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Challenge testing From October 2010 newsletter

Challenge testing

The microbiological quality of a food or drink has a major bearing on the shelf–life that can be assigned to it. However, just because an organism is in a product, it does not mean that it will grow or compromise shelf–life, as Gail Betts, from our Microbiology Department, explains:

"Some pathogens and most spoilage organisms will need to survive and grow in order to reach levels at which the food is compromised. Challenge testing is a practical study to determine the behaviour of relevant organisms, should they be present in a product. It involves deliberate inoculation of the product with these organisms, typically a specific pathogen or group of spoilage organisms, after which the product is stored and tested during shelf life."

Key factors

"The key factors are to select organisms that could realistically be in the food or gain access to it, inoculate them into the food in a form in which they could realistically be expected to occur, and then store appropriately. These factors will vary from product to product, and decisions over what microorganisms and storage conditions are 'relevant' are not always straightforward; pathogens such as Salmonella and Escherichia coli O157 should not be present in any food, even at the lowest level, whilst others such as Listeria monocytogenes or psychrotrophic Bacillus cereus usually need to grow in foods in order to become a risk to consumers. We offer expert advice and guidance on the risks of micro–organisms to food products and consumers, and advice on challenge testing, and can undertake tests with a wide range of organisms, including E. coli O157 and Clostridium botulinum."


To back up our services in this area, we recently published a guideline (Challenge testing protocols for assessing the safety and quality of food and drink – Guideline No. 63), which contains the necessary information for companies wishing to follow a standardised protocol for challenge testing their food products.

We also offer a microbial growth prediction service, based on mathematical growth models. These allow cost effective testing of different 'what if' scenarios when reformulating or developing new products, in which salt, pH, and other factors can be varied, and microbial growth predicted. This can then be used to determine when challenge testing might be necessary. Both are discussed in a podcast by Roy Betts, see our podcasts page to hear this podcast.

For a free fact sheet on Predictive Microbiology, send an e–mail to with the subject line: send prediction.

Technical contact: Gail Betts
+44(0)1386 842071

Publication sales: Carol Newman
+44(0)1386 842048