On the surface of it, an egg white isn’t a very interesting thing. It’s colorless with very little flavor and is made up of about 90% water. However there’s quite a lot of magic in that last 10%. How so? Because that’s where the proteins are. The white contains about half the total proteins of the egg, most of which are different from those of the yolk, and which do some pretty interesting biological jobs. Some of them bind up vitamins, others digest cell walls, still others bind to digestive enzymes rendering them useless. All combined they serve to make the white of an egg a very unfriendly place for invading microbes. They’re a big part of the reason an egg can be stored for so long.
When they aren’t busy protecting an embryo, egg white proteins are great for bakers. They’re rather unique in that they can be uncoiled with just a whisk and a little elbow grease. At that point they’re fabulous for creating foams. The reason they’re so good at this is because they have regions along their length that love water (are hydrophilic) and other regions that hate water (are hydrophobic). The end result being that they love the surface of bubbles, as all the sections of the protein can get their needs met. They can stick their water-loving parts in water, their water-hating parts in the air, and they’re happy.
The nice thing is that once all those proteins are arrayed at the surface of the bubble, they begin bonding to each other, if rather weakly. The network they create reinforces the wall of the bubble, preventing it from popping, even if other substances are subsequently introduced (fat, sugar, etc.). The end result is a foam, but not just any foam: one that will continue to maintain its volume after other delicious components have been added. So say hello to meringues, buttercreams and all sorts of other lovely things I couldn’t live without. So thank you, boring old egg white. My kitchen would be a much duller place without you.