Which Pies Work

• When thickening fruit pies, your best options are starch and root thickeners, such as cornstarch, tapioca and flour. These thickeners can be combined with sugar and mixed directly with fruit. As the pie bakes, these products will thicken the juices. Don't use egg yolks to thicken fruit pies, which will likely result in a scrambled, sticky mess. You can use egg yolk to help thicken custard pies, such as lemon meringue, chocolate or banana cream pies. Although egg yolk doesn't have enough thickening power alone to thicken custard pies, it works well in tandem with cornstarch or arrowroot. It also adds color and flavor to custard pies. Egg yolks -- and whole eggs -- are also used in pumpkin pie to set the filling.

How to Do It

• If you'd like to use egg yolk to thicken a custard pie, start by heating liquid in a saucepan. If you're making a lemon meringue pie, combine cornstarch, sugar and water in a pan and heat it to a simmer. For chocolate or banana pie, heat milk in a saucepan just until bubbles form around the edges. In the meantime, whisk the egg yolks in a bowl until they're smooth. For a chocolate or banana pie, you'll whisk the sugar and flavorings with the egg yolks. Pour the hot liquid into the egg yolks in a slow, steady stream, whisking constantly. This technique slowly cooks the egg yolks without scrambling them. Pour the mixture back into the saucepan and heat it to a low simmer, stirring constantly until it thickens. Allow the custard to cool slightly before pouring it into a baked crust. Making pumpkin pie is simpler. Combine the pureed pumpkin, cream or milk, spices and egg yolks in a bowl and pour into an unbaked crust.

Potential Pitfalls

• Making a pumpkin pie with egg yolks is almost foolproof, but custard pies can be tricky. If you add the hot liquid to the eggs too quickly, they'll scramble. If you cook the custard too long, the cornstarch or arrowroot thickener may break down or become chunky. If you don't cook the mixture long enough, it may fail to thicken. Additionally, the eggs may not reach 160 degrees Fahrenheit -- the temperature required to kill any bacteria that can cause illness. A whisk is your best friend when making custards with egg yolks. Use it to break down any lumps if you overcook the custard, or pour the custard through a sieve. To ensure proper cooking, bring the custard just to a simmer and cook for one minute -- no more, no less.

Serve and Store

• Because both pumpkin pies and custard pies contain eggs and possibly dairy products, they must be kept refrigerated. Immediately after you make a custard pie, cover it with plastic wrap so it doesn't develop a skin. Allow the pie to cool completely before slicing it. Cover and store any leftovers in a refrigerator set at 40 F within two hours of serving. Use the leftovers within two or three days.