10 
BULLETIN 303, U. S. DEPABTMEXT OF AGBICULTUBE. 
acid and coagulate the milk and those which simply form acid within 
14 days. Since the reaction is recorded after 2, 5, and 14 days, the 
rapidity of the growth of the acid-forming bacteria can be deter- 
mined, and this serves as an additional means of separating the 
group. In Table 5 the percentages of the acid-coagulating and the 
simple acid-forming groups of bacteria are shown, based on the 2, 5, 
and 14 day reaction in litmus milk. 
Table 5. — Changes in the percentage of the tctal-acid group of bacteria in ice cream vshen 
determined by litmus-milk reactions after various lengths of incubation. 
Bacterial group. 
Per cent reacting after 
incubation for— 
2 days. 5 days. 14 day 
Average of 71 summer samples 
Acid-coagulating 
Acid-forming 
Average of 28 winter samples: 
Acid-coagulating 
Acid-forming 
Per cent. 
26.31 
35.43 
8.20 
44.51 
Per cent. 
41.52 
25.58 
25.02 
41.30 
Per cent. 
49.82 
20.72 
30.84 
38.03 
An examination of the table shows that among the summer sam- 
ples 49.82 per cent of the bacteria produced acid and coagulated 
the milk after 14 days. After 2 days 26.31 per cent produced this 
reaction. This shows that a little more than half, or 52.81 per cent, 
of the bacteria which were in the ice cream produced the reaction 
within 48 hours. The remaining 47.19 per cent coagulated milk 
more slowly and may represent a different variety of acid-forming 
bacteria. Turning again to the table and considering the acid- 
coagulating group of the winter series, it will be seen that of the 
30.84 per cent which produced the reaction only 8=20 per cent pro- 
duced acid and coagulated milk in 2 days. Therefore only 26.69 
per cent of the acid-coagulating group of the winter samples were 
active enough to produce the reaction in 48 hours, while 52.81 per 
cent of this group in the summer samples brought about the change 
in 2 days. 
There is little to be said regarding the acid-forming bacteria which 
simply produce acid. Many of them grow slowly and do not show 
an acid reaction for several days in litmus milk. The milk-tube 
method furnishes a means of determining the difference in the rapidity 
with which the bacteria produce acid. As may be seen in Table 5, 
ihe percentage of the acid-forming group of bacteria was highest 
-when determined by the 2-day reactions and lowest when based on 
the 5 and 14 day reactions. This is explained by the fact that many 
bacteria have simply formed acid after two days in litmus milk and 
later may coagulate or peptonize the milk, and are therefore thrown 
into another group. 
