THE BACTERIOLOGY OF ICE CREAM 183 



much more frequently used as the basis for determinations 

 than the gram, since it offers a big advantage in the saving 

 of time that is possible in the securing of the portion to be 

 examined. Samples can be conveniently collected in sterile 

 petri dishes or sterile flasks and can then be slowly and care- 

 fully melted down and mixed. The thorough mixing of the 

 sample is very essential as it results in the working out of 

 the incorporated air that otherwise interferes with the accurate 

 measuring of the material. Ice cream can be collected from 

 freezers with a sterile spoon or can be allowed to fall directly 

 into a petri dish and can be removed from containers of 

 hardened ice cream by the use of a sterile spoorLor a sterile 

 butter trier. Under ordinary conditions it seems desirable to 

 discard the ice cream at the surface of containers, and where 

 a trier is used this can be conveniently done by cutting the 

 drawn core over the edge of the bottom half of a petri dish 

 in such a way as to allow as much of the core as is desired 

 to fall into the dish. A wide range of media has been used 

 in the plating of ice cream but, since milk derivatives are the 

 greatest source of the bacteria in ice cream, there seems to 

 be a very good reason for using the same medium for this 

 product that is used for milk. The incubation conditions 

 most commonly used are 37 C. for 48 hours; Ayers & John- 

 son 1 found however, that 30 C. for 5 days gave them prac- 

 tically double the count they secured at 37 C. for 48 hours. 

 The bacterial counts that have been reported in the liter- 

 ature vary widely and are in general surprisingly high. Counts 

 of only a few thousand per c. c. have been occasionally re- 

 corded but those running into the hundreds of thousands or 

 millions are much more common. A count of 8,000,000,000 2 

 per c. c. was secured in Milwaukee. The average of a series 

 of samples examined in our American cities at the present 

 time is almost certain to run into the millions per c. c. unless 

 unusual conditions prevail. Seasonal differences of consider- 



1 U. S. Dept. of Agr., Bulletin 563, p. 12. 



2 Bulletin of the Milwaukee Bureau of Economy and Efficiency, No. 13, 

 p. 35. 



