8 BULLETIN 303, U. S. DEPARTMENT OF AGRICULTURE. 



an average sample of ice cream during the summer season may be 

 divided into five groups in the following proportions : 



Per cent. 



Acid-coagulating group 49. 82 



Acid-forming group 20. 72 



Inert group 13. 98 



Alkali-forming group 1. 86 



Peptonizing group 13. 62 



The total acid group comprises, therefore, 70.54 per cent of the 

 bacteria in the average ice cream during the summer season. In 

 order to make this picture more nearly complete, we have calculated 

 the number of bacteria of each group which would be found in 1 c. c. 

 of an average sample of summer ice cream. These bacterial numbers 

 were obtained by calculation from the average bacterial count of 

 1 c. c. of ice cream. Since the average commercial ice cream during 

 the summer months contained 37,859,909 per cubic centimeter, the 

 calculated number of each group would be as follows: 



Bacteria per c. c. 



Acid-coagulating bacteria 18, 861, 805 



Acid-forming bacteria 7, 844, 575 



Inert bacteria 5, 292, 815 



Alkali-forming bacteria 704, 195 



Peptonizing bacteria 5, 156, 519 



Perhaps the most important observation in this connection is the 

 high number of peptonizing bacteria. 



In the same manner are shown in figure 3 the bacterial groups in 

 an average sample of winter ice cream. The bacteria may be divided 

 into five groups, as follows: 



Per cent. 



Acid-coagulating group 30. 84 



Acid-forming group 38. 03 



Inert group 4. 81 



Alkali-forming group 5. 42 



Peptonizing group 20. 90 



A comparison of the total percentage of the acid groups shows 

 70.54 per cent during the summer and 68.87 per cent during the 

 winter. It will be seen, however, that during the summer 49.82 per 

 cent were acid-coagulating bacteria, while in the winter only 30.84 

 per cent were of the acid-coagulating group. The increase in the per- 

 centage of the alkali and peptonizing groups is particularly notice- 

 able. From the average group percentages and the average bacte- 

 rial count the calculated bacterial composition of the 10,388,222 in 

 1 c. c. of winter ice cream would be as follows: 



Bacteria per c. c. 



Acid-coagulating bacteria 3, 203, 728 



Acid -forming bacteria 3, 950, 641 



Inert bacteria 499, 673 



Alkali-forming bacteria 563, 042 



Peptonizing bacteria 2, 171, 138 



