Brine is known as salt brine, is made from seawater or salt lake water salt, residual in the salt pond mother liquor, the main components are magnesium chloride, calcium sulfate, calcium chloride and sodium chloride, etc., taste bitter. After evaporation and cooling precipitation of magnesium chloride crystals, known as brine block. It is the coagulant commonly used in the north of China for making tofu, which can make the protein in soybean milk coagulate into gel and analyze the water out. Salt brine as coagulant made of tofu, hardness, elasticity and toughness, known as old tofu, or northern tofu, hard tofu.