Norway.- Controlled freezing and thawing of cod give consumers a product all year round that rivals fresh fish. A new research project shows that the way in which cod is frozen and thawed makes all the difference to the quality of the product.
‘Our research means that producers and consumers can have a steady supply of top-quality cod throughout the year,’ says Irja Sunde Roiha, scientist at the National Institute of Nutrition and Seafood Research (NIFES). Roiha has led a research project that has looked at different methods of thawing cod, and studied which method gives the best result.
Fresh quality outside the season
Cod fisheries are seasonal, and fresh cod can only be found in the shops during two or three months of winter. The rest of the year, the cod you can find in the shops is either frozen or thawed, or has even been frozen and thawed several times. This affects the quality and shelf life of the fish.
Careful handling of the fish when it is caught and immediately freezing it on board the boat, followed by thawing in a controlled manner, means the producers can supply cod throughout the year that has been thawed, but still maintains a high quality and long shelf life.
The cod used in the trial had been frozen on board the boat immediately after catch, and was therefore frozen before rigor mortis had set in. Rigor mortis therefore took place while the cod was frozen, and in this way, did not affect the product after thawing.
Thawing in aerated water
The scientists then carried out two trials, the first of which was to discover whether it was best to thaw the cod in water or with heated plates using a plate thawing cabinet with shelves that maintained a temperature of 10°C. They discovered that the quality of the fish was better if it was thawed in water. The trial also showed that the result was slightly better if the water was aerated, which helped to maintain a steady water temperature.
In the second trial, the scientists looked at which type of water thawing method gave the best quality fish. The trial showed that there was little difference between thawing the fish at a constant 10°C, and first thawing the fish at 10°C and then reducing the temperature to -0.5°C. By reducing the temperature during thawing, the process took close to 28 hours, compared to five or six hours for thawing at a constant 10°C.
‘Time is an important factor in food production, and there is little to be gained in relation to quality by defrosting the fish slowly,’ says Roiha.
Long shelf life
It can often take several days before fresh cod that has not been frozen reaches supermarket shelves. Cod that has been frozen straight after being caught and then thawed just before being put in the fish counter, can therefore be of at least as good quality as fish that has not been frozen.
The cod used in the NIFES trials was still of high quality ten days after being thawed, and the expert panel found that the quality of the fish was good in relation to consistency, colour, odour and firmness. The microbiology and chemical analyses also showed good results for the thawed fish.
‘The most interesting aspect of our trials in my mind is that controlled freezing and thawing gave a product that maintained the quality of fresh fish. I hope that our research will benefit consumers and enable them to buy high-quality cod throughout the year and not just during the few months of the cod fisheries. With the right production method, it is possible to develop top-quality products from thawed cod,’ says Roiha.
Irja Sunde Roiha
Department:Seafood safety and health