BACTERIOLOGICAL STUDY OP EETAIL ICE CBEAM. 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. 
