Double frozen fish is generally frozen whole, thawed out for processing and then re-frozen before sold to the end consumer. Because of the varying widths and heights along the body, whole fish does not thaw uniformly. When you add to that the fact that processing facilities do not have time to slow thaw the fish, they are left with one solution, “rapid thaw”. The whole fish is force thawed in chilled water. This rapid thaw breaks down the cellular structure of the meat before it is filleted. With the cell walls already broken down, the filleting and subsequent freezing then causes further damage to the cellular structure of the fish. This can be easily observed during the thawing process prior to cooking. A double frozen fish having so many damaged cells has an enormous amount of “purge” after thawing. Frozen at sea fillets will purge 5-6% of their weight, very good land frozen fillets might purge 8-12% while soft textured land frozen fillets will purge over 15% of their weight!
How exactly is the cellular structure damaged? What happens is that ice particles pierce the membranes of the cell walls causing them to lose moisture and worse, possibly much of their water-soluble proteins and nutrients.
The majority of fin-fish processed like this is cod and Pollock, often from China. Even top quality frozen at sea fish caught in Canada or Alaska is brought to China where it often undergoes the cell damaging rapid thawing, processing and re-freezing.