To clean mussels, scrub with a stiff brush, then pull off the beards or remove them with a sharp knife or kitchen shears. Place the mussels in very cold water and rub the shells together to remove any remaining debris. Or scrub with a vegetable brush. Rinse in several changes of cold water.
If a shell isn't tightly closed, test to see if the mussel is alive. Tap the shell on a firm surface: it should close immediately. Discard any mussels that stay open, have cracked shells or float.
Mussels clean themselves out to some extent if you soak them in cold water with 2 tablespoons flour, 2 hours before cooking them.
If you want to serve the cooking liquor as soup, strain it through a muslin-lined sieve to remove any grit.
