The egg is a self-contained biological system designed to support the development of a potential embryo. It is organized into three main physical divisions: the protective shell, the liquid egg white (albumen), and the nutrient-dense egg yolk. Each division has a unique chemical composition that determines its specific function, collectively offering both protection and sustenance.
The Outer Protective Layer
The hard shell provides the primary physical defense for the egg’s contents, composed of nearly 95% to 97% calcium carbonate (\(\text{CaCO}_3\)). Although this crystalline structure provides rigidity, the shell is not entirely solid; up to 17,000 microscopic pores cover the surface, making it a semipermeable barrier. These pores allow for the exchange of gases, such as oxygen and carbon dioxide, while the outermost layer, called the cuticle or bloom, helps prevent bacterial entry.
Immediately beneath the shell are the inner and outer shell membranes, fibrous, transparent layers composed primarily of protein, including collagen and keratin. These two membranes lie closely together, separating only at the blunt end of the egg to form the air cell. The air cell develops as the egg cools and its internal contents contract after laying, providing a pocket of air for the developing embryo.
Composition of the Egg White
The egg white, or albumen, is characterized by its high water content, making up nearly 90% of its total mass. The remaining portion is mostly protein, with only trace amounts of minerals, glucose, and negligible fat. This aqueous, protein-rich solution surrounds the yolk, helping to cushion and protect it within the shell.
The most abundant protein in the albumen is ovalbumin, constituting approximately 54% of the total protein content. Ovotransferrin is another significant protein, making up about 13%; it binds to metal ions, such as iron, to limit bacterial growth. Lysozyme is also present, accounting for roughly 3.5% of the total protein, and functions as an enzyme that dissolves the cell walls of certain bacteria.
The physical consistency of the egg white, which includes both thick and thin layers, is largely due to the presence of ovomucin. This glycoprotein contributes to the jelly-like texture and viscosity of the thicker albumen, helping to keep the yolk centered within the egg. Overall, the protein concentration is about 10.9 grams per 100 grams of albumen, offering a highly digestible source of amino acids.
Composition of the Egg Yolk
The egg yolk is the most chemically complex component, serving as the primary reservoir of nutrients for the potential growing chick. Although the yolk is about 48% water, its solid content is highly concentrated, consisting of about 32.5% lipids and 16% to 17.5% protein. This high concentration of fats and fat-soluble compounds makes the yolk responsible for three-quarters of the egg’s total calories.
The lipid component of the yolk is a sophisticated mixture, composed primarily of triglycerides (62% of total fat), phospholipids (33%), and cholesterol (5%). The phospholipids, particularly lecithin (phosphatidylcholine), act as naturally occurring emulsifiers. These emulsifiers allow the fat and water-soluble components of the yolk to remain in a stable, blended suspension.
The yolk contains all of the fat-soluble vitamins, along with the essential nutrient choline.
Key Nutrients in the Yolk
- Vitamin A
- Vitamin D
- Vitamin E
- Vitamin K
The yolk also provides the majority of the egg’s iron and phosphorus, often bound to proteins or lipids in a highly bioavailable form. The distinct yellow color is due to carotenoid pigments, such as lutein and zeaxanthin, incorporated from the hen’s diet.