The Wide Open Spaces

• Modern mills process wheat through a series of high-speed steel rollers, and then sift it repeatedly to ensure a very light flour. Unfortunately, it doesn't stay that way. Between the mill and your pantry, the flour is stacked, jolted, shaken, bumped and vibrated many times during shipping and storage. The fine granules of flour originally have lots of open air spaces between them, but under this treatment those air spaces collapse like a house of cards. The net result is a relatively dense and compact flour, especially by the time you reach the bottom of the bag. That can have a significant impact when you're measuring it for a recipe.

Turn Up the Volume

• The problem is that most U.S. cookbooks and recipes measure their ingredients by volume. That isn't a problem with water and other liquids, because they always take up a consistent amount of space. Dry ingredients such as flour and brown sugar are more variable, and a cup can hold more or less depending how tightly it's packed. Just scooping flour from the bag in your measuring cup can compress it seriously. To ensure more consistent results, many recipes specify that the flour be sifted first. This makes it consistently light, and provides more predictable results.

A Better Weigh

• Professional bakers typically bypass the whole question by measuring their ingredients by weight, rather than volume. It's a practical solution, because a pound of flour will always weigh a pound regardless of whether it's sifted or tightly compacted. If you're working from a recipe that lists its ingredients by weight, you won't need to sift the flour to get an accurate measurement. However, sifting isn't only used for accuracy.

Sift It Out

• For one thing, sifting can remove any debris or foreign objects from your flour. That's not the concern it was a few centuries ago, though it remains pertinent for parents of small children. More conventionally, some recipes call for dry ingredients to be sifted together so they're evenly combined. If you don't have a flour sifter, just whisk the ingredients by hand or pulse them in a food processor. Sifted flour's light and even texture is also useful for some delicate tasks in the kitchen, such as folding the flour into a sponge or angel food cake. Sifted flour is easier to incorporate, so you lose less of the air from your egg whites while folding it in.