26 BULLETIN 420, U. S. DEPARTMENT OF AGEICULTTJRE. 



room for the purpose of holding a low temperature in the room, the 

 milk being cooled by running over some form of cooler before it is 

 bottled and placed in the room. 



It seems probable, from our results on a small experimental scale, 

 that with outside air at a temperature of 40° F. (4.4° C.) or below, 

 the process of cooling hot bottled milk can be successfully accom- 

 phshed by forced circulation. No artificial refrigeration of the air is 

 then necessary. 



EFFECT OF SLOW COOLING ON THE BACTERIAL FLORA OF MILK AFTER 



PASTEURIZATION. 



It is beheved 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 (1) because sudden changes in temperature were 

 believed to have a destructive effect on the bacterial cells and (2) 

 because it has been supposed that bacteria left after pasteurization 

 would immediately begin to grow unless the milk were cooled at once. 

 It has been shown by Ayers and Johnson ^ that sudden cooling plays 

 no part in the destruction of bacteria. One question, therefore, 

 remains to be answered: How quickly must pasteurized milk be 

 cooled in order to check bacterial growth ? 



From 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 be expected. These observa- 

 tions natm-aUy gave rise to the idea that pasteurized milk might be 

 cooled directly in bottles by a forced-air circulation, provided the 

 cooling period did not extend over more than a few hours. 



In order to obtain data on this question 10 experiments were made, 

 in which milk was first pasteurized and bottled hot in steamed bottles. 

 In each experiment one of two bottles was then cooled to 45° F. 

 (7.2° C.) within haK an hour in ice water and held at that tempera- 

 ture in a refrigerator for 17| hours, the other bottle being cooled 

 slowly at room temperature for 4 hours and. placed in a refrigerator 

 at 45° F. (7.2° C.) for 14 hours. At the end of that time each bottle 

 of milk was 18 hours old; one had been cooled slowly and had been 

 at 45° F. (7.2° C.) 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 for 6 hours at 

 temperatures of from 75° to 85° F. (23.9° to 29.4° C). The bacte- 

 rial results are seen in Table I. 



1 Ayers, S. Henry, and Johnson, W. T., jr. A study of the haeteria which survive pasteurization. Bui. 

 161, Bureau of Animal Industry. 1913. 



