BACTEEIOLOGICAL STUDY OF EETAIL ICE CEEAM. 9 



A comparison of these figures with those of the average summer 

 samples shows that while the percentages of the alkali and peptoniz- 

 ing groups are higher in winter there were actually smaller numbers 

 of them in ice cream during that season, on account of a lower total 

 number of bacteria during cold weather. 



No study was made of the cultural reactions of the bacteria of the 

 various groups, but from our methods of analysis a few points mav 

 be brought out. 



THE TOTAL-ACID GROUPS. 



As we have shown before, of the average bacterial flora of summer 

 ice cream 70.54 per cent is made up of the total group of acid-forming 

 bacteria, and during the winter 68.87 per cent. While using the 



/ILK/iU /A/EtfT 

 5.43% 4.3/% 



Fig. 3.— Average bacterial groups found in one cubic centimeter of ice cream during the winter months, 



showing percentages of groups. 

 Group. • Bacteria per c. c. 



Acid-coagulating : 3, 203, 728 



Acid-forming. 3,950,(141 



Inert 499, 673 



Alkali-forming 563 , 042 



J -cp tonizing 2, 171, 138 



Total 10, 3S8 , 222 



milk-tube method of differentiation tho reactions of the litmus milk 

 tube- arc recorded after 2, 5, and 14 days, and the total acid-forming 

 group is composed of those bacteria which produce acid in litmus 

 mill; during the II days of incubation. Those bacteria which form 

 acid and peptonize the milk are included in the peptonizing group. 

 The totfll-acid group can be further divided into those which produco 

 5020° Bull. 303 -16 2 



