Whether fish is processed before or after rigor mortis can be significant for the quality of the fish product. What is best depends on which product is to be made.

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Lidunn Mosaker Boge  

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Just as everything else that dies, fish also undergo a period of rigor mortis, known as rigor, after slaughter. Once rigor sets in, the muscle contracts, the fish becomes stiff, inflexible and its texture hardens. A fish in this condition is difficult to process without a producing a product of lower quality. Rigor lifts eventually, but it takes time. A codfish may remain stiff for up to two or three days.

The period before the fish becomes stiff – the pre-rigor period – can vary in duration from shorter than two hours to over a day after death. This is very significant for the fishing industry, which prefers a pre-rigor period that is as long as possible, giving greater production flexibility.

Both biology and technology are crucial

“When rigor occurs, how long it lasts and how strongly the rigor is depends on several conditions,” says Tatiana Ageeva, a doctoral research fellow.

Tatiana Ageeva measuring the temperature of the fish.
Tatiana Ageeva measuring the temperature of the fish. Photo: Silje Kristoffersen © Nofima

“The biological factors such as the species of the fish, gender maturity, spawning, hunger and grazing periods, cannot be controlled, but one should be aware of them. For example, haddock is easily stressed, and rigor can quickly set in. To avoid strong cleavage of the fillets, haddock should be processed within a day after death.”

The technological factors, such as type of equipment, catch method, treatment on board and killing method, can on the other hand be controlled. The more gently the fish is handled before harvesting, the longer the pre-rigor period lasts and the weaker rigor the fish will experience.

“In addition it is important to rapidly cool the fish to a temperature lower than four degrees (C).  The higher the temperature, the faster rigor will set in and the more severe it will be,” says Ageeva.

“All of this is decisive for the fish’s characteristics, such as shape, colour, taste, smell and consistence.”

When should fishery enterprises begin processing the fish? “This depends on the product you want, and what characteristics you wish it to have.”

Tatiana Ageeva and her colleagues have investigated the quality of the fish that is processed before, during and after rigor, and has arrived at the following results:

Fresh fish

  • Pre-rigor fillets:
    Fillets produced before the fish goes into rigor are uniquely fresh. The muscle is flexible, naturally firm in texture and easy to process by machine without compromising, visible marks. Such fillets have less gaping and they reach the shop while they are very fresh.  They taste a little different than post-rigor fillets and have a longer shelf life. The challenge of pre-rigor fillets is that they can shrink in length, since there is no mechanical resistance from the skeleton. They will then have a shorter, wider shape. The earlier the fish is filleted, the more it will shrink. But the fish species, the muscle’s physiological state (the impact of machinery, stress), storage temperature and packing method also determine how much a pre-rigor fillet will shrink.
  • Rigor:
    Fish ought not to be processed while it is in rigor. Strong rigor can lead to incorrect cutting and less yield from the gutting machine, the neck cutter and filleting machine.  Fish in rigor often have a curved form and must be straightened out by hand before they can be run through the filleting machine. The muscle is then physically torn up and mechanically damaged and there is strong filet gaping. In addition, the trimming of fillets from a fish in rigor is challenging since the muscle is very hard and it is difficult to remove pinbones.
  • Post-rigor fillets:
    It is post-rigor fillets that we most often find in shops. They are processed two to four days after death, when the fish is once again flexible. These fish do not retain fluid in the muscle as well and therefore can lose more weight (drip loss) during storage. The texture of the fillets is softer, so they can easily be torn in both the filleting and skinning machines. They also have more gaping which gives a reduced price.
  • Round gutted fish:
    Fresh, good raw material is obviously necessary to produce whole, gutted fish of the best quality. The state of rigor is less significant.

Frozen products

  • If the fish is handled and thawed correctly, it can be difficult to tell the difference between fresh and frozen product made from the same raw material. Fish frozen on board is often marketed as “fresh”. The product is best when the fish is frozen pre-rigor, but if it is rapidly thawed at a high temperature, the muscle may contract unusually forcefully, so-called thaw rigor, where there is significant drip loss, the fillet suffers gaping and it has less taste.

Salted fish and dried and salted cod

  • Full salting:
    Pre-rigor fish go rapidly into a strong rigor when it is salted. The muscles contract and the drip loss occurs, so that the yield is strongly reduced. Pre-rigor salted cod fillets have a very hard texture, and can result in up to ten per cent less yield after rehydration than rigor or post-rigor produced fillets. Fillets that are salted pre-rigor can have a more yellow colour than fillets salted post-rigor. We recommend delaying salted fish production for about two days to avoid all the quality disadvantages of pre-rigor muscle.
  • Light salting:
    Pre-rigor fillets do not absorb salt as fast as post-rigor fillets, and it is therefore difficult to achieve an even salt distribution and the desired level of salt with them. They will also shrink significantly more than fillets produced using rigor and post-rigor fish.
  • Stockfish:
    Both the quality of the raw material and drying conditions are crucial for the quality of the finished product; but also the time at which the processing takes place before it is hung up to dry plays a role. We recommend the use of the freshest possible raw material for stockfish that is produced gutted, with the head removed, but including the spine bone. Pre-rigor raw material will give a better colour and consistency to the end product than post-rigor fish. With respect to Stockfish without the spine bone, be aware that any shrinkage can result in a ridged and cracked surface.

Be quick and gentle

“The optimal starting point for processing depends on what end product is to be made. But the main rule is that the faster and gentler the fish is caught, killed and processed, the better the quality of the end product will be,” says Ageeva.

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