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25.11.2019 - Foods

Stress tests or challenge tests with Listeria monocytogenes in food

Foodborne illnesses and even deaths from Listeria monocytogenes have increased in Germany and the EU in recent years (www.bav-institut.de).

Accordingly, the number of public recalls due to contamination of food with these bacteria has also increased. The consequences for affected consumers but also for the companies are usually severe. In some cases, these crises have also led to the bankruptcy of the food company.

To avoid such cases, companies must conduct appropriate product and environmental investigations as part of quality control (see our special newsletter on Listeria). For perishable foods, it may also be necessary to conduct challenge tests with Listeria monocytogenes.

What are challenge tests?

In these tests, the food is contaminated with certain bacteria. In this case, this is done with various strains of Listeria monocytogenes. The product is then stored under real-world conditions for the duration of its shelf life and regularly tested for these bacteria.

Why are challenge tests required?

These tests provide information on whether Listeria monocytogenes can multiply in the food. This information is very important for manufacturers of ready-to-eat, perishable foods for two reasons:

1. As part of their duty of care, every food business operator must ensure that their products pose no health risk until the end of their shelf life. Since Listeria monocytogenes is widespread in the environment, contamination with these bacteria cannot be completely ruled out during the manufacture of many foods. It is all the more important to know whether Listeria monocytogenes can also multiply in these foods during their shelf life. If the growth of Listeria monocytogenes is favored, the hygiene and safety level in production must be even higher.
2. The Regulation (EC) No 2073/2005 on microbiological criteria requires compliance with certain food safety criteria for Listeria monocytogenes in Annex I, Chapter 1. If ready-to-eat foods favor the growth of Listeria monocytogenes, Listeria monocytogenes must not be detectable in 5 samples of a batch in 25g each at the manufacturing level (Criterion 1.2 in Chapter 1 of Annex I). Failure to comply with this or if the food safety criterion is exceeded and the ready-to-eat food is already on the market, a public recall is imminent.

However, if the ready-to-eat food does NOT favor the multiplication of Listeria monocytogenes, according to this regulation it is sufficient not to exceed the food safety criterion of 100 CFU/g in 5 subsamples of the batch at the manufacturing level.

For ready-to-eat foods with a pH value of ≤ 4.4 or an aw value of ≤ 0.92, the food safety criterion of 100 CFU/g at the manufacturing level directly applies. The same applies if the ready-to-eat products have a pH value of ≤ 5.0 and an aw value of ≤ 0.94 or the shelf life is less than 5 days. This is stated in the footnotes of Chapter 1 of Annex I of the Regulation (EC) No 2073/2005, as in these cases it can be assumed that Listeria monocytogenes will not multiply under these conditions or that the food safety criterion of 100 CFU/g for Listeria monocytogenes in ready-to-eat foods at the trade level will not be exceeded.

The food business operator must provide the authority with evidence of the growth potential of Listeria monocytogenes in ready-to-eat foods. Otherwise, the authority must assume in case of doubt that the stricter criteria for Listeria monocytogenes apply to the products ("not detectable in 25g in each of 5 subsamples of the batch).

To provide this evidence to the authority, challenge tests are required for many perishable foods unless the ready-to-eat food meets the above-mentioned chemical-physical properties or has a shelf life of less than 5 days.

What should be considered when conducting challenge tests?

The EU Commission has published a technical guideline (EURL Lm TECHNICAL GUIDANCE DOCUMENT for conducting shelf-life studies on Listeria monocytogenes in ready-to-eat foods) that describes the procedure for conducting challenge tests with Listeria monocytogenes.

Some important aspects include:

• Conditions during the shelf life must be taken into account (including foreseeable changes compared to packaging information such as refrigerator temperatures in households).
• A mixture of at least 2 strains of Listeria monocytogenes should be used for contamination. Not only laboratory strains should be used, but also strains isolated from similar foods or even from the respective food company.
• The food should be contaminated at the start of the challenge test with a microbial concentration of approximately 100 CFU/g.
• DIN EN ISO 20976-1:2019-09 (https://www.beuth.de/de/norm/din-en-iso-20976-1/297296778)

Conclusion

If it is demonstrated through an appropriate challenge test that the food does not at any time and until the end of its shelf life exceed the number of Listeria monocytogenes of 100 CFU/g, then according to Annex I, Chapter 1, the less stringent food safety criterion of 100 CFU/g for 5 subsamples of a batch applies at the manufacturing level. This evidence can be very important for the food business operator in case of doubt if a positive detection of Listeria monocytogenes occurs at the manufacturing level in 25g.

For questions, our customer advisors are very happy to assist you.