Today when I investigated a small spike in my stats, I found a story on Digg that was similar to my post on chocolate chip cookies. One person linked to my article in the comments, and somebody else joked about me using chicken eggs. When I read that, I decided that it was time to write a little bit about one of my favorite ingredients: the chicken egg.
Unless a recipe with eggs specifies otherwise, it probably calls for large chicken eggs. In most supermarkets that I have been in, I have also seen "extra large", "medium" and sometimes "small". While my focus is going to be on large eggs, let me give you the breakdown on egg sizing, according to my notes from bakeshop in cooking school:
Jumbo 30 oz per dozen (by weight)
Extra Large 27 oz per dozen (by weight)
Large 24 oz per dozen (by weight)
Medium 21 oz per dozen (by weight)
Small 18 oz per dozen (by weight)
Pee Wee 15 oz per dozen (by weight)
That means that the large egg averages about 2 oz. Of that 2 oz, approximately 1 oz is egg white, 2/3 oz is yolk, and the rest of it is shell. Now before you bread out your scales and start trying to prove anything, let me remind you of a few things. First of all, these are approximations and averages. Every egg is different, and every egg producer is different. When I buy my eggs at CostCo, they tend to all be pretty much the same size. When I buy my eggs from my favorite supermarket, they tend to differ radically in size, but I have little doubt that one dozen of them weighs about 24 oz. Obviously, since I prefer a little more consistency in my cooking, I try to buy my eggs from CostCo when possible.
Speaking of their eggs, something else important to note is that while they are cheaper and more consistently sized, theirs tend to be a brown color, while the norm at my supermarket is white. You can buy brown at my supermarket, but at a premium. When I lived in New Hampshire, brown seemed to be the norm. Obviously, this is a very regional thing. As far as I know (and I don't believe anyone has definitively proven otherwise), there is absolutely no difference between the contents of brown vs white eggs. They taste the same to me, and as I'm sure you've guessed, I pay a lot of attention to things like taste. But I have noticed that the brown eggs that I used seem to have a sturdier shell. And since I tend to use these a lot, I've gotten used to how they crack. The white eggs feel funny to me now when I crack them, because the shell is so thin in comparison. But I can't say that this is always the case.
Something else that I noticed in New Hampshire was that all of the eggs were "Grade A". When I mentioned this to my friends out there, they all gave me kind of a "well, duh" response. Of course they were Grade A. It's not like they'd be selling Grade B, and what's this Grade AA thing that I keep talking about? Well, I seem to have the upper hand on this one, since I have never seen anything lower than Grade AA sold in Utah. This baffles me because I have also never seen an egg farm in Utah (though I know they're here), and I drove by one on the way to school in New Hampshire. Obviously it's not a question of distance.
What is the difference between the different grades? That's a good question, and one with many answers. An egg that is Graded AA is pretty fresh. The way to keep an egg fresh is to store it at the right tempurature, ideally 36F. If you can manage to store your eggs at exactly this tempurature, they will last for a good five weeks. During this time, they will gradually drop from Grade AA to Grade A to Grade B. Any of these grades is perfectly edible, but at five weeks, you probably want to toss any that are left. As the eggs age and pass through these grades, they will undergo several physical changes. For instance, eggs have an air gap. At Grade AA, that gap will be about 1/8-inch. As it drops to Grade A, that gap will increase to about 3/16-inch. By the time it hits Grade B, that gap will have increased to somewhere around 3/8-inch.
During this time, the yolk will also begin to steal a little moisture from the white. This is very valuable knowledge for a couple of reasons. Once you have cracked an egg, you can tell roughly what grade an egg is after a little practice. If you crack the egg onto a level surface, you can look at how high the yolk and white is. A Grade AA egg will have a very firm yolk and white, causing it to stand pretty tall. As the yolk leeches moisture from the white, the structural integrity of both will decrease, and the egg won't stand as tall.
This means something else important. The higher the grade of the egg, the easier it is to seperate, because both the yolk and the white have a lot of structural integrity. But this also means that the older the egg is, the easier it is to mix into things. Because of this, Grade B eggs are actually eaiser for bakers to use. This is not to say that your local bakery is using Grade B eggs, but they might be. If they are, I wouldn't worry. If they decide to drop below that, I would.
Eggs serve a lot of purposes in cooking. For instance, while the white is mostly water, the rest of it is a protein called albumin. This protein can be whipped, which is a topic for a whole other post. It is also used sometimes in stocks and broths to clarify the liquid, and is in fact a key component in making a very clear and flavorful soup called consomme for this very reason. The yolk contains fat, water, protein and a small amount of cholesterol. The jury is still out on how healthful the yolk really is, but it does have its culinary uses. One very handy application is as an emulsifier. In a nutshell, emulsification is a process which helps force a stable mixture of two substances that would normally repel each other. Without this power, mayonnaise would be a very difficult recipe to put together.
Since an egg contains somewhere about 73% water, it can also bring a good bit of moisture to the party. Because of the emulsification power of the yolk and the structural power of the white, things like custards are made possible. Custards generally contain at least one egg, some kind of dairy (for additional moisture and/or protein), and a certain amount of fat (usually butterfat). This ranges from stirred custards (such as lemon curd) to baked custards (such as cheesecake).
Eggs also offer a lot of flavor. While I don't generally like eating eggs on their own, I can't deny that I love a lot of breads that are enriched with eggs. Does challah need to have all those eggs for structure? I doubt it. But they do add an excellent flavor that really defines the bread. Speaking of challah, I have never seen a recipe for it that doesn't call for an egg wash before baking. While the dough itself doesn't need any help with color, the egg wash does deepen the color even more, often making it a little brighter and always adding that characteristic shine.
This eggwash is applied to a lot of baked goods for a lot of other reasons too. I have baked many a pie using an egg wash to help it hold onto a generous sprinkling of coarse sugar. Protein is nature's glue, and eggs have plenty of it. Egg washes are also used by some bakers to stick pieces of dough together, for various artistic effects. While I'm at it, I might as well touch on the fact that since the yolk contains some fat, it can also be used for some shortening of gluten strands, though I'm sure much of the water in the eggs helps out with the formation of gluten enough to cancel that out a little. And of course, since egg whites can be whipped, they can be used for leavening.
So that's a nice little overview of eggs for those of you who are interested. If you really want to get into all sorts of nifty historical and scientific facts about eggs, I recommend On Food and Cooking by Harold McGee for additional reading.