Project Year 2016
Rapid freezing and thawing
To limit liquid loss cod should be frozen and thawed rapidly. Rapid thawing has a greater impact on quality than previously thought.
All fish, fresh or thawed, loses fluid during storage. Until recently we had limited knowledge about how freezing and thawing affects fluid loss, and thus the quality of frozen fish.
Many fishing vessels deliver their catch frozen, and the fish are thawed on land for processing.
Nofima researches have conducted tests to study how the rate of freezing and defrosting influence liquid loss in cod.
Cod fillets were vacuum-packed and frozen, using two different methods of freezing: rapid freezing in circulating air at -40°C and slow freezing without air circulation at -20°C. The fish were then thawed at 4°C, either rapidly in circulating water, or slowly in air.
The researcher also studied the effect of double freezing. Fillets were frozen and thawed again as described above.
Consequently, the scientists had four different groups of fish, and 16 combinations of freezing and thawing for the double frozen fish.
The results are clear:
“We found a major difference in fluid loss between the various combinations of freezing and defrosting fish. Rate is a clearly decisive factor. For a twice-frozen product, only two of the 16 combinations significantly limited drip loss, and the final defrosting is the most important,” says scientist Svein Kristian Stormo.
Rapid defrosting in water
Fluid loss in fish that was frozen once ranged between 4.3% and 10%. The best fillets were those that were frozen rapidly at -40°C and defrosted in circulating water.
Speed has an even greater impact when the fish is frozen twice. But since it is the final defrosting that is decisive for the quality of the fillets, it is not necessarily the fish industry’s fault if your cod dinner is a little dry.
“Many consumers are used to defrosting frozen food slowly in the fridge. Based on this research, we will now recommend rapid defrosting in water,” says Stormo.