Tofu (豆腐, doufu) production is one of the foundational food technologies of Chinese civilization — a process of extracting protein from soybeans, forming it into a stable gel, and pressing it to varying degrees of firmness. The process was developed in China approximately 2,000 years ago (with the legendary attribution to Liu An, a Han dynasty prince) and spread to Japan, Korea, Southeast Asia, and eventually the world. Chinese tofu varies from silken (嫩豆腐, nen doufu — extremely smooth, the texture of barely-set custard) through soft (soft but holds shape) to firm (can be sliced and pan-fried without crumbling) to extra-firm (pressed until dense enough to be shredded or cut into noodles).
The soy milk step: Soak 200g soybeans in cold water overnight. Drain. Blend with 1L fresh cold water until completely smooth. Strain through cheesecloth — squeeze to extract all liquid (the residue is okara, 豆渣, used separately as a food ingredient). The resulting liquid is soy milk (豆浆, dou jiang). The coagulation step: Heat the soy milk to just below boiling (80-90C). Remove from heat. Add coagulant while stirring gently. Two coagulants: (1) Gypsum (calcium sulfate, 石膏, shi gao): produces a softer, silkier coagulation — used for silken and soft tofu. Dissolve 5g gypsum in 3 tbsp cold water. Pour the hot soy milk onto the gypsum solution (not the reverse). The curds form slowly and gently — the result is silken tofu. (2) Nigari (bittern, 盐卤, yan lu — magnesium chloride): produces a firmer, grainier coagulation — used for firm and extra-firm tofu. Dissolve 7g nigari in 50ml cold water. Add to the hot soy milk gradually while stirring. Curds form rapidly and more dramatically.
Eileen Yin-Fei Lo, Mastering the Art of Chinese Cooking (2009); Fuchsia Dunlop, Every Grain of Rice (2012)