How do I cook mussels safely?

The alive-or-dead check before cooking, the open-or-shut check after, and the four minutes they actually need.

Difficulty: intermediate
Time: 25 min
Read: 3 min
Safety: warning

Short answer

Before cooking, discard any mussel that stays open when tapped or has a cracked shell. Steam them in a covered pan with a splash of liquid for three to four minutes until they open, then discard any that stayed shut.

Mussels are filter feeders, so they must be alive right up to the moment they cook. Almost all the safety in mussels is in the sorting, and the cooking itself is over in four minutes.

Safety

Mussels must be alive when you cook them. Discard any that stay open when tapped, any with cracked shells, and any that smell of ammonia. After cooking, discard any that have not opened. Buy from a reputable source, keep them cold, and cook them the day you buy them.

Which mussels to keep and which to throw away

Step by step

Buy them fresh and cook them the same day.They should smell of clean sea, not ammonia. A reputable fishmonger or supermarket counter, and straight home into the fridge.
Store them alive and breathing.A bowl in the fridge covered with a damp cloth. Never sealed in a plastic bag and never sitting in fresh water — both kill them.
Sort them under cold running water.Closed mussels are alive: keep. Open ones, tap firmly on the worktop — if they close within a few seconds they are alive: keep. If they stay open: discard.
Discard anything damaged.Cracked or broken shells go, whether or not they close. So does any mussel that smells wrong, and if several in the bag do, discard the lot.
Debeard them.Pull the wiry tuft towards the hinge end and it comes away. Do this shortly before cooking — debearding kills the mussel.
Scrub off barnacles and grit.A stiff brush under cold water. Rope-grown mussels are usually very clean and need little.
Cook them in a wide covered pan.Soften shallots and garlic, add a large glass of wine, cider or stock, bring it to a hard boil, tip the mussels in and clamp the lid on.
Give them three to four minutes.Shake the pan once or twice. They are done as soon as they open. Longer makes them shrink and go rubbery.
Discard any that stayed shut.Do not prise them open. A mussel that has not opened after the rest have was probably dead before it went in.
Serve immediately with the liquor.Pour the cooking liquid over — that is most of the flavour. Leave the last spoonful in the pan in case there is grit.

Tips

Tap, do not squeeze. An open mussel that closes is alive and perfectly good.
Rope-grown mussels are cleaner, gritless and take almost no preparation.
Four minutes maximum. They are done the moment they open.

Common mistakes

Discarding every mussel that is open before cooking — Mussels relax and gape when cold. Tapping is the test — one that closes is alive and fine. Throwing all open ones away wastes a large part of the bag.
Storing them in water or a sealed bag — Mussels are alive and need air. Fresh water kills them and a sealed bag suffocates them, so both turn a good bag into a bin job overnight.

If it doesn't work

Several will not open after cooking

Cause: Were dead before cooking — Fix: Discard them. If a large proportion stayed shut, do not eat any of them.

Gritty

Cause: Sediment in the shells — Fix: Rinse more thoroughly, and pour the liquor off leaving the last spoonful behind.

Rubbery and shrunken

Cause: Overcooked — Fix: Take them off the heat as they open, at three to four minutes.

Smell of ammonia

Cause: Not fresh — Fix: Do not cook them. Discard the whole bag.

Questions people ask

Do mussels need to be soaked in flour or oatmeal?

Not for farmed rope-grown mussels, which are already clean. Wild or dredged mussels can benefit from an hour in cold salted water to purge grit, but never fresh water for long, which kills them.

Can I cook frozen mussels?

Yes — frozen mussels are usually already cooked and shelled, or cooked in the shell. They only need heating through, and the alive-or-dead checks do not apply.