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.
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.
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.
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.
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.