Many baking recipes require you to sift the dry ingredients either before or after you measure them. The purpose of sifting dry ingredients like powdered sugar, also known as confectioners' sugar, is to break up any lumps that might have formed and to add air to the ingredient, according to David Joachim, author of "Brilliant Food Tips and Cooking Tricks: 5,000 Ingenious Cooking Hints." Aerating the sugar ensures that the wet ingredients will moisten the sugar evenly and prevent clumping during the mixing process. If you do not have a sifter on hand, there are alternate methods that you can use to achieve a sifted quality.
Wire Mesh Strainer Technique
Measure the powdered sugar, then put it into a small bowl or leave it in a measuring cup.
Hold the wire mesh strainer by its handle over a bowl. Spoon the powdered sugar into the strainer.
Tap the side of the strainer with your free hand very lightly until all of the sugar has fallen through the mesh. Repeat this technique until you have "sifted" all of your powdered sugar.
Wire Whisk Technique
Measure the powder sugar.
Pour the sugar into a bowl that contains no other ingredients.
Stir the powdered sugar with the whisk. Continue stirring the sugar until it has a smooth, fluffy appearance.
Measure the powdered sugar.
Add powdered sugar to your empty mixing bowl.
Mix and fluff the sugar with a fork to add air and break up any clumps of sugar. Although this method is not as effective as a sifter, wire mesh strainer or whisk, it will make the sugar less compact than it was straight out of the bag.