12 
BULLETIN 303, U. S. DEPARTMENT OF AGRICULTURE. 
Table 6. — Changes in the 'percentage of the alkali group of bacteria in ice cream when 
determined by litmus-milk reactions after various lengths of incubation. 
Alkali group. 
Per cent reacting after 
incubation for— 
2 days. 
5 days. 
14 days. 
Average of 71 summer samples 
Per cent. 
0.15 
17 
Per cent. 
1.03 
4 00 
Per cent. 
1 86 
Average of 28 winter samples 
5 42 
At present we are unable to state the significance of this group of 
bacteria in milk and ice cream, but it is evident that they are not 
present in ice cream in large numbers, as are the bacteria of other 
groups. The average numbers of alkali-forming bacteria in ice 
cream from 16 manufacturers during the summer season and from 
7 during the winter season are shown in Table 7. 
Table 7. 
-Average number of alkali-forming bacteria per cubic centimeter of ice cream 
from various manufacturers. 
Summer. 
Winter. 
Manufacturer. 
Number 
of 
samples. 
Average 
number of 
alkali bac- 
teria per 
cubic 
centimeter. 
Number 
of 
samples. 
Average 
number of 
alkali bac- 
teria per 
cubic 
centimeter. 
A 
11 
3 
3 
1 
3 
2 
4 
1 
1 
1 
4 
1 
1 
3 
1 
1 
56,945 
1, 155, 327 
1, 739, 007 
386, 000 
462, 871 
340, 820 
136, 198 
251, 509 
1,316,800 
902, 400 
18,348 
419,240 
22, 464 
118,219 
1, 044, 900 
47,104 
3 
5 
9,192 
B... 
2, 326, 068 
C 
D. . 
3 
265, 545 
E 
F 
2 
1 
14, 146 
G 
31, 815 
H... 
I 
J 
2 
1 
93, 867 
111,377 
Ki 
L 
M 
N. 

P... 
41 
526, 134 
2,998,700 
4,681 
17 
407,430 
10, 400, 000 
1,901 
Alkali-forming bacteria were not found in each sample examined, 
but this does not prove that there were none present in the ice cream. 
Since these organisms are present in small numbers compared to the 
rest of the bacteria, it is not surprising that none should be found on 
plates in which the dilution had to be high in order to take care of 
the large total number of organisms. 
