The Make-up of an Egg
The egg came first! At least it does in Culinary IQ. I hope you find the information about the egg and how to use it in the kitchen this week useful. If you have any other foods or kitchen items you would like me to discuss in Culinary IQ, please let me know.
Eggs have six distinct parts: thin white, thick white, yolk, shell, air cell, and chalazae. About two-thirds of the weight of the edible part of an egg is egg white; about one-third is the yolk. Overall, most of the whole egg is moisture, with smaller amounts of protein, fat, and emulsifiers.
White
Another name for the egg white is egg albumen. Other than small amounts of mineral ash and glucose, egg whited consists entirely of protein and water. While the special mix of proteins in egg white is extremely important to its functionality, egg white is actually mostly water, about 90 percent water and only 10 percent protein.
Compared to the yolk, egg white has very little flavor or color. It has both thick and thin portions, with the thick portion thinning as the egg ages. As it thins, egg white loses its ability to form foams that are stable.
Yolk
Egg yolks are about half moisture and half yolk solids. As eggs age, yolks pick up additional moisture from the white. When this happens, the yolk thins out and flattens when the egg is cracked into a smooth surface. It has a protective membrane that weakens as it ages, making it more difficult to separate the yolk from the white. The weakening of this membrane also increases the possibility that bacteria will pass into the nutrient-rich yolk where they can multiply if the egg is not kept cold.
The solids in egg yolks consist of proteins, fats, and emulsifiers, with small amounts of mineral ash and yellow-orange carotenoids. Egg yolk proteins are not the same as egg white proteins. Many egg yolk proteins are lipoproteins, proteins bound to fats and emulsifiers, which are both lipids.
The most well-known emulsifier in egg yolk is lecithin. Egg yolk contains an incredibly high amount of lecithin, about 10 percent. Like most of the lipids in egg yolk, lecithin is bound as lipoproteins. The emulsifying lipoproteins perform many functions in foods, most notably bonding to both water and oil. By bonding to both, emulsifiers and emulsifying lipoproteins hold together, or bind, complex mixtures of ingredients, such as cake batters.
An important factor in the color of egg yolk is the hen’s feed. The more carotenoids in the feed, the more yellow-orange the yolk. Alfalfa and yellow corn, which are both high in carotenoids, produce deeply colored yolks. Wheat, oat, and white corn produce lighter yolks. When feed is naturally low in carotenoids, marigold petals, which are a rich source of carotenoids, can be added for color.
The hen’s feed also affects the flavor of the yolks. This explains why some brands of eggs taste different from others. Sometimes, for example, organic eggs tasted different from regular eggs. It isn’t that being organic necessarily gives them a different flavor; more likely, the growers are using a particular feed that - organic or not - has a distinct flavor that passes into the egg.
Omega-3-fatty acids are sometimes added to feed for hens so that the eggs are high in this healthful oil. Eggs that contain omega-3-fatty acids will have a different flavor from regular eggs.
Shell
Eggshell represents about 11 percent of the weight of eggs. Although it serves as a hard protective covering, the eggshell is porous. This means that odors penetrate eggshells, and moisture and gases (primarily carbon dioxide) can escape. In commercial practice, shell eggs are washed and sanitized to remove dirt and to reduce the likelihood of salmonella contamination. They are also lightly coated with mineral oil to delay moisture loss.
Eggshell color can be either brown or white, depending on the breed of hen. Hens with white feathers and white earlobes lay white eggs, hens with red feathers and red lobes lay brown eggs. While most (95 percent) commercial breeds produce white eggs, hens bred in parts of New England produce brown eggs. Shell color has no effect on flavor, nutrition, or functionality of eggs.
Air Pocket
Eggs contain two protective membranes between the shell and the white. Soon after an egg is laid, an air cell forms between the membranes at the egg’s larger end. As the egg ages, loses moisture, and shrinks, the air pocket increases in size. This is why older eggs float in water while fresh ones sink.
Chalazae
The chalazae are twisted white cords that hold the yolk to the center of the egg. They disintegrate as the egg ages. Chalazae are an extension of the egg white and are completely edible, although pastry chefs typically us a chinois or sieve to strain them from certain products such as custards.
Figoni, P. (2008). How Baking Works: Exploring the Fundamentals of Baking Science. Hoboken, New Jersey: John Wiley & Sons, Inc.
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