These custards are cooked over direct heat and stirred continually to ensure that they remain smooth. They are thickened with cornstarch, flour, or other starches, with or without the addition of eggs. When eggs are a component of a starch-bound custard, they are first beaten together with the starch and sugar before cooking. The starch coats the eggs and prevents curdling when the custard is cooked. As with stirred custards, the cooking time and stirring process is crucial to a perfect ending. Overheating may cause the corners of the pan to burn, and both overheating and overstirring causes the custard to become runny; undercooking results in a raw starchy flavor. Crème pâtissière is the primary starch-bound custard.