THE DETERMINATION OF BACTERIA IN ICE CREAM. 



15 



Table VIII. — Colony counts on duplicate plates from the same dilution of a sample of 



ice cream. 



Sample 

 No. 



Number 



of colonies 



on duplicate plates. 



Varia- 

 tion. 



1 



243 



450 

 321 

 319 

 144 



2 



3 



4 



5 



6 



7 



8 



1 

 2 

 3 



4 

 5 



229 

 460 

 302 

 310 

 152 



225 

 510 

 307 

 267 

 153 



208 

 470 

 300 



298 

 178 



232 



480 

 310 



280 



178 



218 

 470 







Percent. 

 16.8 

 26.6 

 7.0 

 19.4 

 23.6 



500 



570 





















INTERPRETING DIFFERENCES IN BACTERIAL COUNTS. 



When expressing bacterial counts the mere statement of the dif- 

 ference in the number of bacteria between two samples is not suffi- 

 cient; in fact, it is impossible to interpret such differences properly 

 without taking into consideration the total number of bacteria 

 found in each sample. If, for example, it is stated that 1 cubic 

 centimeter of ice cream contains 75,000,000 more bacteria than 

 another sample, the difference of so many millions conveys to the 

 mind a marked disparity in the two samples. If, on the other hand, 

 there is said to be a difference of only 750 or 7,500 bacteria per cubic 

 centimeter, we should immediately think of the samples as being 

 practically the same. 



Table IX. — Hypothetical statement showing variable differences in bacterial counts of 

 two samples of ice cream having a fixed ratio between colony counts and a fixed per cent 

 of variation. 



Colony 



375 

 300 

 375 

 300 

 375 

 300 

 375 

 300 

 375 

 300 

 375 

 300 

 375 

 300 



Dilution. 



ltolO 



ltolOO 



1 to 1,000 



1 to 10,000 



1 to 100,000.... 



1 to 1,000,000.. 



}l to 10,000,000 . 



Bacteria 

 per c. c. 



3,750 



3,000 



37,500 



30,000 



375,000 



300,000 



3,750,000 



3,000,000 



37,500,000 



30,000,000 



375,000,000 



300,000,000 



3,750,000,000 



3,000,000,000 



Difference 

 in count. 



} 750 



1 7,500 



| 75,000 



750,000 



7,500,000 



75,000,000 



750,000,000 



Variation 

 between 

 counts. 



Per cent. 

 25.00 



25.00 



25.00 



25.00 



25.00 



25.00 



25.00 



Let us consider a hypothetical case, as shown in Table IX. We 

 will assume that one sample of ice cream shows an average colony 

 count of 375 and another 300. With a dilution of from 1 to 10, 

 there would be a total count of 3,750 in one sample and 3,000 in 

 the other, a difference of 750 bacteria per cubic centimeter, and a 

 variation of 25 per cent between counts. If the same colony count 

 were from a dilution of from 1 to 100 the difference in the bacteria 

 in the samples would be 7,500, but the variation would be still the 



