Uncertainty distribution associated with estimating a proportion in microbial risk assessment

International audienceThe uncertainty associated with estimates should be taken into account in quantitative risk assessment. Each input's uncertainty can be characterized through a probabilistic distribution for use under Monte Carlo simulations. In this study, the sampling uncertainty associated with estimating a low proportion on the basis of a small sample size was considered. A common application in microbial risk assessment is the estimation of a prevalence, proportion of contaminated food products, on the basis of few tested units. Three Bayesian approaches (based on beta(0, 0), beta , and beta(l, 1)) and one frequentist approach (based on the frequentist confidence distribution) were compared and evaluated on the basis of simulations. For small samples, we demonstrated some differences between the four tested methods. We concluded that the better method depends on the true proportion of contaminated products, which is by definition unknown in common practice. When no prior information is available, we recommend the beta prior or the confidence distribution. To illustrate the importance of these differences, the four methods were used in an applied example. We performed two-dimensional Monte Carlo simulations to estimate the proportion of cold smoked salmon packs contaminated by Listeria monocytogenes, one dimension representing within-factory uncertainty, modeled by each of the four studied methods, and the other dimension representing variability between companie

Growth potential assessment of listeria in milk fat products by challenge testing.

Milk fat products (MFP), including butter and low-fat dairy spreads, are a specific type of ready-to-eat food known as water-in-fat emulsions, in which the behavior of microbial foodborne pathogens such as Listeria onocytogenes is not clearly known. This study investigated the growth and survival of L. monocytogenes, and of Listeria innocua as a surrogate for L. monocytogenes, in these foods using challenge testing. Three commercial MFPs with various fat contents (butter, half butter and low-fat dairy spread) and two samples of traditional churned butter with various water droplet sizes were artificially contaminated with Listeria. Total mesophilic microflora including lactic acid bacteria, pH and Listeria were monitored throughout the shelf life. The growth potential of Listeria was calculated in the course of the shelf life and remains below the limit value of 0.5 log cfu/g during the whole shelf life in any of thebutter. However, the concentration of Listeria remained stable during the shelf life in the tested MFPs (commercial and churned) except in the commercial low-fat dairy spread in which Listeria decreased gradually

Modelling the growth kinetics of Listeria monocytogenes in pasta salads at different storage temperatures and packaging conditions

The aim of this study was to model Listeria monocytogenes growth kinetics in ready to eat full meal pasta salads, containing fresh and cooked ingredients. With this aim, laboratory prepared salads, representing two formulations of commercial pasta salads, were spiked with L. monocytogenes and tested under categorised packaging and storage temperature conditions. L. monocytogenes enumeration results collected in 15 different laboratory prepared salad datasets were analysed with primary and secondary models. The models showing the best fit to describe L. monocytogenes growth kinetics in the laboratory prepared salads were then validated within commercial pasta salads. Baranyi no-lag was the best primary model fitting datasets collected at 12 \ub0C, whereas the exponential model gave the best results for datasets collected at 4 \ub0C. The maximum microbial specific growth rate (\u3bcmax) mean values obtained at 4 and 12 \ub0C for salads packaged under air packaging conditions were 0.008 \ub1 0.003 and 0.036 \ub1 0.006 log10(cfu/g) h 121, respectively. At the same temperatures, the \u3bcmaxmean values obtained under modified atmosphere were 0.005 \ub1 0.005 and 0.026 \ub1 0.005 log10(cfu/g) h 121, respectively. The Gamma secondary model predicted the growth kinetics of L. monocytogenes at both temperatures and packaging conditions and the \u3bcmaxat the optimum temperature and the optimum pH for Listeria growth (\u3bcopt) estimated by the model corresponded to 0.247 \ub1 0.009 log10(cfu/g) h 121. Baranyi model without lag phase was used to generate growth kinetics under different scenarios. In the comparison of the predicted log10concentrations respect to the observed ones the residues rarely exceeded 1 Log10cfu/g. The selected models can be applied to describe the growth kinetics of L. monocytogenes in similar types of pasta salads with comparable pH, shelf life and storage conditions

Matière condensée : organisation et dynamique

International audienceno abstrac

Matière condensée : organisation et dynamique

International audienceno abstrac