16 BULLETIN 563, U. S. DEPARTMENT OF AGRICULTURE. 



same, or 25 per cent. As the total count becomes still higher, neces- 

 sitating higher dilutions, the difference in bacteria per cubic centi- 

 meter increases until at a dilution of from 1 to 1,000,000 the difference 

 would be 75,000,000 per cubic centimeter,- — - 



A study of this assumed case merely serves to show that even 

 though there is a difference of 75,000,000 between two samples of 

 ice cream, there is no greater percentage of variation between the 

 samples than when the difference was only 750 per cubic centimeter. 

 It also shows, however, that a difference between counts expressed 

 in bacteria per cubic centimeter should never be considered by itself 

 but should be interpreted in relation to the total number of bacteria 

 per cubic centimeter in each sample. 



SUMMARY AND CONCLUSIONS. 



The method of collecting samples and making bacterial counts 

 used in our experiments gives results which indicate that bacteria 

 in commercial ice cream are distributed quite evenly and that an 

 analysis of one sample from a gallon of ice cream gives results which 

 will hold for any other similar sample from the same gallon. 



Storage of ice cream for 1 1 days in a commercial ice-cream cabinet 

 or in a hardening room for a period of two months did not seem to 

 cause an uneven distribution of bacteria. 



In a series of from 5 to 10 samples taken directly from a large 

 commercial freezer the bacterial counts on each sample checked 

 within the usual limits of error of bacterial analyses. 



No greater variation in bacterial counts between samples was 

 observed when the plates were incubated at 37° C. (98.6° F.) for 48 

 hours than when incubated at 30° C. (86° F.) for a period of five 

 days. 



When dilutions were such that about 200 colonies were present 

 on the plates a lower variation between counts of samples of ice 

 cream was found than when there were 50 or fewer colonies per plate. 



The variation between a series of plates made from the same sample 

 and dilution was found to range from 7 to 26.6 per cent. Among du- 

 plicate plates a variation as high as 41 per cent was observed. This 

 must be remembered in connection with the fact that the variation 

 found in our experiments between average counts of different samples 

 of ice cream from the same gallon lot ranged, generally speaking, 

 between 20 and 30 per cent. To this variation between duplicate 

 plates or a series of plates from the same dilution must be added the 

 error introduced in removing 1 c. c. portions of ice cream from different 

 samples. 



When interpreting bacterial counts, differences in the number of 

 bacteria per cubic centimeter should never be considered except in 

 relation to the total count of each sample. 



W. . JTON : GOVERNMENT PRINTING OFFICE : 1917 



