Bilde av lakseskiver.
Supercooling lengthens the shelf life of salmon enormously. Image: Thinkstock

Supercool salmon stay fresh for a month

Hundreds of tonnes of food are thrown away in Norway every year. Supercooling could keep more fish out of the trash, and even extend the grilling season.

There’s perhaps nothing tastier on the grill than a juicy piece of sizzling salmon, its skin blackening against the hot grill while its pink flesh slowly cooks. But for any fish to be good, it has to be fresh, or carefully handled to keep it as fresh as possible.

Scientists have now perfected a process that supercools salmon, where the fish is preserved by cooling it to a point between freezing and refrigeration temperatures. Preserving salmon using supercooling offers a host of benefits:

  • The fish stays fresh for longer
  • There are less CO2 emissions
  • Less fish is thrown away because it can keep for up to 30 days

“Every person in Norway throws out on average a kilo of food a week. That’s too much,” says Trygve Magne Eikevik, a professor of cooling and food technology at NTNU who has helped develop the process.

Seasonal products

People want fresh food because it tastes best. One dilemma faced by fish processors and slaughterhouses is that during the grill season they fill their cold storage units with fresh food, and then only half of it is sold if the weather isn’t sunny and people don’t want to grill. Consequently, they may end up throwing away a lot of food.

When the heat of the summer finally arrives, fresh meat and fish for the grill sell out too quickly.

Salmon on the menu

This is where supercooling comes in, because it allows food to stay fresh for a very long time. Supercooling has been used for pork, lamb and beef for several years. Now it’s time for salmon to become supercool.

Bilde av laksefilet.

Salmon stays fresher for longer when it is kept at a temperature between frozen and refrigerated.
Image: Thinkstock

Salmon stays fresher for longer when it is in a state that’s between frozen and refrigerated.

Supercooling means cooling the salmon to about -2.5 degrees, which is below the point at which freezing begins. At -2.5 degrees, the fish isn’t completely frozen, so it will keep its fresh quality and won’t taste like frozen fish.

A recent PhD thesis from NTNU documented for the first time how the ice crystals that form in parts of the fish during supercooling develop during storage. The thesis shows that the fish retains its quality, especially with respect to water content and the structure of the fish flesh.

A layer of ice on the outside

The research for the thesis employed a method that involved cooling the fish quickly in -30 degree C. air for four minutes. During the rapid cooling, a layer of ice forms on the outside of the fish. When the fish is moved into cold storage at -2.5 degrees C., this layer grows inward into the fish. The process doesn’t reduce the quality of the fish.

Big boost for the seafood industry

Fresh grill meat for Norwegians is all well and good, but supercooling has larger repercussions.

The marine sector represents 60-70% of the value of Norway’s exports, and 30-40% of the country’s GDP. According to rector Gunnar Bovim at NTNU, seafood and other marine biological resources contribute to better global food safety and health.

“Food is increasingly in short supply. An analysis of the seafood industry in Norway shows the possibility of a turnover of NOK 550 billion by 2050, which is six times today’s revenue,” says Bovim.

Norway exports one million tonnes of salmon per year. According to Eikevik, the salmon industry hopes to export two million tonnes of salmon per year over the next 15-20 years.

Reduces CO2 emissions

Nowadays, fish processing facilities can typically handle 150 – 300 tonnes of salmon per day. They are predicted to increase their capacity to as much as 1000 tonnes per day.

“When a trailer truck carries 20 tonnes of fish, the trailer has to be cooled as well,” says Eikeveik. “A quick method of supercooling is therefore important in being able to take as many tonnes of salmon as possible. Now it has been documented that fast supercooling works well, and an important part of the foundation has been laid.”

Fresh salmon is typically transported in boxes filled with 30 per cent ice. The fish is then transport by truck throughout Europe, or flown to countries like Japan. By using supercooling, the amount of ice can be reduced, because the ice is inside the fish. This will reduce the total shipment weight, and allow less fuel to be used during transport, thereby reducing CO2 emissions.

Supercooling should be in stores

“Grocery stores should invest in cold storage that can store both fish and meat at stable, supercooled temperatures. This isn’t done today because the stores would rather have the cheapest solution, but I hope it will come,” says Eikevik.

“Then less food would end up in the trash. Stores could advertise that they have supercooled food as a mark of quality, as well as demonstrating that the store is environmentally friendly and works to throw away less food.”