Shelf  Life Assessment : Shelf Life and Challenge Testing

by Prof Chris Griffith

The last article considered some of the more general aspects of shelf life assessment. This article will look in a little more depth at some of the methods that may be used in shelf life determination. The previous article mentioned  3 approaches, two involving microbiological testing and one using simulated growth predictions using specialised software. It is the first two methods using testing that will be considered here in more detail.

Before looking at these methods it is perhaps useful to discuss and define some of the relevant terminology. Food can be considered as spoilt when it is no longer safe and / or attractive/desirable/acceptable to eat. In some ways both of these aspects are relative terms i.e. depending on the individual consuming the food. Whilst food desirability is more likely to be considered as subjective even food safety can vary depending upon the dose response curve and the immunity or susceptibility of the individual consumer. However for ease of discussion they will both be considered as more absolute attributes. The term “best before“ is usually applied to relatively stable food with a longer shelf life and is more a measure of quality than safety. Thus this is linked more to presence and growth of spoilage organisms. “Use by“ is applied more to perishable items and generally is more concerned with safety aspects and the ability of the food to allow pathogens to grow. A third but less desirable term is “sell by“ which  is of more use to retailers and does not consider how the product will be treated by the consumer nor does not it give consumers any indication of how long the food should be retained by them after purchase. Shelf life determinations are therefore designed to answer the question, depending upon the food, how long is it possible to keep the food until it deteriorates or becomes unsafe to eat. As discussed in the previous article this should initially be considered at the design stage. After production has started shelf life trials (microbiological and organoleptic)  are carried out on batches of the product produced under routine manufacturing conditions and treated in a manner representative of normal distribution, storage and use.  Batches may also be stored under conditions of abuse (e.g. for perishable foods several degrees above the normal recommended storage temperature) to provide an additional margin of error. “Standardised” allowances for consumer use have also been suggested  e.g.  for refrigerated foods at 22 °C for 2 hrs to represent purchase and 7 °C  for storage. However, these were designed for temperate cooler climates and may need to be modified for South African conditions. Samples from the batches being evaluated are taken at regular intervals (depending on product and shelf life) and examined for key spoilage, indicator or marker organisms as well as pathogens and, if safe, sensory properties until the end of the projected shelf life is reached. An additional 1or 2 days may be added on to the projected shelf life to anticipate consumer retention  and use beyond stated use by dates. This whole process needs to be replicated at least 3 or 4 times to generate sufficient reliable data for decision making. The shelf life of frozen foods is usually undertaken by physical or chemical methods. Frozen foods should not allow the growth of any microorganisms, freezing may even decrease the number of microorganism but will not destroy any toxins that maybe present.  Accelerated shelf life testing is often used for canned or bottled foods (anticipated longer shelf life especially with products that can possibly support microbial growth). In accelerated shelf life testing a portion of the batch is incubated at an often much higher temperatures (up to  55 °C for thermophiles in canned foods). The product can be examined visually and microbiologically for signs of spoilage e.g. blowing due to gas production. The growth of the organism being tested is “accelerated“ by using more favourable growth conditions .

Microbial challenge testing is mostly used for assessing food safety but can also be used for specific spoilage organisms and to answer the questions – is the food capable of allowing or preventing the growth of pathogens (identified by the HACCP plan) or known problematic spoilage organisms, over the course of its shelf life, to a level that is unacceptable e.g. Listeria monocytogenes in ready to eat foods. Such pathogens maybe in the product raw materials or gain access during processing and distribution e.g. Salmonella or STEC in raw milk used for unpasteurised cheeses or Clostridium botulinum in vacuum packaged fish product. This type of testing must NOT be undertaken near production facilities due to the potential risk of importing a pathogen into a food plant. Challenge testing will use a range of organisms and at a range of concentrations  – often representative of minimal, realistic or worst case scenario circumstances. The test organism being used to challenge the food and usually a pathogen, at the selected concentration, is deliberately inoculated into a sample of a pathogen free product and incubated alongside test samples of the pathogen free product as controls. The presence and number of pathogens in the product at selected time intervals over the course of the product’s shelf life is then monitored. Additional days may be added on to represent consumer retention beyond its “stated“ shelf life. As such challenge testing provides data that would not normally be available from the more routine shelf life testing of a hopefully pathogen free product.

Assessing shelf life is an increasingly important element that needs to be considered objectively and scientifically by all food producers  – from design through to routine production – and can be used to inform about the consequences of product abuse from production to usage. The results should not only be used to provide a shelf life determination  for packaging  but to act as a basis for production,  logistics and distribution and advise producers on possible or necessary product reformulations.


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