22 



BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE. 



temperature dropped from 145° to about 100° F. The milk was then 

 allowed to stand at a temperature of from 100° to 80° F. for five 

 hours, after which it was placed in a refrigerator at 45° F., where it 

 cooled slowly in still air. The remaining 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. through the entire experiment. The 

 results of these experiments are given in Table 7. 



Table 7. — Effect of different methods of cooling on the bacterial content of 



pasteurized milk. 



Method of cooling. 



Sample No. 



1 



2 



3 





Bacteria per c.c. 

 9,050,000 



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 



Bacteria per c.c. 



Bacteria per c.c. 

 11,900,000 



8,500 



Bottle No. 1, cooled quickly:] 



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 



Held at 45° F. for 22 hours 



2S, 400 



Held at 75° F. for 6 hours 



76,500 



Held at 75° F. for 24 hours 



Bottle No. 2, cooled slowly: 



11,900 





29,000 



Held at 45° F. fcr 17 hours 



192,000 



Held at 75° F. for 6 hours 



348, 000 







Bottle No. 3, cooled at room temperature: 



8,500 





25,000 



Held at 75° F. for 22 hours .. 



83,400,000 



Held at 75° F. for 28 hours 



269,000.000 





460,800,000 











A study of the table shows that there was no increased bacterial 

 growth with samples 1 and 2 caused by holding the pasteurized milk 

 for five hours after bottling hot, even though the temperature dur- 

 ing that period ranged from 100° to 80° F., which is the most favor- 

 able temperature for bacterial development. With sample 3 there 

 was an increased growth over that in the milk cooled quickly. It 

 must be remembered that these experiments represent extreme con- 

 ditions in slow cooling, but the fact is apparent that the cooling 

 process should not extend over five hours. The effect of not cooling 

 milk to low temperatures is plainly shown in the table by a compari- 

 son of the bacterial counts with those of milk cooled both quickly 

 and slowly. It is believed from these- experiments that it is possible 

 to cool hot bottled pasteurized milk by a blast at room temperature 

 followed by a blast of cold air without any more bacterial develop- 

 ment than would take place if the milk were immediately cooled, 

 provided the milk is cooled to 50° F. gradually within five hours. 

 This is not made as a definite statement, because different results 

 may, of course, be obtained when milk is thus cooled on a commercial 

 scale. 



Again let the fact be emphasized that pasteurized milk or raw milk 

 must be kept at low temperatures after cooling in order to check bac- 

 terial development. 



