In conversation with Aldo Evers about Listeria and challenge tests
Listeria has regularly been in the news lately. The reporting highlights the significant consequences for public health, the importance of having control over Listeria monocytogenes properly in place, and the essential role that challenge tests play in this. In this article, we speak with Aldo Evers – Technical Director Microbiology – about Listeria and challenge tests.
At Normec Foodcare, our laboratories conduct in-depth microbiological analyses to ensure the safety and quality of food: from pathogen research and investigations into various spoilage organisms to chemical and physical tests. Our laboratory specialists are asked all sorts of questions daily about microbiological analyses and legislation, particularly those specifically focused on Listeria.
QA with Aldo Evers
Someone who knows all about this is Aldo Evers. As a Technical Specialist, he has dealt with the most complex situations around Listeria monocytogenes and challenge tests for many years. Recently, he became Technical Director Microbiology at Normec Foodcare. In a conversation with Aldo, we put to him various questions that are frequently asked about challenge tests.
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What is a challenge test?
A challenge test is a specialised laboratory investigation in which a product is deliberately contaminated with a known amount of a bacterium, such as Listeria monocytogenes. We then closely monitor how this bacterium behaves in the product over the full shelf life. This gives us a clear picture of the microbiological risk and allows us to assess whether the product remains safe and complies with legal requirements, as set out in EU Regulation 2073/2005. These tests are particularly important for ready-to-eat chilled products that are consumed without further heating. In addition to Listeria, we also conduct such tests for other pathogens, such as Salmonella or Bacillus cereus.
Why are challenge tests for Listeria monocytogenes important?
Listeria monocytogenes is a notorious bacterium because it can continue to grow even at refrigerator temperatures and can cause severe infections in vulnerable groups, such as pregnant women, the elderly and people with weakened immune systems. For producers of, for example, meat products, fish products and dairy, it is therefore crucial to demonstrate that their product does not allow this growth, or that the bacterium remains below the threshold of 100 cfu/g until the end of shelf life. This is not only a legal requirement according to EU Regulation 2073/2005, but also essential for food safety and consumer confidence. Without this substantiation, a product may be considered unsafe or certain claims may not be permitted.
What do companies struggle with in Listeria challenge tests?
We notice that companies often struggle with several aspects. Firstly, setting up a correct test is not straightforward: you have to select the right strains, determine a realistic contamination level and choose suitable storage conditions. All these choices directly influence the value of the results. In addition, finding a good balance between a realistic yet strict enough ‘worst-case scenario’ is a challenge.
Furthermore, practical factors play a role. A challenge test is intensive: they often last weeks, are costly and may only be carried out in specialised laboratories. For smaller companies, this can be quite a hurdle. The interpretation of the results also often raises questions: what does a small increase in Listeria actually mean for my shelf life and labelling?
Product variation also comes into play. Small differences in pH, water activity or packaging can influence growth, meaning one test is not always representative for the entire production. Finally, European legislation adds extra pressure to execute correctly, as you are required to demonstrate that the product remains below the threshold until the end of shelf life.
Are there alternatives to using a challenge test?
There are certainly alternatives, such as predictive models that simulate the behaviour of micro-organisms like Listeria (for example, ComBase or FSSP). A product can also be developed in such a way that bacterial growth is excluded, for instance, by low pH, low water activity, or the use of preservatives. This approach can be valuable, but authorities do not always regard it as full proof of safety. Especially for ready-to-eat products where Listeria poses a real risk, a challenge test is usually still necessary.
What can companies do to make this process easier?
Our experience shows that good preparation makes all the difference. Work together with specialised laboratories experienced in Listeria challenge tests and international guidelines (such as ISO 20976-1). Use predictive models in advance to map out risks and refine the test design – this saves time and costs. Sometimes, a combination of models with limited, targeted challenge tests is the most efficient approach.
We also recommend that companies build up enough in-house knowledge so that results are not only interpreted correctly but can also be effectively integrated into the food safety plan (HACCP or FSSC 22000). A laboratory that collaborates with QA experts brings enormous benefits. By working smartly together and involving the right expertise, the process can not only proceed faster and more cost-effectively but also more reliably and better aligned with legislation and the specific company situation.
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