For fish lovers, ideally it would always be possible to get freshly caught fish right off the boat. Unfortunately that's not always practical. However, the factory ships of the modern fishing industry have made it possible to enjoy top quality fish anywhere, even hundreds of miles inland. They do this by processing fish such as haddock and cod as soon as they're caught, then blast-freezing them to preserve their delicate flavor and texture. Once defrosted, a frozen haddock can be cooked as if freshly caught.
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The U.S. Department of Agriculture's Food Safety and Inspection Service recognizes four methods of safely defrosting fish and other frozen foods. The best is allowing it to thaw slowly in your refrigerator. Refrigerators are designed to keep foods below 40 degrees Fahrenheit, the threshold of the temperature "danger zone." By never allowing the haddock to rise above 40 F as it thaws, refrigerator defrosting preserves the maximum degree of food safety.
A second method is cold-water thawing. This is often used by restaurants when an item sells better than expected, making it necessary to safely but quickly thaw more. Place the haddock fillets in a plastic bag, or thaw them in the original packaging if it is waterproof. Put the haddock in a large bowl or basin and fill it with cold water. Leave the bowl in your sink with a slow trickle of cold water running into it, if you can. If not, change the water every 30 minutes until the haddock is fully defrosted.
Microwave defrosting is a less-desirable option. Microwaves heat unevenly, creating areas where the haddock might reach temperatures in the food safety danger zone, between 40 F and 140 F. There is also a possibility that some portions of each fillet might cook during defrosting, meaning they'll be overcooked when the dish is finished. Minimize the risk by turning the fillets frequently, especially if your microwave lacks a turntable. To maintain food safety, only defrost in the microwave if you're cooking the fillets immediately.
Cooking From Frozen
A fourth alternative is to simply cook the haddock from a frozen state, so defrosting is simply part of the cooking process. Restaurateurs often use this method for deep-fried fish and chips, because it allows them to minimize the risk of lost profit due to spoilage. At home, baking or grilling the haddock is a better option. Arrange the fillets in a single layer in a lightly oiled casserole dish, or on a parchment-lined baking sheet. Bake them until the thickest part of the fillet is slightly opaque. Grilling haddock requires a special fish basket, since it's delicate and will fall apart without one.