EFFECT OF AGING ON BACTERIAL COUNT 7 



No definite increase in bacterial count occurred up to 10 hours. Of the mixes 

 held for 10 hours, 3 showed an increase and 4 were the same. After 12 hours, 

 the number of organisms increased in 7 cases, decreased in 1 case, and remained 

 the same in 2 cases. In all but one case the bacterial count showed a marked 

 increase after 24 hours at 68° F. The results obtained in this experiment indicate 

 that when a pasteurized mix is held at 68° F., a definite increase in bacterial count 

 does not occur until after the tenth hour. 



Effect of Aging Treatment on the Bacterial Count of 

 Unpasteurized Ice Cream Mixes 



Although the majority of commercial ice cream mixes are pasteurized, un- 

 pasteurized mixes were used in this experiment for comparison. The data are 

 given in Table 4 and summarized in Table 2. Both high and low count mixes 

 were used. 



Table 4. --Effect of Aging Treatment on the Bacterial Count of 

 Unpasteurized Mixes. 



*After Homogenization. 



Table 4 shows that when a raw. mix is held at 68° F., a definite increase occurs 

 after 6 hours; and this increase, as would be expected, becomes more marked 

 after 12 and 24 hours. The summary in Table 2 indicates clearly that the 

 bacterial counts of raw mixes aged at 68° F., whether followed by low-temperature 

 aging or not, are definitely higher than the bacterial counts of the control mixes 

 aged at 38° F. 



GENERAL DISCUSSION 



It has been generally assumed that if an ice cream mix is not cooled immediately 

 after homogenization to approximately 40° F., substantial increases in bacteria 

 will occur. In a previous study (3) as well as in this study, it has been shown 

 that pasteurized ice cream mixes may be aged for 6 hours at 68° F. with no 

 marked increase in bacterial count. In all probability the effect of temperature 

 shock on the organisms and also the type of organisms surviving pasteurization 

 are chiefly responsible for the absence of any marked increase in bacteria when a 

 pasteurized mix is held at 68° F. for 6 hours or even for 10 hours. 



Throughout this experiment most of the mixes were handled so as to prevent 

 recontamination after pasteurization. In some preliminary experiments, how- 



