Challenge tests (microbiological stress tests)

How to check the microbiological stability and safety of your products and production processes

June 2023. By means of a challenge test, you provide evidence that your product meets the microbiological requirements over a process section or a defined period of time and that certain microbial counts remain within the tolerance range or that no microbial proliferation takes place.

In product challenge tests, a food or its components are artificially contaminated with microorganisms in the laboratory. Under conditions that are as close to reality as possible, the influence of the production process, packaging and storage, the use of protective and starter cultures as well as food additives and preservatives on the germ growth of relevant microorganisms can be examined.

Challenge tests are used for product safety, allow the determination and verification of the the use-by date of a product and serve as evidence in official controls or trade requirements.

Health hazard due to microorganisms during shelf life

The calculation of the shelf life and thus also the safety of a foodstuff over the entire trading period depends on the physical chemistry of the product and the success of the technological processes for preservation. The survival of health-critical microorganisms from the raw material in the product or their introduction during the process though secondary contamination, may develop a food infection or intoxication in the consumer. This applies in particular to ready-to-eat products without safe methods of preservation, such as raw meat (carpaccio or minced meat), fish (salmon/tuna with sushi quality) or raw sausages. However, heated products can also contain heat-stable, preformed toxins released by microorganisms, such as staphylococcal enterotoxins, the emetic toxin of Bacillus cereus or histamine.

Procedure of a challenge test for Listeria monocytogenes

For the challenge test for Listeria monocytogenes, at least two characterised strains of the test germ are selected according to EU recommendation^[1]. It is examined whether these reproduce at the correspondingly declared storage temperature in or on the product to be tested. In addition to the two known strains, a test strain directly from the production environment should also be used.

At the beginning of the test, a basic microbiological examination and a test for the absence of "natural" contamination with the germ species to be tested are carried out. Then, depending on necessity, parallel samples of the same batch are inoculated with the freshly prepared suspension of the germs. The packages are hermetically sealed again, if necessary under a protective atmosphere. Subsequently, the samples are stored at a declared temperature in a calibrated cooled incubator and examined at fixed times according to a strictly monitored experimental plan. To simulate a possible temperature excess, the samples are transferred to a slightly higher temperature. At the end of the challenge test, the basic microbiological examination is repeated to detect any possible influence on the test result.

The aim of the challenge test is to be able to make a statement about the growth potential of the strains. If a typical growth curve has been determined, it can be determined at which point in time which germ count is to be expected. Currently, the results of a challenge test for Listeria monocytogenes must be submitted to the monitoring authorities in the form of a test report due to the coupled testing obligations of the manufacturer, control activities of the authorities and the importance for possible recall scenarios. This is also frequently requested by retail chains.

The concept described can also be applied to other pathogenic germs and non-pathogenic spoilage agents in the sense of a challenge test in order to test the stability of a product by means of sensor technology. If there are no ISO or guideline specifications associated with a challenge test, it can also be customised for other test procedures depending on the product or process in question.

Legal and corporate obligations – Recommendations and requirements of Regulation (EC) No 2073/2005

The biggest obligation of producers and distributors are safe products that meet the EU-wide legal criteria of Regulation (EC) No. 2073/2005^[2] for self-monitoring and the requirement of Regulation (EC) No 178/2002^[3], Article 14. In order to meet the legal requirements of Regulation (EC) No. 2073/2005^[2] the establishment of a HACCP concept and other hygiene concepts is required. These include, for example, environmental monitoring with trend analysis in the case of the production of ready-to-eat products with growth potential for Listeria monocytogenes. Scientific literature on microbial growth and its control as well as the use of predictors (e.g. ComBase, FSSP)^[4] are recommended.

If zoonotic pathogens are detected in the end product, but also in certain environmental samples, the company has a reporting obligation. This underlines the importance of the legal requirements. Careful monitoring of raw materials, specifications between trading partners and audits of the origin and logistics of raw materials supplement the technological and laboratory control measures.

Consultation and analysis

We can assist you in the planning, implementation and evaluation of your challenge test. For general information and background on the tests, please contact your personal account manager or our experts Gabi Bauckmann and Hans-Georg-Leusch.

Sources

[1] EURL Lm Technical Guidance Document on challenge tests and durability studies for assessing shelf-life of ready-to-eat foods related to Listeria monocytogenes, Version 4 of 1 July 2021
[2] Consolidated text: Commission Regulation (EC) No 2073/2005 on microbiological criteria for foodstuffs
[3] Consolidated text: Regulation (EC) No 178/2002 laying down the general principles and requirements of food law, establishing the European Food Safety Authority and laying down procedures in matters of food safety
[4] Food Spoilage and Safety Predictor (FSSP)