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Shelf-life From February 2010 newsletter

Challenge testing and shelf-life

Shelf life is defined as the time after production during which a product remains acceptable for consumption. Any product that can support the growth of pathogenic microorganisms must be assigned a 'use-by' date, after which the product must not be sold, and should not be consumed.The manufacturer is responsible for setting this date - and so must be aware of all the factors that could affect microorganism growth, for example when developing a new product.

However, just because an organism is in a product, it does not mean that it poses a problem or will 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.

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 straight forward; pathogens such as Salmonella and E.coli O157 should not be present in any food, even at the lowest level, whilst others such as Listeria monocytogenes or psychrotrophic Clostridium botulinum usually need to grow in foods in order to become a risk to consumers. We offer expert advice and guidance on the risks of microorganisms to food products and consumers, and advice on challenge testing, and can undertake tests with a wide range of organisms, including Escherichia coli O157 and Clostridium botulinum "

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.


Gail Betts
+44(0)1386 842071

Linda Everis
+44(0)1386 842063