e same has become heated to the proper temperature, change the
inflow to full-cream milk, continuing at the same rate. Note the exact
time of change and also when first evidence of milkiness begins to
appear at outflow. If samples are taken from first appearance of milky
condition and thereafter at different intervals for several minutes, it
is possible, by determining the amount of butter-fat in the same, to
calculate with exactness how long it takes for the milk to entirely
replace the water.

Tests made by the writer[143] on the Miller pasteurizer showed, when fed
at the rate of 1,700 pounds per hour, the minimum period of exposure to
be 15 seconds, and the maximum about 60-70 seconds, while about
two-thirds of the milk passed the machine in 40-50 seconds. This
manifest variation in the rate of flow of the milk through the machine
is undoubtedly the reason why the results of this type of treatment are
subject to so much variation. Naturally, even a fatal temperature to
bacterial life can be reduced to a point where actual destruction of
even vegetating cells does not occur.

~Bacterial efficiency of reservoir pasteurizers.~ The bacterial content of
pasteurized milk and cream will depend somewhat on the number of
organisms originally present in the same. Naturally, if mixed milk
brought to a creamery is pasteurized, the number of organisms remaining
after treatment would be greater than if the raw material was fresh and
produced on a single farm.

An examination of milk and cream pasteurized on a commercial scale in
the Russell vat at the Wisconsin Dairy school showed that over 99.8 per
cent of the bacterial life in raw milk or cream was destroyed by the
heat employed, i. e., 155 deg. F. for twenty minutes duration.[144] In
nearly one-half of the samples of milk, the germ content in the
pasteurized sample fell below 1,000 bacteria per cc., and the average of
twenty-five samples contained 6,140 bacteria per cc. In cream the germ
content was higher, averaging about 25,000 bacteria per cc. This milk
was taken from the general creamery supply, which was high in organisms,
containing on an average 3,675,000 bacteria per cc. De Schweinitz[145]
has reported the germ content of a supply furnished in Washington which
was treated at 158 deg. to 160 deg. F. for fifteen minutes. This supply
came from a single source. Figures reported were from 48-hour-old agar
plates. Undoubtedly these would have been higher if a longer period