Cooking fish isn't exactly rocket science: As long as you don't overcook it or overseason it, you can't go too far wrong. Still, there's a degree of art to it that can make the difference between an acceptable meal and a truly memorable one. One notable piece of fish-cooking finesse consists of soaking your portions in a suitable liquid before cooking them.
The types of liquid you can use differ and range some something as gentle as buttermilk to as pungent as vinegar. Your choice will be determined by the final flavor you want to achieve. There are multiple reasons for doing this, depending on your fish, and the impact on your finished dish can be dramatic.
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Reasons for Soaking Fish
When your recipe calls for soaking the fish, there are generally two reasons to do so. One is to take flavor out, and the other is to put flavor in. If you're starting with some form of salted or cured fish, for example, you might have to take out some of that salt for the fish to be edible. Some fish might also have unpleasant flavors that need to be dealt with, either because they're naturally strong-tasting or because of their diet or habitat.
On the other hand, you might soak fish to complement its own flavors. A marinade can infuse the fish with suitable flavors, like citrus or fresh herbs, and even a simple brine can leave each mouthful pleasantly seasoned and moist.
Soaking Salted or Cured Fish
Some of the world's best-loved traditional dishes start with fish that's been preserved in some way, from the salt cod caught off New England and Atlantic Canada, to the salted and smoked herring the English call kippers. Some of them can be cooked and enjoyed just as they are, while others have to be soaked to extract enough salt to make the fish taste good.
If you're preparing a traditional European or New England dish with salt cod, for example, you'll need to soak the pieces in cold, fresh water for up to 48 hours, depending on the thickness of the portions. Thinner pieces or small broken pieces may take as little as 12 to 24 hours. You should change the water every 8 hours or so as the fish becomes saturated. Kippers don't need to be soaked like salt cod, but some brands are saltier than others. If you find yours are too assertive, even 2 to 4 hours in cold water should be plenty to tame them.
Soaking Fish in Milk
Soaking fish in milk is a preliminary that's called for in a lot of recipes that date back before the days of widespread refrigeration. Fish gets "fishier" the longer it's out of the water, and while it's still perfectly good to eat, a lot of people – especially if they're not enthusiastic fish-eaters to begin with – might be put off by the smell. Milk doesn't mask the smell or soak it up from the fish; instead, it actually reverses the chemical reaction that created the odors in the first place.
Soaking a thin fillet for as few as 10 to 15 minutes can make it taste milder, and for thicker fillets or steaks, you can fearlessly double the soaking time. For example, you might soak a generous cut from the thicker part of a large fillet of salmon in milk for as long as 20 to 30 minutes.
Soaking Fish in Buttermilk
Soaking fish in buttermilk is much the same as using milk, only more so. It's a bit tangier and more acidic, and it's even better at quelling unwanted flavors in your fish. It will not only neutralize the molecules that make the fish smell fishy, it will also go a long way toward hiding the muddy taste some fish retain from the water where they've lived their lives. That's why, for example, many Southern recipes call for soaking catfish in buttermilk.
Buttermilk is also just acidic enough to act as a bit of a marinade. Its tenderizing effect isn't really needed with fish, because fish is generally pretty tender to begin with, but 10 to 15 minutes in buttermilk — with or without other seasonings — gives the fish a pleasantly mild tang that stays with it after cooking even if you've breaded the fish and fried it.
Soaking Fish in Brine
Salt does wonderful things for fish, and but you don't have to make salted fish to find that out. A light, mild brine is plenty if you're just planning to cook a fillet or two for a weeknight meal. For soaking fish in saltwater, a good ratio is about a tablespoon of salt for every cup of cold water. You can add sugar as well if you wish. Use about 3/4 as much sugar as salt, so if you've made a quart of brine, for instance, you'd use 4 tablespoons of salt to 3 tablespoons of sugar.
Brining the fish for 4 to 6 hours in the refrigerator will leave it delicately seasoned all the way through without changing its texture. The fish also will be very tender and juicy when it's done, even if you've overcooked it slightly.
Using Flavorful Marinades
Marinades are a remarkably useful flavor enhancer for most foods, but they're especially effective on fish. That's partly because most fish have a mild flavor of their own, which leaves a relatively "blank slate" for the marinade's herbs and spices. It's also partly because a piece of fish has plenty of cracks and crevices where the marinade can penetrate.
Most marinades are made from a mixture of oil and an acidic ingredient, such as lemon or lime juice or a good vinegar. You can leave red meats in a marinade for hours or even overnight, but that's not a good idea with fish. Soaking fish in vinegar for any length of time will change its texture substantially and overwhelm its mild taste. Usually, 20 to 30 minutes is the longest you'll want your fish to stay in a marinade or even less if it's a thin piece. Thick slabs, or extra-firm fish such as sturgeon or swordfish, can go longer.
Soaking Instead of Cooking
The effect of acidity on fish is so strong that you can actually use a marinade instead of heat to "cook" your fish. The technique has a lot of names, but it's best known as ceviche. If you cut your fish or shellfish into bite-sized pieces and soak them in a marinade of salt and lime juice — essentially, a brine and a marinade in one — for at least 10 to 15 minutes, or up to 25 minutes, the pieces will become opaque and take on the texture of fully cooked fish.
Depending on the thickness and firmness of the fish you're working with, the exact time will vary, so taste it frequently until you're happy with the flavor and texture. Then remove the fish from its marinade and serve. You can use other citrus juices or even a good mild vinegar for your acidity and change up the flavors with fresh herbs, spices and onions or other aromatic vegetables, but the basic process remains the same.