Curdling

In custards, egg proteins tighten and squeeze out liquid when heated too far, leaving small curds; stirred custards begin to curdle above about 180°F (82°C). Gentle heat, constant stirring and tempering the eggs prevent it. A slightly curdled custard can often be saved by straining it or blending it right away.

A cake batter can also look curdled when cold eggs are added too quickly to creamed butter. Adding eggs one at a time at room temperature helps, and slightly split batter usually still bakes well.