Even those who “can’t stand” fish become fish lovers when they taste very fresh, perfectly prepared lean fish. But cooking good fish depends on buying good, fresh fish in the first place. Understanding something of the chemistry of fish can help guide you through both the fish market and the kitchen.
Why fish is so perishable
To get the freshest fish possible, chefs at expensive seafood restaurants buy fish when it’s still “in rigor” because that guarantees the fish is freshly caught and still sterile.
All animals go through rigor mortis at death. The release of lactic acid causes the muscles to contract and stay contracted for a period of time. In beef, rigor lasts about a day, and in pork and fowl, it lasts about six hours. In fish, rigor starts sooner and lasts for a shorter time. Fishermen, however, can delay and prolong rigor (and therefore prolong freshness) by minimizing the fish’s struggle during the catch and by chilling it promptly. Some fish, such as halibut, have a longer rigor and keep better than most. In an ideal world, fish would arrive at our markets still in or just coming out of rigor—at the absolute peak of freshness.
Unfortunately, this isn’t an ideal world, and by the time fish gets to us, it usually has long since passed out of this pristine state. At that point, marine microorganisms on and in the fish begin to cause deterioration. Freezing slows the process down, but some of these microorganisms flourish even at temperatures as low as 18°F, well below freezing. So the deterioration continues even while the fish is frozen.
When you get fish home, try my storing tip to help keep it fresh. Put whole fish or fillets in a large strainer set over a bowl. Pile ice high on top of the fish and refrigerate. The ice keeps the fish close to 32°F, and as it melts, the water continually rinses off bacteria and drains it into the bowl. This will slow down the spoilage process, but it’s nevertheless crucial to buy only very fresh fish and cook and eat it the same day, if possible.
Beware the fishy smell
Besides rigor, appearance and odor can reveal a lot about freshness. Fresh fish may have a clean, sea smell, but a “fishy” or ammonia odor tells you that protein deterioration has begun and the fish is past its prime. If you’re choosing a whole fish, you’ll know that it’s extremely fresh if it has shining, iridescent skin, springy flesh, clear, bright eyes with jet black pupils, and pink to red gills. The signs of progressing spoilage are easy to spot: dull, opaque skin, soft flesh, cloudy, sunken eyes, and gray gills.
Fillets are harder to judge, but you can still get a good sense of freshness by their smell and texture. The flesh should be firm, not gaping, falling apart, or mushy. Keep in mind that some oily fish, such as tuna and mackerel, will taste fishy no matter how fresh they are. If you want completely nonfishy taste, stay with lean, whitefleshed varieties, like sole, flounder, orange roughy, cod, and haddock, to name a few.
Short muscle fibers make fish naturally tender
Muscle fibers in beef, pork, and poultry are thin and long, up to a foot in length, and they’re connected via tough tendons. In contrast, fish muscle fibers are thick and short, one inch at most. They are stacked on top of one another in parallel layers, like cord wood, with very thin, delicate sheets of connective tissue at each end of the stack of short fibers.
These sheets of connective tissue are not only very thin, they’re also more sensitive to heat than meat connective tissue (joints and tendons). It doesn’t take much heat or time for the connective tissue of fish to melt away to gelatin, leaving the short fibers to separate, or flake. This is why fish can literally fall apart when it’s cooked or transferred to a serving platter. You might consider cooking fish on a heatproof serving pan or dish, so that you don’t have to move it once it’s cooked.
How can you tell when fish is done? Although many recipes say to cook fish until it flakes, you might find that the fish has lost so much moisture by then that it tastes dry and bland. Instead, watch for a change in color and translucence. Raw natural proteins (whether in fish or another animal) are separate units. There’s plenty of space for light to pass through these individual proteins, giving raw fish a glassy, translucent appearance. When heated, these independent proteins unwind and link up with other proteins; in other words, the proteins cook or coagulate. Light can no longer pass between them, and the fish gradually becomes opaque. Unless you’re aiming for rare tuna or salmon, opacity is a good indication that the fish is done.