Coagulation is the process by which proteins in a liquid or semi-liquid food denature and clump together, turning it into a firmer, more solid or gel-like structure. It's triggered by heat, acidity, enzymes, or certain salts, and is one of the most common transformations in cooking.
It's essential to dairy: rennet or lactic-acid-producing bacteria coagulate milk proteins (casein) into curds, separating them from the liquid whey, as in cheese and yogurt making. In egg cookery, heat coagulates the proteins in whites and yolks, turning a runny raw egg into a firm hard-boiled egg or a set omelet. The same principle coagulates soy milk into tofu, using a coagulant such as nigari (magnesium chloride) or lemon juice.
Coagulation is distinct from gluten development, which builds wheat dough's elastic structure through hydration and mixing rather than protein denaturation - seitan, a wheat-gluten meat substitute used in Japanese and Chinese cooking, firms up further from coagulation during cooking, but its underlying structure comes from gluten, not coagulation alone.