12 



BULLETIN 303, U. S. DEPARTMENT OP AGRICULTURE. 



Table 6. — Changes in the -percentage of the alkali group of bacteria in ice cream when 

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



Alkali group. 



Per cent reacting after 

 incubation for — 



' days. 



5 days. 



14 days. 



Average of 71 summer samples. 

 Average of 28 winter samples. . 



Per cent. 



0.15 



.17 



Per cent. 

 1.03 

 4.00 



Per cent. 

 1.86 

 5.42 



At present we are unable to state the significance of this group of 

 bacteria in milk and ice cream, but it is evident that they are not 

 present in ice cream in large numbers, as are the bacteria of other 

 groups. The average numbers of alkali-forming bacteria in ice 

 cream from 16 manufacturers during the summer season and from 

 7 during the winter season are shown in Table 7. 



Table 7. — Average number of alkali-forming bacteria per cubic centimeter of ice cream 

 from various manufacturers. 





Summer. 



Winter. 



Manufacturer. 



Number 



of 

 samples. 



Average 

 number of 

 alkali bac- 

 teria per 

 cubic 

 centimeter. 



Number 



of 

 samples. 



Average 

 number of 

 alkali bac- 

 teria per 

 cubic 

 centimeter. 



A 



11 

 3 

 3 

 1 

 3 

 2 

 4 

 1 

 1 

 1 

 4 

 1 

 1 

 3 

 1 

 1 



56, 945 



1,155,327 



1, 739, 007 



386, 000 



462, 871 



340, 820 



136, 198 



251, 509 



1,316,800 



902, 400 



18,348 



419, 240 



22, 464 



118,219 



1,044,900 



47, 104 



3 

 5 



9,192 



B 



2,326,068 



c :. 





D 



3 



265, 545 



E , 





F 



2 

 1 



14, 146 



G 



31, 815 



H 





I 







J 



2 

 1 



93, 867 



Ki. 



111,377 



L 





M 







N 







O 







P 















41 



526, 134 



2,998,700 



4,681 



17 



407, 430 





10,400,000 









1,901 











Alkali-forming bacteria were not found in each sample examined, 

 but this does not prove that there were none present in the ice cream. 

 Since these organisms are present in small numbers compared to the 

 rest of the bacteria, it is not surprising that none should be found on 

 plates in which the dilution had to be high in order to take care of 

 the large total number of organisms. 



