The egg is quite possibly the most versatile basic baking ingredient on the planet. I think it has to do with its having two parts: a fatty yolk and a proteiny, watery white. You can also use just yolks, just whites or a combination of the two. Where to begin? Like Maria, I say let's start at the very beginning...
An egg is "the hard-shelled reproductive body produced by a bird and especially by the common domestic chicken; also : its contents used as food." So sayeth Mirriam-Webster online. Thanks, Mirriam Webster for not bothering to discuss the contents of an egg, because for most of us interested in baking with eggs, we are only interested in what is inside the shell.
Eggs contain several structures including membranes and air pockets and such, but the main parts of the egg we are concerned with as bakers and cooks are the yolk and the albumin or white. I will say here that the albumin can be further divided into thick albumin and thin albumin. I'm sure you've all seen this: there's a thicker, almost jelly-like white around the yolk and a thinner, more watery substance that runs out into your pan or bowl. The fresher the egg--the more recently laid--the more of the thicker part there is and the less of the thinner. That's why older whites are preferable to younger whites for whipping. It's much easier to whip air into a thinner mixture than it is with a thicker one.
How Do Eggs Function in Baking?
To really understand all the myriad ways eggs function in baking, we have to look at the ways eggs are called for: whole eggs, just yolks or just whites. Because nothing is ever easy. Still, this will be Fun and Educational!
The fat/protein/water content of whole eggs is 12% fat, 13% protein, 73% water and 2% minerals and such. A whole large egg has about 80 calories.
- Eggs add structure in the form of protein. As eggs bake in a cake, the proteins denature and coagulate which, along with the starches in flour help form the overall structure of your baked goods. Too much egg not balanced with sugar and fat (which both tenderize) and yield tough, dry or chewy results.
- Egg yolks contain emulsifiers that help to form a thick, luscious batter that doesn't separate. An emulsifier helps two items who don't normally get along (fat and water in this case) get along. That's why adding just a bit of egg yolk to a salad dressing helps to keep the oil and vinegar in solution. In this case, eggs add volume to batter and an even texture to the final product.
- Eggs contribute to browning because of Maillard reactions. Maillard reactions are the set of browning reactions that occur when proteins are heated. As well, they lend a yellowish cast to batters and dough.
- The yolks add some trace minerals and up the nutritional value while whites up the protein content.
- Eggs contribute to the overall flavor of whatever you're making, partly because the fat in the yolks helps to carry other flavors.
- Since eggs are mostly water, they contribute to the overall moisture content of whatever you're making. If you are making an enriched bread for example and you decide you want to only use yolks rather than whole eggs, you will have to increase the amount of water or other water-type liquid in the formula so that the bread won't be too dry. So yes, I realize that I have said eggs both dry and contribute moisture. Both are true. There are many sides to eggs.
- The fat in yolks helps to shorten gluten and tenderize the final product.
Although the yolk makes up roughly 1/3 of the volume of a whole egg, it contains half the protein, all of the fat and almost all the vitamins and minerals. It also contains three times as many of the calories as the white (60 as opposed to 20). Yolks are made up of 49% water, 17% protein and 32% fat along with 2% minerals and such.
- While egg yolks are made up of roughly half water, the actual amount is about 8 grams, or 1 1/2 teaspoons. This is a minimal amount compared to using whole eggs, and with all the emulsifiers and fats in a yolk, they contribute more of a shortening and tenderizing function than whole eggs.
- Baked goods made with yolks only are richer and more tender than those made with whole eggs. Less water equals less gluten development, and the fats in the yolk weaken the gluten that is present.
- Yolks contribute a lot of color, much more so than using whole eggs. Baked goods made with yolks only have a lovely deep golden hue. Their protein content assures lovely browning thanks to Maillard reactions.
- Batters made with yolks only are rich and billowy and luscious, partly due to all the emulsifiers present in the yolks.
- If the only difference between two batters is one is made with whole eggs and one is made with yolks, the yolky batter will contain more vitamins, fatty acids and trace minerals than the whole egg version.
- Yolks add a mellow, "eggy" flavor to baked goods. Either you are a fan of that eggy flavor, or you're not, so consider that when deciding whether to bake with yolks only. Yolks can also muddy the flavor of other ingredients, mellowing and muting them. Again, some folks like this and some folks don't.
The albumin or white of an egg contains 86% water, 12% protein, no fat at all and 2% minerals and such. When you realize whites make up two thirds the volume of the egg but only contain 1/3 the calories, it's easy to understand why folks who are on a diet favor using whites only.
- Baked goods made with whites only are pale in color, especially if they don't contain any other "coloring agents," either natural (such as spices) or artificial (like food coloring). If you're going to make those rainbow cakes, start with a whites-only batter so your colors stay true.
- While whites are mostly water, they have a drying effect on baked goods since all that water activates gluten without the tenderizing influence of the fat in the yolks. Most whites-only cakes (think angel food cake) rely heavily on the addition of extra sugar to tenderize the structure and bring moisture to the final product.
- Whites don't contribute to the overall flavor of whatever you're making. This means that the flavors you do introduce are more pure tasting since yolks bring a mellowness.
- There are no emulsifiers in egg whites, so batters containing whites only as well as other liquids are more prone to breaking or having a slightly curdled look to them. Angel food cake is an exception because the only additional liquid in an angel food cake is maybe a bit of extract.
- While the color of the dough or batter is whiter, you should still be able to achieve a nice brown crust on whatever you're making because of the protein in the whites. Again with the Maillard reactions.
What About in Puddings and Custards?
The function of eggs in pudding and custard is a whole other ball of wax. First, let's talk about what happens to eggs when they cook.
As you heat an egg, it goes from a liquid state that can easily mix with other liquids to a solid state. Both the whites and the yolks do this, right? Think of a "hard" fried eggs. Now think of all that egg "solidity" evenly distributed throughout a batter, and that gives you an idea of the structure that eggs can help add to baked goods. This is an oversimplification of what happens when eggs are heated, especially because I'm not addressing the interactions among other ingredients in the dough or batter, but as far as it goes, it's true.
Now, think what happens when you stir the egg in the pan while it's cooking. You end up with a completely different end product: scrambled eggs. Scrambled eggs are generally more tender than fried eggs because you haven't allowed the proteins to coagulate undisturbed. So what could have been an ice rink (fried egg) turns into a slushie (scrambled eggs). The constant stirring action of making scrambled eggs incorporates a bit of air and keeps the proteins moving so they set up or coagulate in smaller little "clumps."
If you haven't noticed yet, I'm not a scientist. But I think the ice rink and slushie are a good illustration of the differences between letting egg proteins cook undisturbed (as in a cake or fried eggs) versus agitating them during cooking (stirred custards and scrambled eggs).
Not all custards are stirred though. Some are baked so the proteins coagulate undisturbed: cheesecake, pots de creme, flan and creme caramel, quiche and creme brulee, just to name a few.
What is a Custard?
Custard is a thickened mixture of some sort of dairy and some form of egg. By that, I mean that you can use any sort of dairy from skim milk to heavy cream and some form of egg: whole, yolks only and in a very few instances, whites only.
The final texture of the custard depends not only on the other ingredients you use but on the ratio of egg to liquid as well as the way you cook it. Stirred custards--anything from creme Anglais to egg nog to pastry cream to good old vanilla pudding--have a softer set (slushie versus ice rink; scrambled eggs versus fried) than baked or "still" custards. Here's another oversimplification for you: you can have a bowl of pudding or a slice of flan.
Some Interesting Egg Facts
Eggs are one of the leading food allergens. When folks have an egg allergy, generally it is to one (or a combination of) the proteins present in the whites. I'm not saying that people with egg allergies are cleared to eat yolks, of course. I just find it interesting.
Eggs do not like really high temperatures. This is why we often bake custards in a water bath. The part of the batter that is submerged in the water (in a pan, of course) can never rise above the boiling point of water, or 212F/100C. Eggs like gentle cooking, and the more slowly you can cook them, the more silky, creamy and sexy your custard will be. Have you ever had a baked custard that has a weird, grainy texture? This happens when the eggs cook too quickly, the proteins seize up tight and squeeze out all the liquid. And what are you left with? A nubbly, tweedy texture. Ew. I cannot tell you how many pieces of flan I've sent back. Sigh.
Grocery store eggs, at least here in America, have been washed so they're all clean and pretty. This removes the protective coating that they naturally have, allowing them to pick up off flavors in the fridge and to age more quickly. We get our eggs from our own chickens, and I can tell you that we are happy when they come out clean, but we don't expect it. Either way, clean or not, we just wipe them off and keep them on the counter until we need to use them. We wash them right before use, thus protecting the coating on them which allows them to sit out at room temperature in the first place. (And this is all within reason, of course. It's not like we're using eggs that have sat out on the counter for months and months.)
I am sure I have left out some information about eggs, so after reading all of this, please let me know if you still have any questions. You can leave them here in the comments or shoot me an email. Either way, I'm happy to help, especially if helping one can help all.
Thanks for spending some time with me today. I appreciate it.
Have a lovely day.
I did know most of the information presented here, but I want to credit some outside research from my main textbook in culinary school. The link is an affiliate link.
Professional Baking, 4th Edition, Wayne Gisslen, pp47-50