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.
That’s a very pretty, upstanding yolk, no? It’s the mark of a fresh egg. If you’re ever in a position where you need to evaluate the relative age of an egg, a yolk is a good place to start. If it’s fairly roundish and bright yellow, and is sitting high atop a slightly milky-looking mound of almost gel-like egg white, your egg is extremely fresh. Make cake, cookies or muffins out of it. Or better still cook it up, because fresh eggs make mighty good eatin’.
Old eggs are very different in their appearance just out of the shell. The white becomes very clear and watery due to a progressive change in the pH of the egg, which begins as soon as the egg is laid. That change in pH causes the proteins in the white to drift away from each other, dispersing the large aggregations that formerly made the egg white jelly-thick and whitish. As that happens the membrane around the yolk starts to weaken. More water from the white enters the yolk, diluting its pigments and giving it a pale appearance. As that happens the yolk membrane stretches out, causing the yolk to lie almost flat. If you’ve ever tried to separate an old egg, you know just how weak that membrane is after many weeks of sitting. If the egg is sitting at room temperature, the pH change happens many times faster.
A post on eggs could go on almost indefinitely. However since I want to focus on the egg as ingredient, I’ll do my best to keep this short and useful. The logical place to start is: how are eggs used in the pastry kitchen? I can think of three main categories of use: as a structural component in cakes, as a thickener in custards and creams and as a foam in batters, meringues, frostings and the like. It’s a pretty crude taxonomy when you consider how much eggs offer the pastry cook in terms of flavor, enrichment and color, but it seems functional to me.
Eggs come in different colors, sizes and grades. For our purposes I’ll focus on the basic white, large (as opposed to “peewee”, “small”, “medium”, “extra large” or “jumbo” as defined by the US Department of Agriculture) chicken egg, since that’s what most pastry recipes printed in the States call for. They’re also the most commonly available egg for home bakers and commercial bakers alike. Those that use shell eggs, anyway. Large eggs weigh about two ounces. The white weighs about an ounce, the yolk about half an ounce and the shell accounts for the rest.