Listeria infections are on the rise, and around 15% of people in the EU who contract the infection die from it.

Researchers from the DTU National Food Institute are proposing a new labeling scheme designed to give consumers a better opportunity to choose ready-to-eat foods, such as smoked salmon and spiced pork roll, without risking infection with Listeria bacteria.

Listeria accounts for approximately 32% of all deaths associated with foodborne disease outbreaks in the EU. The infection poses a particular risk to older people, those with underlying health conditions and other vulnerable groups, as well as pregnant women.

"A labeling scheme should be introduced for ready-to-eat foods that have been stabilised during production to prevent the growth of Listeria. This will enable us to reduce the number of people who fall ill with Listeria infections. The label should give consumers confidence in eating these ordinarily healthy fish products, which may however, be prone to contain Listeria," says Martin Laage Kragh, a researcher at the DTU National Food Institute.

Products already exist that are produced in a way so that Listeria cannot grow in them. However, it is currently difficult for consumers to determine which products have been stabilised to prevent the growth of Listeria and are therefore safe to eat.

The researchers propose a voluntary labeling scheme using the word "STABILIZED", which should make it easier for consumers to choose foods with a minimal risk of Listeria infection.

The label should only be used by producers who can demonstrate that Listeria cannot grow in their products."

Professor Lisbeth Truelstrup Hansen, DTU National Food Institute

In short, stability means that the product's preservation must be appropriate for the stated shelf life.

"Many products have a shelf life that is too long given the way they are made. But by changing the recipe – and often this requires only a small change – the products can be made safe," says Professor Emeritus Paw Dalgaard from the DTU National Food Institute.

How to prevent Listeria from growing

A shorter shelf life limits the growth of Listeria and generally reduces the risk of Listeria infections.

In the case of smoked and marinated/gravad fish, which account for many cases of listeria infections, one can, for example, add vinegar during the salting process – in such small quantities that it cannot be tasted – thereby preventing listeria from growing in the products.

Another option is to sell frozen products. This will effectively prevent the growth of Listeria.

For other types of food, including cold cuts, heat or high-pressure treatment of products in consumer packaging may be a solution. This kills Listeria if the bacterium has entered the product during packaging.

There will be different approaches to making the products safe, depending on the type of food involved.

Traditionally, efforts to combat Listeria have focused on cleaning in the food industry, but this has proved not to be enough.

"Cleaning at the production stage is very important when it comes to preventing Listeria, but it has not prevented a rise in the number of people falling ill. We suggest maintaining the focus on cleaning, but also placing a new and equally important focus on producing food in such a way that Listeria cannot grow in ready-to-eat products," says researcher Martin Laage Kragh.

The cause of current outbreaks should be analyzed

The researchers also suggest that products which have led to recent outbreaks of Listeria infections should be analysed in a new way, so that the reason Listeria grew in the products can be quickly identified.

As things stand, the authorities often identify the source of infection in outbreaks, but this has frequently failed to prevent new outbreaks.

"We propose that investigations should also look into whether the manufacturer is using a formulation where preservation could be improved, or whether the product's shelf life should, for example, be shortened to prevent new outbreaks," says Paw Dalgaard.

It has in fact been shown that outbreaks of Listeria infections can occur repeatedly and be traced back to the same products – and sometimes even to the same manufacturers. It is therefore important to identify what is going wrong with those specific products during production and distribution.