28 



BULLETIN 420, U. S. DEPARTMENT OP AGRICULTURE. 



We do not wish to convey the idea that pasteurized mUk need not 

 be cooled at all. The cooling of any mUk 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 c.oohng 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° O." 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 coohng 

 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 



Held at 75° F. for 24 hours 





Cooled slowly: 



Directly after pasteurization. .* 



11,900 



Heldat 80° tolOO° F. for 5 hours 



29,000 



Held at 45° F. for 17 hours 



192, 000 



Held at 75° F. for 6 hours 



348,000 



Heldat 75° F. for 24 hours .. 





Allowed to cool naturally in air 75° F.: 



8,500 



Held at 75° F. for 5 hours 



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 



