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Significance of Colon-Aerogenes Group in Ice Cream^*

I. Survival of members of the Esceoerichia-aerobacter group to pasteurizing temperatures in ice cream

Michigan State College, East Lansing, Michigan

ABSTRACT

1. The thermal death point of 33 cultures of Escherichia coli, 7 cultures of Aerobacter aerogenes and 4 lactose-fermenting organisms isolated from water but not definitely classified showed considerable variation when heated in ice cream at temperatures of 60°C. (140°F.), 62.8°C. (145°F.) and 65.5°C. (150°F.) for 30 minutes.
   Four determinations were made at a temperature of 60°C. The number surviving expressed in per cent was 51.2, 39.0, 36.0 and 57.0 respectively for each determination. When the temperature was increased to 62.8°C, the number surviving decreased somewhat being 22.7, 6.8, 2.2 and 22.7 per cent respectively for the successive determinations. At a temperature of 65.5°C. the number surviving is still further decreased and was 0.0, 2.4, 2.4 and 9.1 per cent respectively while at a temperature of 68.3°C. none of the cultures tested survived.
   These data indicate that the critical temperature for the Escherichia-Aerogenes group in ice cream is about 65.5°C.
   
2. Ice cream has a greater protective action than skim milk for members of the Escherichia-Aerogenes group when heated at temperatures of 60°C. and 62.8°C. for 30 minutes. At the former temperature in skim milk there were 22 cultures out of the 44 cultures used which failed to survive any of the four heatings as compared to 11 cultures in ice cream. When the temperature was raised to 62.8°C. all the cultures were killed in skim milk in each instance while in ice cream there were certain cultures still surviving.
   
3. Thermal death point determinations made with the cultures in the different ingredients used in making ice cream, viz. cream, sucrose, milk powder and gelatin failed to show any marked protective action of any one of the ingredients.
   
4. The susceptibility of bacterial cells to heat is greatly influenced by their age, young cells are more readily killed than are the older ones.
   
5. The ability of many strains of the Escherichia-Aerogenes group to survive a temperature of 62.8°C. and even 65.5°C. in ice cream should be taken into consideration when using the colon test as an index of the efficiency of pasteurization.
   
6. From the standpoint of reduction in numbers of the Escherichia-Aerogenes group, pasteurizing the ice cream mix at a temperature of 65.5°C. is more desirable than the lower temperature 62.8°C. which is more commonly used.
   

^* Published by permission of the Director of the Michigan Agricultural Experiment Station as Journal Article No. 28 (N. S.). Data from thesis presented in partial fulfillment of the requirements for the Master of Science degree in Michigan State College by the junior author.