When a recipe calls for sifted flour, you have two choices. You can shake the flour through a large sieve or a wire strainer, or run it through a hand-operated sifter. The purpose of both is to aerate the flour and to eliminate any lumps, solids or impurities it may contain.
Sieves are available in a wide range of sizes, from small enough to drain a single tea bag to large enough for draining a large amount of pasta. They are also used to sift flour, which separates its particles and makes it lighter and fluffier. A cup of sifted or sieved flour weighs less than a cup of unsifted flour used right from the bag or canister, and can change the consistency and texture of baked goods by producing a smaller grain. Sieving can be done by simply shaking it over a large mixing bowl, tapping the sides to make the flour flow more smoothly, or helping it along with a large flat spoon.
A sifter performs the same function of aerating flour, but with the two significant differences. A sifter is equipped with a small hand crank located in the top or the side that operates a beater or paddle set over a small screen inside the receptacle. Most sifters also hold considerably less flour than a large sieve, which necessitates refilling it if a recipe calls for more than it can hold. Once the desired amount of flour is added to the sifter, the crank turns the paddles, forcing the flour through the screen and out the bottom.
The sifting or sieving process can also incorporate other dry ingredients to assure that they are thoroughly mixed into the flour. These can be leaveners, such as baking powder or baking soda; spices; cocoa powder; granulated or powdered sugar; and salt. While you can combine dry ingredients with a spoon directly in the mixing bowl, sifting them together separates then recombines them much more completely. You can do this in steps by sifting the flour once alone to loosen it up and then run it through the sieve or sifter a second time with the addition of the other dry ingredients.
While both methods work equally well, turning a hand sifter may be difficult for someone who has mobility issues such as those associated with arthritis, making the tap-and-shake action of a sieve preferable. Keeping a mechanical sifter clean is also a bit more labor-intensive, as small amounts of flour may become trapped in the gears and around the edges of the screen, so it's important to use a stiff brush during cleaning. After a mechanical sifter is washed, it must be dried completely, as the inside components may rust and ruin any flour it processes.