Gentle Heat and Close Attention

In practice, it’s all too easy to overshoot the ideal temperature range for fish. It takes only a matter of seconds to overcook a thin fillet. Two characteristics of fish add to the trickiness of cooking them well. First, whole fish and fillets are thick at the center and taper down to nothing at the edges: so thin areas overcook while the thick areas cook through. And second, fish vary widely in their chemical and physical condition, and therefore in their response to heat. The fillets of cod, bluefish, and other species often suffer from some degree of gaping, separations of muscle layers through which heat penetrates more rapidly. Such fish as tuna, swordfish, and shark have very dense flesh, crammed full of protein (around 25%), which absorbs a lot of heat before its temperature rises; less active members of the cod family get by with less protein (15–16%) in their muscle, and cook more rapidly. Fat transfers heat more slowly than protein, so fatty fish take longer to cook than lean fish of the same size. And the very same species of fish can be protein- or fat-rich one month, depleted and quickly heated the next.