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. 



Lot. 



Sam- 

 ple 

 No. 



Dilu- 

 tion. 



Number of 



colonies on 



duplicate 



plates. 



Average 



count per 



c. c. 



Varia- 

 tion be 

 tween 

 lowest 



and 

 highest 

 counts. 



Dilution. 



Num- 

 ber of 

 colonies 

 on du- 

 plicate 

 plates. 



Average 



count per 



c. c. 



Varia- 

 tion be- 

 tween 

 lowest 

 and 

 highest 

 counts. 



A.... 



1 

 2 

 3 

 4 

 5 

 6 

 7 

 8 

 9 



TSTinn! 



243 

 219 

 242 

 231 

 250 

 264 

 230 

 243 

 271 



229 

 198 

 243 

 245 



233 



245 

 235 



2,360,000 

 2,085,000 

 2,425,000 

 2,380,000 

 2,500,000 

 2,485,000 

 2,300,000 

 2, 440, 000 

 2,530,000 



Per ci. 

 21.34 



115570 OS 



27 

 22 

 24 

 36 

 36 

 30 

 37 

 36 

 34 



30 

 43 

 40 

 24 

 39 

 25 

 28 

 22 

 25 



2,850,000 

 3,250,000 

 3,200,000 

 3,000,000 

 3,750,000 

 2,750,000 

 3,250,000 

 2,900,000 

 2,950,000 



Per ci. 

 36.36 



B.... 



1 

 2 

 3 

 4 

 5 

 6 

 7 

 8 

 9 



T00.0OU 



341 

 274 

 284 

 294 

 284 

 301 

 325 

 321 

 314 



334 

 266 



302 

 270 

 303 

 331 

 329 



33,750,000 

 27,000,000 

 28,400,000 

 29,800,000 

 27,700,000 

 30,200,000 

 32,800,000 

 32,500,000 

 31,400,000 



25.00 



TTBS0705TI 



41 

 31 

 30 

 30 

 33 

 38 

 38 

 35 

 35 



31 

 29 



36' 

 29 

 44 

 43 

 46 



36,000,000 

 30,000,000 

 30,000,000 

 30,000,000 

 34,500,000 

 33,500,000 

 41,000,000 

 39,000,000 

 40,500,000 



35.00 



C... 



1 

 2 

 3 



4 

 5 

 6 

 7 

 8 

 9 



TTTsW 



286 

 360 

 270 

 250 

 314 

 251 

 268 

 311 

 261 



290 

 254 

 289 

 270 

 250 

 252 

 250 

 287 

 306 



288,000 

 307,000 

 279, 500 

 260, 000 

 282, 000 

 251,500 

 259, 000 

 299, 000 

 283, 500 



22.31 



T0.07T5 



42 

 41 

 40 

 31 

 38 

 30 

 32 

 32 

 35 



53 



33' 

 42 

 39 

 33 

 38 

 44 

 52 



475,000 

 410,000 

 365,000 

 365,000 

 385,000 

 315,000 

 350,000 

 380,000 

 435,000 



50.79 



