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Continuing Multiplication of Salmonella enteritidis Strains in Egg Yolk During Refrigeration

21 February 2014

The results of a USDA Agricultural Research Service experiment support the prompt refrigeration of eggs to 7.2 degrees Centigrade to control Salmonella enteritidis but even that did not halt multiplication of some of the strains of this potential foodborne pathogen tested.

The continuing attribution of human illness caused by Salmonella enteritidis to the consumption of contaminated eggs has led to widespread implementation of risk reduction programmes for commercial egg production, often emphasising prompt refrigeration of eggs to prevent bacterial multiplication to dangerously high levels, according to Richard K. Gast and Rupa Guraya of the USDA Agricultural Research Service's Egg Safety and Quality Research Unit at the Russell Research Center in Athens, Georgia.

However, they report in a paper in International Journal of Poultry Science that microbial growth may not cease immediately inside warm eggs after transfer to refrigerated storage.

Their recent study compared the abilities of eight S. enteritidis strains (of four phage types) to continue multiplying in experimentally contaminated egg yolk during the first 24 hours after transition from warm to refrigeration temperatures.

After 15-mL samples of egg yolk were inoculated with 10 colony-forming units (CFU) per ml of S. enteritidis, they were incubated at 37°C for 16 hours and then transferred into refrigeration at 7.2°C for 24 hours.

Bacterial cell concentrations were determined following 37°C incubation and again after both eight and 24 hours at 7.2°C.

All eight S. enteritidis isolates multiplied significantly during 16 hours of incubation, reaching an overall mean of log10 8.790CFU per ml.

After refrigeration, the observed mean values for cell concentrations in yolk samples were log10 8.780CFU per mL at eight hours and log10 8.849CFU per mL at 24 hours.

For three of the eight strains tested, a significant (p<0.05) increase in cell concentrations in egg yolk occurred during 24 hours of refrigeration.

These results support the importance of prompt egg refrigeration for minimising the numbers of S. enteritidis in marketed table eggs, concluded Gast and Guraya. They added that, nevertheless, refrigeration at 7.2°C may not immediately or completely arrest multiplication by all strains.


Gast R.K. and R. Guraya. 2013. Continuing multiplication of Salmonella enteritidis strains in egg yolk during refrigeration at 7.2°C. International Journal of Poultry Science. 12(11):622-627.

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February 2014

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