The significance of the psychrotrophic foodborne pathogen, L. monocytogenes in the egg industry requires considerable investigation as the industry continues to diversify its product range to include minimally processed products retailed under cold storage.
In this study, a comparative evaluation of standard methodologies, frequently used to detect L. monocytogenes in dairy and meat products, was undertaken to determine their suitability for recovery and isolation of non-stressed and heat-stressed L. monocytogenes in various egg products. The modified USDA protocol utilising UVM I and Fraser broths for enrichment followed by isolation on Oxford or PALCAM agars proved to be the most effective and robust procedure for recovery of stressed and non-stressed L. monocytogenes from raw liquid whole egg, pasteurised liquid whole egg, sugared yolk and albumen.
Due to the psychrotrophic nature of L. monocytogenes and its ability to withstand adverse environmental conditions, the survival of L. monocytogenes during frozen and chilled storage in various egg products was examined. Another behavioural aspect studied was the heat resistance of L. monocytogenes in pasteurised liquid whole egg and sugared yolk before and after a heat shock process.
L. monocytogenes survived the short (two day) chilled shelf life of all the products and nine months of frozen storage in all the egg products except albumen. The determined heat resistance of L. monocytogenes indicated the adequacy of the industrial pasteurisation regimes used for liquid whole egg and sugared yolk. A marginal heat shock response was observed amongst tempered L. monocytogenes cells in sugared yolk.
Finally, a study was undertaken to
determine the prevalence of Listeria spp. at the egg farm, in products at different stages of processing and in the factory environment. The isolates were ribotyped to identify sources of contamination within the processing environment. L. monocytogenes was detected in one product (shelled egg) however, L. innocua was readily isolated from the farm to factory environment. Farm producer pulp was the main external source of contamination while transmission within the processing environment was via floor drains and mobile factory equipment. The low prevalence of L. monocytogenes throughout the egg processing environment indicated this microorganism was not a significant threat to the egg industry. However, the observed ability of L. monocytogenes to survive environmental stresses (i.e. frozen and chilled storage) common to the manufacture of egg products and the ubiquitous presence of
Listeria spp. (i.e. L. innocua) in the production environment, dictates the need for a risk management approach by the egg industry to ensure Listeria- free products.