Most pastry dough recipes include a solid shortening as well as dry ingredients, and in order to combine them properly, you need to mix them a certain way. This is referred to as "cutting in." The term means to work the two elements with two knives or a pastry blender until well mixed. You will often see the direction to "cut in" in recipes for biscuits, scones, pie crust, and other pastry that is meant to be flaky when baked.
Why to Cut In
The purpose of cutting butter or solid shortening into flour is to create a flaky texture in pie pastry and cookies. This flaky texture is developed by coating the flour proteins with shortening, interrupting the gluten formation. Small pieces of shortening will remain whole, keeping the shortening separate from the dry ingredients when baked—this separation is what creates the flakiness in the finished product.
How to Cut In
When making a pastry, solid shortening, lard, or butter is cut into a flour mixture until the particles are the size of small peas. (A recipe for a pastry dough may read as follows: "Cut the butter into the flour and sugar mixture until the particles are the size of small peas.") This isn't a difficult process, but it takes time and some patience.
To cut in, you can use either two knives or a pastry blender. If you are using knives, hold a knife in each hand and cut across the shortening in opposite directions, working it into the flour—this may take a bit of time. To cut in a little quicker, you will want to use a pastry blender, which is somewhat circular with an arched handle connected to about four narrow arched blades. To use the pastry blender, hold the handle and press the blades into the shortening while rotating your wrist from side to side; repeat this mixing technique while moving the pastry blender around the bowl to incorporate all of the shortening. You can also use your fingers, mixing the fat with the flour lightly with your fingertips—just make sure your hands are not warm.
Don't work the shortening into the flour so much that it becomes a solid mass. You should stop when the pieces of shortening coated with flour are about the size of small peas. Sometimes the recipe will tell you to cut the shortening into the flour until the pieces are the size of crumbs—no matter how it is described, follow the recipe while applying this cutting in technique and your recipe should come out flaky.
Cutting In Needs Cold
Besides cutting in properly, there is one important thing you can do to improve your chances of achieving a really flaky pastry or pie crust: Make sure the shortening you are using is cold. (Some bakers even chill the remaining ingredients and cooking tools). If the butter is too warm, it will not remain whole and will simply meld into the flour and other ingredients, resulting in a dough that is far from flaky.