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How to avoid food poisoning from this Scandinavian fermented fish dish
A bacterial food culture inhibits the disease-causing bacteria Listeria without affecting the taste.
Published
Rakfisk is a traditional Norwegian product made by fermenting trout or char in brine. In recent years, there have been several outbreaks of food poisoning due to the bacterium Listeria monocytogenes.
Nofima has led a three-year research project, Safe Rakfisk, aimed at making rakfisk safer.
Researchers have tested several different solutions to inhibit Listeria during production and have found two that work well.
"The first is a so-called protective culture, and the second is an inhibitory salt of the buffered vinegar type," says Lars Axelsson.
He is a senior researcher at Nofima and has led the project.
Both of these are often considered biosolutions, meaning they are produced by natural processes.
Protective cultures are beneficial lactic acid bacteria that can prevent the growth of unwanted bacteria. Buffered vinegar is vinegar that has been made less acidic by adding buffer solutions such as sodium or potassium hydroxide. It can be both liquid and in powder form.
"In this case, we used the protective culture Lyoflora and the inhibitory salt Verdad. Interestingly, the effect of Lyoflora was enhanced by using a small amount of sugar when the fish was placed in the brine. This even resulted in some killing of Listeria," says Axelsson.
Ensuring the taste remains the same
Researchers have conducted surveys and interviewed people who eat rakfisk. Most of them are not very aware that Listeria can be a problem.
At the same time, many understand that rakfisk is associated with risk and that some groups, such as pregnant women, should avoid the product.
As long as the taste is as expected, participants accept that the production method is changed if it makes the rakfisk safer.
Nofima's professional sensory assessors have tested untreated rakfisk, rakfisk treated with inhibitory culture, and rakfisk treated with inhibitory salt.
They evaluated various properties related to taste, smell, and texture.
Rakfisk treated with the bacterial culture Lyoflora was very similar to untreated rakfisk, while the inhibitory salt Verdad gave a slightly more pungent and sharp taste.
During the Rakfisk Festival in Valdres, Norway, Nofima's experts conducted a sensory test. 220 people tasted the same variants as the professional tasting panel.
The differences were small, but the test participants preferred rakfisk treated with Lyoflora over both untreated rakfisk and Verdad-treated rakfisk.
The rakfisk producers who tested the different treatments were also more satisfied with using Lyoflora than Verdad. The rakfisk maintained its usual quality compared to regular production.
For fish treated with Verdad, the producers noted the same differences as Nofima's sensory panel.
Better control over bacteria
Rakfisk is stored in plastic tubs. Producers regularly sample and analyse these tubs to prevent rakfisk containing Listeria from reaching the market.
During one such check, a producer found Listeria in a fairly large production batch. This gave researchers an opportunity to study how Listeria spreads in the rakfisk tubs. They could also examine which methods were best suited for sampling.
Part of the project was carried out by the Norwegian Veterinary Institute (NVI). They quickly discovered that Listeria does not distribute evenly in the tubs.
"Samples taken from the bottom always contained Listeria when the bacterium was present in the tub, while the upper layers could be completely free of Listeria," says senior researcher Taran Skjerdal at NVI.
The recommendation is therefore to take samples from the bottom. Experts at NVI suggest using either extra-long pipettes to take samples from the bottom or lowering a gauze strip that absorbs brine from all levels.
These and other recommendations are useful for rakfisk producers involved in the project.
"This has been a very concrete, interesting, and educational project. We have great faith that bacterial protective cultures can contribute to even safer production and are now starting a larger test production to ensure that it provides the same good taste to the fish," says Nils Noraker at Noraker Gård og Rakfisk.
"We at Nofima and the NVI have worked on the two most important areas relevant to the Listeria issue in rakfisk production: the ability to inhibit or kill Listeria in the process and improved methodology for detecting Listeria in the production tubs before the rakfisk is distributed to the market and consumed," concludes Axelsson.
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