This is the fourth in a series of 7 “Cake Batter” classes. We’ve been using the traditional “quatre quarts” pound cake recipe as a tool to understand the science of cake batter.
In the 3rd class we looked at one of the cake structure-builders, flour. We learned that low protein, chlorinated cake flour produced a cake with a light and tender texture. Now it’s time to learn about the other structure-builder for cake batter – eggs.
The Incredible Egg!
It’s not really possible to overstate how important the simple chicken egg is in the kitchen. Eggs are the base for a myriad of dishes, from rich custards to ethereal meringues. Eggs can be scrambled, fried or poached for a nutritious meal. The yolk emulsifies mayonnaise and the white is indispensable for the pastry chef’s glue, royal icing. In fact, the egg holds such culinary importance it is said that each of the 100 folds of a chef’s toque represents one way the chef can cook an egg.
An egg’s culinary superpower comes from the way its proteins bond together. Eggs are comprised mostly of water, with protein molecules dispersed throughout. The water molecules outnumber the protein molecules 1000 to 1. In a raw egg, the proteins take the form of long chains that are folded onto themselves and are dispersed among the many water molecules, so a raw egg flows as a liquid. When an egg is heated, salted, comes in contact with an acid, or is whipped into a foam, the protein chains break the bonds that form the compacted shapes, open up, then form new bonds. This creates a three dimensional network of proteins that trap the water. The trapped water is unable to flow so the egg becomes a solid.
Whites & Yolks
There is more to an egg than just water and protein, especially within the yolk. The yolk is about a third of a shelled egg’s weight. It packs a load of nutrition and most of the calories. The yolk is comprised mostly of water and protein, but also some fat and lecithin. It’s the lecithin that gives the yolk it’s emulsifying powers.
The egg white is about two thirds of an egg’s weight. The white is about 90% water, with the remainder made up of protein and traces of vitamins, minerals, and glucose. Unlike the yolk, egg whites have no fat, so they tend to dry a cake more than the yolk. But they have great capacity to create structure.
When separated from the yolks and whipped to a foam, egg whites can be used to leaven a cake. As mentioned above, whipping egg whites has the same effect as cooking whites- the proteins unfold, reattach and trap water. Since the whipped whites are already partially “cooked” they don’t contribute as strongly to the structure of the cake. In my testing I found that a cake made with the same proportion of yolks and whites had a softer texture when the whites were whipped and folded into the batter.
Eggs in Cake Batter:
The most important job of eggs in a cake batter is to contribute structure building proteins from both the yolk and the white. The protein coagulates as the cake bakes and, along with the starch from the flour, forms the cake crumb.
So what does all this mean for our pound cake recipe? It means we can alter the cake significantly by manipulating the number of eggs, yolks and whites in the batter.
The yolk contributes protein, but also some fat, flavor, and emulsifying lecithin. Because emulsifiers hold water and fat together, adding extra egg yolks to the batter enables the batter to hold extra liquid and, consequently, extra sugar. This helps create a moister and sweeter cake that will still bake up with a good structure rather than falling into a gooey mass.
If you find your cake recipe tends to be on the dry side, try swapping out some of the egg whites for extra yolks. If you find your cake has a poor structure or is gummy, you can add some extra whites. Want to lighten up the texture without adding more baking powder? Separate and whip the whites and fold them into the batter.
More Cake Tests!
I wanted to see how changing the number of yolks and whites in our pound cake recipe would affect the final product. So I baked six cakes, changing the number of yolks and whites for each test. I kept the other ingredients (butter, sugar, flour) to 8 oz each. Though I varied the number of yolks and whites, I kept the total weight of eggs for each test at 8 oz. For the last two tests I separated the eggs, whipped the whites with 2 oz of the sugar, and folded them into the batter before baking.
There is no “right” answer as to which outcome is the best. It’s a matter of taste and what works for your purposes. But knowing how to use eggs to tweak your recipes is a valuable tool for every baker.
Our next “Cake Batter” class is all about the sweet stuff, sugar!