EFFECT OF AGING ON BACTERIAL COUNT 3 



Also, pasteurized and raw mixes were held at 68° F. for varying periods of time 

 in order to determine when the bacterial counts start to increase. Although raw 

 mixes are rarely used in commercial practice, they were included in this experiment 

 for comparison with pasteurized mixes, in order to obtain if possible some informa- 

 tion as to why the bacterial content of the pasteurized mixes does not increase 

 markedly with holding at 68° F. for 6 hours. 



Experimental Methods 



Preparation of the pasteurized mixes. Most of the ice cream mixes were pre- 

 pared and processed in the college creamery. A few raw and processed mixes 

 were obtained from nearby commercial ice cream plants, the raw mixes being 

 subsequently pasteurized at the college creamery. The mixes represented a 

 variety in formulas, ingredients, and size of batch. All of the ingredients, with 

 the exception of the flavoring material, were mixed, pasteurized, and homogenized 

 together. 



Pasteurization. The ice cream mixes were pasteurized in most instances at 

 145° to 150° F. for 30 minutes. In some instances other combinations of tem- 

 perature and time were used in order to secure higher initial bacterial counts. 



Homogenization. Most of the mixes were homogenized at 2500 pounds pres- 

 sure per square inch, at the temperature of pasteurization. In most instances 

 the homogenizer was thoroughly washed and sterilized before being used. In 

 some instances, in order to secure higher initial bacterial counts, the homogenizer. 

 was not thoroughly cleaned. 



Cooling and Aging. Immediately after homogenization the mixes were cooled 

 to their respective aging temperatures either over the surface cooler, or in sterile 

 flasks set in cold brine. Aging was done in sterile flasks which were placed in 

 either a thermostatically controlled water bath or a thermostatically controlled 

 temperature box. Samples from these flasks were plated at various times during 

 the aging period. 



Preparation of the unpasteurized mixes. The mixes used represented a variety 

 in formulas, ingredients, and size of batch. All of the ingredients, with the 

 exception of the flavoring material were mixed, heated to approximately 100° F., 

 and immediately homogenized at 2500 pounds pressure per square inch. After 

 homogenization the unpasteurized mixes were treated in the same manner as 

 the pasteurized mixes. 



Bacteriological methods. Samples for the bacteriological analyses were weighed, 

 and the number of bacteria per gram of sample were determined by the agar 

 plate count method recommended by the American Public Health Association (4). 



EXPERIMENTAL RESULTS 



Effect of Aging Treatment on the Bacterial Count 

 of Pasteurized Ice Cream Mixes 



Table 1 gives the original data obtained when pasteurized ice cream mixes 

 were subjected to various aging treatments. It will be observed that the bacterial 

 counts of the raw mixes covered a wide range, the highest being 58,000,000 per 

 gram and the lowest 1,000. When the counts before and after pasteurization 



