By Harold McGee
We sometimes eat fish and shellfish very fresh indeed, just minutes or hours after their death, and before they pass through the chemical and physical changes of rigor mortis. This stiffening of the muscles may begin immediately after death in a fish already depleted by struggling, or many hours later in a fat-farmed salmon. It “resolves” after a few hours or days when the muscle fibers begin to separate from each other and from the connective-tissue sheets. Fish and shellfish cooked and eaten before rigor has set in are therefore somewhat chewier than those that have passed through rigor. Some Japanese enjoy slices of raw fish that are so fresh that they’re still twitching (ikizukuri); Norwegians prize cod held in tanks at the market and killed to order just before cooking (blodfersk, or “blood-fresh”); Chinese restaurants often have tanks of live fish at the ready; the French prepare freshly killed “blue” trout; and many shellfish are cooked alive.