If you pay attention to the situation of making tofu in tofu shops, you will find that people always soak soybeans in water, grind them into soybean milk, boil them, and then add brine (mainly magnesium chloride) to soybean milk. At this time, a lot of shiny things are separated and filtered once, and the tofu is ready.

Since brine can't be drunk, why use brine to make tofu?

It turns out that the main chemical component of soybean is protein. Protein is a polymer compound composed of amino acids, and the surface of protein has free carboxyl and amino groups. Due to the action of these groups on water, a layer of water film colloidal substance with the same charge is formed on the surface of protein particles, so that the particles are isolated from each other and will not sink due to collision.

When bittern is ignited, because bittern is electrolyte, it will split into many charged small particles-positive ions and negative ions. Due to the hydration of these ions, the water film in the protein was seized, so there was not enough water to dissolve protein. In addition, the positive and negative ions of salt inhibited the repulsion generated by the surface charge of protein, reduced the solubility of protein, and the particles condensed into precipitates. There are many shiny things in soybean milk at this time.

There are many electrolytes in brine, mainly metal ions such as calcium and magnesium, which will solidify protein in human body. Therefore, if people drink more brine, their lives will be in danger.

Tofu workshops sometimes use gypsum instead of brine, and the same is true.