Eggs play a very important role in everything they are put in. Whether that be in a cookie or a cake, they create stability and structure within a batter, they add moisture to baked goods, and many other things for different types recipes. However, which part of the egg you use has the most affect of a recipe. Depending on if you use the yolk, the white, or the whole egg, the egg will have a different function and that can vary widely from recipe to recipe.
The Role of Egg Yolks: Fat
You would want to use just the yolk of an egg typically for the yolks emulsifying abilities and fat content. The fat gives baked goods become extra-rich in flavor and a velvety texture. Most importantly it has the unique ability to bind liquids and fats together, creating an emulsion that prevents them from separating. This emulsion process helps create a homogenous mix of ingredients, which aid in an even distribution of liquid and fats throughout a recipe for smooth batters.
The Role of Egg Whites: Foam
Typically egg whites are used in recipes for a soufflé or cake, as a leavening agent. However, you would want to use them in a recipe for a cookie, but with the egg yolk, not by itself. When you are using an egg white it is normally whipped, creating a foam consistency. Whipping egg whites incorporates millions of little bubbles within the white. Now in the heat of the oven, the air trapped in the foam start to expand, causing the recipe to rise. You normally want your cookies to rise some so thats why you need egg whites somewhere in your recipe for baking cookies.
The Role of Whole Eggs: Fat and Foam
There is a huge benefit from using a whole egg in a recipe, because you get the best properties of both the yolk and the white. Although a whole egg isn’t as good as straight a yolk at creating emulsion, they are still great binding agents. Also, a whole egg will firm up and solidify when heated, giving structure to the recipe. However, at the same time a whole egg makes cookies, and other baked goods more tender and give them a lighter texture.
Side Note: When an egg is mixed with sugar in an ingredient, the egg helps trap and hold air to give some lightness and lift. Also, it adds a whole lot moisture and flavor to the recipe.