I bought 3lbs if muscles yesterday, kept them in the fridge in ice to cook tonight. Almost all are open some bit. If it's a tiny bit opened is that ruined?
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Chops is a trusted home cook.
I would throw away all of the Mussels.
Susan W is a trusted source on General Cooking.
Tap them. If they shut, they are fine. They are breathing. If they don't shut, Chuck them. I gather muscles at the coast and they are often open a little. They don't get much fresher than when attached to a rock.
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SMSF is a trusted home cook.
As Susan wrote, tap the mussels - and if they close, they're perfectly fine.
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