A sifter is a cooking and baking tool that comes in various shapes, sizes and forms. Its purpose, though, is always the same—to push a powdered substance through a fine wire mesh in order to make sure that the substance has an even, smooth consistancy. Most sifters have hand cranks and some type of fan apparatus that forces the powder through the mesh.
Refine lumpy flour. If your flour has a lot of lumps, you can run it through a sifter one or more times. As the lumps are forced through the fine wire mesh, they will disintegrate. Any lumps that cannot be crumbled should be thrown away.
Mix multiple powdered ingredients. When baking cakes, muffins and other breads, you may have to add a lot of flour but just a little bit of baking soda or various spices. A sifter helps you fully blend dry ingredients before you mix them in with wet ones so that you do not end up with a high concentration of the spices or baking soda in part of the cake and nothing at all in other portions.
Remove flavoring elements that should not remain in a powdered mixture. Many chefs store vanilla beans or other flavoring spices with their flour or sugar to give it a hint of exotic flavor. These flavoring elements are too large and hard to actually bake into a dish. Use a sifter to make sure that all of these spices are out of the flour or sugar before you use it in a recipe. Then, just put the flavoring back into in the storage bin.
Make sure that you wash and hand dry your sifter immediately after using it, as the wire mesh is very prone to rust if allowed to air dry.