BACTERIOLOGICAL STUDY OF RETAIL ICE CREAM. 11 



THE INERT GROUP. 



The inert group of bacteria in ice cream comprises those which 

 produce no change in litmus milk during the 14 days' incubation at 

 30° C. (86° F.). By this method of grouping there are, of course, 

 included in the inert group those cultures which fail to grow in milk 

 and tubes of litmus milk, and which would also be included even 

 though the lack of growth were caused by failure to inoculate the 

 tubes properly. However, this last possibility is small. The inert 

 group is of little interest, on the whole, since the bacteria produce no 

 apparent change in milk, and in all probability the same is true of 

 ice cream. 



THE ALKALI GRO^F. 



The alkali-forming group of bacteria is made up of organisms 

 capable of producing an alkaline reaction and no other apparent 

 change in litmus milk during the 14 days' incubation at 30° C. (86° 

 F.). This group does not include bacteria which produce an alkaline 

 reaction together with visible signs of peptonization. While there 

 are in the literature references which deal with types of alkali-forming 

 bacteria, this group has rarely, if ever, been considered when the flora 

 of milk has been under discussion. The authors in some previous 

 work on bacteria in milk showed that considerable numbers of this 

 group were present in milk. In a later piece of work we have shown 

 the numbers of this type of bacteria in milk, together with some of 

 the cultural reactions of the alkali-forming bacteria. These bacteria, 

 however, give very few positive reactions with the usual cultural 

 media, and it is impossible to give much information regarding this 

 group. A detailed bacteriological and chemical study of these 

 organisms is under way in the research laboratories of the Dairy 

 Division. 



It will be seen from Table 6 that during the summer series of ice- 

 cream samples the average sample contained 1.86 per cent of the 

 alkali group of bacteria, and during the winter series 5.42 per cent. 

 In general, the alkaline reaction is not noticeable until after four or 

 five days' incubation in litmus milk. Occasionally, however, the reac- 

 tion is in evidence in 48 hours. The group percentage for the sum- 

 mer season was 1.86 after 14 days and only 0.15 per cent based on the 

 2-day reaction. Therefore only 8.06 per cent of the bacteria of the 

 alkali group produce an alkaline reaction within 48 hours. Among 

 the samples collected during the winter season only 3.13 per cent of 

 the bacteria of this group were capable of producing the reaction 

 within two days. Whether this indicates a different variety of 

 organism can not be said with assurance. 



