BACTERIOLOGICAL STUDY OF EETAIL ICE CREAM. 3 
samples were taken directly to the laboratory and portions from vari- 
ous parts of each sample were transferred to a sterile Erlenmeyer 
flask. After the ice cream had melted it was thoroughly shaken to 
remove as much air as possible and at the same time mix the sam- 
ple. One cubic centimeter was then plated by the usual methods on 
plain infusion agar prepared according to standard methods prescribed 
by the Committee on Standard Methods. The plates were incubated 
at 30° C. (86° F.) for five days, then counted. 
Special methods for the determination of colon bacilli will be dis- 
cussed later in this paper. 
In order to divide the bacteria into groups the " milk- tube method" 
of differentiation was used. This method, devised by the authors, 
is fully described in a previous publication. 
Briefly described, the milk-tube method consists in picking each 
colony from an infusion agar plate and inoculating into tubes of 
litmus milk. The reactions produced in the litmus milk are recorded 
after 2, 5, and 14 days' incubation at 30° C. (86° F.). Knowing the 
14-day reaction produced by bacteria from each colony on the original 
plate, it is possible to determine the number of bacteria in the original 
material plated and to divide them into groups, namely, the acid- 
coagulating, acid-forming, inert, alkali-forming, and peptonizing. 
An examination of samples collected as mentioned gives the bacterial 
flora of ice cream just as the consumer would receive it. 
THE ACIDITY OF ICE CREAM. 
Before entering upon a discussion of the bacteriological results of 
this work it seems advisable to discuss a few miscellaneous analyses 
which were made during this study. All the samples of vanilla ice 
cream were tested for acidity by titration with tenth-normal sodium 
hydroxid, using phenolphthalein as an indicator. Most of the ice 
cream of other flavors could not be tested on account of the color. 
The maximum acidity found was 0.387 per cent and the minimum 
0.09 per cent, calculated as lactic acid. The average acidity of 65 
samples was 0.206 per cent. Many of the samples were distinctly sour 
to the taste, and evidently some manufacturers used either old sour 
cream or else aged the cream in their plants at temperatures suffi- 
ciently high to allow lactic-acid bacteria to produce acidity. The 
acidity did not seem to bear any definite relation to the bacterial 
count, however, as the samples showing an acidity of 0.387 per cent 
contained 217,000 bacteria per cubic centimeter, of which 74,51 per 
cent were acid-forming bacteria, while ice cream with an acidity of 
0.09 per cent contained 49,000,000 bacteria per cubic centimeter, of 
which 89.79 per cent were acid-forming bacteria. 
