Starches

Most starches—flour, cornstarch, potato starch, arrowroot, and tapioca—prevent egg and other proteins from curdling even when a mixture is brought to a boil. The theory is that before the egg protein coagulates, the starch swells and blocks the protein molecules from bonding. As a result, starch-based custards can be exposed to higher heat. In fact, it is essential to reheat a starch-based custard mixture after adding the egg yolks to deactivate an enzyme in the yolks (alpha-amylase) that destroys starch gels. Moreover, the more sugar the mixture contains, the higher the temperature needed to deactivate the enzyme. If the enzyme is not deactivated, it can turn a thick custard or cream pie to soup overnight in the refrigerator.