28 
BULLETIN 420, U. S. DEPARTMENT OF AGRICULTURE. 
We do not wish to convey the idea that pasteurized milk need not 
be cooled at all. The cooling of any milk is absolutely essential in 
order to restrain bacterial growth, and we wish to emphasize the fact 
that the process of cooling pasteurized milk slowly does not eliminate 
the cooling process, but simply makes use of a slower cooling process 
than is in use at present. 
In order to show, respectively, the effect of cooling quickly, cooling 
slowly, and not cooling to low temperatures at all, three experiments 
were made. Milk was pasteurized in bulk, and three steamed and hot 
quart bottles were filled with hot milk. One bottle was cooled in ice 
water in half an hour to 50° F. (10° C.) and refrigerated at 45° F. 
(7.2° C). Another bottle was cooled in a blast of air at room 'tem- 
perature for half hour, during which time the temperature dropped 
from 145° F. (62.8° C.) to about 100° F. (37.8° C). The milk was 
then allowed to stand at a temperature of from 100° to 80° F. (37.8° 
to 26.7° C.) for five hours, after which it was placed in a refrigerator 
at 45° F. (7.2° C), where it cooled slowly in still air. The other bottle 
was cooled for half an hour in an air blast at room temperature and 
allowed to remain at a temperature of about 75° F. (23.9° C.) 
throughout the experiment. The results of these experiments, in 
which bacterial counts were made at different stages of the cooling 
process, are given in Table III. 
Table III. — Effect of different methods of cooling on the bacterial content of pasteurized 
milk {bacteria in 1 c. c. of milk.) 
Sample No. 
1 
2 
3 
9, 050, 000 
11,900,000 
Cooled quickly: 
6,450 
5,050 
4,800 
1,370,000 
7,150 
6,100 
6,200 
9,600 
2, 760, 000 
4,950 
6,850 
700,000 
2, 750, 000 
460,800,000 
2,110 
1,720 
2,340 
885,000 
2,580 
1,600 
2,400 
2,740 
850,000 
2,180 
2,890 
2,420,000 
13, 400, 000 
8,500 
Held at 45° F. for 22 hours 
28,400 
Held at 75° F. for 6 hours 
76,500 
Cooled slowly: 
11,900 
Held at 80° to 100° F. for 5 hours 
29,000 
192, 000 
Held at 75° F. for 6 hours 
348,000 
Allowed to cool naturally in air 75° F.: 
8,500 
25,000 
Held at 75° F. for 22 hours 
83, 400, 000 
Held at 75° F. for 28 hours 
269,000,000 
A study of the table shows that there was no increased bacterial 
growth in experiments 1 and 2 caused by holding the pasteurized 
milk for five hours after bottling hot, even though the temperature 
during that period ranged from 100° to 80° F. (37.8° to 26.7° C), 
which is the most favorable temperature for bacterial development. 
In experiment 3 there was an increased growth compared with that 
