ndled per hour. It is evident under these conditions that the milk
must be heated for only a short time, and hence a higher temperature
must be employed. These machines are called "continuous flow"
pasteurizers since the milk passes through them in a constant
stream. The period of exposure is very short, in some only a few
seconds; hence, they are sometimes called "flash" pasteurizers.

[Illustration: Fig. 29.--A Continuous Pasteurizer.

The milk is exposed but a short time since it flows through the
heater in a constant stream.]

All machines of this type possess the obvious disadvantage that it
is impossible to heat all of the milk for a uniform period. The milk
in contact with the walls of the machine flows much more slowly than
in the middle of the stream, just as the current near the bank is
less rapid than in mid-stream. In none of the machines yet devised
have the designers been able to overcome this disadvantage. In a
test of one of the most widely used pasteurizers of this type, it
was found that some of the milk passed through the machine in 15
seconds, while the larger part of it was held for about 30 seconds,
and some as long as forty-five to sixty seconds. If the temperature
employed had been such as to destroy the bacteria in that part of
the milk heated for the minimum time, hygienic safety would be
assured, but in order to avoid injuring the physical properties of
the milk, the tendency is to use as low a temperature as possible,
so that the milk heated for the minimum time may often contain
organisms that have passed through the machine uninjured.

Many devices have been proposed for the heating and cooling of the
milk. In many of the pasteurizers, the milk flows in a thin stream
over a metal surface, on the opposite side of which is the heating
agent, usually steam; while in others, the milk is allowed to flow
through a vat in which revolve a series of discs into which steam is
passed. The discs are of considerable size; thus, making a large
heating surface; the milk is thus heated quickly, and is constantly
stirred by the rotation of the heating discs. In other types the
milk passes into the bottom of a chamber in which a dasher revolves
at a rapid rate. This catches the milk, throwing it in a thin film
onto the wall of the chamber, which is heated with steam on the
opposite side. From such machines, of which the Fjord, the Jensen,
and the Reid machines are types, the milk may be forced to a
cons