Sifting flour is a way of aerating your flour and making sure that there are no large lumps in it. Flour is very finely milled and it is typically packed in small bags, where it gets compressed easily. This is especially true of cake flour, which has an exceptionally fine texture. Depending on the climate you live in, your flour might also develop lumps due to very high humidity and it can even attract small bugs, and sifting eliminates both of these problems. When a recipe calls for sifting flour together with other ingredients – such as cocoa powder, leavening agents or salt – it is to help disperse those ingredients into one mixture before adding them to a recipe.
Aerated flour – as opposed to packed-down flour – is easier to mix in to recipes. For most recipes, giving your flour a few gentle stirs with a knife or whisk to aerate it while it is in your storage container is enough agitation to break up any big lumps that might be present and prepare the flour for use in a recipe. As long as you are gentle and don’t pack the flour very firmly into your measuring cup you shouldn’t have any problems when you use it, even if the recipe calls for sifting your flour after measuring.
When a recipe calls for flour to be sifted before measuring (i.e. “1 1/2 cups sifted cake flour”) - you should take care to sift your flour before measuring. Sifted flour does have more air dispersed in it than unsifted flour, and there are some delicate recipes