In the past, tofu was made by soaking soybeans in water, foaming and softening, grinding them into soybean milk in a stone millstone, filtering off bean dregs and boiling them. At this time, the protein granules in the soybean were surrounded by water and kept moving, as if dancing in a group in a soybean milk bucket, and they could not get together, forming a "colloid" solution. To turn the colloidal solution into tofu, you must order brine. Bittern or gypsum is used to light bittern, and bittern is bitter bittern and bittern alkali. It is the mother liquor left in the salt pond after making salt from seawater or salt lake water. The main components are magnesium chloride, calcium sulfate, calcium chloride and sodium chloride, which are bitter and toxic. After evaporation and cooling, magnesium chloride crystals are precipitated, which are called halogen blocks. Bittern blocks dissolved in water are called bittern, which is a commonly used coagulant for making tofu in northern China, and can make protein solution coagulate into gel. Tofu made of bittern as coagulant has strong hardness, elasticity and toughness, and is called old tofu, or north tofu and hard tofu.

Edible salt refers to processed edible salt with sodium chloride as the main component obtained from seawater, underground rock (mineral) salt deposits and natural brine (salty) water, excluding low sodium salt.

The main component of edible salt is sodium chloride (NaCl), and it also contains a small amount of water, impurities and other elements such as iron, phosphorus and iodine, which are different chemical components ~ ~ Tofu made with edible salt will not solidify like tofu made with bittern ~ ~

When using bittern as coagulant to make tofu, the concentration is generally18 ~ 22 be', and the dosage is about 2% ~ 3.5% of the weight of raw soybean. The way to order bittern is as follows: while stirring the cooked soybean milk, the bittern trickle is continuously added; Salt and bittern can also be added to cooked soybean milk intermittently with a certain time interval.

Brine has a strong stimulating effect on skin and mucous membrane, and has an inhibitory effect on the central nervous system. If people accidentally take it by mistake, they will feel nausea and vomiting, dry mouth, stomach pain, burning sensation, abdominal distension, diarrhea, dizziness, headache, rash, etc. In severe cases, they will stop breathing, go into shock and even cause death.