THE DETERMINATION OF BACTERIA IN ICE CREAM. 
13 
THE NUMBER OF COLONIES MOST DESIRABLE ON PETRI PLATES. 
When a sample of ice cream is plated it is of course necessary to 
make several dilutions, since the bacterial content is unknown. It 
is sometimes a question as to which dilution will give the most 
accurate count. 
Some interesting facts regarding this point are shown by reference 
to Table VII. Seven 1 -gallon lots of ice cream were obtained, and 
nine samples from each were plated in the usual manner. Three 
dilutions were made, but only two are recorded in the table. The 
aim was to obtain plates with about 200 colonies, as in bacterio- 
logical work that number is believed to give the most nearly accurate 
count. The two recorded, therefore, are the counts obtained from 
the dilution which gave about 200 colonies per plate and the counts 
from the next highest dilution. 
An examination of the table shows that the variation between 
samples taken from the same lot of ice cream ranged from 15.10 to 
105.32 per cent when the dilution was such that from 100 to 300 
colonies were on the plates. The same samples at the next higher 
dilution, in which the number of colonies was less than 50, varied 
from 35 to 1,014.28 per cent. In every case in which there was a 
small number of colonies on the plates the variation between the 
samples from each gallon lot was decidedly greater than when a lower 
dilution was used. This fact is by no means new, but it should be 
kept in mind when interpreting the results obtained from a bacterial 
analysis of duplicate samples of ice cream. 
Table VII. — Comparison of bacterial counts obtained from the same samples of ice cream 
but with different dilutions. 
Varia- 
Num- 
ber of 
colonies 
on du- 
plicate 
plates. 
Varia- 
Lot. 
Sam- 
ple 
No. 
Dilu- 
tion. 
Number of 
colonies on 
duplicate 
plates. 
Average 
count per 
c. c. 
tion be 
tween 
lowest 
and 
highest 
counts. 
Dilution. 
Average 
count per 
c. c. 
tion be- 
tween 
lowest 
and 
highest 
counts. 
Per ct. 
Per ct. 
A.... 
1 
T<575in5 
243 
229 
2,360,000 
21.34 
tgxs:h 5S 
27 
30 
2,850,000 
36.36 
2 
219 
198 
2,085,000 
22 
43 
3,250,000 
3 
242 
243 
2,425,000 
24 
40 
3,200,000 
4 
231 
245 
2,380,000 
36 
24 
3,000,000 
5 
250 
2,500,000 
36 
39 
3,750,000 
6 
264 
**233' 
2,4§5,000 
30 
25 
2,750,000 
7 
230 
2,300,000 
37 
28 
3,250,000 
8 
243 
"245" 
2,440,000 
36 
22 
2,900,000 
9 
271 
235 
2,530,000 
34 
25 
2,950,000 
B.... 
1 
TOO. 666 
341 
334 
33,750,000 
25.00 
T7(5??j:5U5 
41 
31 
36,000,000 
35.00 
2 
274 
266 
27,000,000 
31 
29 
30,000,000 
3 
284 
28,400,000 
30 
30,000,000 
4 
294 
"362" 
29,800,000 
30 
30,000,000 
5 
284 
270 
27,700,000 
33 
"36" 
34,500,000 
6 
301 
303 
30,200,000 
38 
29 
33,500,000 
7 
325 
331 
32,800,000 
38 
44 
41,000,000 
8 
321 
329 
32,500,000 
35 
43 
39,000,000 
9 
314 
31,400,000 
35 
46 
40,500,000 
C... 
1 
T77>Vo" 
286 
290 
288,000 
22.31 
ts.wsts 
42 
53 
475,000 
50.79 
2 
360 
254 
307,000 
41 
410,000 
3 
270 
289 
279, 500 
40 
'33' 
365,000 
4 
250 
270 
260, 000 
31 
42 
365,000 
5 
314 
250 
282,000 
38 
39 
385, 000 
6 
251 
252 
251, 500 
30 
33 
315,000 
7 
268 
250 
259,000 
32 
38 
350,000 
8 
311 
287 
299,000 
32 
44 
380, 000 
9 
261 
306 
283, 500 
35 
52 
435,000 
