THE BACTERIOLOGY OF ICE CREAM “185 
cently studied the variation between results secured on 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 iee 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 types of bacteria 
can be found in ice cream, while the prevailing types are 
determined by various factors. I£ 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. Jee 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’ 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- 
lating 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 
7U. S. Dept. of Agriculture, Bulletin 303. 
