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Cross-contamination from the external surface of eggs in relation to risk of exposure of Salmonella
Project Code: B03016
London Metropolitan University
This project has been undertaken to provide the Food Standards Agency with the data required to analyse the risk to the consumer posed by eggshell contamination with Salmonella. This combines the basic question of the ability of Salmonella to survive on egg shells with studies of the extent to which transfer from the shells occurs. It was not practicable to investigate all aspects of transfer and effort was concentrated on certain key areas. These were transfer into the contents of the egg on breaking, transfer to hands during simulated culinary manipulations and transfer onto three types of simulated food contact material. The long-term risk associated with accumulation of Salmonella when storage containers are subject to repeated use was also investigated.
The outcome of survival experiments, based on viable counts, showed a high level of variability. Extensive statistical analysis was made by Dr Peter Cripps, Consultant Statistician of the University of Liverpool Veterinary School. In summary, he concluded that, while numbers tended to decline with storage, it is never possible to state that Salmonella will not be present. The outcome confirmed that, based on the prevalence of Salmonella across the whole of egg production, risk of exposure is very low. There is, however, likely to be a small risk associated with contamination of eggs at particular farms. Simulations showed that, in most cases, exposure would involve numbers of Salmonella unlikely to cause symptomatic infection in healthy persons. In a domestic situation, however, susceptible persons, including those at the extremes of age and pregnant women, are also potentially liable to exposure
Investigation of the possibility that cells were entering a viable non-recoverable state posed significant technical difficulties. It was possible to conclude, however, that entry into the viable non-recoverable state is not the general pattern of behaviour and that the cultural methods used provided a true measure of the numbers of viable bacteria present.
Studies concerning transfer of Salmonella from the egg shell into egg contents on breaking, to gloved hands and onto simulated work surfaces demonstrated the potential for contamination of foods by a number of routes. In all cases, transfer rates were relatively high, with transfer to gloved hands being in the order of 90 to 100% under moist conditions. Transfer into the egg contents on breaking and onto simulated work surfaces was also high, although rates were generally lower than onto gloved hands.
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