which may be
present, and the pasteurising temperature should therefore be in excess
of the thermal death-point of all such organisms.

Pasteurisation owes its origin to Pasteur, and has become an adopted
method throughout the dairy industry, and there are many mechanical
devices termed "pasteurisers" (see Fig. 8) which are used for the
carrying out of this particular operation. The form of one of these is
that of a vertical jacketed cylinder with paraboloidal surface, around
which steam is made to pass, so as to maintain the temperature at about
176 deg. F. Milk is allowed to flow in at the bottom of the paraboloidal
surface, and is caught by mechanical agitating arms, which revolve at a
given speed, and by this action milk is distributed centrifugally over
the paraboloidal surface, and is forced out by the same action, at the
top of the apparatus, after being heated.

[Illustration: PASTEURISER

FIG. 8.--The milk enters from the bottom and circulates to
the top of the inside cylinder, which is paraboloidal in
construction. It is heated as it passes through the
apparatus, and is discharged at the top at a temperature
of 176 deg. F.]

The centrifugal action is sufficient to raise the milk some three to
four feet, through a tube, and this is taken advantage of so as to cause
the milk to flow over a conical cooler, described as a primary cooler,
and in which water is made to circulate. As the hot milk descends over
the conical cooler it gives up most of its acquired heat to the water,
and, in practice, is reduced in temperature to within 4 deg. of the
temperature of the water.

Below this primary cooler is fixed a cooler of the same size and shape,
which is termed a secondary cooler. In it, brine at a temperature of
about 35 deg. F. is circulated from a refrigerating machine, and, as the
milk falls over the secondary cooler, it is cooled to a temperature of
about 40 deg. F., when it may be looked upon as being pasteurised and free
from all pathogenic organisms, in which state it will keep for a
considerable length of time.

It is desirable that the milk should, as soon as possible after the
cooling takes place, be delivered to the consumers, and be kept under
cool conditions, either in bottles or in a closed vessel covered over
with muslin, so as to keep out specks of germ-laden dust.

Briefly speaking, the foregoing is an outline of what is carried on in
the ordinary dairy practice.

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