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. GibsonDepartment of Dairy and Food Science

University of SaskatchewanSaskatoon, Saskatchewan

Samples of ice cream for bacteriological analyses arecommonly 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 24hours after sampling.

After pasteurization and before aging, ice cream mixwas inoculated with a nutrient broth culture of Escherichiacoli (non-hemolytic) isolated from ice cream. Before freez­ing, the mix was plated with Violet Red Bile (VRB) agar todetermine the initial number of coliforms (ranging from360 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 eachsample was plated in duplicate according to the proceduredescribed in "Standard Methods for the Examination ofDairy Products" (American Public Health Association,1972).

The results of three trials showed that the freezingprocess caused a sharp decrease in coliforms. This is con­trary to some reports (Foster et ai., 1957) (Prucha andBrannon, 1926) which indicate an increase in plate countsdue 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 weregrown in the mix before freezing, in this study a well sha­ken broth culture with few or no clumps of cells was addedto the mix. It is believed that the latter method moreclosely represents practical conditions where contaminantsenter the mix from improperly cleaned equipment, but arerestricted in their growth there due to low storage tempera­tures ~nd relatively short periods of time prior to freezingthe mIX.

!he decrease in the coliform population during the!re~zmg process was such that only about 15 to 25% of the1~ltlal number was still viable (Figure 1). It is possible thatdifferent types of coliforms may show variations in the rateof destruction in the freezer. Further reductions during 15

57

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Fig. I P~rccntage of coliforms remaining viable in ice ,,:ream stored al -25°C. (Average of threetnals.)

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

ReferencesAmerican 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