Thickeners help prevent fruit pies from being too runny. All-purpose flour, cornstarch and quick-cooking tapioca are the thickeners commonly used in fruit pies. Flour gives the filling an opaque appearance, cornstarch gives a clear to almost gel-like appearance.One thickener can be substituted for another; however, the thickening power of each is different and you may need to make adjustments. Equal amounts of quick-cooking tapioca and cornstarch can be substituted for each other. When replacing flour in a recipe, use half the amount of cornstarch or use 2 teaspoons of quick-cooking tapioca for every 1 tablespoon of flour.When using tapioca, mix it with the filling ingredients and allow the mixture to stand for 15 minutes before proceeding with the recipe.