Egg (food)

2007 Schools Wikipedia Selection. Related subjects: Food and agriculture

An egg is an ovum produced by a female animal for reproduction, often prepared as food.

Most edible eggs, including bird eggs and turtle eggs consist of a hard, oval outer eggshell, the "egg white," or albumen, the egg yolk, and various thin membranes. Every part of these eggs is edible, although the eggshell is generally discarded.

Roe and caviar are edible eggs produced by fish.

Egg uses as food ingredients

A carton of free-range chicken eggs
A carton of free-range chicken eggs
Ostrich egg
Ostrich egg

Bird eggs are a common food source. The most commonly used bird eggs are those from the chicken, duck, and goose, but smaller eggs such as quail eggs are occasionally used as a gourmet ingredient, as are the largest bird eggs, from ostriches. Most commercially produced chicken eggs intended for human consumption are unfertilized, since the laying hens are kept without any roosters. Fertile eggs can be purchased and eaten as well, with little nutritional difference. Fertile eggs will not contain a developed embryo, as refrigeration prohibits cellular growth.

Chicken eggs are widely used in many types of cooking. Dishes that use eggs range from both sweet to savoury dishes. Eggs may be pickled; hard-boiled and refrigerated; or eaten raw, though the latter is not recommended for people who may be susceptible to salmonella, such as the elderly, the infirm, or pregnant women.

Separated eggs

Eggs are easily separated by cracking the shell and pouring off the egg white while carefully holding the egg yolk in the shell. The egg yolk and the albumen behave quite differently when cooked, so recipes often require separating the egg white from the yolk.


Egg yolks are used to make mayonnaise and other dishes high in fat. Egg yolks are important as binding agents in many preparations in European cooking due to the emulsifying action of lecithin. This property is crucial for sauces such as mayonnaise and Hollandaise; custards such as crème anglaise, crème brûlée, flan, and lemon custard; and meat dishes such as sausages and pâté.


The albumen, or egg white contains protein but little or no fat. It is used in cooking separately from the yolk, and can be aerated or whipped to a light, fluffy consistency known as soft peaks and stiff peaks. Beaten egg whites are used in desserts such as meringues and mousse.


Ground egg shells are sometimes used as a food additive to deliver calcium.

Problems when cooking eggs

If a boiled egg is overcooked, a greenish ring sometimes appears around egg yolk. This is a manifestation of the iron and sulfur compounds in the egg. It can also occur when there is an abundance of iron in the cooking water. The green ring does not affect the egg's taste; overcooking, however, harms the quality of the protein.

When eggs become rotten, the yolk will turn green, and the egg will emit a pungent sulfurous odour when broken.

Egg substitutes for baking

For those who choose not to or are unable to consume eggs, alternatives used in baking include other rising agents, such as "Ener-G" egg replacer; or binding materials, such as ground flax seeds. Tofu can also act as a partial binding agent, since it is high in lecithin due to its soy content. Extracted soybean lecithin, in turn, is often used in packaged foods as a cheap substitute for egg-derived lecithin.

Egg characteristics

The shape of an egg is an oval with one end larger than the other end. The egg has cylindrical symmetry along the long axis.

An egg is surrounded by a thin, hard shell. Inside, the egg yolk is suspended in the egg white by one or two spiral bands of tissue called the chalazae (from the Greek word khalazi, meaning hailstone or hard lump.)

Air cell

The larger end of the egg contains the air cell that forms when the contents of the egg cool and contract after it is laid. Chicken eggs are graded according to the size of this air cell, measured during candling. A very fresh egg has a small air cell and receives a grade of AA. As the size of the air cell increases, and the quality of the egg decreases, the grade moves from AA to A to B.

Shell and its colour

Egg shell colour is caused by pigment deposition during egg formation in the oviduct and can vary according to breed, from the more common white or brown to pink or speckled blue-green. Although there is no significant link between shell color and nutritional value, there is often a cultural preference for one colour over another. For example, in most regions of the United States, eggs are generally white; while in the northeast of that country and in the United Kingdom, eggs are generally light-brown. Regarding chicken eggs, the color of the egg depends on the color of the bird. According to the Egg Nutrition Centre, hens with white feathers and earlobes will lay white eggs, and chickens with red feathers and earlobes will lay brown eggs (see also [ ])

White (Albumen)


The yolk in a newly laid egg is round and firm. As the yolk ages it absorbs water from the albumen which increases its size and causes it to stretch and weaken the vitelline membrane (the clear casing enclosing the yolk). The resulting effect is a flattened and enlarged yolk shape.

Yolk colour is dependent on the diet of the hen; if the diet contains yellow/orange plant pigments known as xanthophylls, then they are deposited in the yolk, coloring it. A colorless diet can produce an almost colorless yolk. Farmers may enhance yolk colour with artificial pigments, but in most locations, this activity is forbidden.


Three eggs frying, two of which are double-yolked eggs.
Three eggs frying, two of which are double-yolked eggs.

Some hens will lay double-yolked eggs as the result of unsynchronized production cycles; although heredity causes some hens to have a higher propensity to lay double-yolked eggs, these occur more frequently as occasional abnormalities in young hens beginning to lay. Usually a double-yolked egg will be longer and thinner than an ordinary single-yolk egg. Double-yolked eggs only rarely, and even then only with human intervention, lead to the successful development of two embryos .

It is also possible for a young hen to produce an egg with no yolk at all. Yolkless eggs are usually formed about a bit of tissue that is sloughed off the ovary or oviduct. This tissue stimulates the secreting glands of the oviduct and a yolkless egg results.

Nutritional value

Eggs provide a significant amount of protein to one's diet, as well as various nutrients.

Chicken eggs are the most commonly eaten eggs, and are highly nutritious. They supply a large amount of complete, high-quality protein (which contains all essential amino acids for humans), and provide significant amounts of several vitamins and minerals, including vitamin A, riboflavin, folic acid, vitamin B6, vitamin B12, choline, iron, calcium, phosphorous and potassium. They are also one of the least expensive single-food sources of complete protein. One large chicken egg contains approximately 7 grams of protein.

3 egg yolks in a glass
3 egg yolks in a glass

All of the egg's vitamin A, D and E is in the yolk. The egg is one of the few foods which naturally contain vitamin D (although this nutrient is naturally produced in humans when their skin is exposed to sunlight). A large egg yolk contains approximately 60 calories (250 kilojoules); the egg white contains about 15 calories (60 kilojoules). A large yolk contains more than two-thirds of the recommended daily intake of 300 mg of cholesterol (although one study shows that your body does not absorb much cholesterol from eggs ). The yolk makes up about 33% of the liquid weight of the egg. It contains all of the fat in the egg and slightly less than half of the protein. It also contains all of the choline, and one yolk contains approximately half of the recommended daily intake. Choline is an important nutrient for development of the brain, and is said to be important for pregnant and nursing women to ensure healthy fetal brain development .

Recently, chicken eggs that are especially high in Omega 3 fatty acids have come on the market. These eggs are made by feeding laying hens a diet containing polyunsaturated fats and kelp meal. Two brands available in the UK are "Columbus Eggs" and "The Hearty Egg" . Nutrition information on the packaging is different for each of the brands.

Health issues of eating chicken eggs

Cholesterol and fat

Chicken egg yolks contain a small amount of fat. People on a low- cholesterol diet may need to cut down on egg consumption, although most of the fat in egg is unsaturated fat and may not be harmful. The egg white consists primarily of water (87%) and protein (13%) and contains no cholesterol and little, if any, fat.

Some people try to avoid eggs in their diet because they are high in cholesterol, which is concentrated in the yolk. This issue is sometimes addressed by eating only some or none of the yolk. People sometimes remove the yolk themselves, or may use prepared egg substitutes such as Egg Beaters.

There is debate over whether egg yolk presents a health risk. Some research suggests it may lower total Low density lipoprotein ("bad" cholesterol) while raising High density lipoprotein ("good" cholesterol) levels. Some people advocate the eating of raw eggs and egg yolks for this reason, claiming that uncooked cholesterol in the yolk is healthier than when it is cooked.

The United States egg industry launched its continuing "Incredible Edible Egg" campaign, which touts eggs as a healthy part of a balanced diet. The American Egg Board publicizes modern research which shows that dietary cholesterol has less effect on blood cholesterol than previously thought.


A health issue associated with eggs is contamination by pathogenic bacteria like Salmonella enteritidis. Contamination of eggs exiting a female bird via the cloaca may also occur with other members of the Salmonella group, so care must be taken to avoid the egg shell becoming contaminated with fecal matter. In commercial practice, eggs are quickly washed with a sanitizing solution within minutes of being laid.

Most health experts advise people to cook their eggs thoroughly before eating them, as the heat is necessary to kill any infectious micro-organisms that may be present. Raw and undercooked eggs have been associated with salmonella infection. As with meat, containers and surfaces that have been used to process raw eggs should not come in contact with ready-to-eat food.

The risk of infection from raw or undercooked eggs is dependent in part upon the sanitary conditions under which the hens are kept. Some smaller egg producers make a point of keeping their hens in cleaner (and, in their view, more humane) conditions, and observe few or no cases of salmonella in the birds themselves.

Recent evidence suggests the problem is not as prevalent as once thought. A study by the U.S. Department of Agriculture earlier this year (Risk Analysis April 2002 22(2):203-18) showed that of the 69 billion eggs produced annually, only 2.3 million of them are contaminated with salmonella - equivalent to just one in every 30,000 eggs.

Egg shells act as Hermetic seals which guard against bacteria entering, but this seal can be broken through improper handling or if laid by unhealthy chickens. Most forms of contamination enter through such weaknesses in the shell.

Food allergy

One of the most common food allergies is eggs. Infants usually have the opportunity to grow out of this allergy during childhood, if exposure is minimized. Generally, physicians will recommend feeding only the yolk to infants because of the higher risk of allergic reaction to the egg white.

Edwina Currie, Salmonella and the UK Lion Mark

The Lion Mark was introduced to the UK by the British Egg Information Council (the equivalent of the American Egg Board) in 1998. British Health Minister Edwina Currie sparked a controversy in 1988 after she issued a warning about salmonella in British eggs that was criticised for being hysterical and over-cautious. The amount of eggs infected by salmonella was a minority and Currie resigned after the then UK Prime Minister Margaret Thatcher stated, "I had eggs for breakfast".

The Lion Mark was launched as an attempt to restore UK public confidence in eggs, which had been smashed by Currie's claims. The Lion Quality Code of Practice includes compulsory vaccination against Salmonella Enteritidis of all pullets destined for Lion egg-producing flocks, independent auditing improved traceability of eggs and a "best-before" date stamped on the shell and pack which shows that they are fresher than required by law, as well as on-farm and packing station hygiene controls.

Since its introduction in 1998, the Lion Mark has been extremely successful so much so that, in 2001 a UK Government committee (the Advisory Committee on the Microbiological Safety of Food) produced a report highlighting the effectiveness of poultry vaccination in reducing human salmonella cases by half.

This has since been reinforced by the Food Standards Agency which has confirmed the success of the UK egg industry in overcoming salmonella in eggs. In its survey, published in 2004, it tested more than 28,000 UK-produced eggs and no salmonella was found inside any of them.

Approximately 85% of UK eggs are now produced to Lion Quality Standards.

The Lion Mark was itself well known in the UK from the 1950s, which was one reason why it helped to restore public confidence. The British Egg Industry also brought back the popular advertising strapline from the 1950s, Go To Work On An Egg.

Chicken egg sizes

Chicken eggs are graded by size, for the purpose of sales. The United States Department of Agriculture grades them by weight per dozen. The following egg masses have been calculated on the basis of the USDA grades:

Modern Sizes
Size Mass per egg
Jumbo Greater than 2.5 oz. or 71 g
Very Large or Extra Large (XL) Greater than 2.25 oz. or 64g
Large (L) Greater than 2 oz. or 57g
Medium (M) Greater than 1.75 oz. or 50g
Small (S) Greater than 1.5 oz. or 43g
Peewee Greater than 1.25 oz. or 35g

Traditional Sizes
Size Mass
Size 0 Greater than 75 g
Size 1 70g-75g
Size 2 65g-70g
Size 3 60g-65g
Size 4 55g-60g
Size 5 50g-55g
Size 6 45g-50g
Size 7 less than 45g

The most common size of chicken egg is 'Large' and is the egg size commonly referred to for recipes.

Issues in mass production

Commercial factory farming operations often involve raising the hens in small crowded cages, preventing the chickens from engaging in activities such as wing-flapping, dust-bathing, scratching, pecking, perching and nest-building. Laying hens are often slaughtered after twelve months when their egg productivity starts to decline. All hens confined to battery cages, and many raised in cage-free conditions, are de-beaked, typically with a hot laser, to prevent cannibalistic pecking. According to critics of the practice, this can cause hens severe pain to the point where some may refuse to eat and prefer to starve to death. Some hens may be force molted to increase egg quality and production level after the molting . Due to modern selective breeding, laying hen strains differ from meat production strains. As male birds of the laying strain do not lay eggs and are not suitable for meat production, they are generally killed at one to three days old .

Many animal welfare advocates object to the industrial agriculture conditions, such as those stated above, under which laying hens are raised. Animal welfare advocates claim that the resulting frustration and stress on the hens can lead to aggressive behaviour such as feather-pecking and cannibalism.

Free-range eggs are considered by some advocates to be an acceptable substitute to factory farmed eggs. Free range laying hens are given outdoor access instead of being contained in crowded cages. Questions on the actual living conditions of free range hens have been raised as there is no legal definition or regulations for eggs labeled as free range in the US .


  • If a raw egg is spun, abruptly stopped and then quickly released, it will start to spin again as the liquid inside continues to rotate. This technique can be used to reliably determine whether an egg is raw or already boiled — a solid egg will remain stationary once halted. Additionally, if a cooked egg and a raw egg are spun with the same force, the cooked egg will spin much faster. Another way of determining if an egg is raw or already boiled is to spin it quickly. An already boiled egg will spin into an upright position after a few seconds, but the raw egg will continue to spin on its wide side.
  • It is more difficult to break a chicken egg by squeezing it from end-to-end than on its long axis.
  • An ostrich egg can make omelets for ten people, and takes forty-five minutes of boiling to harden through.
  • An egg that floats in water is likely to have been spoiled by bacteria that entered through the pores in the shell and created gas inside it. The increased pressure due to the gas production may push some of the white through the shell, which may also signify a bad egg.
  • An alternative theory to the floating egg is that because the shell is porous, liquid can slowly evaporate out. Thus an older egg has more evaporated liquid and therefore a bigger air gap. This is used as a test of the freshness of an egg, as a very fresh egg will sink in a bowl of water, and lie on its side. An older egg with a bigger air gap will tilt up slightly, as the air acts as a float. A very old egg will point straight up or may even float to the surface.
  • The world's fastest egg thrower is American Brandon L. Shellhorn, who holds the world record of throwing 136 eggs per minute for 30 minutes in 2001.
  • The world's fastest omelette maker is American Howard Helmer who rustled up 427 omelettes in 30 minutes in 1990.
  • The longest throw of a fresh egg — without breaking it — is 98.51 metres. The record was achieved in Texas, USA in 1978.
  • The world's largest omelette was made in Madrid from 5,000 eggs by chef Carlos Fernandez. It weighed 1,320 lbs.
  • An egg will age more in 24 hours at room temperature than it would in a week under refrigeration.
  • Eggs contain naturally occurring Vitamin D, a rarity with foods.
  • It takes a hen 24–26 hours to lay an egg, with the cycle repeating 30 minutes later.
  • A large egg contains 75 calories and 5 grams of fat.
  • A typical egg shell contains up to 17,000 pores, through which odors and flavours can pass.
  • Keeping eggs stored in their cartons keep eggs fresher by isolating them from the rest of the refrigerator.
  • The colour of a yolk varies with a hen's diet; although artificial colors are not allowed in chicken feed, naturally occurring items such as yellow-orange marigold petals may be used.


A popular Easter tradition in some parts of the world is the decoration of hard-boiled eggs (usually by dyeing). Adults often hide the eggs for children to find, an activity known as an Easter egg hunt.


Although a food item, eggs are sometimes thrown at people or things such as houses and cars. This act, known commonly as egging in the United States, is a minor form of vandalism and therefore usually a criminal offence. On Halloween, for example, trick or treaters have been known to throw eggs (and sometimes flour) at property or people from whom they received nothing. Furthermore, egg white can degrade (and sometimes remove) certain kinds of automotive paint. Eggs are also often thrown in protests, as they are cheap, nonlethal and, at the same time, very messy when broken. There is also an element of humiliation associated with being covered in an egg's contents, referenced by the phrase "egg on one's face". The act is often used on October 30, sometimes referred to as Mischief night, or on March 31 on the day before April fools day.

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