Friend of The Food Geek, Marc, asks via the telephone, "How do eggs provide lift in various baked goods?"
We've discussed previously the notion of an egg foam, which are egg whites beaten until they become a bunch of vaguely stable bubbles, which are then cooked until they are larger, very stable bubbles. Read the article for the full info, but the short of it is that the action of whisking spreads the proteins out, and the water provides a medium into which the proteins can spread out. The proteins will then connect to each other in various ways, and will trap the water in between the strands of protein. While all that is going on, air gets trapped in between the layers of protein and water, and everything spreads out and becomes bubbles. When you bake that mixture, the air expands, but can't escape before the proteins set.
That's all well and good, but what if you haven't made bubbles out of your egg whites? How do just the eggs themselves, mixed willy-nilly into a batter, give the batter more lift? Some of it has to do with the fact that any whisking that you do will mix air into the batter, even if it's not as well-organized and easily-seen as in the egg foam. The eggs themselves provide structure, from the protein, and liquid (among many other things, but for today it's structure and liquid that are important). The structure helps to hold the air into the batter when it's expanding. The liquid makes the mixture a little more loose, which makes it easier for air to make it in during mixing.
The more you mix the eggs and liquids together before adding it to the rest of the batter, the more lift you'll get. If you mix after you've incorporated the flour, then you're going to end up making gluten, which will toughen the texture of whatever you're making. That may be better, but probably not, depending on what you're making. But you can mix all day before you add the flour. This is one of the secrets of the genoise cake method, which makes a foam out of the eggs before adding them to the rest of the mixture. Still, that's just a little farther down the spectrum, with more emphasis on the rise.
The spectrum of baked goods goes something like this:
And, of course, there are all sorts of tricks to making things lighter and fluffier on the far end, each requiring more and better technique than the last. The basic mechanism, however, is the same: mix air into structure, and you get lift.