What happens inside rakfisk?

From tradition to modern production of rakfisk

Rakfisk is a distinctively Norwegian specialty with long and rich traditions in local communities, perhaps especially in the Valdres area. From August and throughout the autumn, mountain trout were handpicked and carefully processed according to ancient methods in order to produce a delicacy that could be enjoyed in the run-up to Christmas. 

Fish weighing approximately half a kilo gave the best results. It was cleaned, thoroughly washed and salted in the belly and head. It was then stored at cool temperatures, and packed tightly and under pressure in a bucket with its belly facing upwards. The fish was ready for consumption after about three months. 

The production of rakfisk relied on experience-based knowledge that was often passed on to the next generation of the family. Each producer had their own special secret tricks to produce the best rakfisk.

Many producers currently have their own trout farms in connection with the production facility so that they can control the quality of the raw product. The actual rakfisk production process takes place in modern facilities using modern equipment. These facilities also provide important jobs in rural areas, although the work can be seasonal.

From being a tradition in the local community, rakfisk production has become a serious processing industry. Just over 400 tonnes of rakfisk are sold every year. Rakfisk is served at many Norwegian Christmas parties and is popular restaurant food during a season that now lasts from the end of October all the way to February. 

Although this is the main season, some producers will probably say that rakfisk has become a year-round product, as an increasing number of people also eat this delicacy at other times of the year.

Quality, hygiene, salt and temperature are important for rakfisk

While rakfisk lovers get excited every time the season approaches, many still wonder what really happens in the process from fish to rakfisk. There is also a question that gets asked often: Is rakfisk safe to eat?

The answer is yes: as long as rakfisk is produced according to the correct principles, there is no reason not to enjoy this delicacy for your Christmas dinner or on other occasions!

Good quality raw produce, good hygiene, the right amount of salt and the correct temperature are the decisive factors for making good and health-safe rakfisk. These factors also help ensure that certain lactic acid bacteria grow rapidly early on in the process and produce lactic acid that lowers the pH.

Good quality raw fish, good hygiene, the right amount of salt and the right temperature are crucial for making good and safe rakfisk.

Good tips: How to rak (ferment) fish?

Most manufacturers use dry salting for rakfisk, but there are recipes where the fish is placed directly in a brine. In both cases, after an equilibrium is established, the fish reach a salt content of approximately 5 percent. After just a few days of dry salting, a lot of brine is formed. Alternatively, the bucket is filled up with brine to cover the fish. Some manufacturers add sugar in addition to the salt, and small amounts of vinegar are also mentioned as a possible additive. The temperature profile during storage varies slightly between manufacturers. A temperature of 4-7 °C is maintained.

The salting process is probably of great importance for the development of the bacterial flora and what happens next to the fish. Initially, the salt content of the fish is low compared to the amount of salt surrounding the fish. Water is drawn out of the fish and spontaneously forms brine. After just three days, the salt content in the fish is relatively high, but it takes about another a week before an equilibrium is achieved. An equilibrium should be achieved as soon as possible, especially from a health perspective, since the salt has the least inhibitory effect against unwanted bacteria in the fish before equilibrium has been achieved. The speed of the salting process may depend on the type of salt used (solubility and purity).

The pH development is about the same in the brine as it is in the fish, but the pH of the fish is about 0.5 units higher. Depending on the recipe, the pH of the brine decreases to approximately 5.3 after about 3-4 weeks. This is the result of a fermentation process in which lactic acid bacteria are involved. At some producers, especially those that use slightly higher amounts of salt (over 5%) and lower temperatures (below 5°C), the pH decreases less and the bacterial community is not dominated as much by lactic acid bacteria.

In rakfisk where the pH decreases the most, lactic acid bacteria dominated after 3-4 weeks until the rakfisk process was complete (about 13 weeks), based on an initially low number. They produce acid and cause the pH decrease. Other bacteria inhibit some of the acid production and generally remain at a relatively low level. Some yeast may also appear, especially on the surface that is most exposed to oxygen, but the amount is low.

Few lactic acid bacteria are able to grow at a low temperatures and high salt levels. One of the species capable of this is Latilactobacillus sakei (formerly Lactobacillus sakei). This species is most abundant in rakfisk that is dominated by lactic acid bacteria. In rakfisk processes that involve high salt levels and low temperatures, where lactic acid bacteria are not as dominant, bacteria from the genus Psychrobacter dominate instead, but the total number of bacteria is usually lower. Since excellent rakfisk exists that has not really been through what we believe to be a ‘proper’ lactic acid fermentation process, it seems that it is the later maturation of the fish that produces the typical ‘rakfisk taste’. This is most likely an enzymatic process originating in the fish itself.

Some of the manufacturers we have worked with use Arctic char instead of trout as their raw material. It doesn’t seem that the fish species has much impact on how the process generally takes place, it is the amount of salt and the temperature that determine the results. Of course, the raw material is important in terms of how the product turns out in the end. Among other things, the colour difference between the two fish species is relatively large.

Refrigeration is important for rakfisk food safety

Although rakfisk is a popular delicacy, especially at Christmas time, it is also a product that is associated with certain scepticism because mistakes in the process can lead to the growth of bacteria that are hazardous to health. Rakfisk is based on raw fish, and no heat treatment is involved before it is eaten. This means that the product is somewhat vulnerable. The Norwegian Food Safety Authority recommends certain population groups, such as pregnant women and people with weakened immune systems, to avoid certain types of food. On these lists are rakfisk, especially due to the Listeria risk, and also much more common products such as cured meats and soft cheeses.

The bacteria mentioned in connection with rakfisk are primarily Clostridium botulinumand Listeria monocytogenes. Both of these bacteria can cause serious illnesses such as botulism and listeriosis, and it is therefore important to secure the process to avoid them growing. Since these bacteria are quite common in nature, one must assume that they are found in the environment around facilities (soil, sludge, etc.) and therefore can contaminate the fish. C. botulinum may also be present in the intestines of fish. Classical botulism is actually a type of poisoning, caused by a very potent nerve toxin, the botulinum toxin. The toxin must have started forming in the food before it is eaten. The bacterium itself does not normally infect humans and does not produce the toxin in the human body (exceptions include a form of botulism that can affect infants, and wound botulism, which can affect people with a weakened immune system). C. botulinum is a spore-forming bacterium that grows anaerobically, i.e. without oxygen being present. The spores are very resistant and can survive in various harsh conditions. Many strains have the ability to produce the dreaded toxin, which can be fatal if eaten in sufficiently large quantities. The toxin is not heat-stable, and heating to 60°C for over 10 minutes kills it. As mentioned, the toxin must have started forming in the food, i.e. the bacterium must grow. C. botulinum does not belong to the most cold- and salt-tolerant bacteria, and growth is usually inhibited by a salt concentration of about 5% and temperatures lower than 8°C.  A properly performed rakfisk process should therefore be enough to inhibit the growth of the bacterium.

Nevertheless, over the past 25 years, rakfisk has been involved in most cases in Norway where botulism has been suspected. However, the number is low, and they are always due to homemade rakfisk where poor hygiene has led to contamination of the fish with C. botulinum, and where the abovementioned principles of the process have not been followed. This has led to the growth of the bacterium. Rakfisk produced by reputable producers at quality-controlled facilities is considered safe in relation to C. botulinum. However, it is important to note that the temperature requirements must also be followed by stores and consumers after the fish leaves the manufacturers. Rakfisk is a product that must be kept refrigerated!

Listeria monocytogenes causes the disease listeriosis. The bacterium is invasive, i.e. it penetrates the intestinal barrier and infects the bloodstream and eventually the nervous system. However, the potential for L. monocytogenes to cause disease is relatively low compared to many other pathogenic bacteria. Healthy adults are rarely affected, but an infection could be life-threatening for vulnerable groups such as the elderly, people with weak immune systems, infants and foetuses.

Similar to C. botulinum, L. monocytogenes usually has to grow in a product in order to cause disease. This is because disease first occurs when a relatively high number of bacteria are eaten. The difference is that, theoretically speaking, L. monocytogenes is NOT inhibited by salt and temperature conditions during the rakfisk process. The listeria bacterium is able to grow at temperatures as low as 2 °C and can tolerate salinity of over 10 %, although growth is slow. L. monocytogenes has been detected several times in rakfisk and serious outbreaks of disease have also occurred, although they are few. There have recently been two known outbreaks. In the autumn of 2013, there was a small outbreak where just a few people fell ill. Unfortunately, one person died (who had underlying immunodeficiency). The 2018-19 season saw a much larger outbreak, probably affecting up to 20 people, possibly even more, fortunately without the most serious consequences. In addition, large batches were recalled from stores.  In the aftermath of this outbreak, a relatively large rakfisk producer ran into financial problems and was eventually declared bankrupt. The risk of listeriosis outbreaks is considered low if production is carried out in accordance with approved hygiene and processing principles. Something that also distinguishes the Listeria bacterium from C. botulinum is its ability to establish itself and grow in facilities (surfaces, equipment) and then get transferred to the fish.  The outbreak in 2013 is most likely due to contaminated equipment used to fillet the fish before the process is initiated. Good general hygiene in facilities is absolutely essential. In this respect, the Listeria problem here is no different from other types of fish processing, such as smoked salmon and gravlax.

Our studies indicate that growth of Listeria in rakfisk can largely be avoided at temperatures of 4 °C. Newer methods for combating Listeria (using so-called protective cultures of lactic acid bacteria that inhibit Listeria, special salts with inhibitory effects or bacteriophages, i.e. viruses that kill bacteria) may also be applicable. Improved sampling methodology, with the aim of identifying and thereby excluding contaminated batches before they reach the market, may also be an opportunity to minimise the risk of Listeriain products for human consumption. It is a question of costs and production logistics. However, as mentioned: good quality raw fish, good hygiene, the correct amount of salt and the correct temperature are the decisive factors for making good and health-safe rakfisk.

Rakfisk research

Nofima has been involved in several networking, commissioned and research projects on rakfisk. The outbreak history, and possible consequences, as mentioned above, show just how important knowledge is regarding the survival of the industry. During the period 2021-2024, we are leading a major research project with support from the Research Council of Norway. The project is owned by Norway’s largest rakfisk manufacturer, and a further five rakfisk producers with slightly different rakfisk production routines are partners in the project and contribute with their own efforts. Other project partners are the Norwegian Veterinary Institute and a supplier of industrial decontamination equipment. The project aims to find methods that can be used in practice to increase safety with regard to Listeria in rakfisk.

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