2 BULLETIN 563, U. S. DEPARTMENT OF AGRICULTURE. 



living organisms, the bacteriological method of analysis must take 

 into account their distribution and development into colonies on the 

 Petri plates. In this part of the method we encounter the difficulty 

 of separating the bacterial cells and distributing them evenly. Their 

 development is more or less influenced by the growth of different 

 kinds of bacteria, one of which may retard the development of those 

 near it. 



All these points are recognized by bacteriologists and are men- 

 tioned here merely to call attention to the many difficulties which 

 ^^^ arise in making accurate bacterial counts and to point 

 .f^^^V ou t that there must be variations in the result over 

 if I which the analyst has no definite control. This con- 



1/ 3 i dition has been recognized, consequently duplicate 



ij^Nssggs^i! plates are made and results reported from the average 



ii 5 counts of both plates. The variation in bacterial 



counts is particularly important and must be taken 

 into consideration when a study is made of various 

 samples of any material containing bacteria. If these 

 variations are not considered, mistakes are easily made 

 in the study of the distribution of bacteria. 



If the bacteria in ice cream are unevenly dis- 

 tributed, and a bacterial analysis of a sample does not 



Fig. 1.— Showing ' . J c 



location of nine give results which will represent the whole mass of the 

 from Pl6S i-gaUon cream, this fact will greatly complicate any study of 

 can of ice the bacteria in the product. Consequently, before 

 starting any further studies on the subject it was con- 

 sidered advisable to carry out some experiments to throw more 

 light on this point. 



METHOD OF SAMPLING AND PLATING THE ICE CREAM. 



Ice cream from various manufacturers was delivered in 1 -gallon 

 cans at the laboratory. As soon as received, the can of cream was 

 removed from the tub, the ice and salt wiped off, the cover removed, 

 and the top layer taken off with a large sterile spoon. 



Three samples were taken from the topmost third of the gallon, three 

 from the middle, and three from the bottom third, making a total of 

 nine samples from each gallon, as shown in figure 1. Each sample 

 contained about 30 grams and was removed with a small sterile scoop 

 and placed in a sterile flask. After removing samples from positions 

 1, 2, and 3, about one-third of the ice cream was removed with a 

 sterile spoon and three more samples taken from positions 4, 5, and 6; 

 similarly the three remaining samples were taken from positions 

 7, 8, and 9. 



The flasks containing the samples were then placed in water at 

 40° C. (104° F.) for 15 minutes in order to melt the ice cream, the 



