|Smoking is one of the oldest methods of preserving fish, or any other meats for that matter. Long before there were refrigerators and freezers, our fishing ancestors learned to use a combination of salt and smoke to keep fish from spoiling. Today, smoking is no longer "necessary," but it remains popular for the flavor it gives to such fish as salmon, tuna, trout, sturgeon, bluefish, and sablefish.
Smoking methods vary, but all are based on a few common principles. First, the fish is treated with salt, either in the form of a strong brine or a surface coating of dry salt. During this curing stage (which can last for anywhere from a few minutes to many hours depending on the size and density of the fish), a two-way exchange takes place, with much of the moisture drawn out of the fish and some salt soaking in. This combination of reduced moisture and salt inhibits the growth of spoilage bacteria, a basic principle of all cured meats.
Most fish are given a second cure after the initial salting to add additional flavors. Recipes for this second cure are often proprietary secrets, but they typically include sugar in some form; like salt, sugar both draws moisture out of and soaks into the fish. Often the second cure will include spices or herbs, and sometimes a bit of rum or whisky.
After curing, the fish is rinsed to remove the salt and other curing ingredients from the surface, then allowed to dry in cool flowing air until a shiny, slightly tacky skin (pellicle) forms on the surface. The pellicle serves several functions: it provides an ideal surface for the smoke flavor to adhere, it helps seal in the remaining moisture through the smoking process, and it prevents the fats in the fish from rising to the surface and spoiling.
The actual smoking takes place inside a chamber filled with smoke from smoldering hardwood. At this point the process diverges; fish are either "hot-smoked" or "cold-smoked," depending on the temperature of the smoking chamber. The choice determines the texture, flavor, and potential uses of the fish.
In cold-smoking, the fish is slowly smoked, often for 24 hours or more, at a temperature from 60 to 110 degrees F. Commercial smokers use a system of liquid-filled cooling tubes, a sort of reverse radiator, to remove excess heat inside the smoking chamber. In this temperature range, the fish is not cooked, just dried a little further and infused with smoke flavor, so it remains especially moist and tender.
The best known cold-smoked fish is the type of smoked salmon variously known as "lox" or "Nova." Lightly cured, tender, and moist, it can be sliced thin without breaking apart. (The name Nova, by the way, dates from an earlier time when much of the salmon in New York came from Nova Scotia; today, the fish is likely to be either Pacific Chinook or coho salmon or Atlantic salmon raised on farms.) Sturgeon also takes beautifully to the cold-smoke treatment.
Hot-smoked fish actually cooks during the smoking process, in which the smoke gradually reaches a temperature of up to 180 degrees F. Smoking time is generally less than for cold-smoking. The result is firmer and flakier than cold-smoked fish, yet moister than grilled or "barbecued" fish. Temperature control is just as critical in hot-smoking; if the heat gets too high, the fish will lose all its moisture before the smoking is complete. This is the kind of smoked fish produced by most home-sized smokers.
Many types of fish can be hot-smoked. In addition to salmon, commercially available choices include trout, various tunas, and the Northwestern fish properly known as sablefish but usually marketed as "black cod." The golden-colored whole smoked whitefish sold in delis also fall into this category, as do smoked mackerel and bluefish.
Don't expect to slice hot-smoked fish the way you can lox; it will crumble if cut too thin. Cut hot-smoked fish into cubes, chunks, or thick slices, or just flake it apart along the natural seams between the muscles. In this form, the fish is ready to use in salads and other cold dishes, or gently warmed in a pasta sauce.
Ideally, smoked fish should get its flavor and mahogany color from the smoke, but many cheaper smoked fish have smoke flavor added, and some varieties, like some hot-smoked and garishly colored "kippered" salmon, use artificial food color as well. In fact, some of the brightly-colored cured fish sold in supermarkets are not smoked at all, simply flavored with a cure that includes smoke flavoring.
In pre-refrigeration days, smoked fish were heavily cured and smoked fairly dry, for storage at room temperature or in a cellar. Today's cures are lighter, so most forms need to be refrigerated. They will last longer than fresh fish, but you should still plan to use them within a couple of days of purchase. Both hot-and cold-smoked fish can be frozen, and in fact a lot of delicatessen lox is stored and shipped in this form.
For longer storage, some smokers pack their fish in cans, or in the newer vacuum-sealed foil packaging known as a retort pouch. The latter is like "canned fish without the can"; just like canned fish, the fish in a retort pouch has been fully cooked and sterilized in the packing process, so it can be stored at room temperature for up to seven years. Don't confuse retort pouches with vacuum-packed plastic packages, however; vacuum-packed product is just as perishable as lox bought from a deli. Read and follow package labels carefully with these as with any seafood product.