14 
BULLETIX 303, U. S. DEPARTMENT OF AGRICULTURE. 
Among this group there are a large number of different types of 
organisms. Many rapidly peptonize the casein of milk and render 
milk alkaline or slightly acid, while others first attack the lactose 
and only produce a slight peptonization after several, days' growth. 
From the milk-tube method of differentiation of the bacterial groups 
it was possible to gain some information as to the extent of these 
different classes of peptonizers. In Table 9 are shown the average 
percentages of the peptonizing group in summer and winter samples 
of ice cream. Based on the 14-day reaction among the summer 
so 
I 
i 
* 
/C 
1 
; 1 
. 
! i 
5 ! 
6 { 
3 * 
5 ^ 
i 1 
2 s 
) <: 
J 
1 i 
u e 
1 s 
h s 
I 5 
0GRCE/V7?IG£ Of 7VTAL e/^CT£rfi?M 
Fig. 4.— Frequency curve showing percentage of peptonizing bacteria per cubic centimeter of ice cream, 
summer and winter series combined. 
samples, 13.62 per cent of the bacteria belonged to the peptonizing 
group. According to the 2-day reaction, there were 5.93 per cent. 
Therefore 43.54 per cent of the peptonizing bacteria were sufficiently 
active to produce a peptonization within two days. Among the 
winter samples 34.06 per cent of the peptonizing bacteria were suffi- 
ciently active to peptonize milk within 48 hours. These active 
peptonizing bacteria are more important than the slower-acting 
varieties, since their peptonizing action is usually more complete 
than that of the latter-named varieties, and if any harm is produced 
by this group, they are most likely to be the organisms concerned. 
Table 9. — Changes in the percentage of the peptonizing group of bacteria in ice cream 
vjhen determined by litmus-milk reactions after various lengths of incubation. 
Peptonizing group. 
Fer cent reacting after incu- 
bation for— 
2 days. 5 days. 14 day-. 
Average of 71 summer samples. 
Average of 2S winter samples. . 
Per cent. 
5.93 
7.12 
Per cent. 
9.76 
13.58 
Per cent. 
13.62 
20.90 
