Whether fish is processed before or after rigor mortis can be significant for the quality of the final fish product. What is best depends on which product is being made.
As with everything else that dies, fish also undergo a period of stiffness, known as rigor mortis, after slaughter. Once rigor mortis sets in, the muscles contract, the fish becomes stiff and inflexible, and its texture hardens. A fish in this condition is difficult to process without it resulting in a product of lower quality. Rigor mortis passes eventually, but it takes time. Cod may remain stiff for up to two or three days.
The period before the fish becomes stiff – the pre-rigor mortis period – can vary in duration from less than two hours to over a day after death. This is very significant for the fishing industry, which prefers the pre-rigor mortis period to be as long as possible, giving greater flexibility in the production process.
Both biology and technology are crucial
“When rigor mortis occurs, its duration and severity depends on several factors,” says scientist Tatiana Ageeva.
“Biological factors such as the species of the fish, biological maturity, spawning, starvation and grazing periods cannot be controlled, but one should be aware of them. For example, haddock are easily stressed, and rigor mortis can set in quickly. To avoid severe fillet gaping, haddock should be processed within a day after death.
However, the technological factors, such as the type of equipment, catch method, treatment on board and killing method, can be controlled. The more gently the fish is handled before harvesting, the longer the pre-rigor mortis period lasts, and the rigor will be less severe.
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 final characteristics, such as shape, colour, taste, smell and consistency.”
When should the fish be processed? “That depends on the final 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 mortis, and have arrived at the following conclusions:
- Pre-rigor mortis fillets:
Fillets produced before the fish goes into rigor mortis are uniquely fresh. The muscle is flexible, naturally firm in texture and easy to process by machine without leaving visible marks. Such fillets have less fillet gaping and reach the shop while they are still very fresh. They taste a little different than post-rigor mortis fillets and have a longer shelf life. The challenge of pre-rigor mortis 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 mortis fillet will shrink.
- Full rigor mortis:
Fish should not be processed while it is in a state of rigor mortis. Severe stiffness can lead to incorrect cutting and less yield from the gutting machine, the neck cutter and filleting machine. Fish in rigor mortis 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 fillet 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 pin bones.
- 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 fillet gaping, which results in a reduced price.
- Whole gutted fish:
Fresh, high-quality raw material is obviously necessary to produce whole, gutted fish of the best quality. The state of rigor mortis is less significant.
- 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 mortis, but if it is rapidly thawed at a high temperature, the muscle may contract with unusual force – known as “thaw rigor” – where there is significant drip loss and fillet gaping, and the fish has less flavour.
Salted fish and dried and salted cod
- Full salting:
Pre-rigor fish rapidly go unusually stiff when it is salted. The muscles contract and shed a lot of water occurs, so that the yield is greatly 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 full-rigor or post-rigor produced fillets. Fillets that are salted pre-rigor mortis can have a more yellow colour than fillets salted post-rigor mortis. We recommend delaying salted fish production for about two days to avoid all the quality disadvantages of pre-rigor mortis muscle.
- Light salting:
Pre-rigor mortis fillets do not absorb salt as fast as post-rigor mortis 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 full and post-rigor mortis fish.
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 and spine intact. Pre-rigor mortis raw material will give a better colour and consistency to the end product than with post-rigor mortis fish. With respect to stockfish without the spine, 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 being 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.