To the casual observer, eggs are simple enough. The pale whites cook up to a firm, gelatinous texture with little flavor, while the yolks are rich and can be either creamy and thick or firm and grainy, depending on how long they're cooked. To scientists, bakers and other curious observers, eggs are marvels of compact chemistry. Their whites and yolks act in a number of ways to bind ingredients and change their textures.
Anatomy of an Egg
The yolk of an egg is the portion that would produce a new chicken if it were fertilized and left to mature. It's mostly made of water, but also contains large quantities of protein, fat and nutrients to feed the embryonic chick -- or opportunistic humans. The yolk accounts for about a third of the egg's volume, with whites making up the rest. The egg's proteins, and the emulsifiers in the yolk, act in different ways to bind ingredients.
There is an old saying that "oil and water don't mix," and the separated ingredients in a bottle of vinaigrette provide a useful visual illustration of its accuracy. However, eggs contain substances -- notably lecithin -- that help water-based and fat-based ingredients bind together. Their molecules have one end that bonds with fats and one end that bonds with fluids, just as you'd use a special cable to connect your computer with a phone or tablet. Egg yolks in a batter or dough help incorporate the ingredients smoothly, binding them together in a smooth emulsion that results in tender, even-textured baked goods.
Proteins Stand Strong
Emulsifiers bind ingredients chemically, while the proteins in eggs do it physically. The eggs in a batter or custard sauce try to become firm when they're heated, just as they would in a skillet. But because they're diluted by other ingredients, they aren't able to set to the signature consistency of a cooked egg. Instead, the proteins form a fine web resembling a microscopic sponge or 3-D fish net. This web immobilizes the other ingredients, holding them in place and forming a soft gel. Custards are done at that point, while cakes and other baked goods must still set to a firm texture as the remaining ingredients finish cooking.
If you're omitting eggs from your baking because of food allergies or other concerns, experiment to find the right binder substitute. Ground flax seed soaked in water produces a similar gelling effect, and pectin or starch thickeners, such as cornstarch, can replace the thickening power of eggs in some recipes. Vegan bakers and others coping with food allergies may substitute commercial products such as guar gum and xanthan gum to recreate the texture of egg-based baked goods. Lecithin granules from soy and other sources, which are readily available at bulk-food stores, can replicate the eggs' ability to emulsify other ingredients.
- On Food and Cooking: The Science and Lore of the Kitchen; Harold McGee
- Kids with Food Allergies: Cooking and Baking Without Egg Ingredients
- Photo Credit Jupiterimages/Comstock/Getty Images