ICE CREAM BACTERIA 435 
cream was frozen in the usual type of freezer and stored in quart bricks. 
The method of analysis is given in another place. They found that 
there was little change in the number of bacteria as shown by litmus 
lactose gelatin plates when cream was kept frozen for a month. 
Ayers and Johnson (1915) undertook an investigation on ice cream 
with the following objects: 
1. To determine the number of bacteria in commercial ice cream 
during the summer and winter seasons. 
2. To determine what groups of bacteria are found in commercial 
ice cream. 
3. To determine the relative value of different methods for the deter- 
mination of Bacillus coli in ice cream. 
They found that the average number of bacteria in ice cream is 
lower during the winter months. A summary of their results is given 
in the following table: 
Summer Series Winter Series 
Item. (94 Samples). | (91 Samples). 
Average number of bacteria per cubic centimeter. 37,859,807 10,388,222 
Maximum number... ....... 02.0 cece eee ee eees 510,000,000 114,000,000 
Minimum number.........-0ccee cece eee renee 120,000 13,000 
Studies by means of the milk-tube method showed that the bacterial 
groups bore about the same relation to each other in the average summer 
and winter samples. During the summer months the averages based on 
71 samples were as follows: 
Acid coagulating............. 00. cee eee eee eee 49.82 
Acid forming. . 2.2.0... 2.0... cece cece eee eee nes 20.72 
Inert group........- 2.0. eee eee ee tenes 13.98 
Alkali forming groups..........- 0.02.00 ee eee eeee 1.86 
Peptonizing group... ........0 0c cece eee e eens 13 .62 
The averages for the winter samples are: 
Acid coagulating group... .... 0.2.0... ..00 000s 30.84 
Acid forming group........2.. cece cece cence eae 38 .03 
Inert QTOUP.. 6.0... cece eee tenes 4.81 
Alkali forming group..........-.20. eee eee eee 5.42 
Peptonizing group... 1... 0... eee cece eee ee ees 20.90 
The peptonizing group is of most significance since it is these bac- 
teria which cause poisoning from ice cream. 
