MICROBIOLOGY OF MILK AND MILK PRODUCTS 



refrigerator temperatures. Streptococci were found in fifty-five (80 per 

 cent) of the sixty-eight samples examined. It was found that at refrig- 

 erator temperatures the relative growth of these organisms was greater 

 than at higher temperatures, a fact which may account, in part at least, 

 for the frequency with which these organisms occur in ice cream. 



Frequently ice cream is held for a considerable time in a frozen con- 

 dition before it is sold. It has generally been supposed that there is no 

 bacterial growth in material which is held below the freezing tempera- 

 ture. This, however, did not seem to be the case in samples examined 

 by the investigators already mentioned. They found in samples held 

 about a month that there was normally a decrease in the bacterial count 

 and also in the amount of gas production for a number of days, after 

 which there was frequently a marked increase in the bacterial counts. 

 These results would seem to indicate that even in the frozen condition 

 there may be some increase in the number of bacteria present. Ellen- 

 berger* found the same general conditions as these earlier investigators 

 as shown by the following chart. 



Per cent of 



bacteria 



150 



80 

 70 



60 



ad 



40 



30 



Days in 

 storage 



\ 



10 



20 30 



40 



50 



60 70 



80 



90 



100 



120 



FIG. 151. Average percentage increase and decrease of bacteria in fourteen sam- 

 ples of ice cream during a storage period of i2oMays. 



* Cornell Memoir No. 18. 



