20 BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE.. 
heat units which can be removed from the milk by an air blast at 
room temperature the cheaper the cost of cooling, since refrigeration 
would be saved and about the only cost would be the operation of a 
blower. 
These experiments, although by no means conclusive as to the value 
of this method of cooling by an air blast on a practical scale, since 
many complications may arise in the practical application, indicate 
great possibilities for such a system. 
THE EFFECT OF QUICK AND SLOW COOLING ON THE BACTERIAL FLORA OF THE 
MILK. 
It is believed that any system of pasteurization in which the milk 
is not cooled immediately after heating will be looked upon with 
suspicion and will excite comment. It has always been supposed 
that immediate cooling was an indispensable part of the process of 
pasteurization, first, because sudden changes in temperature were 
believed to have a destructive effect on the bacterial cells, and second, 
because it has been supposed that bacteria left after pasteurization 
would immediately begin to grow unless the milk was cooled at once. 
As stated earlier in this bulletin, it was shown in Bulletin 161 (3) 
that sudden cooling played no part in the destruction of bacteria. 
There remains, therefore, one question to be answered, How quickly 
must pasteurized milk be cooled in order to check bacterial growth ? 
From the writers' former studies of pasteurization it seemed 
apparent that the bacteria which survived heating were somewhat 
weakened or at least did not begin to grow as might theoretically 
be expected. These observations naturally gave rise to the idea that 
pasteurized milk might be cooled directly in bottles by a cold air 
blast, provided the cooling period did not extend over a few hours. 
In order to obtain data on this question 10 samples of milk were 
pasteurized and bottled hot in steamed bottles. Two bottles for each 
sample were cooled as follows : One bottle was cooled within half an 
hour in ice water and placed in a refrigerator at 45° F. for 17 J 
hours; the other bottle was cooled slowly at room temperature for 
4 hours and placed in a refrigerator at 45° F. for 14 hours. At 
the end of that time each bottle of milk was 18 hours old; one was 
cooled quickly and had been at 45° for 17J hours; the other had 
been cooled slowly and had been at 45° for probably a very short 
time, because, although it had been in the refrigerator for 14 hours, 
the milk was warm when placed there, and cooling in still air is a 
slow process. Both bottles after the 18-hour cooling period were 
allowed to stand at temperatures of from 75° to 86° F. for a period 
of 6 hours. The bacterial results are shown in Table 6. 
