An improperly thickened sauce can leave you with a watery mess on the plate. Any time you're thickening a sauce with flour, you need to create a roux, which is a thickening paste made from flour and fat. Bread flour can be a suitable substitute for all-purpose flour in your roux, but using it means your final product will have a different texture.
All-purpose flour is made from a combination of soft and hard wheat that is milled together. It has a medium protein content, ranging from 9 to 12 percent. All-purpose flour can be substituted for other types of flour -- cake, pastry and bread -- in most recipes. Bread flour is higher in protein than all-purpose flour -- it typically has 12 to 13 percent -- which increases gluten and creates a chewy, elastic texture. In most recipes, bread flour cannot be used in place of all-purpose flour because it creates a chewy texture.
A roux is a thickening mixture made by combining equal parts flour and fat. When using flour to thicken a sauce, a roux must be produced to incorporate the flour without adding clumps to the sauce. Either bread flour or all-purpose flour can be used to make a roux.
Mix a roux using equal parts bread flour and fat. The fat can be melted butter, shortening, oil or fat drippings from meat you've already cooked for the meal. Cook the roux over medium-high heat until it reaches the desired color. A blond or white roux is cooked until the flour taste has gone, while a brown roux is cooked until the roux begins to brown and takes on a nutty flavor. Since a roux made with bread flour thickens faster due to its extra protein, use less roux than you would if using an all-purpose flour roux. Allow the sauce to cook for 30 minutes after adding the roux to fully thicken.
A sauce thickened with bread flour may have a gummy, gooey texture because of the extra protein. If you don't want to use bread flour, try cornstarch, potato flour or rice flour instead. Cornstarch doesn't need to be incorporated into a sauce through a roux and is faster thickening solution if you're in a hurry. Mix cornstarch with enough water or milk to create a smooth paste and slowly whisk it into your sauce. Allow the sauce to thicken by simmering for 5 to 10 minutes.