First off, there is nothing better than fresh seafood you caught/dug yourself.....maybe a fed ex of clams for the helpful chef?
To answer your question I wanted to explain the "hows and whys" of purging clams. The cornmeal is not really needed, it just doesn't hurt.
The cornmeal method, and all methods of purging clams, are based off of the fact that clams are filter feeders and will cycle surrounding water in and out of their shell, separating out nutrients before expelling the water. This water commonly contains things like sand that are not expelled and remain in the clam (this is the way we get pearls actually) The thought behind using cornmeal is that it is an irritant that causes the clam to "cough" and expel all sand that may be in its shell. There is also a thought that it helps to change the flavor of the meat and make them less fishy (I can't say I have noticed a flavor difference between a clam purged with cornmeal and one without). Whether you use cornmeal or not you will need to use saltwater. Soaking the clams in saltwater for an hour or two will allow them to cycle clean saltwater in and out of the shell and purge the sand. Using cornmeal to purge is common and is thought to aid in the complete expulsion of the sand, but it really is not the real reason the purging will happen. Putting a clam in fresh water is like putting a man on the moon in a space suit, both are taking an organism and putting it in a foreign environment. The astronaut is really going to want to remain in his suit the same way the clam will want to keep that shell nice and tight. Without the saltwater to make the clam feel at home you will likely not see much purging, if any, occurring.
It is the salt that will allow purging to occur, not the cornmeal....so to finally answer your question, you need to use saltwater and not freshwater to purge. Add 3oz of kosher or sea salt to 1 gallon of cold water, stir and allow a few minutes for the salt to dissolve. Make sure not to use iodized salt, clams don't like iodine! It will likely be about 1-2 hours they will need to purge. The easiest way to verify is to crack on open and check(then eat with a little lemon and tabasco!)
Hope this helps your next clam bake!
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