THE DETERMINATION OF BACTERIA IN ICE CREAM. 3 



melting being hastened by frequent shaking with a circular motion. 

 At the end of the 15-minute period each flask containing the melted 

 ice cream was again shaken 30 times, with a circular motion, in order 

 to mix the sample thoroughly and to shake out as much air as possible. 



One cubic centimeter of melted ice cream was then removed from 

 each flask and placed in 99 c. c. of sterile water at a temperature of 

 40° C. (104° F.). The water in all the dilution bottles was at a 

 temperature of 40° C. (104° F.) in order to keep the fat in a melted 

 condition. 



The pipettes were so graduated as to deliver 1 cubic centimeter 

 between two marks. This avoids the necessity of blowing out the 

 pipette or immersing the end in the dilution water, and therefore 

 eliminates the introduction of varying quantities of melted cream 

 which adhere to the pipette. 



The dilutions were made in the usual way, using 99 c. c. and 9 c. c, 

 respectively, of sterile water. Each dilution bottle or tube was shaken 

 25 times, and great care was taken to measure the quantity accu- 

 rately in the pipettes. Standard beef-infusion agar was used, and 

 sufficient medium was prepared to last through the work; conse- 

 quently no factor of variation was introduced by the plating medium. 

 The plates were incubated at 30° C. (86° F.) for a period of five days, 

 after which the duplicate plates were counted. 



VARIATION IN THE BACTERIAL CONTENT OF COMMERCIAL ICE CREAM. 



VARIATION IN DUPLICATE SAMPLES FROM VARIOUS PARTS OF THE SAME LOT. 



In our first experiment twenty- two 1 -gallon lots of ice cream were 

 obtained from seven different manufacturers. This cream, as in- 

 tended, was of different flavors, was made in different ways, and 

 included products containing different ingredients and varying 

 percentages of butterfat. 



The complete results obtained from a study of these samples are 

 shown in Table I, in which is recorded the percentage of fat in the 

 ice cream from each manufacturer, also the presence or absence of 

 gelatin, the flavor of each lot, the. dilution used in plating, the num- 

 ber of colonies found on each of the duplicate plates, and the calcu- 

 lated average number of bacteria in a cubic centimeter of melted 

 ice cream*. Where there is a blank space in the number of colonies 

 on duplicate plates no count could be made on account of "spreaders," 

 which entirely obscured the colonies. Every count that could be 

 obtained is included in this table of results, and no count was left 

 out as being a "freak' 7 result. 



A study of the table shows that the bacterial counts of the nine 

 samples from as many different positions in each gallon lot of ice 

 cream check remarkably well with one another. 



