The risk of foodborne botulism is still relevant when it comes to a traditional fish product in Norway, especially if it is homemade, according to scientists.

A study assessed the risk of contamination during the process of preparing the fish, and what conditions are needed for neurotoxin to be formed. Botulinum toxin is a neurotoxin produced by Clostridium botulinum.

Norwegian fermented fish, known as rakfisk, is a traditional method of conservation during winter. If wrongly prepared or stored, fish can be contaminated with Clostridium botulinum. Hygiene is one of the main preventive factors to decrease contamination.

Scientists said botulism is a rare disease in Norway but it was possible to become infected after eating inadequately prepared rakfisk. The most common botulinum toxin in the country is type E.

Botulism cases linked to rakfisk
Researchers looked at data from the Norwegian Surveillance System of Communicable Diseases (MSIS). The study was published in the Journal of Microbiology, Biotechnology, and Food Sciences.

A report by the Norwegian Food Safety Authority (Mattilsynet) on the supervision of producers of small-scale freshwater fish recorded 38 cases of foodborne botulism from 1977 to 2015. When the traditional Norwegian rakfisk was implicated, it was because of inadequate standards in a non-professional environment.

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

In 2021, the Norwegian Food Safety Authority inspected 53 rakfisk producers and companies that handle fish as a raw material. Issues found related to risk management systems and a need to improve hazard analysis, documentation and sampling.

Multi-hurdle approach
Norwegian fermented fish is made with attention to four steps and strategies to prevent contamination by bacteria. The temperature and salt concentration are some of the most important hurdles to prevent unwanted microbial growth.

The first step of the processing procedure involves killing the fish and the second covers removal of the abdominal content. Third, the fish is washed under high pressured water to remove contaminating materials before being sorted and put into buckets, where salt is added. It is then placed in cold storage.

In the final step, during the first two days in cold storage, brine is created from the addition of salt to the fish. They are covered with brine and buckets are stored for a variable period. The time the fish is placed in cold storage depends upon the temperature and salt concentration.

“In-depth knowledge about the fermentation process and the potential hazards of wrong storage has resulted in the development of the hurdle principle which effectively put several barriers in place. The hurdle principle effectively prevents botulinum toxin from being developed within the stored fish,” said researchers.

“Strict guidelines and controls from the Norwegian Food Safety Authority have resulted in an improvement of knowledge and generally better quality of the fermentative process. The cases which have been reported are invariably due to homemade rakfisk where the end product has been contaminated with toxin-forming Clostridium botulinum bacteria and conditions allowing the formation of neurotoxin.”

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