STATUS OP PASTEURIZATION OP MILK 
11 
cially for infant feeding, while others 
desire it. It has been the experience 
of numerous milk dealers that the 
labeling of their product has greatly 
increased their trade. 
COST OF PASTEURIZING MILK 
The cost of Pasteurization in 1922 
was estimated by Bowen from the 
cost given in his earlier paper (10) 
on the assumption that the average 
price of coal had increased 2.04 times 
and that milk-plant labor and equip- 
ment had increased 50 per cent over 
the prices of 1913, the year in which 
his paper was written. He obtained 
the information from a series of tests 
in five establishments which were con- 
sidered to represent the average city 
milk plant. The Pasteurizing equip- 
ment in each consisted of a heater, a 
holding tank, a regenerator, and a 
cooler. The cost of operation was 
based on the Pasteurizing cycle, start- 
ing with the initial temperature of the 
raw milk and raising it to the Pasteur- 
izing temperature, then cooling to the 
initial temperature of the raw milk. 
He based the costs on daily interest 
at 6 per cent per annum on capital 
invested in Pasteurizing equipment, 
and depreciation and repairs per day 
at 25 per cent per annum ; interest per 
day at 6 per cent per annum on capi- 
tal invested in mechanical equipment 
for Pasteurizing, such as engines, boil- 
ers, etc., and depreciation and repairs 
per day at 10 per cent per annum. 
Other costs figured were labor, coal 
estimated at $8.16 a ton, cooling water 
estimated at $0.75 per 1,000 cubic feet, 
and refrigeration estimated at $2 a 
ton. With these later estimates sub- 
stituted for the old figures, Bowen 
calculated that the average cost of 
Pasteurizing 1 gallon of milk was ap- 
proximately $0.0049, or a little less 
than one-half cent. 
BACTERIA WHICH SURTIYE PAS- 
TEURIZATION 
It has been stated that about 99 
per cent of the bacteria in milk are 
destroyed by Pasteurization ; conse- 
quently about 1 per cent of the bac- 
teria remain alive, and the kinds left 
depend entirely on the temperature to 
which the milk is heated and the 
number of heat-resistant bacteria in 
the milk. From studies of the bac- 
teria which survive Pasteurization, it 
is possible to show graphically the 
hypothetical relations of the bacterial 
groups in raw milk and in milk 
Pasteurized by the holder process at 
various temperatures under laboratory 
conditions. 
The bacterial flora of the various 
kinds of milk is represented in Figure 
1 by columns of equal length divided 
into sections, which, in a general way, 
show the relative proportion of the 
bacterial groups. 
From the figure it may be seen that 
raw milk contains four principal 
groups of bacteria — the acid, inert, al- 
kali, and peptonizing. The acid group 
is divided into two types — the acid- 
coagulating, which produces sufficient 
acid to curdle the milk within 14 days, 
and a type which produces acid more 
slowly and does not curdle the milk in 
14 days. In raw milk the inert group 
is the largest. 
In milk Pasteurized at 145° F. the 
great increase in the proportion of the 
acid-coagulating and acid groups is 
plainly shown. The per cent of the 
alkali and peptonizing groups is re- 
duced. At 160° F. the total-acid group 
is still the largest, but the acid-coagu- 
lating group is made up of bacteria 
which coagulate very slowly. At this 
temperature the alkali group is greatly 
reduced, and the peptonizing reduced 
to the minimum. At 170° F. the total- 
acid group remains about the same, 
but the organisms produce acid and 
coagulate the milk very slowly. The 
alkali group is practically destroyed, 
although occasionally a sample may 
show a fairly high per cent. The most 
important change is in the peptonizing 
group. At this temperature the ratio 
of this group to the total number of 
bacteria begins to increase. The in- 
crease when milk is Pasteurized at 
180° F. is even more striking. At this 
temperature more than 75 per cent of 
the bacteria which survive are pep- 
tonizers. No organisms of the acid- 
coagulating group are found, and only 
a small per cent of the acid group. 
Occasionally a few of the alkali group 
may be found. At 190° F and 200° F. 
the bacterial groups which survive are 
about the same in their relative sizes 
as at 180° F. 
It is very evident that when the 
bacterial flora of Pasteurized milk is 
under discussion the temperature of 
the process is of fundamental im- 
portance. In Figure 1 the bacteria 
groups left in milk Pasteurized at dif- 
ferent temperatures may be seen at 
a glance. It must be remembered, 
however, that the relations of the bac- 
terial groups represent only average 
conditions and that the bacterial flora 
of every sample of milk must not be 
