Makes about 3½ pounds.
Nigari (magnesium chloride) is naturally derived from seawater and available by mail order through various online sites, including Amazon.com. It is available in both liquid and dry powder form (either works for this recipe), but its concentration varies. Be sure to check the back of the package to ensure that you are using the correct amount of nigari for 2 quarts of soy milk. Many packaged soy milks contain additives like flavorings or sweeteners that can affect coagulation and flavor. Make sure your bottle contains nothing but soybeans.
2 quarts soy milk
If your nigari comes in dry form, dissolve the appropriate amount (as recommended on the back of the package) in 1 tablespoon cold water. Heat the soy milk in a large heavy-bottomed sauce pot over medium-low heat, stirring constantly with a plastic spatula until it reaches 153° as measured by an instant-read thermometer. If you don't have a thermometer, the soy milk should be heavily steaming but not yet simmering.
Transfer the soy milk to a square container just large enough to hold it (a large plastic food storage container works well for this). Stir soy milk clockwise until slight vortex forms. Slowly drizzle in nigari solution and immediately stir once counterclockwise. Cover with plastic wrap and allow to sit for 10 minutes. If you prefer your tofu warm, eat it immediately, drizzled with a bit of soy sauce. Tofu is best eaten the day it's made, but can be chilled and kept tightly wrapped in the refrigerator for up to five days.