Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn't, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you're not sure why, send a tweet to The Food Geek to find out what's happening.
Mary passes along a question for Mollie,
Hi, Mollie and Mary,
Two major things happen when you're sifting flour. Well, two major things are supposed to happen, but only one really does. The first thing is that all of the flour gets spaced out and away from each other, so it takes up a lot more volume than it used to. That definitely happens with sifting. The second thing, which is supposed to happen but doesn't really, is that the other powdered ingredients (baking soda, baking powder, different kinds of flour, or what have you) get mixed up and evenly distributed within each other.
So: gluten formation. This is not the first time that regular readers will have noticed me writing about gluten, because it is vital to most baked goods. Gluten is the web of molecules that form when glutenin and gliadin, proteins found in wheat flour, mix with water. They lend structure to all manner of doughs, sauces, and similarly thickened materials.
With cake, you want enough gluten to provide structure so that the cake rises, but not enough structure to impede chewing at all. if you can spread the flour out before you mix it into the water and fat, you let the fat get in-between a lot of the flour before the water mixes with it. The more fat that coats the flour the less water can get in (after all, oil and water don't mix, and that goes for other fats just as well), and the less water that meets the flour, the less gluten you have.
A sifter spreads out flour admirably. If you put flour through a sifter, you can virtually see in-between every particle of flour as it falls out of the sifter. It's fantastic. What the sifter doesn't do well is mix ingredients.
You want to mix the dry ingredients well because the the dry ingredients will generally be structural, provide flavor, or provide lift. If you have clumps of undistributed flavor or leavener hidden in caches around your cake, then someone will have an unhappy surprise when they bite into it.
With a sifter, you have a mechanism on one end that spreads out the powder as it drops it. When you add ingredients, you tend to do a scoop here and a scoop there, so having something at one end spreading out nearby powders will only mix ingredients that are already pretty well blended.
A better method for incorporating dry ingredients that does pretty well with the aeration of the powder is to mix it with a whisk or, if you're particularly ambitious, a food processor. Particles spread out, different powders mix together, and you get a nice cake without fussing with a big piece of equipment like the sifter. Give it a try if you generally sift, I think you'll be pleased.