BACTERIOLOGICAL STUDY OF RETAIL ICE CREAM. 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 may 
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 
S.42% <?.s/y<> 
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, 641 
Inert 499, 673 
Alkali-forming 563, 042 
Peptonizing 2, 171, 138 
Total 10,388,222 
milk-tube method of differentiation the reactions of the litmus milk 
tubes are recorded after 2, 5, and 14 days, and the total acid-forming 
group is composed of those bacteria which produce acid in litmus 
milk during the 14 days of incubation. Those bacteria which form 
acid and peptonize the milk are included in the peptonizing group. 
The total-acid group can be further divided into those which produce 
5020°— Bull. 303—15 2 
