There are two broad classes of creams, and they demand entirely different handling by the cook. The pourable creams, crème anglaise for example, are meant to have the consistency of heavy cream at serving temperature. They contain the standard eggs, milk, and sugar (sugar is omitted for a savory cream), and are cooked only until they just begin to thicken, far below the boil. The cream fillings—crème pâtissière, banana cream, and so on—are meant to stay put in a dish and hold their shape. They are therefore stiffened with a substantial dose of flour or cornstarch; and this means not only that they can be heated to the boil, they must be boiled. Egg yolks contain a starch-digesting enzyme, amylase, that is remarkably resistant to heat. Unless a starch-egg mix is brought to a full boil, the yolk amylase will survive, digest the starch, and turn the stiff cream into a pourable one.