Protein & Other Nutrients in an Egg
A combination of amino acids, some of which are called essential because the human body needs them from the diet because it can’t synthesize them. Adequate dietary protein intake must include all the essential amino acids your body needs daily. The egg boasts them all: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. These amino acids are present in a pattern that matches very closely the pattern the human body needs, so the egg is often the measuring stick by which other protein foods are measured. In addition to the nine essential amino acids, there are nine other amino acids in an egg.
Many different ways to measure protein quality have been developed. According to the Protein Digestibility Corrected Amino Acid Score (PDCAAS), whole egg, whey protein, casein and soy-protein concentrate all score 1 on a scale of 0 to 1. Whole egg exceeds all other protein foods tested with a score of 1.21 (above human needs) in the Amino Acid Score (AAS) rating system. At 3.8, the Protein Efficiency Ratio (PER) of eggs also outscores other proteins. When Nitrogen Protein Utilization (NPU) is evaluated, whole egg at 98% falls just below whey protein and casein (both at 99%). On a scale with 100 representing top efficiency, the Biological Value (BV) of eggs is rated between 88 and 100, with only whey protein rated higher (100).
Altogether each Large egg provides a total of 6.29 grams of high-quality, complete protein. For this reason, eggs are classified with meat in the Protein Foods Group. One egg of any size equals one ounce of lean meat, poultry, fish or seafood. In addition to about 12.6% of the Daily Reference Value (DRV) for protein, a large egg provides varying amounts of many other nutrients, too.
The yolk, or yellow portion, of an egg makes up about 34% of the liquid weight of the egg. It contains all of the fat in the egg and a little less than half of the protein. The yolk of a large egg contains about 55 calories.
With the exception of niacin and riboflavin, the yolk contains a higher proportion of the egg’s vitamins than the white, including vitamins B6 and B12, folic acid, pantothenic acid and thiamin. All of the egg’s vitamins A, D, E and K are in the yolk. Egg yolks are one of the few foods naturally containing vitamin D. The yolk also contains more calcium, copper, iron, manganese, phosphorus, selenium and zinc than the white.
Double-yolked eggs are often produced by young hens whose egg production cycles are not yet completely synchronized. They’re often produced too, by hens which are old enough to produce extra large-sized eggs. Genetics is a factor, also. Occasionally a hen will produce double-yolked eggs throughout her egg-laying career. It’s rare, but not unusual, for a young hen to produce an egg with no yolk at all.
It’s the yolk which is responsible for the egg’s emulsifying properties.
Yolk color depends on the hen’s diet. If a hen eats plenty of yellow-orange plant pigments called xanthophylls, the xanthophylls will be deposited in the egg yolk. Hens fed mashes containing yellow corn or alfalfa meal lay eggs with medium yellow yolks, while those eating wheat or barley yield lighter-colored yolks. A colorless diet, such as white cornmeal, produces almost colorless yolks. Natural yellow-orange substances, such as marigold petals, may be added to light-colored feeds to enhance yolk color. Artificial color additives are not permitted. Most buyers in this country prefer gold or lemon-colored yolks. Yolk pigments are relatively stable and are not lost or changed in cooking.
Albumen – Also known as egg white.
Depending on the size of the egg, albumen accounts for most of an egg’s liquid weight, about 66%. The white contains more than half the egg’s total protein, a majority of the egg’s niacin, riboflavin, magnesium, potassium and sodium, and none of the fat. The white of a large egg contains about 17 calories.
Albumen color is opalescent and doesn’t appear white until an egg is beaten or cooked. The cloudy appearance comes from carbon dioxide. As eggs age, carbon dioxide escapes, so the albumen of older eggs is more transparent than that of fresher eggs.
The albumen consists of four alternating layers of thick and thin consistencies. From the yolk outward, they are designated as the inner thick or chalaziferous white, the inner thin white, the outer thick white and the outer thin white. As an egg ages, the egg white tends to thin out because its protein changes in character. That’s why fresh eggs sit up tall and firm in the pan while older ones tend to spread out.
When you beat egg white vigorously, it foams and increases in volume six to eight times. Egg foams are essential for making meringues, puffy omelets, soufflés, angel food and sponge cakes.