The harvest of clams is monitored by the Food and Drug Administration through the National Shellfish Sanitation Program. Clams sold to restaurants and retail markets must be tagged with information about where the clams originated, who the packer was and the date they were shipped. You can ask the fish seller to show you this information.
BUYING: Fresh clams should smell just that way -- fresh, even sweet, with no trace of fishiness. Clams are highly susceptible to bacterial contamination once they die; steer clear of open bivalves and those that are cracked or chipped. If an open hard-shell clam closes when tapped on its shell, it is still safe to eat. A soft-shell clam, however, will not close completely when tapped, though its neck will retract.
Be sure the fish seller does not pack the clams in an airtight bag, which could cause them to suffocate. Perforated bags are preferable. Live clams must be kept cold until it is time to cook or serve them, so get them from the market to your refrigerator quickly. (I usually ask for a bag of ice, packed separately, to place next to the clams for the journey home.)
STORING: Live clams should be stored uncovered in the refrigerator (I use a large metal bowl). Do not keep them in water or they will die (soaking them for a couple of hours to remove sand and grit is okay, however). Although clams will last for several days in the refrigerator, it's best to eat them the same day you buy them. Shucked clams in their liquor can be refrigerated for several days or frozen in an airtight container. Defrost frozen clams in the refrigerator, not at room temperature.
CLEANING: Soft-shell clams, which carry a lot more sand and grit than hard-shell ones, should be rinsed in several changes of water, then soaked for a couple of hours in abundantly salted water, in the refrigerator, with a tablespoon or two of cornmeal added to the water (this apparently encourages the clams to spit out their grit). Opinions differ on whether hard-shell clams need to be soaked. Rinse them several times in water, scrub them vigorously with a stiff brush and rinse them again. Soak them as you would soft-shell clams, in salted water with a little cornmeal tossed in. Drain and rinse thoroughly.
STEAMING: Steam clams in one to two inches of liquid such as wine or beer, with fresh herbs added. Be sure to cook them for a minimum of six minutes; remove them as they open to prevent the meat from toughening. Discard any clams that don't open within a reasonable amount of time (about 15 minutes).
-- Domenica Marchetti