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Gentle and rapid handling gives best salmon fillet

Gentle and short-lived crowding and pumping from the sea cage to the processing vessel is best in order to achieve good quality and long shelf life of salmon fillet. This is one of the conclusions of a project carried out at Marine Harvest from 2010-2011 involving scientists from the food research institute Nofima and the Institute of Marine Research.

Marine Harvest leading the way

Marine Harvest has played a leading role in testing out many different harvesting methods for farmed salmon. Nofima has been involved in most of these experiments. The main goal of this project, which was financed by the Norwegian Seafood Research Fund FHF), was to test killing and bleeding of salmon pumped straight from the sea cage.

Central relevant issues were: is salmon harvested straight from the cage more economically viable and of better quality than salmon that is, for instance, harvested via a holding cage or straight from a well boat? In the southern region, there are high temperatures during the summer months. This can create major challenges in relation to disease and weaker fish and at times leads to high mortality when the fish are stored in a holding cage or transported by well boat.

“We wanted to find out whether there was a correlation between the handling the salmon received and softness and gaping in the fillet. We also wanted to see whether the time until the onset of rigor mortis was of significance to the result,” says Project Manager and Nofima Senior Scientist Kjell Ø. Midling.

Processing vessel “Tauranga”

The well boat B/B “Tauranga” was reconstructed just before the project, becoming the first processing vessel in Norway.

The scientists ascertained that salmon harvested on the boat straight from the cage had extremely long “pre-rigor” time, sometimes more than 35 hours. The results also showed that the onset of rigor mortis was slower in salmon stored in refrigerated sea water (RSW) in tanks on board than salmon that was stored on ice in containers. This is of major significance, as salmon that has gone into rigor mortis when it is pumped is at major risk of developing gaping.

“Correct handling of the salmon at the cage gives B/B “Tauranga” greater reach and more time before the salmon needs to be pumped in for further processing. The vessel should be emptied as rapidly as possible. The longer we wait, the greater the chances of damage,” says Midling.

In or out of Rigor mortis?

The onset of rigor mortis is more rapid for salmon subjected to tough handling. This is also the case when electric stunning is used, so consequently only percussive stunning may be used during harvesting straight from the cage. It is difficult to follow the development of the salmon in the RSW tanks and the fish becomes soft regardless of how long it is stored on board.

Different muscle groups in salmon go into rigor mortis at different times. Dark muscle (just under the skin), muscles in the neck and near the bone in the ear go into rigor mortis first. As a result, it is easy to believe that the whole fish has gone into rigor mortis, but if the salmon is provoked out of rigor mortis, other muscle groups will go into rigor mortis if it is stored on ice.

“In order to test this, experiments were carried out with salmon from the storage tanks on B/B “Tauranga”. We ascertained that salmon that had gone into rigor mortis were pumped out of rigor mortis and the further they had entered into rigor mortis, the greater the damage to the fillet,” says Midling.

“We bent various groups of salmon around a 10-inch steel pipe 6, 14, 20 and 32 hours after being killed. The first three groups became stiff again, but the salmon subjected to this handling after 32 hours remained soft. The amount and degree of gaping increased in accordance with the time the salmon had been in rigor mortis, says Midling.

In all likelihood, storage in RSW on board is gentler for the salmon than storage in containers, and salmon that are pumped while in rigor mortis develop gaping. Salmon stored and filleted 20 hours after killing had extremely little gaping, but the consistency of the fillet was softer the longer that elapsed from the time of killing until processing.

“It is worth pointing out that many of the remaining challenges can be related to crowding, tough handling on board and inadequate control of rigor mortis. Consequently, these are also important goals in the optimisation that is now taking place at Marine Harvest,” concludes Midling.

The project report 44/2011 “Slakting direkte fra oppdrettsmerd” (in Norwegian language only) is attached.