New Techniques Could Speed Up Cod Processing

“In this test we have looked at the possibility of immediate gutting and filleting of unbled fish, followed by rinsing of the fillet. The aim is to considerably shorten the processing time for cod without being left with more blood in the fillet. We have also investigated what effect superchilling had on quality,” explains Bjørn Tore Rotabakk, a researcher at Nofima Mat.

Superchilling and new bleeding method provides new opportunities

“Reducing the processing time brings many advantages. Because the whole process can be performed in one place, you save both time and energy, and there is less transporting. This gives both environmental and financial savings,” says Hogne Bleie, Director of Biology and Biosafety at Atlantic Cod Farms AS.

At present, farmed cod that is intended for the EU market is often processed in Denmark, France or Poland. However there are no punitive duties on processed white fish in the EU and with the new method processing can be performed near the fish farms - at slaughterhouses in Norway.

“Filleting directly after death and the use of superchilling removes the need for repacking in other places, which means a great deal of packaging material can also be saved,” says Director Bleie.

Trial with pre-rigor farmed cod

One group of farmed cod was bled in a traditional bleeding vessel, without movement. The fish were cut across the gills and laid in seawater for 30 minutes to bleed out before gutting and filleting. The other group of cod was not bled in the traditional way, but gutted and filleted directly after being killed. After filleting the fish was washed by sprinkling with fresh water for 10 minutes. The processing was carried out at Slakteriet Br. Larsen AS in Bremanger.

Both groups were then divided into two for packing in different ways. One group was superchilled to about -1°C and packed in expanded polystyrene cases without ice. The other group was packed directly into expanded polystyrene cases and 3kg of ice was placed on top. After packing, the four groups were transported and stored at 0°C until the fillets were 6 days old. The cases were sent by refrigerated transport and arrived at their destination after about 48 hours.

“The temperature in the cases was logged during transport and storage. The equalisation temperature of the case of superchilled fish was -0.9°C, which corresponds very well with the theoretical equalisation temperature of cod, which is stated as being -1.0°C,” explains Bjørn Tore Rotabakk.

Lower drip loss with superchilling

The drip loss, colour and texture of the cod fillets was analysed after the cases were opened.

Drip loss was significantly (P<0.001) reduced by superchilling, while traditional bleeding gave significantly (P<0.001) lower drip loss than the new bleeding method. Earlier tests at Nofima Mat (at that time Norconserv AS) showed the same result.

“The reason superchilling reduces drip loss in cod is not yet clear, but increased proteolysis and enzymatic and bacteriological activity because of higher temperatures may be a plausible explanation. One should bear in mind however that the superchilled fillets were still below 0°C at the second weighing and this may have led to free water being fixed in the surface in the form of small crystals. This may mean that the drip loss in superchilled fillet is greater if the temperature rises above 0°C,” Mr Rotabakk points out.

The greater drip loss with the new bleeding method, compared with traditional bleeding, is because the fillets were weighed after 10 minutes of rinsing. The rinsing led to water being absorbed into the surface of the fillets and temporarily bound to the muscle. There will therefore be more water in the surface of the rinsed fillets during the first weighing compared with those that have not been rinsed, and this water will be recorded as drip loss when the water leaves the fillet during storage.

New bleeding method provides good bleeding, but a softer texture

No differences in colour were recorded between superchilled/ice and traditional bleeding and the new bleeding method. It therefore appears that the new bleeding method works at least as well as the traditional one. During rinsing of the fillets, no blood was observed in them and it may be possible to reduce the rinsing time.

“Texture analysis revealed that the new bleeding method gave a significantly (P=0.017) softer surface to the fillet than traditional bleeding. This may be due to water being taken up into the surface of the fillet during rinsing. Even though the other analyses indicate that this water disappeared during storage, it may have led to a softer texture in the surface of the fillet. The superchilled cod was also significantly (P<0.001) softer in the surface of the fillet than cod iced in the normal way. Superchilling causes the surface of the fillet to freeze and then thaw again, which may explain why the superchilled fillet has a softer surface texture,” says Mr Rotabakk.

Promising results

The trials carried out for this project show that the new bleeding method works well. No difference in colour was found, which means that it cannot be distinguished visually from traditionally bled fillets. Further work must be done to optimise the washing process. It may be possible to reduce the rinsing time, since no visual observations of blood were made in the fillet directly after filleting.

“Drip loss was greater with the new bleeding method. This is most probably due to water from the rinsing that becomes bound up in the surface, but this must be verified. Superchilling gave a smaller drip loss than traditional icing, but a somewhat looser texture in the surface of the fillet. All in all, the combination of this new bleeding method and superchilling shows very promising results,” concludes Mr Rotabakk.