The scientific name of the bittern used to make tofu is bittern, which is the mother liquor left in the salt pond after being salted by seawater or salt lake water. Its main components include magnesium chloride, calcium sulfate, calcium chloride and sodium chloride, etc. It tastes bitter and toxic, and magnesium chloride crystals will be precipitated after evaporation and cooling, which is called bittern block. The bittern block is also a coagulant often used to make tofu in northern China, and its function is to coagulate protein in soybean milk.