Eggs are a widely recognized food source globally, valued for their versatility in cooking and their nutritional content. From a biological perspective, an egg functions as a self-contained unit designed to support the development of an embryo. This remarkable structure is a common part of human diets, found in a variety of culinary traditions and daily meals. Its complex composition allows it to provide sustenance and protection.
The Protective Outer Shell
The outermost layer of an egg is its shell, which serves as a protective casing. This rigid structure is composed almost entirely of calcium carbonate crystals, making up 90% to 95% of its dry weight. The shell provides mechanical strength and defense against physical damage and external contaminants.
Despite its solid appearance, the eggshell is semipermeable, containing thousands of microscopic pores. These tiny channels facilitate the exchange of gases and moisture. Covering the shell is a thin outer coating called the cuticle, or bloom, primarily made of glycoproteins. This layer helps to plug the pores, offering a first line of defense against bacterial entry. The color of an eggshell, which can vary from white to brown depending on the chicken breed, does not indicate any difference in its nutritional content.
The Nutrient-Rich Yolk
Within the egg’s interior lies the yolk, a concentrated source of nutrients. It contains all of the egg’s fat and cholesterol, along with a significant portion of its vitamins and minerals. The yolk provides beneficial polyunsaturated and monounsaturated fats, including omega-3 fatty acids. A phospholipid called lecithin is also present, acting as an effective emulsifier in culinary applications.
The yolk is particularly rich in fat-soluble vitamins, such as vitamins A, D, E, and K. It also supplies various minerals, with iron and zinc being notably abundant. Other minerals found in the yolk include phosphorus, selenium, calcium, and magnesium. The yolk contains diverse proteins, including phosvitin, which plays a role in binding and storing iron. The characteristic yellow to orange color of the yolk comes from pigments known as carotenoids, like lutein and zeaxanthin, which also offer antioxidant properties.
The Albumen
Surrounding the yolk is the albumen, commonly known as the egg white. This clear liquid consists primarily of water, making up about 90% of its composition, with the remaining 10% being a complex mixture of various proteins. The albumen contains many different types of proteins, each contributing to its unique functional properties.
Among these proteins, ovalbumin is the most abundant, accounting for around 54% of the total protein content in egg whites. It is well-regarded for its ability to form stable foams and act as an emulsifier, useful properties in cooking. Another important protein is ovotransferrin, representing about 12% to 13% of the albumen’s protein. Ovotransferrin has antimicrobial properties due to its capacity to bind iron, depriving bacteria of an essential nutrient needed for their growth.
Ovomucoid, which makes up about 11% of the egg white protein, is a heat-resistant glycoprotein that acts as a trypsin inhibitor and is recognized as a major egg allergen. Lysozyme, present at approximately 3.5% of the albumen’s protein, is an enzyme with antimicrobial activity. It functions by breaking down the cell walls of certain bacteria, particularly Gram-positive types, offering a defense mechanism against microbial contamination. Collectively, these proteins enable the albumen to exhibit a range of functional attributes, including foaming, binding, emulsifying, and gelling, which are widely utilized in food preparation.