12 



BULLETIN 563, TJ. 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 five 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 

 5 

 6 

 7 

 8 

 9 



77 

 74 

 77 

 76 

 81 

 75 

 73 

 80 

 83 



80 ' 



79 

 83 



82 

 80 



90 



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. 



