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 spoon or 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- 
sont 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 ¢. ¢. 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° 
per e. 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 ec. ¢. unless 
unusual conditions prevail. Seasonal differences of consider- 
i . 22, 
LU. 8. Dept. of Agr., Bulletin 563, p ; 
2 Bulletin of the Milwaukee Bureau of Economy and Efficiency, No. 13, 
p. 35. 
