22 



BULLETIN 303,, U. S. DEPARTMENT OF AGEICULTUEE. 



SUMMARY AND CONCLUSIONS. 



Samples of ice cream as sold at retail were examined in Washington 

 during the summer season from June 20 to November 11, 1912, and 

 also during the winter season in February and March of 1913. 



1 . The average acidity of 65 samples of vanilla ice cream examined 

 during the summer and winter season was 0.206 per cent, calculated 

 as lactic acid. The maximum acidity was 0.387 per cent, the mini- 

 mum 0.09 per cent. No definite relation was found between acidity 

 and the bacterial count. 



2. The average number of bacteria found in 94 samples of ice cream 

 examined during the summer months was 37,859,907 per cubic 

 centimeter. The maximum count was 510,000,000, the minimum 

 120,000. Of the 91 samples examined during the winter months the 

 average count was 10,388,222, the maximum 114,000,000, and the 

 minimum 13,000 bacteria per cubic centimeter. Among the 94 sum- 

 mer samples none contained fewer than 100,000 bacteria per cubic 

 centimeter, while 14.28 per cent of the 91 winter samples were lower 

 than 100,000. Of the summer samples 9.57 per cent contained fewer 

 than 500,000 per cubic centimeter, and of the winter 39.55 per cent 

 contained less than that number. Of the summer samples, 19.14 

 per cent contained fewer than 1,000,000 per cubic centimeter, while 

 41.75 per cent of the winter samples contained fewer than that num- 

 ber and 80.86 per cent of the summer samples and 58.25 per cent 

 of the winter samples contained more than 1,000,000 bacteria per 

 cubic centimeter. There was a wide range in the bacterial content 

 during the summer and winter seasons, and with one exception, the 

 average number of bacteria in ice cream from each manufacturer was 

 distinctly lower during the winter months. 



3. The bacteria in 71 summer samples and 28 winter samples were 

 divided into groups by the milk-tube method. The percentages of 

 the various groups of bacteria, together with the calculated number 

 of bacteria in each group in an average sample of ice cream examined 

 during the summer and winter seasons, are summarized in the fol- 

 lowing table : 



Table 15. — Summary of bacteria in ice cream. 1 



Bacterial groups. 



Summer samples. 



Average 



number of 



bacteria 



per cubic 



centimeter. 



Average 

 group per- 

 centage. 



Winter samples. 



Average 

 number of 



bacteria 



per cubic 



centimeter. 



Average 

 group per- 

 centage. 



Acid-coagulating . 



Acid-forming 



Inert 



Alkali-forming . . . 

 Peptonizing 



18,861,805 



7,844,575 



5,292,815 



704, 195 



5,156,519 



49.82 



20.72 



13.98 



1.86 



13:62 



3,203,728 



3,950,641 



499, 673 



563, 042 



2,171,138 



30.84 

 38.03 

 4.81 

 5.42 

 20.90 



Total. 



37,859,909 



100. 00 



10,388,222 



100. 00 



1 The counts were made from the average percentage of each group and the average total count. 



