Reasons for making tofu with a salt such as gypsum (calcium sulfate):
The protein (glycinin) in soy milk has a free carboxyl group and amino group on the protein due to its structure, so water with the same charge is formed on the surface. The membrane, while the charge on the membrane separates the protein particles from each other and does not condense. The reason why gypsum can coagulate the protein in soy milk is because the gypsum will dissociate the charged particles (calcium ions) when it meets water, and the calcium ions can neutralize the repulsive force caused by the charge on the water film on the surface of the protein, which will promote the protein. When the isoelectric point is reached and the dispersed protein is aggregated and precipitated, a gypsum salt is used as a coagulant.