Dissolve 1 tablespoon of salt in 100 ml of hot water. Once dissolved, add the ice to cool, and then add the 350 ml of water. This should make a slightly less than 3% salt solution.
Put a wire rack in a tray, and put the clams and the salt solution in it. The rack will prevent the clams from reabsorbing the sand that they expel. Cover loosely with a newspaper, sushi mat, etc.
Leave in a cool place, preferably for at least 3 hours. This will let the clams expel their sand, slime, and odor.
If you are worried about the water temperature getting too high, you can put some ice cubes on of the cover.
This is the water after the clams expelled their sand. They expelled a lot of slime as well.
They are too salty as-is, so remove excess salt an hour before cooking. Quickly rinse the clams and leave on a colander, and the clams will expel salt.
Rub the shells together to cleanse it of dirt and slime. They will break if you handle them too roughly, so handle with care.
If you want to freeze them, bag them in plastic bags after you de-salt them, remove air from the bag, and freeze. Do not thaw them when you cook them: just use them as is.
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Story Behind this Recipe
The gritty feel of the sand in clams is quite unpleasant. Let's eat tastier clams by completely removing the sand and the slime!
The Manila clams are sensitive to fresh water and heat, so be careful. It will also weaken if the salt concentration is too low. The salt concentration in sea water is about 3%, so you can mix 30 g of salt to 1 liter of water. A dead clam could contaminate the water and kill the entire batch, so discard promptly.