Coagulation in cooking happens when a liquid ingredient is either dramatically thickened or transformed entirely into a solid. This process can be achieved by the application of heat, such as cooked egg whites, or through the addition of thickening agents, such as adding cornstarch to broth to make gravy.
Coagulation via heat can be observed as a cracked raw egg cooks, with heat providing the catalyst for the clear, runny liquid of the egg white and yolk to transform into a solid. In other proteins such as steak, the heat coagulation happens at the muscle fiber level as heat forces out liquids and alters the structure of the cells inside, solidifying them.
When food coagulation occurs due to added ingredients rather than heat, other considerations such as overall acidity, absorption and agitation guide the mixing process and determine how quickly a liquid will begin to gel and thicken. Other terms for coagulation in cooking include clotting, as in clotted cream and curdling, as observed in certain dairy products through purposeful technique or aging. Heat or ingredient coagulation is a technique applied to ingredients with the intent to solidify them, and is generally used to achieve more dramatic results than simply thickening a liquid ingredient by cooking off moisture.