THE BACTERIOLOGY OF ICE CREAM 185 



cently studied the variation between results secured 011 the 

 same lot of ice cream and found "the general variation among 

 the samples from each gallon of ice cream was from 20 

 to 30 per cent ' '. These investigators consider that the 

 method used by them gives results "which indicate that bac- 

 teria in commercial ice cream are distributed quite evenly 

 and that an analysis of one sample from a gallon of ice cream 

 gives results which will hold for any other similar sample 

 from the same gallon". The variation in results secured 

 on the same lot of ice cream is one of the objections to a 

 bacterial standard that is advanced by commercial men and 

 it seems that it is entitled to much study although there is 

 ample evidence that a bacterial count is a very good index 

 of the sanitary quality of the material employed and the care 

 used in the manufacture. 



TYPES OF BACTERIA IN ICE CREAM 



As would be expected from the wide range of sources of 

 contamination, practically all of the common typos of bacteria 

 can be found in ice cream, while the prevailing types are 

 determined by various factors. If cream which has been held 

 under conditions favorable for the development of Bact. lactis 

 acidi is used, the finished product is practically certain to 

 contain this organism as the predominating type. In the same 

 way, other organisms may be present in excessive numbers if 

 conditions are favorable for their development in any of the 

 materials used or in the utensils employed. Ice cream with 

 a low bacterial count is very likely to have the most of the 

 bacteria of the spore-forming types due to the efficient pas- 

 teurization and holding. 



Avers and Johnson 7 studied the types of bacteria that were 

 present in the ice cream sold at retail in Washington. The 

 group percentages for 71 summer samples were acid-coagu- 

 latiug 49.82 per cent, acid-forming 20.72 per cent, inert 13.98 

 per cent, alkali-forming 1.86 per cent, and peptonizing 13.62 

 per cent, and for 28 winter samples acid-coagulating 30.84 per 



7 U. S. Dept. of Agriculture, Bulletin 303. 



