Starches

The addition of almost any starch—flour, cornstarch, potato starch, arrowroot, tapioca—will prevent eggs and other proteins from curdling, even when the mixture is brought to a boil. The exact reason for this is not known, but it is assumed that the starch swells and blocks the egg protein molecules from finding each other and bonding before they have had the opportunity to unwind. Because of this, starchbound creams can be exposed to much higher heat. In fact, a starchbound custard must be reheated after the eggs have been added to inactivate an enzyme (alpha-amylase) in the yolks that destroys starch gels. In addition, the more sugar the mixture contains, the higher the temperature needed to do this. If this enzyme is not inactivated before refrigerated storage, even the thickest, firmest custard will become liquid once chilled.