In the last post, we explored how eggs are made to support growing chicks. Now, we’ll see how those same eggs can contribute leavening, structure, and flavor in the kitchen. This post will focus on the functions of eggs in bakes such as cakes, muffins, breads, brownies, and cookies, but we will turn to meringues and custards in future posts.
Eggs have the functions of their components.
In the introduction, we broke eggs down into three major components: water, proteins, and lipids (that is, fats and emulsifiers). Since eggs are made of these basic molecules, eggs have their functions. We’ve already discussed most of these roles in detail—water, for example, adds moisture, and protein adds structure—but we’ll review them here in the context of eggs.
Culinary functions of the water in eggs
Water adds moisture.
Remember that eggs are, first and foremost, water. Water makes up 76% of an egg! Thus, eggs increase the moisture content of batters and doughs to make them runnier or softer. Choux pastry is one example of this. After the flour, water, and butter are cooked into a paste, eggs are added one by one until the batter reaches the right consistency. If you add too many eggs, the batter becomes too wet to hold its shape.
Note, however, that extra eggs do not necessarily create a moister final product. As we’ll discuss, egg proteins absorb water to contribute structure and toughness, and this gives eggs (especially the whites) a drying effect despite their high water content.
Water provides leavening.
The water in eggs actually influences the final bake more through leavening than through moistness. Remember that leavening gives baked goods their rise. If you make choux pastry with too few eggs, you might find that the batter is stiff and difficult to pipe, but worse, the choux won’t rise, resulting in a dense, flat pastry. The water in the eggs is crucial to the choux’s “puff.” In the oven, water vaporizes into steam, which generates lift and rise. As with any recipe that relies on steam for leavening (such as puff pastry, which uses steam from the water in butter), accurate oven temperatures are crucial. If the oven is too cool, not enough steam will form before the outer shell sets, and the pastry will not rise. Not only is leavening important for texture by making the final product lighter and airier, but it also increases tenderness by stretching the batter or dough thin.
The water in eggs also affects the rise of cakes and brownies. Cakes in which whole eggs are replaced with yolks contain less water, so one of the effects is a denser, tougher texture. The water in eggs can also make a fudgy brownie cakey. If you add extra eggs, the additional moisture boosts leavening and gelatinizes starches, both of which contribute to a cake-like texture.
Culinary functions of the proteins in eggs
Proteins can contribute to leavening.
Eggs can also provide leavening by holding air, as in soufflés, génoise, or sponge cakes. When the eggs are whipped in these recipes, their proteins rearrange to trap air bubbles. Then, the whipped eggs, along with the air they carry, are folded into the rest of the batter. The air expands in the oven to leaven. We’ve covered the basics of the foaming process here, but we’ll further explore the effects of other ingredients in the next post!
Proteins add structure.
Like gluten, certain proteins in eggs also contribute structure and toughness to our baked goods. In the introduction to eggs, we outlined the biological functions of some egg proteins. In the kitchen, these same proteins allow the egg to solidify with heat or to foam when whipped.
Ovalbumin is the most plentiful protein in egg whites. It has all the properties of the proteins we’ve discussed before, including the ability to set with heat, and it is one of the proteins responsible for egg coagulation. In a raw egg, ovalbumin and other proteins float around in water like tiny tangled wads of string. With heat, these strings unwind in a process called denaturation. As the proteins denature, they grab nearby water molecules to stabilize themselves. They also stick to each other and coagulate into a mesh-like network that traps water. (This is the point at which an egg white becomes opaque.) Any water that’s tied up by the proteins doesn’t contribute moistness, so the proteins in eggs dry our baked goods despite the eggs’ high water content.
This is how eggs set our batters and doughs. As the eggs bake, they coagulate and solidify the structure of our baked goods. Many cake recipes depend heavily on eggs for structure and would not set without them. (Remember that cakes contain little gluten.) Eggs also work alongside gluten to provide structure to muffins and cookies, and as we’ll discuss in a future post, the coagulation of egg proteins helps to thicken custards.
Different proteins behave slightly differently because of variations in their structures. For example, egg yolk proteins coagulate at a higher temperature than egg white proteins, so they set later and create a more tender texture. Even within the egg white, different proteins set at temperatures ranging from 140 to 170°F (60–80°C).
Proteins add browning and shine.
The proteins in eggs also participate in Maillard browning, so egg washes create a desirable brown sheen on the surface of bakes. For a thinner egg wash, you can whisk in some milk or water, or you can add a little salt and let the mixture sit for a few hours. The salt disrupts the proteins‘ structure and allows them to flow more easily, thus thinning the egg.
Culinary functions of the fats in eggs
Tenderness is outweighed by structure.
One of the main functions of fats in baking is to add tenderness. However, it’s important to remember that all of the fat content in an egg is concentrated in the yolk, so egg whites do not tenderize. But neither do whole eggs. In fact, egg yolks aren’t even tenderizers because they contain both fats and proteins. The fats tenderize and the proteins toughen, and ultimately, the toughening effect of the proteins outweigh the tenderizing effect of the fats. So although egg yolks contain tenderizing fats, the net effect of eggs in a recipe is toughening, not tenderizing, due to the eggs’ protein content.
Fats add flavor and yolks add color.
However, the fat in egg yolks still contributes flavor to our baked goods. Cookie and brownie recipes will sometimes incorporate an additional egg yolk for its rich flavor, and eggs are one of the ingredients that distinguish rich brioche from other yeast breads. The carotenoids that give egg yolks their orange color also yellow our baked goods. For this reason, white cakes are made from egg whites only to maintain their white color.
Culinary functions of the emulsifiers in eggs
Emulsified fats add creaminess.
Fats also taste creamy if they are packaged into small droplets by emulsifiers. As we discussed, this occurs naturally in egg yolks, so yolks lend creaminess to ice creams and custards. To review, the emulsifier lecithin in egg yolks holds tiny bubbles of fat and dissolves the fat into the water of the yolk. As a result, egg yolks appear homogenous despite containing both fat and water.
Emulsifiers blend ingredients together.
Beyond contributing creaminess, the lecithin content of egg yolks is crucial to many recipes because it helps the fatty and watery ingredients blend together. Homogenous batters and doughs bake more evenly to create products with consistent rise, taste, and texture.
In the pictures below, I was mixing the wet ingredients together for a chocolate cake. As you can see on the left, even after vigorously whisking milk and oil together for a couple minutes, there are still large oil droplets in the milk. After taking the photo on the left, I added the eggs and whisked for a few seconds more. The mixture quickly homogenized, as shown on the right, thanks to the emulsifiers in the eggs.
Many recipes add eggs to oil or to creamed butter and sugar. This helps the lecithin get a head start on breaking down the fatty ingredients into small globules that will easily dissolve into watery ingredients. Even so, you might notice that it takes a second or two of mixing for the batter to homogenize after you add eggs. That’s because the lecithin is in the egg yolk, and the yolk has to break in order to release the emulsifiers that will mix the fatty ingredients into the watery egg whites. As other ingredients are added, the lecithin from the eggs keeps the fat in small, dissolved droplets.
However, lecithin isn’t the only variable that influences batter homogeneity. For example, many cake and cookie recipes call for room-temperature ingredients because if you add cold eggs or milk to creamed butter, the butter will solidify, and the batter will contain chunks of butter no matter how many emulsifiers there are.
Eggs are versatile.
Eggs are one of the most versatile ingredients for a baker because they are packed with water, protein, fat, and emulsifiers. These components allow eggs to play large roles in many aspects of baking, including leavening, structure, flavor, and batter homogeneity. Eggs can also be whipped into meringue, which we will explore in the next post, or stirred into creamy custards. Because they have so many functions, eggs can be difficult to replace in a recipe. To make a successful substitution, it is important to fully consider the functions you will need to replace.
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Nelson, D. L.; Cox, M. M. Lehninger Principles of Biochemistry, 6th ed.; Freeman, W. H. & Company: New York, 2012.