In an ideal world, a sweet tooth wouldn't have health repercussions. You'd be able to eat anything you wanted, any time you wanted, and you'd never gain a pound or worry about your heart. Unfortunately, you don't live in a perfect world — and that means sometimes having to adjust your favorite recipes to reduce fat, or work with other dietary restrictions. Fortunately, you can make good cakes with just egg whites rather than whole eggs.
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For casual bakers, cakes are simple enough. You add your ingredients to the mixer according to the instructions in your favorite recipe, bake it in the usual pan and it will generally come out right. However, this straightforward approach isn't an option for people with special dietary requirements or restrictions, or for professional bakers looking to fine-tune their recipes to achieve a specific end result. Before you can start changing ingredients, it's useful to know how they interact with each other so you can compensate for undesired changes. Professionals refer to this as "balancing" the changed recipe.
The Role of Eggs
Eggs are a particularly difficult ingredient to experiment with, because they affect a cake in so many ways. Proteins in the egg's whites help the cake to set and hold its shape. In fact, egg whites provided all the structure in those flourless cakes that were so trendy at the turn of the century. Egg yolks contain a degree of fat, which helps soften a cake's texture. They also contain lecithin and other naturally occurring emulsifiers, which help the recipe's fat and milk blend smoothly. The yolks add richness to the flavor and give a gentle golden hue.
Switching to Egg Whites
Some bakers have to adjust their recipes to work with egg whites, rather than whole eggs, for dietary reasons. This is an easier adjustment than removing eggs entirely, because the egg whites can still provide structure to the cake. Sometimes, depending on the recipe, you might be able to make a simple substitution of extra egg whites for the volume of the missing yolks. Two egg whites, or 1/4 cup of egg whites, is the equivalent of one whole egg. You should bake a test cake this way, with just egg whites, to evaluate the effect on your recipe. If it is moist but chewy, or if it is dry, you can compensate with other ingredients.
If your test cake cake is chewy but still moist, it has enough fat content that you won't miss the egg yolks. Your problem is that without the yolks, you need to add another ingredient that will soften the gluten proteins in your flour. Try increasing the sugar, or add a tablespoon of vegetable oil in place of the yolks. If your cake is dry, vegetable oil will also provide moisture to the crumb. You could also add low-fat yogurt or sour cream in place of the egg yolks, which will give a softer and moister cake.