To the uninitiated, flour is simply flour. However, for many pastry chefs and bakers around the world, extremely specialized flours are optimized for very specific uses.
Knowing whether to use pastry flour versus cake flour is a skill that takes both culinary intuition and scientific precision.
Baking Kneads says that pastry flour is a finely ground flour with about 9% protein. It is used to make baked goods like pie crusts, croissants, and tarts.
Cake flour comes in different kinds, and it has less protein than pastry flour. Cake flour is ground very finely, which gives cakes their characteristic crumbly or fluffy texture.
Because the type of flour you use is so critical to the final texture of baked items, swapping pastry flour for cake flour or vice versa won't work.
Protein concentration (and gluten levels) and flour fineness determine bread texture, making them challenging to manage.
MasterClass suggests using a combination of 1/2 cup all-purpose flour and 1/2 cup cake flour for every 1 cup of pastry flour if you need to make a substitution.
In order to achieve a consistency similar to cake flour, Baking Kneads suggests combining 14 tablespoons of all-purpose flour and 2 tablespoons of cornstarch.
Of course, if you're baking, you should always have both flours on available, but these substitutions will work nearly as well as the originals.