Scientists have found a way to stop the growth of Listeria in a traditional Norwegian product and have improved detection methods.

Rakfisk is made by fermenting trout or char in brine. In recent years, it has been linked to several outbreaks of Listeria monocytogenes, as well as botulism cases.

In 2020, two cases of botulism were recorded with the suspected source of infection being homemade rakfisk. Three recent Listeria outbreaks have been reported including one in late 2018 to early 2019 that caused 13 illnesses.

Nofima led a three-year research project, called Safe Rakfisk, aimed at making the product safer.

“We have tested several different solutions to inhibit Listeria during the process and 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,” said Lars Axelsson, senior scientist at Nofima, who led the work.

Support for protective culture
Protective cultures are lactic acid bacteria with activity against unwanted bacteria and are often isolated from food. Buffered vinegar is a liquid or dried product that primarily contains acetic acid and its salts are neutralized.

“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,” said Axelsson.

Nofima conducted surveys and interviews with people who eat rakfisk.

Most of them did not know that Listeria could be a problem. However, the majority were aware 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, people can accept that the production method is changed if it makes the rakfisk safer.

Nofima’s sensory assessors tested untreated rakfisk, rakfisk treated with inhibitory culture, and rakfisk treated with inhibitory salt. They evaluated various properties related to taste, smell and texture. The rakfisk treated with the bacterial culture Lyoflora is very similar to the untreated product, while the inhibitory salt Verdad gave a more pungent and sharp taste.

During the Rakfisk Festival in Valdres, Nofima scientists conducted a sensory test. A total of 220 people tasted the same variants as the professional sensory assessors. Differences were small, but participants preferred rakfisk treated with Lyoflora over both untreated and Verdad-treated rakfisk.

Producers who tested the different treatments were also more satisfied with using Lyoflora than Verdad, as the rakfisk had unchanged quality compared to regular production.

Improved detection
Rakfisk is stored in plastic tubs and producers regularly sample and analyze them to prevent contaminated products from reaching the market. When a producer found Listeria in a fairly large production batch, scientists were able to study how Listeria was distributed in the tubs and which methods were best for sampling.

The Norwegian Veterinary Institute worked on this part of the project. The agency 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,” said Taran Skjerdal, senior scientist.

This led to a recommendation to take samples from the bottom. The Norwegian Veterinary Institute methodology suggests using either extra-long pipettes to take the samples or lowering a gauze strip that absorbs brine from all levels.

“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,” said Nils Noraker at Noraker Gård og Rakfisk.