Some eggs are laid by female animals of many different species, including birdsreptilesamphibiansmammals, and fish, and have been eaten by humans for thousands of years.[1] Bird and reptile eggs consist of a protective eggshell, albumen (egg white), and vitellus (egg yolk), contained within various thin membranes. The most commonly consumed eggs are chicken eggs. Other poultry eggs including those of duck and quail also are eaten. Fish eggs are called roe and caviar.

Egg yolks and whole eggs store significant amounts of protein and choline,[2][3]and are widely used in cookery. Due to their protein content, the United States Department of Agriculture categorizes eggs as Meats within the Food Guide Pyramid.[2] Despite the nutritional value of eggs, there are some potential health issues arising from cholesterol content, salmonella contamination, and allergy to egg proteins.

Chickens and other egg-laying creatures are kept widely throughout the world and mass production of chicken eggs is a global industry. In 2009, an estimated 62.1 million metric tons of eggs were produced worldwide from a total laying flock of approximately 6.4 billion hens.[4] There are issues of regional variation in demand and expectation, as well as current debates concerning methods of mass production. In 2012, the European Union banned battery husbandry of chickens.


Bird eggs have been valuable foodstuffs since prehistory, in both hunting societies and more recent cultures where birds were domesticated. The chicken probably was domesticated for its eggs (from jungle fowl native to tropical and subtropical Southeast Asia and Indian subcontinent) before 7500 BCEChickens were brought to Sumer and Egypt by 1500 BCE, and arrived in Greece around 800 BCE, where the quail had been the primary source of eggs.[5]In Thebes, Egypt, the tomb of Haremhab, dating to approximately 1420 BCE, shows a depiction of a man carrying bowls of ostrich eggs and other large eggs, presumably those of the pelican, as offerings.[6] In ancient Rome, eggs were preserved using a number of methods and meals often started with an egg course.[6] The Romans crushed the shells in their plates to prevent evil spirits from hiding there.[7] In the Middle Ages, eggs were forbidden during Lentbecause of their richness.[7] The word mayonnaise possibly was derived from moyeu, the medieval French word for the yolk, meaning center or hub.[7]

Egg scrambled with acidic fruit juices were popular in France in the seventeenth century; this may have been the origin of lemon curd.[8]

The dried egg industry developed in the nineteenth century, before the rise of the frozen egg industry.[9] In 1878, a company in St. Louis, Missouri started to transform egg yolk and egg white into a light-brown, meal-like substance by using a drying process.[9] The production of dried eggs significantly expanded during World War II, for use by the United States Armed Forces and its allies.[9]

In 1911, the egg carton was invented by Joseph Coyle in Smithers, British Columbia, to solve a dispute about broken eggs between a farmer in Bulkley Valley and the owner of the Aldermere Hotel. Early egg cartons were made of paper.[10]


Quail eggs (upper left), chicken egg (lower left), and ostrich egg (right).

Bird eggs are a common food and one of the most versatile ingredients used in cooking. They are important in many branches of the modern food industry.[7]

The most commonly used bird eggs are those from the chickenduck, and goose eggs. Smaller eggs, such as quail eggs, are used occasionally as a gourmet ingredient in Western countries. Eggs are a common everyday food in many parts of Asia, such as China and Thailand, with Asian production providing 59 percent of the world total in 2013.[11]

The largest bird eggs, from ostriches, tend to be used only as special luxury food. Gull eggs are considered a delicacy in England,[12] as well as in some Scandinavian countries, particularly in Norway. In some African countries, guineafowl eggs often are seen in marketplaces, especially in the spring of each year.[13] Pheasant eggs and emu eggs are edible, but less widely available,[12] sometimes they are obtainable from farmers, poulterers, or luxury grocery stores. In many countries, wild bird eggs are protected by laws which prohibit the collecting or selling of them, or permit collection only during specific periods of the year.[12]


In 2013, world production of chicken eggs was 68.3 million tonnes. The largest four producers were China at 24.8 million of this total, the United States at 5.6 million, India at 3.8 million, and Japan at 2.5 million.[14] A typical large egg plant ships a million dozen eggs a week.[15]

During production eggs usually are candled to check their quality. The size of its air cell is determined and the examination also reveals whether the egg was fertilized and thereby contains an embryo. Depending on local regulations they may be washed before being placed in egg boxes. Washing may shorten their length of freshness.

Anatomy and characteristics

A raw chicken egg within its membrane, with the shell removed by soaking in vinegar.

Schematic of a chicken egg:
1. Eggshell
2. Outer membrane
3. Inner membrane
4. Chalaza
5. Exterior albumen
6. Middle albumen
7. Vitelline membrane
8. Nucleus of pander
9. Germinal disc (nucleus)
10. Yellow yolk
11. White yolk
12. Internal albumen
13. Chalaza
14. Air cell
15. Cuticula

The shape of an egg resembles a prolate spheroid with one end larger than the other and has cylindrical symmetry along the long axis.

An egg is surrounded by a thin, hard shell. Thin membranes exist inside the shell. The egg yolk is suspended in the egg white by one or two spiral bands of tissue called the chalazae (from the Greek word χάλαζα, meaning ‘hailstone’ or ‘hard lump’).

Air cell

The larger end of the egg contains an air cell that forms when the contents of the egg cool down 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. This provides a way of testing the age of an egg: as the air cell increases in size due to air being drawn through pores in the shell as water is lost, the egg becomes less dense and the larger end of the egg will rise to increasingly shallower depths when the egg is placed in a bowl of water. A very old egg will float in the water and should not be eaten.[16]


Eggshell color is caused by pigment deposition during egg formation in the oviduct and may vary according to species and breed, from the more common white or brown to pink or speckled blue-green. Generally, chicken breeds with white ear lobes lay white eggs, whereas chickens with red ear lobes lay brown eggs.[17] Although there is no significant link between shell color and nutritional value, often there is a cultural preference for one color over another (see ‘Color of eggshell’, below). Brown eggs have significantly higher incidence of blood spots due to candling being less effective.[18]


The eggshell membrane is a clear film lining the eggshell, visible when one peels a boiled egg. Primarily, it composed of fibrous proteins such as collagentype I.[19] These membranes may be used commercially as a dietary supplement.


“White” is the common name for the clear liquid (also called the albumen or the glair/glaire) contained within an egg. Clear initially, upon cooking it turns white. In chickens it is formed from the layers of secretions of the anterior section of the hen oviduct during the passage of the egg.[20] It forms around both fertilizedand unfertilized yolks. The primary natural purpose of egg white is to protect the yolk and provide additional nutrition during the growth of the embryo.

Egg white consists primarily of approximately 90 percent water into which is dissolved 10 percent proteins (including albuminsmucoproteins, and globulins). Unlike the yolk, which is high in lipids (fats), egg white contains almost no and the carbohydrate content is less than one percent. Egg white has many uses in food and many other applications, including the preparation of vaccines, such as those for influenza.[21]


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 color is dependent on the diet of the hen. If the diet contains yellow or orange plant pigments known as xanthophylls, then they are deposited in the yolk, coloring it. Lutein is the most abundant pigment in egg yolk.[22] A diet without such colorful foods may result in an almost colorless yolk. Yolk color is, for example, enhanced if the diet includes foods such as yellow corn and marigold petals.[23] In the US, the use of artificial color additives is forbidden.[23]


See Double-yolk eggs and Yolkless eggs

Shell-less or thin-shelled eggs may be caused by egg drop syndrome.

Culinary properties

Types of dishes

A fried chicken egg, “sunny side up”.

Soft-boiled quail eggs with potato galettes.

Chicken eggs are widely used in many types of dishes, both sweet and savory, including many baked goods. Some of the most common preparation methods include scrambledfriedpoachedhard-boiled, soft-boiled, omelettes, and pickled. They also may be eaten raw, although this is not recommended for people who may be especially susceptible to salmonellosis, such as the elderly, the infirm, or pregnant women. In addition, the protein in raw eggs is only 51 percent bioavailable, whereas that of a cooked egg is nearer 91 percent bio-available, meaning the protein of cooked eggs is nearly twice as absorbable as the protein from raw eggs.[24]

As a cooking ingredient, egg yolks are an important emulsifier in the kitchen, and are also used as a thickener, as in custards.

The albumen, or egg white, contains protein, but little or no , and may be used in cooking separately from the yolk. The proteins in egg white allow it to form foams and aerated dishes. Egg whites may be aerated or whipped to a light, fluffy consistency, and often are used in desserts such as meringues and mousse.

Ground egg shells sometimes are used as a food additive to deliver calcium.[25]Every part of an egg is edible,[citation needed] although the eggshell is generally discarded. Some recipes call for immature or unlaid eggs, which are harvested after the hen is slaughtered or cooked, while still inside the chicken.[26]


Instructional video on baking with eggs and cakes.

Half boiled egg dish.

Eggs contain multiple proteins that gel at different temperatures within the yolk and the white, and the temperature determines the gelling time. Egg yolk becomes a gel, or solidifies, between 65 and 70 °C (149 and 158 °F). Egg white gels at different temperatures: 60 to 73 °C (140 to 163 °F). The white contains exterior albumen which sets at the highest temperature.[27] In practice, in many cooking processes the white gels first because it is exposed to higher temperatures for longer.[28][29]

Salmonella is killed instantly at 71 °C (160 °F), but also is killed from 54.5 °C (130.1 °F), if held at that temperature for sufficiently long time periods.[28] To avoid the issue of salmonella, eggs may be pasteurized in-shell at 57 °C (135 °F) for an hour and 15 minutes. Although the white then is slightly milkier, the eggs may be used in normal ways. Whipping for meringue takes significantly longer, but the final volume is virtually the same.[30]

If a boiled egg is overcooked, a greenish ring sometimes appears around egg yolk due to changes to the iron and sulfur compounds in the egg. It also may occur with an abundance of iron in the cooking water.[citation needed] Overcooking harms the quality of the protein.[citation needed] Chilling an overcooked egg for a few minutes in cold water until it is completely cooled may prevent the greenish ring from forming on the surface of the yolk.

Peeling a cooked egg is easiest when the egg was put into boiling water as opposed to slowly heating the egg from a start in cold water.[31]

Flavor variations

A batch of tea eggs with shell intact soaking in a brew of spices and tea.

Although the age of the egg and the conditions of its storage have a greater influence, the bird’s diet affects the flavor of the egg.[8] For example, when a brown-egg chicken breed eats rapeseed (canola) or soy meals, its intestinal microbes metabolize them into fishy-smelling triethylamine, which ends up in the egg.[8] The unpredictable diet of free-range hens will produce likewise, unpredictable egg flavors.[8] Duck eggs tend to have a flavor distinct from, but still resembling, chicken eggs.

Eggs may be soaked in mixtures to absorb flavor. Tea eggs are steeped in a brew from a mixture of various spices, soy sauce, and black tea leaves to give flavor.


Careful storage of edible eggs is extremely important, as an improperly handled egg may contain elevated levels of Salmonella bacteria that may cause severe food poisoning. In the US, eggs are washed. This cleans the shell, but erodes its cuticle.[32][33] The USDA thus recommends refrigerating eggs to prevent the growth of Salmonella.[23]

Refrigeration also preserves the taste and texture, however, intact eggs (unwashed and unbroken) may be left unrefrigerated for several months without spoiling.[34] In Europe, eggs are not usually washed, and the shells are dirtier, however the cuticle is undamaged, and they do not require refrigeration.[33] In the UK in particular, hens are immunized against salmonella and generally, their eggs are safe for 21 days.[33]


Salted duck egg.

The simplest method to preserve an egg is to treat it with salt. Salt draws water out of bacteria and molds, which prevents their growth.[35] The Chinese salted duck egg is made by immersing duck eggs in brine, or coating them individually with a paste of salt and mud or clay. The eggs stop absorbing salt after approximately a month, having reached osmotic equilibrium.[35] Their yolks take on an orange-red color and solidify, but the white remains somewhat liquid. These often are boiled before consumption and are served with rice congee.

Pickled egg, colored with beetroot juice.

Another method is to make pickled eggs, by boiling them first and immersing them in a mixture of vinegar, salt, and spices, such as ginger or allspice. Frequently, beetroot juice is added to impart a red color to the eggs.[36] If the eggs are immersed in it for a few hours, the distinct red, white, and yellow colors may be seen when the eggs are sliced.[36] If marinated for several days or more, the red color will reach the yolk.[36] If the eggs are marinated in the mixture for several weeks or more, the vinegar will dissolve much of the shell’s calcium carbonate and the egg, making it acidic enough to inhibit the growth of bacteria and molds.[35] Pickled eggs made this way generally keep for a year or more without refrigeration.[35]

century egg or hundred-year-old egg is preserved by coating an egg in a mixture of claywood ash, salt, lime, and rice straw for several weeks to several months, depending on the method of processing. After the process is completed, the yolk becomes a dark green, cream-like substance with a strong odor of sulfur and ammonia, while the white becomes a dark brown, transparent jelly with a comparatively mild, distinct flavor. The transforming agent in a century egg is its alkaline material, which gradually raises the pH of the egg from approximately 9 to 12 or more.[37] This chemical process breaks down some of the complex, flavorless proteins and fats of the yolk into simpler, flavorful ones, which in some way may be thought of as an “inorganic” version of fermentation.

Cooking substitutes

For those who do not consume eggs, alternatives used in baking include other rising agents or binding materials, such as ground flax seeds or potato starch flourTofu also acts as a partial binding agent, since it is high in lecithin due to its soy content. Applesauce may be used, as well as arrowroot and banana. Extracted soybean lecithin, in turn, often is used in packaged foods as an inexpensive substitute for egg-derived lecithin. Leguminous broths, such as chickpea brine or green pea canning liquid, also known as aquafaba, can replace egg whites in desserts such as meringues and mousses.

Other egg substitutes are made from just the white of the egg for those who worry about the high cholesterol and content in eggs. These products usually have added vitamins and minerals, as well as vegetable-based emulsifiers and thickeners, such as xanthan gum or guar gum. These allow the product to maintain the nutrition and several culinary properties of real eggs, making possible foods such as Hollandaise saucecustardmayonnaise, and most baked goods with these substitutes.

Nutritional value

Chicken egg
whole, hard-boiled
Nutritional value per 100 g (3.5 oz)
Energy 647 kJ (155 kcal)
1.12 g
10.6 g
12.6 g
Tryptophan 0.153 g
Threonine 0.604 g
Isoleucine 0.686 g
Leucine 1.075 g
Lysine 0.904 g
Methionine 0.392 g
Cystine 0.292 g
Phenylalanine 0.668 g
Tyrosine 0.513 g
Valine 0.767 g
Arginine 0.755 g
Histidine 0.298 g
Alanine 0.700 g
Aspartic acid 1.264 g
Glutamic acid 1.644 g
Glycine 0.423 g
Proline 0.501 g
Serine 0.936 g
Vitamins Quantity%DV
Vitamin A equiv.

149 μg

Thiamine (B1)

0.066 mg

Riboflavin (B2)

0.5 mg

Niacin (B3)

0.064 mg

Pantothenic acid (B5)

1.4 mg

Vitamin B6

0.121 mg

Folate (B9)

44 μg

Vitamin B12

1.11 μg


294 mg

Vitamin D

87 IU

Vitamin E

1.03 mg

Vitamin K

0.3 μg

Minerals Quantity%DV

50 mg


1.2 mg


10 mg


172 mg


126 mg


124 mg


1.0 mg

Other constituents Quantity
Water 75 g
Cholesterol 373 mg

Leave a Reply