sodium chloride, which tastes bitter and toxic. After evaporation and cooling, magnesium chloride crystals are precipitated, which are called halogen blocks. It is a commonly used coagulant for making tofu in northern China, which can make protein in soybean milk coagulate into gel and analyze the water. Tofu made of bittern as coagulant has strong hardness, elasticity and toughness, and is called old tofu, or north tofu and hard tofu. As the proverb goes: Bitter water points to tofu, everything has its vanquisher. When using bittern as coagulant to make tofu, the concentration is generally 18 ~ 22 Be', and the dosage is about 2% ~ 3.5% of the weight of raw soybean. Bittern has a strong stimulating effect on skin and mucous membrane, and has an inhibitory effect on the central nervous system. It cannot be eaten directly. If people accidentally take it by mistake, they will feel nausea and vomiting, dry mouth, stomachache, burning sensation, abdominal distension, diarrhea, dizziness, headache, rash, etc., and in severe cases, they will stop breathing, appear shock and even cause death.