372 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



ICE CREAM. 



Ice cream is one of the important manufactured dairy products and 

 its use seems to be increasing steadily. Its bacterial flora varies with 

 the materials used in its manufacture and the conditions under which it 

 is made. It may be made from fresh cream which is only a few hours 

 old and under good sanitary conditions. On the other hand, it may be 

 made from cream which has been produced and handled under unsani- 

 tary conditions, kept in storage for a number of days and finally manu- 

 factured in surroundings not conducive to a low bacterial content. We 

 are not surprised, therefore, to find a very wide variation in the germ 

 content of ice cream, as it is placed upon the market. 



An examination of 263 samples of ice cream collected in the city of 

 Washington* showed an average germ content of over 26,600,000 per c.c. 

 The lowest count obtained was 37,500 and the maximum was 365,000,000. 

 A similar study of commercial ice cream in Philadelphia! showed the 

 average bacterial content to be very high. The lowest count found was 

 50,000 per c.c., while the highest count was 150,200,000. In this work 

 it was found that the bacterial content of the ice cream was in quite 

 direct relation to the sanitary conditions of the establishment where the 

 ice cream was manufactured. When ice cream is manufactured in a 

 city from materials which have been shipped in from considerable dis- 

 tances and frequently held for several days in cold storage, it is not sur- 

 prising that the germ content of the manufactured product should be 

 high. In some establishments the cream is pasteurized before manu- 

 facturing, while in others it is used in its raw condition. 



In normal cream held for sometime, the lactic bacteria should exist 

 in considerable numbers, but when cream is held at low temperatures 

 these organisms do not develop rapidly. Pennington found that cer- 

 tain species of streptococci developed quite rapidly in cream held at 

 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 



* Results of work done under the direction of Dr. G. W. Stiles, 

 f Work done under the direction of Dr. M. E. Pennington. 



