I met someone once who seemed incredulous about whether sifting was really necessary. The long answer is yes. Sure it's a pain in the toukas, but it's important. And then there's all that confusion it creates in recipes. Should you sift and then measure or the other way around? (Measure a cup and then sift it, unless the recipe author tells you otherwise. A cup of sifted flour rarely weighs the same as an unsifted one. That said, you should never weigh compacted flour. Scoop and lightly drop flour a few times upon itself before weighing it.)
Don't even get me started on the pastry chef I worked for who said everything had to be triple sifted. At the least.
Sifting is important. Especially when it comes to making baked goods that need to be airy and light. When crumb is perfect: even and continuous, it's because all the ingredients were where they needed to be when it was just batter.
How come some recipes tell you everything needs to be room temperature, and some don't?
Unless an ingredient should be cold or frozen, as is the case with pie dough, all ingredients should be room temperature when making baked goods. This is so that crumb is there, not just a concept.
And sifting helps all those dry ingredients marry. Yes, it's also for air/lightness, but in the case of a cake, let's say, with a number of powdered items of different weights and grinds, sifting can help with getting them all evenly distributed.
I think chemical leavening agents should always be sifted, too, because they tend to clump, and, let's face it, no one likes the taste of baking powder.
Pictured above is the sifting process of flour and cocoa powder. Specifically, all purpose flour and natural cocoa powder by Scharffen Berger. This cocoa powder is ground insanely fine. I know this because when I worked at Citizen Cake we were their first guinea pig for this new product experiment of theirs. We learned the hard way just how much moisture their cocoa powder absorbed and after a number of failed recipe tests they said, "Um, yes, yeah. It turns out that our cocoa is ground 100 times finer than most other cocoas."
The properties of cocoa, besides the fact that it helps baked goods taste chocolatey, are not the subject of this hint, but let's just say that understanding cocoa is an important part of baking with it.
In the photo are these things: a tamis sits on top of a bowl and the orange semi-circle is a pliable bowl scraper. A tamis is a large sifter, a "drum sieve," if you will. It is a French word that's pronounced TAM-ee, or Tammy, if you wanna give her a cute name too. It's what anyone who wants to make any of The French Laundry cookbook recipes needs. It is with a tamis that one "passes" potatoes or foie gras or pastry cream to make them more smooth. At work we have 3 sizes, one being about a yard in diameter! If you live in the Bay Area you're really lucky because there's an undercover restaurant supply store on Clement street in the Richmond District called Kamei. Check it out-- there's amazing prices on everything from plates to cutting boards and sheet pans. It's there that I bought my latest friend: a hand-held baby tamis.
As is the case with the above photo, it's important that the bowl have as big a mouth as the tamis' rim. You may use your hand to push the dry goods to pass through the fine mesh, but a bowl scraper is a little neater. You can also use the method wherein you lift the sifter and knock its sides so that said flours fall through themselves. But you can sometimes lose a lot, it's messy, and it's not terribly good for you to inhale so much flour. Your bowl should be deep enough that when you're done passing, the mesh sieve doesn't sit on what you just sifted. Pressing on sifted ingredients can crush, eliminate or deflate the airy mound you just worked so hard to create. Think of it like fresh powder snow as compared to compacted snow.
When sifting a number of drys in a recipe it's good to put a few of them in the sifter at once. In the case of Devil's Food Cake, for example, pour into tamis ingredients in this order:
1. cake flour, 2. cocoa, 3. leaveners, 4. sugar.
I never sift my salt because I use Kosher salt in baked goods and it rarely passes itself through the mesh sieve. But I do sprinkle it in, from a height, and then whisk it into the drys until the color is uniform. (I have also been known to season to taste once the batter is done, but you should only do this if you live in an area where raw eggs are nothing to be afraid of tasting/eating.)
When sifting a number of ingredients together, it's also important that the heaviest substances don't go through the tamis first. Unless you plan on double or triple sifting, because that which ends up on the bottom wants to stay there, thus helping to defeat the purpose of why you sifted in the first place. It's best if your heaviest drys sit on top of the lighter ones, as is the case with the above example: the 4th ingredient, sugar, will help the lighter ones, like cake flour and cocoa to pass and also distribute.
So, nu? Why sift? What if I can be sure my flour is fresh and light already?
Sifting is good with any ingredients that clump, for example: baking soda, confectioner's sugar, cornstarch, and cake flour --------> these are all substances that are ground really really fine. Sugar is hygroscopic, which means it attracts water, so when it rains outside you'll notice that almost any "grind" of sugar clumps. Ground spices also like to be sifted because they tend to be more moist and like stays with like. As is with my famous gingerbread, there are so many different spices, ground to all manner of fine and coarse, if I don't sift all the spices with the flour and sugar, I get not only a very different crumb, but there's a good chance I'd hurt someone's mouth with a dead spicy bite of all the ginger hiding in one spot.
Sifting is also good for taking a look at dried goods better. Sometimes, or often, depending on where you hail from, little things end up in ground "flours." Stones, hay, bugs, or bigger pieces of that which has been ground are some of the things you can find hiding in your bins. Sometimes taking the time to find and use your sifter can save you much embarrassment later on...
When a recipe says sift, do it. Recipe instructions such as this are not generally random. Of course you'll still get cake if you remain sifter-less, but think of the difference one tiny finesse step will make! And no matter which tool you use, whether it be a hand-held sieve or the old-fashioned kind shaped like an over-sized metal mug with a crank on the side, or a gigantor tamis, I think you'll even enjoy the extra effort. If nothing else sifting can conjure the lovely image of snow falling softly.
More on the subject of sifting flour can be found here, with the inimitable Rose Levy Birnbaum.