Also known as syneresis or weeping. When you cook an egg mixture such as a custard sauce too rapidly or for too long, the protein becomes over-coagulated and separates from the liquid, leaving a mixture resembling fine curds and whey. If the curdling in a custard sauce hasn’t progressed too far, you may be able to reverse it if you remove the mixture from the heat and stir or beat vigorously.
To prevent syneresis or curdling in a custard sauce, use a low temperature, stir (if appropriate for the recipe), cook just until the custard tests done, and cool quickly by setting the pan in a bowl of ice or cold water and stirring for a few minutes.
The term curdling is usually used in connection with a stirred mixture such as custard sauce, while weeping or syneresis are more often used with reference to pie meringues or baked custards.
– See Meringue, Soft Meringue