Overheating, Curdling, and the Addition of Starch

There is a fine line (or split second) between thickening and curdling in egg-based sauces. Too little heat will result in a thin and unsterilized sauce. Too much heat will curdle it. Curdling occurs when excessive heat causes the egg proteins in a sauce to bond together too quickly. If, however, a sauce needs to be heated beyond 185°F (85°C), adding the right amount of flour or cornstarch will do the trick. Starch not only absorbs heat, it also interferes with the bonding of egg proteins—which is to say that the sauce will withstand higher temperatures (even up to the boiling point) without curdling. One fairly serious drawback to adding starch, however, is that it alters the texture and dilutes the flavor of a sauce.