UNITED STATES DEPARTMENT OF AGRICULTURE 
BULLETIN No. 563 
Contribution from Bureau of Animal Industry. 
A. D. MELVIN, Chief. 
Washington, D. C. 
PROFESSIONAL PAPER 
June 19, 1917 
THE DETERMINATION OF BACTERIA IN ICE CREAM. 
By S. Henry Ayers and W. T. Johnson, Jr., of the Dairy Division. 
CONTENTS. 
Difficulty of making accurate bacteriological 
analyses 1 
Method of sampling and plating the ice cream . 2 
Variation in the bacterial content of com- 
mercial ice cream 3 
Variation when held in an ice-cream cabinet. 8 
Variation when held in storage 9 
Variation in samples taken directly from 
freezer 10 
Comparison of incubation at 37° C. for two 
days and at 30° C. for five days 12 
Number of colonies most desirable on Petri 
plates 13 
Variation between duplicate counts from 
same sample 14 
Interpreting differences in bacterial counts. . . 15 
Summary and conclusions 16 
DIFFICULTY OF MAKING ACCURATE BACTERIOLOGICAL ANALYSES. 
Statements have been made that the distribution of bacteria in 
ice cream is markedly uneven, that there is great variability in the 
bacterial counts of different portions of the same container, and that 
this variability is so great that any small sample selected for analysis 
will not represent the whole mass of the ice cream. 
It must be remembered that the accuracy of a bacterio- 
logical analysis can never be so great as that of a chemical 
analysis. In making bacterial counts we are dealing with living 
organisms which are distributed in the material under examination. 
The method of analysis follows the assumption that the bacteria, as 
individual cells, are distributed evenly throughout the sample and 
that the portion removed for analysis contains a number in exact 
proportion to the total number in the sample. Having removed a 
definite part, it must then be placed in a medium suitable for plating 
in which the individual bacterial cells can multiply and form visible 
colonies. The inaccuracy of such a method must be evident at once. 
We know that some bacteria are in clumps or chains, and many 
organisms may then develop into one colony which must be counted 
as a single colony. The removal of a quantity of material which will 
contain the same number of bacteria in suspension as another like 
quantity is known to be impossible. Since we are dealing with 
95793°— Bull. 563—17 1 
