Shrimp

Raw shrimp keeps 3–5 days refrigerated — longer than finfish, since shrimp are usually handled frozen and thawed close to sale. Keep them cold and covered, ideally over ice.

Nearly all shrimp sold in the US was frozen on the boat. What you see at the counter is thawed product, which means the 3–5 day clock began before you bought it, and it means buying the frozen bag is often the fresher choice as well as the cheaper one. You control the thaw date.

Thaw frozen shrimp in the refrigerator overnight, or under cold running water for a few minutes if you are in a hurry. Never thaw them in warm water — shrimp are small enough to move through the bacterial growth range fast.

Frozen shrimp keep 6–18 months, an unusually long window for seafood, because they are lean. Freeze in a solid block of water if you are freezing them yourself; the ice shell prevents freezer burn better than any wrapping.

Shrimp give you an unusually clear and trustworthy warning signal, which is worth knowing precisely because most seafood does not.

As shrimp break down, bacteria convert compounds in the flesh into ammonia, and the smell is unmistakable — sharp, chemical, like window cleaner. Fresh shrimp smell faintly of salt water and almost nothing else. An ammonia smell in shrimp means discard, without exception. Cooking does not remove it and does not make the shrimp safe; the smell indicates decomposition that is already well advanced.

Texture is the second signal. Fresh raw shrimp are firm and translucent with a slight snap. Spoiled shrimp turn mushy, and the shells go slimy.

Two things look alarming but are not. Black spots on the shell — melanosis — are an enzymatic browning reaction, the same chemistry as a cut apple turning brown. They are harmless and affect appearance only. And shrimp that seem oddly slippery straight from the bag are usually coated in sodium tripolyphosphate, a water-retention treatment; rinse and pat dry.