Curdle

To curdle means to separate into soft lumps and a thin, watery liquid. It happens to milk, cream, yogurt and egg mixtures such as custard, when their proteins clump together. Usually the cause is too much heat, too much acid, or both. Curdling is wanted when making cheese, paneer or ricotta, but not in a sauce, soup or custard.

Why it works

Milk and eggs contain proteins that float freely in liquid. Heat, acid and salt make those proteins unfold and stick to each other, a process called denaturation. A little of this thickens a custard. Too much makes them form tight clumps that squeeze out water. Low-fat dairy curdles more easily than high-fat dairy, because fat gets in the way of the proteins.

How to use it

Ways to avoid curdling:

• Heat egg sauces and custards gently, ideally in a bain-marie, and stop at 77 to 82°C (170 to 180°F).
• Temper eggs by whisking a little hot liquid into them before adding them to the pot.
• Add milk, yogurt or sour cream to soups and curries off the heat, or at a gentle simmer. Do not boil them.
• Use heavy cream or crème fraîche with acidic ingredients such as wine, lemon or tomato. They are less likely to curdle than milk.
• Stir a teaspoon of flour or cornstarch into yogurt before adding it to a hot dish. Starch helps protect the proteins.

Common mistakes

• Boiling a soup after adding milk or a liaison of yolks and cream.
• Adding cold dairy to a very hot, acidic sauce in one go.
• Walking away from a custard on the stove.

In recipes

To rescue a curdled custard, take it off the heat at once, pour it into a cold bowl and blend it with an immersion blender, or strain it. Slight curdling often smooths out, though the texture will not be perfect. Badly scrambled custard cannot be saved. For an emulsion that has separated into fat and liquid, see split sauce, which is a related but different problem.

Some recipes ask you to curdle milk on purpose, for example adding lemon juice to milk as a stand-in for buttermilk. When a cake batter looks curdled after adding eggs, it usually smooths out when you add the flour.