There are different kinds of flour for different purposes: A baker wouldn't use bread flour to bake cakes, for example. And she'd use pastry flour, which has a slightly more off-white color than cake flour, to bake biscuits, muffins, and cookies. So if a recipe calls for pastry flour and you don't have any on hand, you can use one of these two substitutes instead.
Combine 1/2 cup each of all-purpose and cake flour. This will create a flour with a protein content that is very close to that of pastry flour. If you want a more precise match (and don't mind a bit of measuring), use 3/8 cup of all-purpose flour and 5/8 cup of cake flour.
If you don't have cake flour, use 2 tablespoons of cornstarch combined with enough all-purpose flour to make a cup. Your baked goods will be a bit tougher (due to the extra protein), but they'll still be quite good. Use either substitute to replace 1 cup of pastry flour. Double or triple this substitute as needed to arrive at the amount of flour your recipe calls for.
Uses for Pastry Flour
Pastry flour is similar to regular flour but with 8 to 10 percent protein. King Arthur's pastry flour, for example, has 8 percent protein. Other brands may have a little more.
So, why does any of this matter? Because the amount of protein in the flour you're using plays a big part in how light or dense your baked goods come out. Flours with more protein make denser, chewier baked goods.
Flours with less protein make lighter, airier baked goods. Pastry flour is a relatively low-protein flour that has been specially formulated for use in things like scones and—as the name hints—pastries.
So, while a lot of hardcore bakers will swear by the necessity of pastry flour, most home bakers will probably be just as happy with the results they get from all-purpose flour. You can make perfectly good biscuits from all-purpose flour, and you'll save money doing it that way. Specialty flours, like pastry flour, cost more—often a lot more.
If you want light, airy baked goods without using pastry flour, measure your flour properly. While it's a bit faster to dig a measuring cup into the bag of flour, you'll put a bunch of extra flour into your recipe if you do it that way. And nothing makes a cake or biscuit dense faster than too much flour.
To match the measurement intended in the recipe, use a spoon to scoop the flour into the measuring cup. Then, level off the top before adding it to your recipe. This simple skill can turn a good baker into a pro.
If you are making bread, cakes, or other baked goods, there are flour substitutes you can use when a recipe calls for a type that you don't have on hand. Learn how to use swap-outs for:
While you're at it, take the time to learn how to store flour properly. Indeed, if you're working on any number of recipes and run out of something, there are literally dozens of ingredient substitutions you can turn to—saving you from making a trip to the store.