Coliform Bacteria in Ice Cream Mix and Ice Cream

Research Note

Coliform Bacteria in Ice Cream Mix and Ice Cream G. Blankenagel and D. L. Gibson Department of Dairy and Food Science University of Saskatchewan Saskatoon, Saskatchewan

Samples of ice cream for bacteriological analyses are commonly packed in dry ice for shipping to the laboratory. Several federal and provincial agencies state in their regulations that such samples must be analyzed within 24 hours after sampling. After pasteurization and before aging, ice cream mix was inoculated with a nutrient broth culture of Escherichia coli (non-hemolytic) isolated from ice cream. Before freezing, the mix was plated with Violet Red Bile (VRB) agar to determine the initial number of coliforms (ranging from 360 to 7,000/ g) and again immediately after freezing. After various lengths of time in the hardening room at -25°C, two packages of ice cream were removed and each sample was plated in duplicate according to the procedure described in "Standard Methods for the Examination of Dairy Products" (American Public Health Association, 1972). The results of three trials showed that the freezing process caused a sharp decrease in coliforms. This is contrary to some reports (Foster et ai., 1957) (Prucha and Brannon, 1926) which indicate an increase in plate counts due to breaking up clumps of cells. The reason for this discrepancy lies probably in the history of the test organism. While III the reports mentIOned above, bacteria were grown in the mix before freezing, in this study a well shaken broth culture with few or no clumps of cells was added to the mix. It is believed that the latter method more closely represents practical conditions where contaminants enter the mix from improperly cleaned equipment, but are restricted in their growth there due to low storage temperatures ~nd relatively short periods of time prior to freezing the mIX. !he decrease in the coliform population during the !re~zmg process was such that only about 15 to 25% of the 1~ltlal number was still viable (Figure 1). It is possible that different types of coliforms may show variations in the rate of destruction in the freezer. Further reductions during 15

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days of storage in the hardening room were very sli"ght. This seems to confirm that there is a critical temperature range from -1 to -5°C (Haines, 1938) and that organisms surviving this range may remain viable for a long time in frozen foods. It appears that the time of determining the number of coli forms in frozen ice cream is not too critical as long as the analysis is performed within a week or two after sampling.

References American Public Health Association. 1972. Standard Methods for the Examination of Dairy Prod~ UCIS. I3lh Edition. Foster. E. M.. Nelson. F. E.. Speck. M. L.. Doetsch. R. N.. and Olson. J. C. 1957. Dairy Micro' biology. Prentice-Hall. Inc. Haines, R. B. 1938. The effect of freezing on bacteria. Roy. Soc. (London) Proc. B 124:451. Prucha, M. J., and Brannon, 1. M. 1926. Viability of Bacterium l)phosum in icc ('fearn. J. Bact. 11 :27. Received August 19. 1974

J. Insl. Can. Sci. Technol. Aliment. Vol. 8. No. I. 1975