22 
BULLETIN 303, U. S. DEPARTMENT OF AGRICULTURE. 
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. 
Acid-coagulating . 
Acid-forming 
Inert 
Alkali-forming... 
Peptonizing 
Total. 
Summer samples. 
Average 
number of 
bacteria 
per cubic 
centimeter. 
18,861,805 
7,844,575 
5,292,815 
704, 195 
5,156,519 
37, 859, 909 
Average 
group per- 
centage. 
Winter samples. 
Average 
number of 
bacteria 
per cubic 
centimeter. 
49. 82 
20.72 
13.98 
1.86 
13.62 
3,203,728 
3,950,641 
499, 673 
563,042 
2, 171, 138 
100.00 10,388,222 
Average 
group per- 
centage. 
30.84 
38.03 
4.81 
5.42 
20.90 
100. 00 
The counts were made from the average percentage of each group and the average total count. 
