defect.

~Methods of treatment.~ Two different systems of pasteurization have grown
up in the treatment of milk. One of these has been developed from the
hygienic or sanitary aspect of the problem and is used more particularly
in the treatment of cream and relatively small milk supplies. The other
system has been developed primarily from the commercial point of view
where a large amount of milk must be treated in the minimum time. In the
first method the milk is heated for a longer period of time, about
fifteen minutes at a relatively low temperature from 140 deg.-155 deg.
F.; in the other, the milk is exposed to the source of heat only while
it is passing rapidly through the apparatus. Naturally, the exposure
under such conditions must be made at a considerably higher temperature,
usually in the neighborhood of 160 deg. F.

The types of apparatus used in these respective processes naturally
varies. Where the heating is prolonged, the apparatus employed is built
on the principle of a _tank_ or _reservoir_ in which a given volume of
milk may be held at any given temperature for any given period of time.

When the heat is applied for a much shorter period of time, the milk is
passed in a continuous stream through the machine. Naturally the
capacity of a continuous-flow apparatus is much greater than a machine
that operates on the intermittent principle; hence, for large supplies,
as in city distribution, this system has a great advantage. The question
as to relative efficiency is however one which should be given most
careful consideration.

~Pasteurizing apparatus.~ The problems to be solved in the pasteurization
of milk and cream designed for direct consumption are so materially
different fro