20 BULLETIN 240;, U. S. DEPARTMENT OF AGRICULTUEE. 



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 sj^stem 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 f oIIoays : One bottle was cooled within half an 

 hour in ice water and placed in a refrigerator at 45° F. for 174, 

 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 beeii at 45° for 17.1 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 G hours. The bacterial results are shown in Table 6. 



