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Improved control of Clostridium perfringens

Project Code: B14009

31/05/2004

Institute of Food Research Enterprises Ltd
Peck, M ; Baranyi, J; Plowman, J; LeMarc, Y; Aldus, C

Background

The Food Standards Agency seeks to reduce the burden of foodborne illness, and Clostridium perfringens has been identified as one of five pathogens for which a 20% reduction in foodborne illness is sought over a five-year period. In England and Wales in 2000, C. perfringens was the second most common cause of bacterial foodborne illness (behind Campylobacter), and the second most common cause of death associated with foodborne bacteria (behind Salmonella) [i]. C. perfringens is commonly found in low numbers in many foods especially meat and poultry, and is frequently associated with food poisoning when cooked foods are cooled too slowly. A particular concern is the safe production of large bulked meats. C. perfringens grows very rapidly between 40°C and 50°C, and slow cooling of the meat may permit rapid multiplication to an infectious dose. Once consumed, sporulation and associated enterotoxin production occurs, leading to illness. The application of appropriate effective cooling regimes should lead to a substantial reduction in the incidence of foodborne illness associated with C. perfringens.

Aims and Objectives

The aim is to provide information on appropriate safe cooling regimes for meats. The new approach taken in the current project is to produce a user-friendly computer software tool that can be used to predict growth of C. perfringens based on the temperature during cooling. This approach is followed in order to evaluate the safety of meat of every shape and size, and subject to any temperature cooling regime. The user (e.g. Environmental Health Officer (EHO), food processor) will input a cooling profile for the coolest part of the meat into the new software tool, and the output will be a prediction of whether a critical cell concentration is reached, and also whether this complies with advice from the Food Standards Agency on the safe production of cooked meats. The project objectives were:

Develop a dynamic growth model to predict the growth of C. perfringens during the heating/cooling of meats (for incorporation in the software tool).
Demonstrate that the dynamic growth model provides a good prediction of the growth of C. perfringens during the heating/cooling of meats.
Produce a user-friendly computer software tool to predict the growth of C. perfringens during the heating/cooling of meats. Prepare an easy to use manual explaining how to use the software tool.
Make the user-friendly computer software tool (and user manual) freely available on the internet. Give presentations and make publications to ensure that the availability of the user-friendly computer software tool is widely known.

Key Findings

Using the approach described by Baranyi [ii], a dynamic model has been developed to predict the growth of C. perfringens during the cooling of bulked meats. The model is based on 45 growth curves, and predicts growth within a range of 15°C-52°C. To take account of sub-lethal heat damage received by the spores(during typical heat treatments received by bulked meats (70°C to 95°C for 1.5 to 6h)), a value of α₀ = 10^-3 is used in the model.

The optimal sporulation media have been identified for twenty strains of C. perfringens previously associated with outbreaks of foodborne illness, and used to produce spores of these strains. The most hazardous five strains were then identified by assessing spore heat resistance and by measurement of growth in different heating/cooling profiles, and then used in a mixture, together, to validate the dynamic model for growth of C. perfringens during cooling of bulked meats. The model has been validated for ten heating/cooling regimes representative of those of interest to the food industry for use with bulked meats, three initial spore concentrations, and two meats (beef and turkey). Experimental reproducibility has been demonstrated. The observed increase in viable count from thirteen cooling profiles described in six refereed publications also closely agrees with predictions from the dynamic model for the appropriate cooling curve. This further confirms the validity of the new predictive model. This new model also appears to be superior to an existing model available in the US Pathogen Modeling Program.

A user-friendly software tool (called Perfringens Predictor) has been written. This software interface is an Excel add-in written in Visual Basic. The program enables the user to input temperature profiles (such as temperature logger data) into a spreadsheet. The software provides the user with a prediction of growth of C. perfringens under the specified dynamic cooling conditions, and with interpretation advice about predicted growth. This advice is automatically brought up after the predictions are displayed. Based on risk assessments carried out internationally, the Food Standards Agency recommend that the cooling of the meat should be sufficiently rapid to ensure that growth from heat-resistant spores of C. perfringens is minimal (i.e. does not allow more than a one log increase). The user is able to input three curves with different cooling rates to evaluate their effect on growth of C. perfringens and the risk of harmful events.

A user manual has been written (word document) describing how to use Perfringens Predictor. The user manual and software tool are both freely available within Growth Predictor on the internet (www.ifr.ac.uk/safety/growthpredictor/). Perfringens Predictor and the user manual were sent to ten interested parties (EHOs, UK food industry scientists, overseas experts, and Food Standards Agency scientists) for evaluation. All responses were extremely positive. Several presentations have been made to highlight the availability of Perfringens Predictor and publications in peer reviewed journals are anticipated.

Relevance of Findings to Food Standards Agency Policy

A dynamic predictive model has been developed and validated for predicting growth of C. perfringens during the cooling of bulked meats, and is freely accessible through a user-friendly software tool via the internet. The user-friendly software tool together with a model developed at the University of Reading by Dr B. Mackay (Food Standards Agency project B14608) will enable food processors, Environmental Health Officers and other users to assess the safety of cooling profiles used with meats (e.g. bulked meats) with respect to C. perfringens. If remedial action is necessary, it will also identify what action is required. Adoption of the software tool should bring about a reduction in the number of cases of food poisoning associated with C. perfringens, and contribute to the Food Standards Agency aim to reduce the number of cases of foodborne illness associated with C. perfringens by 20% over a five-year period, and the overall burden of foodborne illness.

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