In Japan, kelp stock is the most basic vegetarian stock. Simple kelp stock is used to prepare vegetarian or delicately flavored dishes. When kelp stock is used as a base for other recipes, it is then usually flavored with a combination of key Japanese seasonings such as sea salt, shoyu, miso, mirin, sake, sugar, and rice vinegar. —Hiroko Shimbo
Fill a large, clean bowl with 8 cups of cold tap water. If using kelp sheets, wipe your sheets with a moist, clean kitchen towel to remove any impurities, and cut the kelp into pieces so it easily fits in your bowl. Add the kelp to the water (there is no need to stir).
Cover the bowl with plastic wrap and store it in the refrigerator overnight, or about 10 hours.
Strain the stock through a fine mesh strainer set over a large container. (I line the strainer with a moist paper towel, so that none of the kelp particles get into the stock.)
Your stock should be full-bodied, with a faintly sweet and salty taste. For the best flavor, use within three days, or freeze what you won't be using immediately.
You can make a second stock by placing the used kombu and 6 cups of water in a large pot and bringing it to a gentle simmer over medium heat. Decrease the heat to medium-low and cook for 15 minutes, then strain through a paper towel-lined sieve. The second stock is somewhat weaker in flavor than the first stock, but it is perfect for making soup and braising fish, poultry, or meat.