14 



BULLETIN 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 



% % % $ 8 18 



° •*) & 

 *0 VO N 



B 



§ 



% % 13 ? § § 



!o P !o 



R 



15 



/='£:f?C£'U7XGE OF 70TAL 



BACTER/A 







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 

 when determined by litmus-milk reactions after various lengths of incubation. 



Peptonizing group. 



Per cent reacting after incu- 

 bation for— 



2 days. 



5 days. 



14 days. 



Average of 71 summer samples . 

 Average of 28 winter samples . . 



Per cent. 

 5.93 

 7.12 



Per cent. 

 9.76 

 13.58 



Per cent. 

 13.62 

 20.90 



