Q: What does sifting flour do? How is it done? Is it necessary?
In earlier days, sifting flour served several purposes. When flour was milled using stone wheels, as opposed to modern steel rollers, sifting removed bits of the millstone and other impurities that might be found in the flour. With modern milling equipment, tighter food regulations and higher quality control, the need to sift to remove impurities is greatly reduced.
Sifting also breaks up clumps, adds air to the flour which helps produce lighter cakes and pastries, and makes measurement more uniform.
As flour sits, it slowly settles, becoming more compacted. A cup of sifted flour may weigh 20% - 25% less than a cup of flour that has settled. This difference can significantly affect the results, making breads and cakes more dense.
The best way to assure uniform results in baking is to weigh flour rather than measure it. In fact, many baking and breadmaking books now give not only flour but all of the ingredients by weight. Now if we could only get reliable protein measures on flour (see Flour Power?)!
Another method, when sifted flour is called for, is to loosen the flour, removing the error caused by settling, and then measure it. Many recipes now use a scoop-and-level technique, which is quicker than sifting. To do this, stir the flour in its container to aerate and lift it. Then scoop out the required amount, getting a heaping measure, and level it off with the back of a knife or other straight edge. The measuring cup should not be tapped or shaken to make the flour settle. This method results in something close to sifted flour in the amount, but isn't very convenient if you keep your flour in the bag. Scoop-and-level works best if you have a bin with a large opening to work with. Some recipes, however, will use a scoop and level technique but clearly tell you not to disturb the flour first. Sometimes they will say not to fill the measuring cup by spooning the flour into it, which would also partially aerate it.
Sifting makes for a uniform measure, plus it adds air which can make the resulting product lighter. It is important to read the recipe however. A recipe that asks for, say, "one cup of flour, sifted" will use more flour than one that asks for "one cup of sifted flour". In the first instance, the flour is measured prior to sifting, while in the second it is sifted first then measured.
Another purpose of sifting is to blend the dry ingredients. In this case, all of the dry ingredients are placed together in a bowl, lightly stirred and then sifted together. This may mean that the flour is sifted twice, once on initial measure and again in mixing the dry ingredients.
To sift flour, either use a sifter designed specifically for the job, following the manufacturer's instructions, or place the flour in a fine mesh sieve and shake it gently to cause the flour to flow through. In either case, sifting onto a piece of wax paper or a flexible cutting sheet makes it easier to move the flour on to the next step than if you sift into a bowl.
Is it necessary to sift flour? Not really, provided that you follow the above suggestions to weigh or scoop-and-level, but it may add a bit to the quality of your cakes and to the crumb of your bread.