BACTERIOLOGICAL STUDY OP RETAIL ICE CREAM. 23 



The bacterial groups bore much the same relation to each other in 

 the average summer and winter samples. There was, however, in the 

 summer samples a higher percentage of the acid-coagulating group of 

 bacteria and a lower percentage of the alkali and peptonizing groups 

 than in the winter samples. In spite of a higher percentage of the 

 two groups last named in the average winter sample, the number of 

 bacteria of these was much lower, owing to a lower average total 

 bacterial count. 



4. Among the summer samples of ice cream 52.81 per cent of the 

 bacteria of the acid-coagulating group were active enough to coagu- 

 late milk in 48 hours when incubated at 30° C. (86° F.). The remain- 

 ing 47.19 per cent coagulated milk more slowly and may represent a 

 different variety of acid-forming bacteria. Only 26.69 per cent of 

 the acid-coagulating group of bacteria in the winter samples coagu- 

 lated in 48 hours. There is therefore a higher percentage of rapid 

 acid-coagulating bacteria in ice cream during the summer months. 



5. The average number of peptonizing bacteria found in the sum- 

 mer samples of ice cream was 1,449,533. The maximum count was 

 21,000,000, the minimum 36,063, peptonizing bacteria per cubic centi- 

 meter. During the winter season the average number was 268,693, 

 which is about one-fifth the average summer count. The maximum 

 number was 2,974,400 and the minimum 1,194 bacteria per cubic 

 centimeter. The majority of the samples during the entire investiga- 

 tion contained from 0.1 to 5 per cent of peptonizers. A large propor- 

 tion contained as high as 25 to 30 per cent, and in a few samples they 

 were present to the extent of from 90 to 95 per cent of the total 

 bacteria. Among the summer samples 43.54 per cent of the pep- 

 tonizing bacteria were sufficiently active to produce a peptonization 

 in milk within 48 hours at 30° C. (86° F.) ; while 34.06 per cent of these 

 bacteria in the winter samples were able to produce this change in the 

 same time. It is probable that these active peptonizers are more 

 important than the slower-acting varieties, since they usually produce 

 a greater decomposition of the milk. 



6. Gas-forming bacteria of the colon-aerogenes group when deter- 

 mined on litmuH-lactose-asparagin agar were found present in one- 

 i nil) <>f a cubic centimeter in 106, or 88.33 per cent, of the 120 sam- 

 ple, examined, and absent in one-tenth of a cubic centimeter in 14, 

 or 1 L.67 per cent, of the samples. Of the 14 negative samples, 13 



of the winter series and 1 was examined during October. The 

 Qumber of gas formers in the entire scries of samples was 

 16,298 per cubic centimeter. Fifty-seven samples examine'd during 

 the summer averaged 29,544 per cubic- centimeter. The 49 winter 

 samples contained an average of 889 per cubic centimeter. Ice cream 

 contained ;i much larger aumber of gas-forming organisms during the 

 summer season. „\ large number of media were used in an attempt 



