This section is from the book "The American Woman's Cook Book", by Ruth Berolzheimer. Also available from Amazon: The Domestic Arts Edition of the American Woman's Cook Book.
The best way to prevent wheying, separating or curdling is to regulate the temperature and time of cooking all custards by cooking them over or surrounded by water slightly below the boiling-point, by removing them from the heat when they are done, and by being sure that milk used in making them is entirely sweet.
If a soft custard begins to whey, separate, or , as it is usually called in this case, curdle, it should be removed immediately from the heat. The pan containing it may be set into a pan of cold water, and the custard may be beaten vigorously to redistribute the particles of egg and milk solids.
Precautions For Custards Made With Acids - If a soft custard mixture is to be made with vinegar or acid juices, such as lemon-juice or tomato-juice, the custard should be removed from the heat the minute the mixture begins to thicken. Hot acid coagulates egg and then very soon begins to digest it. This process makes it thin instead of thick. If an acid custard mixture has become thinned by cooking it for a minute too long, it must be thickened by adding more egg or by thickening it with flour, following the directions for starchy sauces. Custards made with acid require more egg than other custards to secure the same degree of thickness.