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The big chill

A recent edition of New Scientist raised an interesting question about whether bacteria can live or even thrive at very low temperatures, particularly bacteria that can live inside a refrigerator. With the majority of bacteria, cold temperatures – such as the temperature inside a refrigerator – slow the metabolism of bacteria and prevent them from reproducing.
Unlike most other types of bacteria, Listeria monocytogenes, a bacterium responsible for an estimated 450 cases of food poisoning every year, can survive and often multiply to harmful levels in the fridge – even if food is stored properly. Although it may be relatively rare, around a third of cases end in death.

This is why sticking to ‘use by’ dates is important. You’ll find them present on labels for foods that are likely to go off quickly, particularly chilled food such as paté (including vegetable paté), soft mould-ripened cheeses such as Camembert, Brie and Stilton, and smoked fish.  Once you open these foods they can go off quickly. Even if it looks and smells fine, consuming it after the ‘use by’ date could put your health at risk.

Certain groups of people are more at risk of listeria infection, including pregnant women, those over the age of 60, people with compromised immune systems, for instance due to illness or medication, and babies less than a month old.

So, to reduce the risk of listeria infection, ensure that you have followed the storage instructions, if any are included on the food packaging. Don’t use food past its ‘use by’ date, regardless of whether the food looks or smells fine, and ensure the temperature of your refrigerator is below 5°C.


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Because so many people are confused as to the difference between 'use by' and 'best before' dates, I think it would be helpful if the FSA were to point out that a food that is past its 'best before' date is not unsafe and in many cases is still perfectlly OK to eat. You would not want to encourage people to throw food away unnecessarily. There are very few foods that will automatically become unsafe when they reach their durability dates. Even those with use-by dates often become unpleasant to eat before they become unsafe. In other words, their theoretical 'best before' dates would be reached before the 'use-by' ones. For this reason, many use by dates are set at dates when the food is past its best but not necessarily unsafe.

Posted by Clare Cheney on August 17, 2011 at 05:00 PM BST #

Hi Andrew, this is a perfect place for me to ask - WHY?

Why is use by date so improtant?
Listeria, Salmonella. Campylobacter, E. Coli and the other nice friends do not come from the air. They are either present in the foodstuff, or they are introduced by cross-contamination. They don't multiply from 0 to plenty, there must be some present.

and now, the big question. If the foodstuff contains Listeria Mono, it is unsafe for consumption in any amount. Of course, there's a pathogenicy threshold, but it is so variable that the only safe amount is none. So, if the food is contaminated, why is is safe to use it before the use by date? And if the foodstuff entered the fridge clean, why should there be any pathogens by the end of the shelf-life?

This is one dogma of food safety I cannot grok even after all my time in the industry. Expiration date as an indicator for microbial spoilage - this is clear. But why should a product that was safe for consumption a week before shelf life ended should become unsafe just because the time is up? you are the chief scientist, I'll like a scientific explanation.

Posted by Itay Shlamkovich on August 18, 2011 at 06:31 AM BST #

Author comment

These are some good points raised and provides a good opportunity to try to remove any confusion. The ‘use by’ date relates to a food’s safety and indicates the time limit within which the food can be expected to remain microbiologically safe, which will have been assessed scientifically by the producer. After this date we have no guarantee about its safety, so these foods should be eaten, cooked or frozen by this date.

Naturally we are keen that food not be wasted unnecessarily, but it should not be at the expense of our safety. Listeria is a particular risk to specific groups of people (see above). They need to be vigilant to ensure their food remains safe, and stick to the ‘use by’ dates that are indicated.

It’s not accurate to say that any level of contamination of food is unsafe. Food hygiene legislation requires the absence of, or sets a maximum limit for, harmful microorganisms in certain foods to ensure that consumers are not exposed to levels that would cause illness. A number of factors are taken into consideration when deciding the levels of bacteria that can be allowed in food including, in the case of listeria in chilled foods, whether the organism will grow during the shelf-life of the food in its recommended storage conditions, and by how much. Some important pathogens such as salmonella and E.coli O157 can pose a risk when present in foods at low levels whereas others need to be present in higher levels. And contamination can be unevenly distributed across a batch of food and may not be detected by testing, particularly if bacteria such as listeria are present at low levels.

The foods we buy, now as in the past, are rarely entirely uncontaminated and will undergo some degree of microbiological and chemical changes during storage. Some changes affect quality aspects (flavour, colour or texture) but the ‘use by’ date indicates to consumers that microbiological contamination could reach harmful levels after this time.

Posted by Bob Martin, FSA Microbiologist on August 19, 2011 at 05:33 PM BST #

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