A quantitative assessment of the exposure to Listeria monocytogenes from cold-smoked salmon (CSS) consumption in France is developed. The general framework is a second-order (or two-dimensional) Monte Carlo simulation, which characterizes the uncertainty and variability of the exposure estimate. The model takes into account the competitive bacterial growth between L. monocytogenes and the background competitive flora from the end of the production line to the consumer phase. An original algorithm is proposed to integrate this growth in conditions of varying temperature. As part of a more general project led by the French Food Safety Agency (Afssa), specific data were acquired and modeled for this quantitative exposure assessment model, particularly time-temperature profiles, prevalence data, and contamination-level data. The sensitivity analysis points out the main influence of the mean temperature in household refrigerators and the prevalence of contaminated CSS on the exposure level. The outputs of this model can be used as inputs for further risk assessment.
The natural contamination of foods with a bacterial pathogen frequently consists of a mixture of strains with their own characteristics of survival, growth potential and virulence. Quantitative Microbial Risk Assessment (QMRA) must account for this genetic diversity to reflect the variability of the pathogen risk and to identify the genetic groups present at key stages of the food pathway. To describe the transmission dynamics of a heterogeneous population of B. cereus, we developed an exposure model that covers a food processing chain from "farm to table". The studied food was a cooked, pasteurised and chilled courgette purée used as an example of Refrigerated Processed Food of Extended Durability (REPFED). The B. cereus population consists of a continuum of genetic groups ranging from mesophilic and highly heat resistant, to psychrotrophic and moderately-heat resistant ones. At each step in a processing chain comprising cooking, blending, mixing with ingredients providing a secondary contamination, pasteurisation and chilling for several weeks, the prevalence of contaminated units (batches or packages) and the spore load within the units was determined for each genetic group, as well as their proportion to the total B. cereus population in the units. The model predicted that all packages contain mesophilic groups just after partitioning. The addition of mesophilic strains by the ingredients during the process of the courgette purée was an important contribution. At the end of the domestic storage, the model predicted a dominance of the mesophilic groups, while only some psychrotrophic groups were present.
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