An egg is a hard-shelled reproductive body produced by a bird, widely consumed as food. While its primary function is reproduction, the majority of eggs from domestic fowl are unfertilized and intended for human consumption. Chickens are the primary source of eggs, followed by ducks, geese, turkeys, guinea fowl, pigeons, pheasants, and quail. This article will focus on chicken eggs, which constitute the majority of egg production in the United States and Europe.
Understanding the Egg’s Structure and Composition
The egg’s structure can be broken down into three primary components: the shell and shell membranes (approximately 10% of the egg’s weight), the albumen or white (60%), and the yolk (30%). The albumen is further divided into the thick albumen, outer thin albumen, inner thin albumen, and the chalazae.
In fertilized eggs, the yolk provides essential nutrients, while the albumen supplies the necessary water for embryo development. The albumen layers also act as a protective cushion against jarring movements, and the chalazae maintain the embryo’s orientation.
Nutritional Powerhouse: What Makes an Egg So Good for You?
The table below showcases the nutritional composition of a fresh chicken egg:
| Nutrient | Whole Egg (per 100g) | Yolk (per 100g) | White (per 100g) |
|---|---|---|---|
| Energy (kcal) | 149 | 358 | 50 |
| Water (g) | 75.33 | 48.81 | 87.81 |
| Protein (g) | 12.49 | 16.76 | 10.52 |
| Fat (g) | 10.02 | 30.87 | 0 |
| Cholesterol (mg) | 425 | 1,281 | – |
| Carbohydrate (g) | 1.22 | 1.78 | 1.03 |
| Vitamin A (IU) | 635 | 1,945 | – |
| Riboflavin (mg) | 0.508 | 0.639 | 0.452 |
| Calcium (mg) | 49 | 137 | 6 |
| Phosphorus (mg) | 178 | 488 | 13 |
Note: 100g is approximately equal to two large whole eggs. Source: U.S. Department of Agriculture, Composition of Foods, Agriculture Handbook no. 8-1.
Eggs are a high-quality protein source, containing all the essential amino acids required in the human diet. They’re also packed with essential vitamins (excluding vitamin C) and minerals like phosphorus and zinc.
The yolk is where all the fats, or lipids, including cholesterol, are found. The yolk lipids are rich in unsaturated fatty acids, typically with a 2:1 ratio of unsaturated to saturated fatty acids. Some producers manipulate the hen’s diet to produce eggs with a higher ratio of unsaturated to saturated fatty acids, focusing on increasing omega-3 fatty acids by adding fish oil to the hen feed. These omega-3 fatty acids are vital for normal growth, development, and disease prevention. A large egg contains roughly 216 milligrams of cholesterol, which is lower than previously reported due to improved analytical techniques and potentially genetic selection and improved diets in the egg industry.
Freshness and Safety: From Farm to Table
Eggs rolling down a conveyor belt in preparation for flash candling, ensuring quality control.
Most eggs are free from contamination when laid, with Salmonella and spoilage organisms primarily occurring afterward. Washing and sanitizing the eggs can eliminate most of these surface contaminants. Salmonella enteritidis, a cause of gastroenteritis, can be transferred through the hen’s ovary in less than 1% of eggs. Thorough cooking, ensuring no runny whites or yolk, effectively controls this.
Spoilage organisms such as Alcaligenes, Proteus, Pseudomonas, and certain molds can cause various rots in eggs after extended storage. However, with rapid market turnover, consumers rarely encounter spoiled eggs.
Grading and Selection: What to Look for in Fresh Eggs
Eggs being sorted by size and quality on a conveyor belt.
Fresh eggs are gathered automatically and stored at around 7°C (45°F) before being transported to processing plants for washing, sanitizing, and grading. Automated machines are used to sort eggs by size and quality. Flash candling, which involves passing eggs over a strong light, detects abnormalities like cracks, blood spots, or other defects. Higher-grade eggs feature a thick, upstanding white, an oval yolk, and a clean, smooth, unbroken shell.
In the United States, eggs are sized based on minimum weight per dozen in ounces: extra large (27 ounces), large (24 ounces), and medium (21 ounces).
Most supermarket eggs are approximately four to five days old. When refrigerated, they maintain good quality and flavor for about four weeks.
