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.
In the States eggs found in stores are graded either AA (top quality) or A. There are grade B eggs which are perfectly usable, but they’re generally not sold in the shell, rather they’re used for packaged carton yolk and white products. Below grade B are so-called “dirty” eggs which aren’t fit for human consumption and go to various sorts of animal feeds. Eggs are graded by “candling” i.e., shining a bright light on the egg an inspecting the shell and contents for defects (this is done by machine these days).
What do egg grades measure? Aside from shell cracks, blood spots and the like, which consign eggs to lower grades, the grade is a measure of the viscosity of the white and air cell size, which are both indicators of age. Older eggs have thinner whites and bigger air cells. Of course most of us reflexively gravitate to the freshest possible ingredients when we cook. Indeed we’re increasingly trained to do so. However in the pastry kitchen excellent arguments can be made for eggs that are, well, old. At least to some degree.
Why? Because old eggs have thinner whites, and that thinness makes them easier to whip. Imagine a whisk trying to cut through a bowl full of water versus one full of corn syrup and you’ll have a sense for what I mean. You can apply a lot more shearing force to the bowl full of water since the thickness of the medium slows your stroke down a whole lot less. All of which means that older eggs whip up faster and also higher. So if your focus is foam for meringues, macarons, soufflés and the like, grade A eggs will be just fine for your purposes. Even old grade A eggs. Also if you like to make hard boiled eggs old eggs are better since they’re easier to peel once they’re cooked (the membranes have pulled away from the inside of the shell).
What are some applications for very fresh eggs? Some bakers will say none. Most of those are Europeans, who tend not to refrigerate their eggs, so theirs are always “old” by American standards. However very fresh eggs are quite useful for some things we Americans like to bake: layer cakes and muffins for instance, where the thicker, more viscous whites create more viscous batters which tend to create more volume. Fresher eggs can also be nicer when you need to separate an egg, since old yolks have very thin membranes which frequently break when you try to separate them.
There is so much more to be said about eggs and how they perform, indeed a whole lot more than one post can contain. What I will say on that point is that we mostly value eggs for their proteins, and further the ability of those proteins to bind other elements together, be those elements sugar and flour (in the case of cakes), fat and water (in the case of custard) or cells of air (in the case of foams). Those proteins are like strings, and they occur in little balls or clumps inside egg yolks and whites in their natural state. We use heat — and in the case of foams, agitation — to coax those egg proteins to uncoil, at which point they’re available for all sorts of creative uses.
I’ll get into more of that when I talk about whites and yolks individually.