You can’t smell it, it’s invisible, and the food looks completely fine. The Listeria bacteria doesn't make infected food appear spoiled, which is why it's especially feared in the food industry. Even in small amounts Listeria can cause illness and, in worst cases, death.

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Anne-May Johansen  

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The food industry and researchers are therefore constantly exploring new ways to combat the frustratingly robust food bacteria Listeria monocytogenes.

In the research project DeList, researchers will investigate various measures to reduce and eliminate the presence of Listeria in salmon and trout.

The project is funded by FHF – The Norwegian Seafood Research Fund.

Not yet approved 

The question researchers asked themselves was: Can you dip the fish or fish fillet in a bath and thus remove the bacteria?

“In our experiments, we’ve used several methods that aren’t yet approved by Norwegian authorities. That’s the nature of research. If you don’t try new methods, you won’t know what works and what doesn’t. We need to conduct experiments to get results that will either lead to a new method being approved – or not,” says senior researcher Tone Mari Rode, who leads the DeList research project.

In the experiments, brown trout is first exposed to Listeria bacteria, then dipped in various treatment baths to determine what can remove or reduce the bacteria in the fish fillet. 

“We’ve used acetic acid, low pH, and antimicrobial – bacteria-inhibiting – substances. Nisin, which is approved in Norway for use in dairy products, is one of them,” explains Tone Mari Rode.

Effect on shelf life 

Listeria bacteria occur naturally in the environment. Several studies in recent years show findings of Listeria monocytogenes in up to ten percent of Norwegian salmon slaughterhouses and processing facilities. Researchers are therefore working to test which measures can be implemented, for example, at a slaughterhouse. When it comes to farmed fish, Listeria can be present on boats, facilities, and slaughter equipment. And the bacteria can spread.

“We compare the test fish with trout that hasn’t been dipped in Listeria, to compare what bacteria-inhibiting methods do to color, texture, and microbiology – in other words, shelf life. The fish is also stored over time to check the effect the treatment has on the development of both quality and bacterial growth,” explains the project leader.

Any reduction or prevention of the feared bacteria, big or small, provides valuable information.

“Listeria can grow – even at refrigerator temperature. Even if there are only ten bacteria in a product, and these can be removed, that represents a significant potential gain,” the researcher states firmly.

Broadly combined expertise 

Where in the value chain the bacteria are detected is also of great significance, both economically and from a health perspective.

“The product’s expiration date plays a major role here. There can be recalls and media coverage, and illness and death in the worst case, if the bacteria is discovered late,” says Rode.

She and her colleagues in the project combine different methods such as high pressure and baths with various additives.

“Nofima is unique in such an approach because we have such broadly combined expertise in different fields. We have expertise in both microbiology, process technology, heat treatment, and alternative processing methods. And by combining different treatment methods, there’s often much to gain,” says Tone Mari Rode.

The project will conclude at the end of January 2025.

Facts about Listeria monocytogenes:

  • Listeria consists of several species, where Listeria monocytogenes is the one that causes illness in humans.
  • Listeria monocytogenes is among the foodborne bacteria that pose the greatest burden, both health-wise and economically for actors in the food chain.
  • Most people occasionally eat food with Listeria without getting sick. However, the bacteria can be dangerous for pregnant women, immunocompromised individuals, and the elderly. If you do get sick, there is relatively high mortality. Of those who actually become ill with listeriosis, 15-20 percent die.
  • It can be difficult to eliminate Listeria once it enters facilities where food is produced, and Listeria can contaminate food during production.
  • Listeria grows even during refrigerated storage. It survives and grows at high salt concentrations and can survive drying and freezing. Listeria is killed by heat treatment (pasteurization). 

Source: Nofima

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