Calculating Food Expiry Dates

Food expiration dates help us to plan meals and reduce food waste. Discover the science behind those all-important best-before dates and use-by dates

Studying Food Degradation 

To measure the presence of potentially food-spoiling microbes, food scientists analyse food samples under different conditions. For these tests, products are stored in conditions similar to what they expect during storage, transport and in the consumer’s home. They will even simulate the temperatures and rough handling the product could experience as it is transported. Samples are tested at regular intervals to see how the food's chemistry, microbiome, appearance, taste and smell changes. This is called static testing.¹ 

The next stage, accelerated testing, sees changes made to temperature, oxygen concentration and other key factors to see how much this causes the food to deteriorate. Once researchers have calculated the estimated shelf life in accelerated testing, this will inform their decision of use by date.

‘Use by date’ and ‘Best Before date’ – What’s The Difference?  

There is an important difference between a ‘use by date’ and a ‘best before date’. ‘Use by’ indicates the date by which food is considered safe to eat. It especially appears on highly perishable foods like meat and pre-packaged vegetables. ‘Best before’, on the other hand, indicates the maximum date to which food is expected to retain its best quality. It can still be safe to eat after this date, but it may have begun to lose its flavour or texture.² 

Predicting The Journey Of Food

Understanding how food has been prepared and packaged helps researchers decide what microorganisms it could come into contact with during its journey to the consumer's home. In microbial challenge testing, scientists add a pathogenic microorganism to a product. This is generally a microorganism holding the potential to either degrade a product, or make a consumer sick. Samples are taken at regular intervals to measure how much the microorganism has grown in the expected conditions for the food. This starts to give an indication of how long that food is likely to be viable to eat if it was infected with the microorganism at some point in the processing.¹

Every harmful microorganism has an infective dose, or the amount of that organism that will typically make someone sick. For example, some people do not react to the presence of Salmonella bacteria, whereas in others, it causes diarrhoea, fever and cramps. It takes around 10,000 Salmonella bacteria to make someone ill, which is a relatively low infective dose when compared with other pathogenic bacteria.^{3, 4} Although Salmonella poisoning is often associated with poultry, it can also be transmitted through pork and beef as well as consuming unpasteurised milk and eggs from contaminated animals; but thorough cooking will kill Salmonella bacteria.⁵ 

Predicting Shelf Life and Use By Dates

Microbial challenge studies aim to understand how microbes grow in different food samples. Based on this information, scientists can calculate how long food will remain safe to eat under expected storage and transport conditions. This enables them to predict food expiration dates.

Food scientists develop mathematical models based on numerous microbial challenge studies to predict food expiry dates. These models can be accessed by all types of food manufacturers, including small businesses. By entering information like the type of product, moisture content, acidity and expected storage temperatures into the model, it will predict how long that product is likely to be safe to eat. It will give the manufacturer a good benchmark of shelf life, but it cannot be used in isolation, as each product will be exposed to different packaging and conditions, which could impact the product’s longevity. Usually, the expiry date is conservatively set to several days earlier than when the product will no longer be safe for consumption. Despite this commonly used process, there is no single standard safety margin applied when setting food expiration dates. The safety margin used is decided at the manufacturers' discretion.¹