Cod

Cod keeps 1–3 days refrigerated and, as a lean whitefish, a generous 6–8 months frozen — roughly triple what oily fish like salmon manages.

Cod is one of the wettest fish at the counter, and moisture is what shortens its life. Take it out of the wrapping, pat it dry, and store it on a rack over ice in a covered dish so meltwater drains away rather than pooling around the fillet.

Most cod sold in the US was frozen at sea within hours of the catch — the North Atlantic and Pacific fleets have done it that way for decades. That is a quality advantage rather than a compromise, but it means counter cod should not be refrozen, and its clock started before you saw it.

Cod's thick, large flakes make it prone to gaping, where the muscle segments separate into visible gaps. Handle fillets gently and support them from underneath.

Cod is very lean — under one percent fat — which shapes everything about how it keeps. With almost no fat to go rancid, oxidation is barely a factor, and that is why the freezer window stretches to 6–8 months where salmon gets 2–3.

What cod does have is a great deal of water and very fragile connective tissue. The muscle is arranged in short blocks separated by thin sheets of collagen, and as those sheets break down the flakes pull apart into the ragged gaps called gaping. That structural collapse is the first thing you will notice, and it is a reliable indicator that the fish is aging even before it smells.

The chemistry is the standard fish story: bacteria convert trimethylamine oxide into trimethylamine, producing the sharp ammoniac smell. Fresh cod should smell of clean seawater and essentially nothing else.

Cod also loses water readily, so an older fillet weeps a milky liquid into the dish and turns dull and chalky where it was translucent. Press it — fresh cod springs back, and older cod holds the dent.