Risk Assessment or Assessment of Risk? Developing an Evidence-Based Approach for Primary Producers of Leafy Vegetables To Assess and Manage Microbial Risks.

Over the last 10 years, some high-profile foodborne illness outbreaks have been linked to the consumption of leafy greens. Growers are required to complete microbiological risk assessments (RAs) for the production of leafy crops supplied either to retail or for further processing. These RAs are based primarily on qualitative judgements of hazard and risks at various stages in the production process but lack many of the steps defined for quantitative microbiological RAs by the Codex Alimentarius Commission. This article is based on the discussions of an industry expert group and proposes a grower RA approach based on a structured qualitative assessment, which requires all decisions to be based on evidence and a framework for describing the decision process that can be challenged and defended within the supply chain. In addition, this article highlights the need for evidence to be more easily available and accessible to primary producers and identifies the need to develop hygiene criteria to aid validation of proposed interventions.

Use of global sensitivity analysis in quantitative microbial risk assessment: application to the evaluation of a biological time temperature integrator as a quality and safety indicator for cold smoked salmon.

The aim of this study was to apply a global sensitivity analysis (SA) method in model simplification and to evaluate (eO)®, a biological Time Temperature Integrator (TTI) as a quality and safety indicator for cold smoked salmon (CSS). Models were thus developed to predict the evolutions of Listeria monocytogenes and the indigenous food flora in CSS and to predict TTIs endpoint. A global SA was then applied on the three models to identify the less important factors and simplify the models accordingly. Results showed that the subset of the most important factors of the three models was mainly composed of the durations and temperatures of two chill chain links, out of the control of the manufacturers: the domestic refrigerator and the retail/cabinet links. Then, the simplified versions of the three models were run with 10(4) time temperature profiles representing the variability associated to the microbial behavior, to the TTIs evolution and to the French chill chain characteristics. The results were used to assess the distributions of the microbial contaminations obtained at the TTI endpoint and at the end of the simulated profiles and proved that, in the case of poor storage conditions, the TTI use could reduce the number of unacceptable foods by 50%.

Applicability of biological time temperature integrators as quality and safety indicators for meat products.

The objective of this study was to evaluate (eO), a biological time temperature integrator (TTI) as a quality and safety indicator for ground beef packed under modified atmosphere and spiced cooked chicken slices packed under modified atmosphere. Storage trials and challenge tests were thus performed on several batches of the studied food to monitor and model the behavior of Listeria monocytogenes, Salmonella, Staphylococcus aureus and the indigenous food flora. Then, two different prototypes of the TTI (eO) were set and manufactured according to the studied products shelf lives. The TTI evolution with time at static and dynamic temperatures was monitored and modeled. Finally, exposure assessment models were set and used under several realistic storage profiles to assess the distributions of the concentration of the indigenous food flora and the distributions of the increase in the pathogens populations obtained at the end of the product shelf life or at the end point of the TTI, taking into account the TTIs batch variability. Results showed that in case of poor storage conditions, TTI can reduce the consumer exposure to altered or hazardous foods.

Modelling the behaviour of Listeria monocytogenes in ground pork as a function of pH, water activity, nature and concentration of organic acid salts.

AIMS: to study and model the effect of sodium acetate, sodium lactate, potassium sorbate and combination of acid salts on the behaviour of Listeria monocytogenes in ground pork. METHODS AND RESULTS: Water activity (a(w)), pH and concentration of acid salt of the meat were adjusted. The behaviour of inoculated L. monocytogenes was studied and modelled according to physicochemical parameters values. Whatever the acid salt concentration used, we observed an inhibition of the growth of L. monocytogenes at pH 5.6 and a(w) 0.95. At pH 6.2 and a(w) 0.97, addition of 402 mmol l(-1) of sodium lactate or 60 mmol l(-1) of potassium sorbate was required to observe a slower growth. CONCLUSIONS: The inhibitory effect of acid salts was a function of pH, a(w), as well as of the nature and concentration of acid salts added. When one acid salt was added, the Augustin's model (Augustin et al. 2005) yielded generally correct predictions of either the survival or growth of L. monocytogenes. SIGNIFICANCE AND IMPACT OF THE STUDY: The suggested model can be used for risk assessment concerning L. monocytogenes in pork products.

Automated image analysis of bacterial colony growth as a tool to study individual lag time distributions of immobilized cells.

A method to determine the individual lag time (lag) distributions of immobilized bacteria was presented. The method was based on the image analysis of the bacterial colony growth. The lag distributions were retrieved from the distributions of the detection times (Td) required to form macroscopically visible colonies. Using this method, the lag distributions on agar for Listeria monocytogenes cells previously subjected to two situations reproducing conditions encountered during the contamination of cheese, were determined. The results were presented and compared with lag distributions obtained with an established method based on the time to detection of turbidity in broth. An original method to retrieve lag in broth and agar without any knowledge of the growth rate was also proposed. In order not to bias the distributions of lag on agar the impact of spatial separation between colonies on colony growth rates was quantified. Means and standard deviations of lag distributions for the two different stresses were found to be similar in broth and on agar. Extreme Value type II distributions fitted the best the different datasets of lag distributions.

Growth rate and growth probability of Listeria monocytogenes in dairy, meat and seafood products in suboptimal conditions.

AIMS: To evaluate the performances of models predicting the growth rate or the growth probability of Listeria monocytogenes in food. METHODS AND RESULTS: Cardinal and square root type models including or not interactions between environmental factors and probability models were evaluated for their ability to describe the behaviour of L. monocytogenes in liquid dairy products, cheese, meat and seafood products. Models excluding interactions seemed sufficient to predict the growth rate of L. monocytogenes. However, the accurate prediction of growth/no-growth limits needed to take interactions into account. A complete and a simplified form (preservatives deducted) of a new cardinal model including interactions and parameter values were suggested to predict confidence limits for the growth rate of L. monocytogenes in food. This model could also be used for the growth probability prediction. CONCLUSIONS: The new cardinal model including interactions was efficient to predict confidence limits for the growth rate of L. monocytogenes and its growth probability in liquid dairy products, meat and seafood products. In cheese, the model was efficient to predict the absence of growth of the pathogen. SIGNIFICANCE AND IMPACT OF THE STUDY: The suggested model can be used for risk assessment and risk management concerning L. monocytogenes in dairy, meat and seafood products.

Influence of stress on individual lag time distributions of Listeria monocytogenes.

The effects of nine common food industry stresses on the times to the turbidity (T(d)) distribution of Listeria monocytogenes were determined. It was established that the main source of the variability of T(d) for stressed cells was the variability of individual lag times. The distributions of T(d) revealed that there was a noticeable difference in response to the stresses encountered by the L. monocytogenes cells. The applied stresses led to significant changes of the shape, the mean, and the variance of the distributions. The variance of T(d) of wells inoculated with single cells issued from a culture in the exponential growth phase was multiplied by at least 6 and up to 355 for wells inoculated with stressed cells. These results suggest stress-induced variability may be important in determining the reliability of predictive microbiological models.

Mathematical modelling of the growth rate and lag time for Listeria monocytogenes.

Growth data for Listeria monocytogenes were collected from the literature and a global model built with existing secondary models describing independently the effects of environmental factors on the growth rate and lag time was based on these data. The growth rates calculated with this model were consistent with the published ones but the fit was poor near the limits of growth of the micro-organism. The model was also less accurate to describe the lag time. It seems then that reliable predictions of the growth rate of L. monocytogenes could be obtained in a wide range of growth conditions, but models should take into account interactions between environmental factors. Furthermore, it is necessary to better model the lag phase duration and particularly to model the effect of the history of the inoculum on the subsequent lag time.

Modelling the growth rate of Listeria monocytogenes with a multiplicative type model including interactions between environmental factors.

A multiplicative secondary model previously published to describe independently the effects of environmental factors on the growth rate of Listeria monocytogenes (Augustin and Carlier, 2000) was improved by taking into account interactions between these environmental factors. The proposed model allowed to decrease the rate of fail-safe growth predicted from 13.5% to 12.1% and the rate of fail-dangerous no growth predicted from 16.1% to 7.1%.

A model describing the effect of temperature history on lag time for Listeria monocytogenes.

A model was built to describe the influence of the temperature and the duration of pre-incubation on the lag time for regrowth of Listeria monocytogenes at low temperature. This model is consistent with the usual procedure used to calculate lag times of cultures growing under fluctuating temperatures. It also describes the effect of prolonged starvation conditions on the regrowth lag time and takes into account the influence of the physiological state of inocula in predictive models.

Significance of inoculum size in the lag time of Listeria monocytogenes.

The lag time of Listeria monocytogenes growing under suboptimal conditions (low nutrient concentrations, pH 6, and 6.5 degrees C) was extended when the inoculum was severely stressed by starvation and the inoculum size was very small. Predictive microbiology should deal with bacterial stress and stochastic approaches to improve its value for the agro-food industry.

Estimation of temperature dependent growth rate and lag time of Listeria monocytogenes by optical density measurements.

An automated turbidimetric system, Bioscreen C, was used to monitor growth of ten strains of Listeria monocytogenes at different temperatures. Several methods for estimation of maximum specific growth rate (mu(max)) and lag time (lag) from turbidimetric data were compared to values estimated from viable count data. By using a calibration factor, reliable estimations of mu(max) could be obtained from turbidimetric measurements. On the other hand, accurate estimations of lag required some viable count data.

Mathematical modelling of the heat resistance of Listeria monocytogenes.

The heat resistance of Listeria monocytogenes phagovar 2389/2425/3274/2671/47/108/340 (1992 French outbreak strain) in broth was studied at 55, 60 and 65 degrees C. Experiments were carried out on bacterial cultures in three different physiological states: cultures at the end of the log phase, cultures heat-shocked at 42 degrees C for 1 h, and subcultures of cells resistant to prolonged heating. Survivor curves were better fitted using a sigmoidal equation than the classical log-linear model. This approach was justified by the existence of heat resistance distributions within the bacterial populations. Peaks (log10 of heating time) of heat resistance distributions of untreated, heat-shocked, and selected cultures at 55, 60 and 65 degrees C were 0.34, -0.90 and -1.84 min, 0.74, -0.51 and -1.24 min, and 0.17, -0.94 and -1.45 min, respectively. The widths of the distributions are proportional to 0.29, 0.36 and 0.41 min0.5, 0.26, 0.36 and 0.41 min0.5, and 0.34, 0.44 and 0.41 min0.5. An increase in the thermal tolerance could then be induced by sublethal heat shock or by selection of heat resistant cells.

[Resistance of Listeria monocytogenes to physical exposure]. / Résistance de Listeria monocytogenes aux traitements physiques.

The resistance of Listeria monocytogenes to physical processing, particularly heat resistance and radioresistance, is widely dependent on the method involved, the physiological state of the strain used, and, obviously, the substrate in which the organism is. HTST pasteurization of milk would allow at least 11 decimal reductions of the potentially present population of L. monocytogenes, and then greatly minimizes the risks of survival of the organism. On the other hand, high and low pasteurizations of egg products may involve only 4 to 5 decimal reductions and appear then not very reliable towards Listeria. Similarly, meat products cooking can, in some conditions, be inadequate to allow the total inactivation of contaminant L. monocytogenes. A 3 kGy irradiation of meat products should allow, on an average, 6 decimal reductions. These results must incite the manufacturers to take into account factors present in their products which allow L. monocytogenes to better resist and this in order to adapt processing to these conditions of increased resistance.