EFFECT OF AGING TREATMENT ON THE BACTERIAL 

 COUNT OF ICE CREAM MIXES 



By W. S. Mueller, Assistant Research Professor of Dairy Industry and 

 R. L. France, Assistant Research Professor of Bacteriology 



INTRODUCTION 



It is a general belief in the ice cream industry that after the mix has been homo- 

 genized it must be cooled at once to a temperature of approximately 40° F. in 

 order to avoid marked increases in bacterial growth during the aging period. 

 However, to our knowledge, there is no experimental evidence to support this 

 general assumption. No doubt aging an ice cream mix above 50° F. for 24 hours 

 would result in a marked bacterial increase. Olson and Fay (1) reported that 

 the aging, without agitation, of improperly cooled mix (65° F.) in cans in a cold 

 room (38° F.), resulted in a marked increase in bacterial content in 24 hours. 



The question remains as to whether or not an increase in bacteria would occur 

 if a pasteurized mix were held for a relatively short time at a temperature above 

 50° F. The comparatively recent advocacy of a reduction of the aging period 

 from 24 hours, or longer, to 2 to 4 hours makes it appear more feasible to use 

 higher aging temperatures. 



In previous publications (2, 3) by the senior author, it was shown that when an 

 ice cream mix contained gelatin the efficiency of this ingredient was increased 

 by aging the ice cream mix for 4 hours at 68° F., regardless of whether or not this 

 was followed by aging at a lower temperature. The efficiency of the gelatin was 

 increased to such an extent that the gelatin could be reduced approximately 

 one-fourth without impairing the quality of the finished product. Also, a limited 

 number of trials showed that aging a pasteurized mix at 68° F. for 2, 4, and 6 hours 

 had no apparent effect on the bacterial growth, pH, or titratable acidity, which 

 would be of any commercial significance. 



The purpose of the work herein reported was: to check the results previously 

 reported concerning the effect on the bacterial count of aging ice cream mixes 

 at 68° F. for 6 hours; to study the effect on the bacterial count of aging ice cream 

 mixes at 68° F. for 6 hours, followed by aging at 38° F. for 18 hours; to determine 

 at what time significant increases in bacterial count occur in ice cream mixes 

 held at 68° F. 



PLAN OF EXPERIMENT 



The general plan of this study was to subject both pasteurized and raw ice 

 cream mixes to the following aging treatments, and compare the total bacterial 

 counts of the mixes so treated: 



(1) Unaged. 



(2) Held 6 hours at 38° F. 



(3) Held 6 hours at 68° F. 



(4) Held 24 hours at 38° F. 



(5) Held 6 hours at 68° F., plus 18 hours at 38° F. 



