12 
BULLETIN 563, U. S. DEPARTMENT OF AGRICULTURE. 
It will be seen from the table that the highest variation among the 
samples taken from any freezer was 40.47 per cent, the minimum count 
in that case being 63,000 and the maximum 88,500 bacteria per cubic 
centimeter. The lowest variation was 7.84 per cent, the counts in 
this instance ranging from 102,000 to 118,000 per cubic centimeter. 
It is interesting to note that the two sets of samples marked A 
were taken on the same day from two different freezers, which were 
probably filled with the same "mix." The same is true of the sets 
marked B. In each of these cases the samples check well with each 
other. From these results it seems evident that the bacterial con- 
tent of one sample from a freezer may be said to represent for prac- 
tical purposes the bacterial content of the rest of the ice cream in the 
freezer. 
COMPARISON OF INCUBATION OF PLATES AT 37° C. FOR TWO DAYS AND 
30° C. FOR FIVE DAYS. 
The plates in our experiments were incubated at 30° C. (86° F.) for 
five days. Since in general practice plates are incubated at 37° C. 
(98.6° F.) and counts made after 48 hours, it was considered advisable 
to make a comparison between these two methods of incubation. 
A gallon of ice cream was obtained and nine samples taken from 
nine different positions in the usual manner. Two sets of duplicate 
plates were made; one set was incubated at 37° C. (98.6° F.> and 
counted after 48 hours. The other set was incubated at 30° C. 
(86° F.) and counted after five days. The results of this experi- 
ment (see Table VI) show that the variation among the samples 
from the same lot of ice cream was 16.89 per cent after the 48-hour 
count at 37° C. (98.6° F.) and 35.11 per cent after incubation at 
30° C. for five days. 
From this experiment it seems evident that incubation at 37° C. 
(98.6° F.) for 48 hours does not give counts which show any greater 
variation than those obtained by incubation at 30° C. (86° F.) for 
five days. It is interesting to observe, however, that the count 
obtained by incubation for five days at 30° C. (86° F.) is practically 
double that obtained by incubation at 37° C. (98.6° F.) for 48 hours. 
Table VI. — Variation in counts obtained by incubation of plates at 37° C.for two 
days and 30° C.for Jive days. 
Incubated at 37° C for 48 hours. 
Incubated at 30° C. for 5 days. 
Sam- 
ple 
No. 
Number of 
bacteria on 
duplicate 
plates. 
Average 
count 
per c. c. 
Variation between 
lowest and highest 
counts. 
Number of 
bacteria on 
duplicate 
plates. 
Average 
count 
per c. c. 
Variation between 
lowest and highest 
counts. 
1 
2 
3 
4 
| 
8 
a 
77 
74 
77 
76 
81 
75 
73 
SO 
83 
80 
"79" 
83 
82 
80 
"96" 
785,000 
740,000 
770,000 
775,000 
820,000 
785,000 
765,000 
800,000 
865,000 
Samples 2 and 9, 
16. 89 per cent. 
170 
161 
161 
179 
162 
174 
131 
184 
157 
176 
183" 
165 
192 
143* 
1,730,000 
1,610,000 
1,720,000 
1,720,000 
1,770,000 
1,740,000 
1,310,000 
1,635,000 
1,570,000 
Samples 5 and 7, 
35. 11 per cent. 
