If you are like me, the idea of having to sift an ingredient is anathema, and possibly cause for skipping a recipe entirely. My mother taught me that flour comes pre-sifted, and therefore sifting is a waste of time. To a certain extent, she was right -- supermarket flour is usually marked "Pre-sifted" and you can skip this step. But that applies only to flour, it turns out, and only to that from the supermarket. The 50-lb. bags of flour that we buy for the bakery are not pre-sifted, so sifting is essential to smooth out lumps. The one time I tried to skip the sifting, I regretted it -- I had to put the whole batch of batter through a sieve, and that was a whole lot messier than just sifting the flour would have been.
But you probably aren't buying flour in huge amounts, so do you need to sift? It depends. If your flour has been packed down and compressed, it is a good idea. If you are making a recipe that insists you should sift -- do it. Place the dry measuring cup on a piece of waxed paper or flexible cutting mat, sift the flour into it until it is piled up over the edge, and gently level with a straight knife. Repeat until you have the right amount of flour for your recipe. Otherwise, you can just fluff up the flour with a spoon, and use the dip/level/pour method of measuring. (Later posts will discuss weighing ingredients, and why we prefer that to volume measurements.)
Cocoa, on the other hand, almost always needs to be sifted. The tiny amount of cocoa butter left in cocoa seems to make it clump up, and you will have unpleasant lumps in your batter if you don't sift. The same is true for confectioner's (powdered) sugar, though I admit that confectioner's sugar seems to get lumpy again as soon as it is through the sieve. Even so, sift!
My baking results took a giant step forward when I started sifting leavenings, such as baking powder, baking soda, and cream of tartar. I have tea strainers -- tiny sieves -- that I keep just for sifting teaspoons or tablespoons of ingredients. Sift them right into the other dry ingredients, and mix in. You'll get better distribution of your leavening and a better product.
Betty reminded me about brown sugar and the need to sift it. You may not have this problem if you buy it in one-pound amounts in the supermarket; Trader Joe's organic brown sugar, besides being a very nice product, seems to be completely lump-free. This isn't the case when we buy big bags -- either the 7-lb bags at the warehouse club, or the 25-lb bags at the food wholesaler. The lumps in these products, though they are the same brand as the supermarket kind, range from lentil-sized to walnut-sized -- and they don't dissolve in batters or doughs. So you can either push the sugar through a sieve or sift through it with your fingers and try to pinch out the lumps. The sieve, obviously, is more thorough, but the pinching is adequate for cookies. If you realize, too late, that you've forgotten to sift and there are lumps in your batter, you can try to sieve or pinch then -- but it's going to be quite messy. It's easier to do it ahead of time.
If you use a multi-screen flour sifter (as opposed to a simple sieve, which is like a bowl made of mesh), don't ever wash it -- first, you will end up with clumps of pasty flour in between the screens, which you will never get out until they dry and flake into your next mixture. And the screens, which are probably not stainless steel, will rust. Simply knock the sifter gently against the side of the sink so that the flour, cocoa or sugar comes out, and seal the sifter in a zippered plastic bag to keep out bugs. Sieves can usually go in the dishwasher, but make sure they are completely dry before you use them again, or you'll have that pasty flour mess.