10 



BULLETIN 303, U. S. DEPARTMENT OF AGRICULTURE. 



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 tcial-acid group of bacteria in ice cream when 

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



Bacterial group. 



Per cent reacting after 

 incubation for — 



2 days. 



5 days. 



14 days. 



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 haL°, 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 t 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. 



