Coagulation

Coagulation is the stage of cooking where unfolded (denatured) proteins link together into a network that sets into a solid or gel. It's what turns liquid egg into an omelette, milk into cheese curds, and a runny custard base into a silky set cream.

From denatured to set

Coagulation always follows protein denaturation. Heat, acid, or salt first unfolds proteins. The exposed parts are attracted to each other and start forming bonds, trapping water inside a mesh. The result depends on how far it goes:

• Gentle coagulation forms a loose, tender network that holds water. Think soft-scrambled eggs or wobbly crème caramel.
• Over-coagulation pulls the network so tight that water is wrung out. You get rubbery eggs, weeping custards, and curdled sauces.

Egg coagulation temperatures

Diluting eggs with milk, cream, or sugar spreads the proteins apart, raising the setting point and widening the safe window. A stirred custard like crème anglaise is done around 80–82 °C (176–180 °F), and it curdles not far above that.

Coagulation beyond heat

• Acid. Lemon juice or vinegar in hot milk coagulates casein into ricotta-style curds or paneer.
• Enzymes. Rennet snips casein proteins so they clump, the first step in most cheesemaking.
• Salts. Calcium sulfate or magnesium chloride (nigari) coagulates soy milk into tofu.

How to control it

• Use gentle, indirect heat. A bain-marie keeps baked custards from overshooting.
• Temper eggs. Whisk hot liquid into eggs slowly so they warm evenly instead of scrambling (see tempering).
• Pull early. Carryover heat continues to set proteins after cooking stops. Take custards and eggs off the heat slightly underdone.
• Add starch for insurance. Flour or cornstarch in pastry cream gets between egg proteins, which lets you boil the mixture without curdling.

Signs you've gone too far

• Custard looks grainy or separated, with thin liquid at the edges.
• Baked cheesecake or flan has bubbles, cracks, or a weeping layer.
• Eggs squeak and leak water on the plate.

Mild graininess can be rescued by straining and blending. Once a custard separates into curds and whey, the structure is gone; start over and keep the temperature lower.