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Humans and human ancestors have scavenged and eaten animal
egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...
s for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especially chickens. Eggs of other birds, including ostriches and other
ratite A ratite () is any of a diverse group of flightless, large, long-necked, and long-legged birds of the infraclass Palaeognathae. Kiwi, the exception, are much smaller and shorter-legged and are the only nocturnal extant ratites. The systematics ...
s, are eaten regularly but much less commonly than those of chickens. People may also eat the eggs of
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s,
amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
s, and fish. Fish eggs consumed as food are known as roe or caviar. Bird and reptile eggs consist of a protective
eggshell An eggshell is the outer covering of a hard-shelled egg and of some forms of eggs with soft outer coats. Diversity Worm eggs Nematode eggs present a two layered structure: an external vitellin layer made of chitin that confers mechanical ...
, albumen (
egg white Egg white is the clear liquid (also called the albumen or the glair/glaire) contained within an egg. In chickens it is formed from the layers of secretions of the anterior section of the hen's oviduct during the passage of the egg. It forms arou ...
), and vitellus (
egg yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...
), contained within various thin membranes. Egg yolks and whole eggs store significant amounts of protein and choline, and are widely used in cookery. Due to their protein content, the United States Department of Agriculture formerly categorized eggs as ''Meats'' within the Food Guide Pyramid (now
MyPlate MyPlate is the current nutrition guide published by the USDA's Center for Nutrition Policy and Promotion, and serves as a recommendation based on the Dietary Guidelines for Americans. It replaced the USDA's MyPyramid guide on June 2, 2011, endin ...
). 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 Mass production, also known as flow production or continuous production, is the production of substantial amounts of standardized products in a constant flow, including and especially on assembly lines. Together with job production and batch ...
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. 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.


History

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 BCE. Chickens 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. 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 Pelicans (genus ''Pelecanus'') are a genus of large water birds that make up the family Pelecanidae. They are characterized by a long beak and a large throat pouch used for catching prey and draining water from the scooped-up contents before s ...
, as offerings. In ancient Rome, eggs were preserved using a number of methods and meals often started with an egg course. The Romans crushed the shells in their plates to prevent evil spirits from hiding there. In the Middle Ages, eggs were forbidden during
Lent Lent ( la, Quadragesima, 'Fortieth') is a solemn religious observance in the liturgical calendar commemorating the 40 days Jesus spent fasting in the desert and enduring temptation by Satan, according to the Gospels of Matthew, Mark and Luke ...
because of their richness, although the motivation for forgoing eggs during Lent was not entirely religious. An annual pause in egg consumption allowed farmers to rest their flocks, and also to limit their hens' consumption of feed during a time of year when food stocks were usually scarce. Egg scrambled with acidic fruit juices were popular in France in the seventeenth century; this may have been the origin of lemon curd. The dried egg industry developed in the nineteenth century, before the rise of the frozen egg industry. In 1878, a company in
St. Louis, Missouri St. Louis () is the second-largest city in Missouri, United States. It sits near the confluence of the Mississippi River, Mississippi and the Missouri Rivers. In 2020, the city proper had a population of 301,578, while the Greater St. Louis, ...
started to transform egg yolk and egg white into a light-brown, meal-like substance by using a drying process. The production of dried eggs significantly expanded during World War II, for use by the United States Armed Forces and its allies. 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.
Polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is a ...
egg cartons became popular in the latter half of the twentieth century as they were perceived to offer better protection especially against heat and breakage, however by the twenty-first century environmental considerations have led to the more
biodegradable Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradati ...
paper cartons (often made of recycled material) once again becoming more widely used. Whereas the wild Asian fowl from which domesticated chickens are descended typically lay about a dozen eggs each year during the breeding season, several millenia of selective breeding have produced domesticated hens capable of laying more than three hundred eggs each annually, and to lay eggs year round.


Varieties

Bird eggs are a common
food Food is any substance consumed by an organism for nutritional support. Food is usually of plant, animal, or fungal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals. The substance is inge ...
and one of the most versatile ingredients used in cooking. They are important in many branches of the modern
food industry The food industry is a complex, global network of diverse businesses that supplies most of the food consumed by the world's population. The food industry today has become highly diversified, with manufacturing ranging from small, traditiona ...
. The most commonly used bird eggs are those from the chicken, duck, and goose. Smaller eggs, such as
quail egg Quail eggs are eaten and considered a delicacy in many parts of the world, including Asia, Europe, and North America. In Japanese cuisine, they are sometimes used raw or cooked as ''tamago'' in sushi and often found in ''bento'' lunches. In ...
s, 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. The largest bird eggs, from ostriches, tend to be used only as special luxury food. Gull eggs are considered a delicacy in England, 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. Pheasant eggs and
emu The emu () (''Dromaius novaehollandiae'') is the second-tallest living bird after its ratite relative the ostrich. It is endemic to Australia where it is the largest native bird and the only extant member of the genus ''Dromaius''. The emu' ...
eggs are edible, but less widely available; 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.


Production

In 2017, world production of chicken eggs was 80.1 million tonnes. The largest producers were China with 31.3 million of this total, the United States with 6.3 million, India at 4.8 million, Mexico at 2.8 million, Japan at 2.6 million, and Brazil and Russia with 2.5 million each. A typical large egg factory ships a million dozen eggs per week. For the month of January 2019, the United States produced 9.41 billion eggs, with 8.2 billion for table consumption and 1.2 billion for raising chicks. Americans are projected to each consume 279 eggs in 2019, the highest since 1973, but less than the 405 eggs eaten per person in 1945. 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, eggs may be washed before being placed in egg boxes, although washing may shorten their length of freshness.


Anatomy and characteristics

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, especially if a foul odor can be detected if the egg is cracked open.


Shell

Eggshell color is caused by pigment deposition during egg formation in the oviduct and may vary according to species and
breed A breed is a specific group of domestic animals having homogeneous appearance (phenotype), homogeneous behavior, and/or other characteristics that distinguish it from other organisms of the same species. In literature, there exist several slig ...
, 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. Although there is no significant link between shell color and nutritional value, often there is a cultural preference for one color over another (see below). As candeling is less effective with brown eggs, they have a significantly higher incidence of blood spots.


Membrane

The
eggshell membrane Eggshell membrane or shell membrane is the clear film lining eggshells, visible when one peels a boiled bird egg. Chicken eggshell membranes are used as a dietary supplement. Eggshell membrane is derived commercially from the eggshells of industri ...
is a clear film lining the eggshell, visible when one peels a boiled egg. Primarily, it composed of fibrous
proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
such as
collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...
type I Type 1 or Type I or ''variant'', may refer to: Health *Diabetes mellitus type 1 (also known as "Type 1 Diabetes"), insulin-dependent diabetes * Type I female genital mutilation * Type 1 personality *Type I hypersensitivity (or immediate hypersensit ...
. These membranes may be used commercially as a dietary supplement.


White

"White" is the common name for the clear liquid (also called the albumen or the glair/glaire) contained within an egg. Colorless and transparent initially, upon cooking it turns white and opaque. In chickens, it is formed from the layers of secretions of the anterior section of the hen oviduct during the passage of the egg. It forms around both fertilized and unfertilized yolks. The primary natural purpose of egg white is to protect the yolk and provide additional nutrition during the growth of the
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
. Egg white consists primarily of approximately 90 percent water into which is dissolved 10 percent proteins (including albumins,
mucoprotein A mucoprotein is a glycoprotein composed primarily of mucopolysaccharides. Mucoproteins can be found throughout the body, including the gastrointestinal tract, reproductive organs, airways, and the synovial fluid of the knees. They are called mucopr ...
s, and globulins). Unlike the yolk, which is high in lipids (fats), egg white contains almost no fat 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.


Yolk

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. 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 Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. Th ...
and marigold petals.Shell eggs from farm to table
. USDA Food Safety and Inspection Service, (2011).
In the US, the use of artificial color additives is forbidden.


Abnormalities

Abnormalities that have been found in eggs purchased for human consumption include: * Double-yolk eggs, when an egg contains two or more yolks, occurs when ovulation occurs too rapidly, or when one yolk becomes joined with another yolk. * Yolkless eggs, which contain whites but no yolk, usually occurs during a pullet's first effort, produced before her laying mechanism is fully ready. * Double-shelled eggs, where an egg may have two or more outer shells, is caused by a counter-peristalsis contraction and occurs when a second oocyte is released by the ovary before the first egg has completely traveled through the oviduct and been laid. * Shell-less or thin-shelled eggs may be caused by
egg drop syndrome Egg drop syndrome '76 (EDS '76) is a viral disease that affects birds, notably chickens, ducks, geese and swans. It is characterised by a sudden drop in production of eggs as well as its eggshell quality in apparent healthy laying birds. Previou ...
.


Culinary properties


Types of dishes

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
scrambled Scrambled eggs is a dish made from eggs (usually chicken eggs) stirred, whipped or beaten together while being gently heated, typically with salt, butter, oil and sometimes other ingredients. Preparation Only eggs are necessary to make scrambled ...
, fried, poached, hard-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 bioavailable, meaning the protein of cooked eggs is nearly twice as absorbable as the protein from raw eggs. As a cooking ingredient, egg yolks are an important
emulsifier An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Altho ...
in the kitchen, and are also used as a thickener, as in
custard Custard is a variety of culinary preparations based on sweetened milk, cheese, or cream cooked with egg or egg yolk to thicken it, and sometimes also flour, corn starch, or gelatin. Depending on the recipe, custard may vary in consistency fro ...
s. The
albumen Egg white is the clear liquid (also called the albumen or the glair/glaire) contained within an egg. In chickens it is formed from the layers of secretions of the anterior section of the hen's oviduct during the passage of the egg. It forms arou ...
(egg white) contains protein, but little or no fat, 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 A mousse (; ; "foam") is a soft prepared food that incorporates air bubbles to give it a light and airy texture. Depending on preparation techniques, it can range from light and fluffy to creamy and thick. A mousse may be sweet or savory. as e ...
. Ground eggshells sometimes are used as a
food additive Food additives are substances added to food to preserve flavor or enhance taste, appearance, or other sensory qualities. Some additives have been used for centuries as part of an effort to preserve food, for example vinegar (pickling), salt (salt ...
to deliver calcium. Every part of an egg is edible, 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.


Cooking

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 A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state, although the liquid phase may still di ...
, or solidifies, between . Egg white gels at different temperatures: . The white contains exterior albumen which sets at the highest temperature. In practice, in many cooking processes the white gels first because it is exposed to higher temperatures for longer. Salmonella is killed instantly at , but also is killed from , if held at that temperature for sufficiently long time periods. To avoid the issue of salmonella, eggs may be pasteurized in-shell at 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. If a boiled egg is overcooked, a greenish ring sometimes appears around egg yolk due to changes to the iron and
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
compounds in the egg. It also may occur with an abundance of iron in the cooking water. Overcooking harms the quality of the protein. 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.


Flavor variations

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. 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. The unpredictable diet of free-range hens will produce likewise, unpredictable egg flavors. 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, a common snack sold from street-side carts in China, are steeped in a brew from a mixture of various spices, soy sauce, and black tea leaves to give flavor.


Storage

Careful storage of edible eggs is extremely important, as an improperly handled egg may contain elevated levels of ''
Salmonella ''Salmonella'' is a genus of rod-shaped (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' is the type species and is fur ...
'' bacteria that may cause severe
food poisoning Foodborne illness (also foodborne disease and food poisoning) is any illness resulting from the spoilage of contaminated food by pathogenic bacteria, viruses, or parasites that contaminate food, as well as prions (the agents of mad cow disease) ...
. In the US, eggs are washed. This cleans the shell, but erodes its
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...
. The USDA thus recommends refrigerating eggs to prevent the growth of ''Salmonella''. Refrigeration also preserves the taste and texture, however, intact eggs (unwashed and unbroken) may be left unrefrigerated for several months without spoiling. In Europe, eggs are not usually washed, and the shells are dirtier, however the cuticle is undamaged, and they do not require refrigeration. In the UK in particular, hens are immunized against salmonella and generally, their eggs are safe for 21 days.


Preservation

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. The Chinese salted duck egg is made by immersing duck eggs in
brine Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for br ...
, or coating them individually with a paste of salt and
mud A MUD (; originally multi-user dungeon, with later variants multi-user dimension and multi-user domain) is a Multiplayer video game, multiplayer Time-keeping systems in games#Real-time, real-time virtual world, usually Text-based game, text-bas ...
or clay. The eggs stop absorbing salt after approximately a month, having reached
osmotic equilibrium Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in a pure s ...
. 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 Congee or conjee ( ) is a type of rice porridge or gruel eaten in Asian countries. It can be eaten plain, where it is typically served with side dishes, or it can be served with ingredients such as meat, fish, seasonings and flavourings, most o ...
. 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 Ginger (''Zingiber officinale'') is a flowering plant whose rhizome, ginger root or ginger, is widely used as a spice A spice is a seed, fruit, root, bark, or other plant substance primarily used for flavoring or coloring food. Spices ...
or
allspice Allspice, also known as Jamaica pepper, myrtle pepper, pimenta, or pimento, is the dried unripe berry (botany), berry of ''Pimenta dioica'', a Canopy (forest), midcanopy tree native to the Greater Antilles, southern Mexico, and Central America, ...
. Frequently,
beetroot The beetroot is the taproot portion of a beet plant, usually known in North America as beets while the vegetable is referred to as beetroot in British English, and also known as the table beet, garden beet, red beet, dinner beet or golden beet ...
juice is added to impart a red color to the eggs. 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. If marinated for several days or more, the red color will reach the yolk. 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 penetrate the egg, making it acidic enough to inhibit the growth of bacteria and molds. Pickled eggs made this way generally keep for a year or more without refrigeration. A
century egg Century eggs (), also known under a wide variety of names (see infobox), are a Chinese egg-based culinary dish made by preserving duck, chicken or quail eggs in a mixture of clay, ash, salt, quicklime, and rice hulls for several weeks to severa ...
or hundred-year-old egg is preserved by coating an egg in a mixture of clay, wood 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 In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a base (chemistry), basic, ionic compound, ionic salt (chemistry), salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as ...
material, which gradually raises the pH of the egg from approximately 9 to 12 or more. 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 Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food ...
.


Nutrition


Egg sizes


Nutritional value

A medium/large chicken egg provides approximately 70 kilocalories (290 kJ) of food energy and 6 grams of protein."Full Report (All Nutrients): 01129, Egg, whole, cooked, hard-boiled"
USDA Branded Food Products Database
"Full Report (All Nutrients): 45174951, EGGS"
USDA Branded Food Products Database
Eggs (boiled) supply several vitamins and minerals as significant amounts of the Daily Value (DV), including (per 100g) vitamin A (19 percent DV),
riboflavin Riboflavin, also known as vitamin B2, is a vitamin found in food and sold as a dietary supplement. It is essential to the formation of two major coenzymes, flavin mononucleotide and flavin adenine dinucleotide. These coenzymes are involved in ...
(42 percent DV), pantothenic acid (28 percent DV), vitamin B12 (46 percent DV), choline (60 percent DV), phosphorus (25 percent DV), zinc (11 percent DV) and vitamin D (15 percent DV). The diet of laying hens also may affect the nutritional quality of eggs. For instance, chicken eggs that are especially high in omega-3 fatty acids are produced by feeding hens a diet containing polyunsaturated fats from sources such as fish oil,
chia seeds Chia seeds are the edible seeds of ''Salvia hispanica'', a flowering plant in the mint family (Lamiaceae) native to central and southern Mexico, or of the related ''Salvia columbariae'' of the southwestern United States and Mexico. Chia seeds a ...
, or
flaxseeds Flax, also known as common flax or linseed, is a flowering plant, ''Linum usitatissimum'', in the family Linaceae. It is cultivated as a food and fiber crop in regions of the world with temperate climates. Textiles made from flax are known in W ...
. Pasture-raised free-range hens, which forage for their own food, also produce eggs that are relatively enriched in omega-3 fatty acids when compared to those of cage-raised chickens. A 2010 USDA study determined there were no significant differences of
macronutrients A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
in various chicken eggs. Cooked eggs are easier to digest than raw eggs, as well as having a lower risk of salmonellosis.


Health effects


Cholesterol and fat

More than half the calories found in eggs come from the fat in the yolk; 50 grams of chicken egg (the contents of an egg just large enough to be classified as "large" in the US, but "medium" in Europe) contains approximately five grams of fat.
Saturated fat A saturated fat is a type of fat in which the fatty acid chains have all single bonds. A fat known as a glyceride is made of two kinds of smaller molecules: a short glycerol backbone and fatty acids that each contain a long linear or branched c ...
(
palmitic Palmitic acid (hexadecanoic acid in IUPAC nomenclature) is a fatty acid with a 16-carbon chain. It is the most common saturated fatty acid found in animals, plants and microorganisms.Gunstone, F. D., John L. Harwood, and Albert J. Dijkstra. The Li ...
,
stearic Stearic acid ( , ) is a saturated fatty acid with an 18-carbon chain. The IUPAC name is octadecanoic acid. It is a waxy solid and its chemical formula is C17H35CO2H. Its name comes from the Greek word στέαρ "''stéar''", which means tallow. ...
, and myristic acids) makes up 27 percent of the fat in egg. The
egg white Egg white is the clear liquid (also called the albumen or the glair/glaire) contained within an egg. In chickens it is formed from the layers of secretions of the anterior section of the hen's oviduct during the passage of the egg. It forms arou ...
consists primarily of water (around 90 percent) and protein (around 10 percent) and contains no cholesterol and little, if any, fat. There is debate over whether egg yolk presents a health risk. Some research suggests dietary cholesterol increases the ratio of total to HDL cholesterol and, therefore, adversely affects the body's cholesterol profile; whereas other studies show that moderate consumption of eggs, up to one a day, does not appear to increase
heart disease Cardiovascular disease (CVD) is a class of diseases that involve the heart or blood vessels. CVD includes coronary artery diseases (CAD) such as angina and myocardial infarction (commonly known as a heart attack). Other CVDs include stroke, hea ...
risk in healthy individuals. Harold McGee argues that the cholesterol in the egg yolk is not what causes a problem, because fat (particularly saturated fat) is much more likely to raise cholesterol levels than the consumption of cholesterol.


Type 2 diabetes

Studies have shown conflicting results about a possible connection between egg consumption and type 2 diabetes. A meta-analysis from 2013 found that eating four eggs per week was associated with a 29 percent increase in the relative risk of developing diabetes. Another meta-analysis from 2013 also supported the idea that egg consumption may lead to an increased incidence of type two diabetes. A 2016 meta-analysis suggested that association of egg consumption with increased risk of incident type 2 diabetes may be restricted to cohort studies from the United States. A 2020 meta-analysis found that there was no overall association between moderate egg consumption and risk of type 2 diabetes and that the risk found in US studies was not found in European or Asian studies.


Cancer

A 2015 meta-analysis found an association between higher egg consumption (5 a week) with increased risk of breast cancer compared to no egg consumption. Another meta-analysis found that egg consumption may increase ovarian cancer risk. A 2019 meta-analysis found an association between high egg consumption and risk of upper aero-digestive tract cancers in hospital-based case-control studies. A 2021 review did not find a significant association between egg consumption and breast cancer. A 2021 umbrella review found that egg consumption significantly increases the risk of ovarian cancer.


Cardiovascular health

One systematic review and meta-analysis of egg consumption found that higher consumption of eggs (more than 1 egg/day) was associated with a significant reduction in risk of coronary artery disease. Another systematic review and meta-analysis of Dietary Cholesterol and Egg Consumption found that egg consumption was associated with an increased all-cause mortality and CVD mortality. These contrary results may be due to somewhat different methods of study selection and the use primarily of observational studies, where
confounding In statistics, a confounder (also confounding variable, confounding factor, extraneous determinant or lurking variable) is a variable that influences both the dependent variable and independent variable, causing a spurious association. Con ...
factors are not controlled. A 2018 meta-analysis of randomized clinical trials found that consumption of eggs increases total cholesterol (TC),
LDL-C Low-density lipoprotein (LDL) is one of the five major groups of lipoprotein that transport all fat molecules around the body in extracellular water. These groups, from least dense to most dense, are chylomicrons (aka ULDL by the overall densit ...
and
HDL-C High-density lipoprotein (HDL) is one of the five major groups of lipoproteins. Lipoproteins are complex particles composed of multiple proteins which transport all fat molecules (lipids) around the body within the water outside cells. They are t ...
compared to no egg-consumption but not to low-egg control diets. In 2020, two meta-analyses found that moderate egg consumption (up to one egg a day) is not associated with an increased cardiovascular disease risk. A 2020
umbrella review In medical research, an umbrella review is a review of systematic reviews or meta-analyses. They may also be called overviews of reviews, reviews of reviews, summaries of systematic reviews, or syntheses of reviews. Umbrella reviews are among the hi ...
concluded that increased egg consumption is not associated with cardiovascular disease risk in the general population. Another umbrella review found no association between egg consumption and cardiovascular disorders. A 2013 meta-analysis found no association between egg consumption and heart disease or stroke. A 2013 systematic review and meta-analysis found no association between egg consumption and cardiovascular disease or cardiovascular disease mortality, but did find egg consumption of more than once daily increased cardiovascular disease risk 1.69-fold in those with type 2 diabetes mellitus when compared to type 2 diabetics who ate less than one egg per week. Another 2013 meta-analysis found that eating four eggs per week increased the risk of cardiovascular disease by six percent. Eggs are one of the largest sources of phosphatidylcholine (
lecithin Lecithin (, from the Greek ''lekithos'' "yolk") is a generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues which are amphiphilic – they attract both water and fatty substances (and so ar ...
) in the human diet. A study published in the scientific journal, '' Nature'', showed that dietary phosphatidylcholine is digested by bacteria in the gut and eventually converted into the compound TMAO, a compound linked with increased heart disease. Another study found that type 2 diabetes mellitus and kidney disease also increase TMAO levels and that evidence for a link between TMAO and cardiovascular diseases may be due to
confounding In statistics, a confounder (also confounding variable, confounding factor, extraneous determinant or lurking variable) is a variable that influences both the dependent variable and independent variable, causing a spurious association. Con ...
or reverse causality.


Other

Egg consumption does not increase
hypertension Hypertension (HTN or HT), also known as high blood pressure (HBP), is a long-term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure usually does not cause symptoms. Long-term high bl ...
risk. A 2016 meta-analysis found that consumption of up to one egg a day may contribute to a decreased risk of total
stroke A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...
. Two recent meta-analyses found no association between egg intake and risk of stroke. A 2019 meta-analysis revealed that egg consumption has no significant effect on serum biomarkers of inflammation. A 2021 review of clinical trials found that egg consumption has beneficial effects on macular pigment optical density and serum lutein.


Contamination

A health issue associated with eggs is contamination by
pathogenic In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
bacteria, such as ''
Salmonella enteritidis ''Salmonella enterica'' subsp. ''enterica'' is a subspecies of ''Salmonella enterica'', the rod-shaped, flagellated, aerobic, Gram-negative bacterium. Many of the pathogenic serovars of the ''S. enterica'' species are in this subspecies, includi ...
''. Contamination of eggs with other members of the genus ''Salmonella'' while exiting a female bird via the
cloaca In animal anatomy, a cloaca ( ), plural cloacae ( or ), is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles and birds, a ...
may occur, so care must be taken to prevent the egg shell from becoming contaminated with fecal matter. In commercial practice in the US, eggs are quickly washed with a sanitizing solution within minutes of being laid. The risk of infection from raw or undercooked eggs is dependent in part upon the sanitary conditions under which the hens are kept. Health experts advise people to refrigerate washed eggs, use them within two weeks, cook them thoroughly, and never consume raw eggs.Eggs – No Yolking Matter
." ''Nutrition Action Health Letter,'' July/August 1997.
As with
meat Meat is animal flesh that is eaten as food. Humans have hunted, farmed, and scavenged animals for meat since prehistoric times. The establishment of settlements in the Neolithic Revolution allowed the domestication of animals such as chic ...
, containers and surfaces that have been used to process raw eggs should not come in contact with ready-to-eat food. A study by the U.S. Department of Agriculture in 2002 (Risk Analysis April 2002 22(2):203-18) suggests the problem is not so prevalent in the U.S. as once thought. It showed that of the 69 billion eggs produced annually, only 2.3 million are contaminated with ''Salmonella''—equivalent to just one in every 30,000 eggs—thus showing ''Salmonella'' infection is quite rarely induced by eggs. This has not been the case in other countries, however, where ''Salmonella enteritidis'' and '' Salmonella typhimurium'' infections due to egg consumption are major concerns. Egg shells act as hermetic seals that 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. In the UK, the British Egg Industry Council awards the lions stamp to eggs that, among other things, come from hens that have been vaccinated against ''
Salmonella ''Salmonella'' is a genus of rod-shaped (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' is the type species and is fur ...
''. In 2017, authorities blocked millions of eggs from sale in the Netherlands, Belgium, and Germany because of contamination with the insecticide
fipronil Fipronil is a broad-spectrum insecticide that belongs to the phenylpyrazole chemical family. Fipronil disrupts the insect central nervous system by blocking the ligand-gated ion channel of the GABAA receptor and glutamate-gated chloride (GluCl ...
.


Food allergy

One of the most common food allergies in infants is eggs.Egg Allergy Brochure
, distributed by Royal Prince Alfred Hospital
Infants usually have the opportunity to grow out of this allergy during childhood, if exposure is minimized. Allergic reactions against egg white are more common than reactions against egg yolks. In addition to true allergic reactions, some people experience a food intolerance to egg whites. Food labeling practices in most developed countries now include eggs, egg products, and the processing of foods on equipment that also process foods containing eggs in a special allergen alert section of the ingredients on the labels.


Farming

Most commercially farmed chicken eggs intended for human consumption are unfertilized, since the laying hens are kept without roosters. Fertile eggs may be eaten, with little nutritional difference when compared to the unfertilized. Fertile eggs will not contain a developed
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
, as refrigeration temperatures inhibit cellular growth for an extended period of time. Sometimes an embryo is allowed to develop, but eaten before hatching as with balut.


Grading by quality and size

The U.S. Department of Agriculture grades eggs by the interior quality of the egg (see
Haugh unit The Haugh unit is a measure of egg protein quality based on the height of its egg white (albumen). The test was introduced by Raymond Haugh in 1937 and is an important industry measure of egg quality next to other measures such as shell thickness an ...
) and the appearance and condition of the egg shell. Eggs of any quality grade may differ in weight (size). *U.S. Grade AA **Eggs have whites that are thick and firm; have yolks that are high, round, and practically free from defects; and have clean, unbroken shells. **Grade AA and Grade A eggs are best for frying and poaching, where appearance is important. *U.S. Grade A **Eggs have characteristics of Grade AA eggs except the whites are "reasonably" firm. **This is the quality most often sold in stores. *U.S. Grade B **Eggs have whites that may be thinner and yolks that may be wider and flatter than eggs of higher grades. The shells must be unbroken, but may show slight stains. **This quality is seldom found in retail stores because usually they are used to make liquid, frozen, and dried egg products, as well as other egg-containing products. In
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
and the European Union, eggs are graded by the hen raising method,
free range Free range denotes a method of farming husbandry where the animals, for at least part of the day, can roam freely outdoors, rather than being confined in an enclosure for 24 hours each day. On many farms, the outdoors ranging area is fenced, ...
, battery caged, etc. Chicken eggs are graded by size for the purpose of sales. Some maxi eggs may have double-yolks and some farms separate out double-yolk eggs for special sale. File:Egg and maxi egg 1.jpg, File:Egg and maxi egg 2.jpg, File:Egg with two yellows.jpg,


Color of eggshell

Although eggshell color is a largely cosmetic issue, with no effect on egg quality or taste, it is a major issue in production due to regional and national preferences for specific colors, and the results of such preferences on demand. For example, in most regions of the United States, chicken eggs generally are white. However, brown eggs are more common in some parts of the
Northeastern United States The Northeastern United States, also referred to as the Northeast, the East Coast, or the American Northeast, is a geographic region of the United States. It is located on the Atlantic coast of North America, with Canada to its north, the Southe ...
, particularly New England, where a television jingle for years proclaimed "brown eggs are local eggs, and local eggs are fresh!". Local chicken breeds, including the
Rhode Island Red The Rhode Island Red is an American breed of domestic chicken. It is the state bird of Rhode Island. It was developed there and in Massachusetts in the late nineteenth century, by cross-breeding birds of Oriental origin such as the Malay with ...
, lay brown eggs. Brown eggs are preferred in
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
,
Costa Rica Costa Rica (, ; ; literally "Rich Coast"), officially the Republic of Costa Rica ( es, República de Costa Rica), is a country in the Central American region of North America, bordered by Nicaragua to the north, the Caribbean Sea to the no ...
, Ireland, France, and the United Kingdom. In Brazil and Poland, white chicken eggs are generally regarded as industrial, and brown or reddish ones are preferred. Small farms and
smallholdings A smallholding or smallholder is a small farm operating under a small-scale agriculture model. Definitions vary widely for what constitutes a smallholder or small-scale farm, including factors such as size, food production technique or technology ...
, particularly in economically advanced nations, may sell eggs of widely varying colors and sizes, with combinations of white, brown, speckled (red), green, and blue (as laid by certain breeds, including araucanas, heritage skyline, and cream leg bar) eggs in the same box or carton, while the supermarkets at the same time sell mostly eggs from the larger producers, of the color preferred in that nation or region. These cultural trends have been observed for many years. ''The New York Times'' reported during the Second World War that housewives in Boston preferred brown eggs and those in New York preferred white eggs. In February 1976, the '' New Scientist'' magazine, in discussing issues of chicken egg color, stated "Housewives are particularly fussy about the colour of their eggs, preferring even to pay more for brown eggs although white eggs are just as good". As a result of these trends, brown eggs are usually more expensive to purchase in regions where white eggs are considered "normal", due to lower production. In France and the United Kingdom, it is very difficult to buy white eggs, with most supermarkets supplying only the more popular brown eggs. By direct contrast, in Egypt it is very hard to source brown eggs, as demand is almost entirely for white ones, with the country's largest supplier describing white eggs as "table eggs" and packaging brown eggs for export. Research conducted by a French institute in the 1970s demonstrated blue chicken eggs from the Chilean
araucana The Araucana ( es, Gallina Mapuche, italic=no) is a breed of domestic chicken from Chile. Its name derives from the Araucanía region of Chile where it is believed to have originated. It lays blue-shelled eggs, one of very few breeds that do so. ...
fowl may be stronger and more resilient to breakage. Research at Nihon University,
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
in 1990 revealed a number of different issues were important to Japanese housewives when deciding which eggs to buy and that color was a distinct factor, with most Japanese housewives preferring the white color. Egg producers carefully consider cultural issues, as well as commercial ones, when selecting the breed or breeds of chickens used for production, as egg color varies between breeds. Among producers and breeders, brown eggs often are referred to as "tinted", while the speckled eggs preferred by some consumers often are referred to as being "red" in color.


Living conditions of birds

Commercial
factory farming Intensive animal farming or industrial livestock production, also known by its opponents as factory farming and macro-farms, is a type of intensive agriculture, specifically an approach to animal husbandry designed to maximize production, while ...
operations often involve raising the hens in small, crowded cages, preventing the chickens from engaging in natural behaviors, such as wing-flapping, dust-bathing, scratching, pecking, perching, and nest-building. Such restrictions may lead to pacing and
escape behavior Escape response, escape reaction, or escape behavior is a mechanism by which animals avoid potential predation. It consists of a rapid sequence of movements, or lack of movement, that position the animal in such a way that allows it to hide, freeze ...
. Many hens confined to battery cages, and some raised in cage-free conditions, are debeaked to prevent them from harming each other and engaging in
cannibalism Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is well documented, b ...
. According to critics of the practice, this can cause hens severe pain to the point where some may refuse to eat and starve to death. Some hens may be forced to molt to increase egg quality and production level after the
molting In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
. Molting can be induced by extended food withdrawal, water withdrawal, or controlled lighting programs. Laying hens often are slaughtered when reaching 100 to 130 weeks of age, when their egg productivity starts to decline. 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 generally are killed soon after they hatch. Free-range eggs are considered by some advocates to be an acceptable substitute to factory-farmed eggs.
Free-range Free range denotes a method of farming husbandry where the animals, for at least part of the day, can roam freely outdoors, rather than being confined in an enclosure for 24 hours each day. On many farms, the outdoors ranging area is fenced, ...
laying hens are given outdoor access instead of being contained in crowded cages. Questions regarding the living conditions of free-range hens have been raised in the United States of America, as there is no legal definition or regulations for eggs labeled as free-range in that country. In the United States, increased public concern for
animal welfare Animal welfare is the well-being of non-human animals. Formal standards of animal welfare vary between contexts, but are debated mostly by animal welfare groups, legislators, and academics. Animal welfare science uses measures such as longevity ...
has pushed various egg producers to promote eggs under a variety of standards. The most widespread standard in use is determined by United Egg Producers through their voluntary program of certification. The United Egg Producers program includes guidelines regarding housing, food, water, air, living space, beak trimming, molting, handling, and transportation, however, opponents such as The Humane Society have alleged UEP certification is misleading and allows a significant amount of unchecked animal cruelty. Other standards include "Cage Free", "Natural", "Certified Humane", and " Certified Organic". Of these standards, "Certified Humane", which carries requirements for stocking density and cage-free keeping and so on, and "Certified Organic", which requires hens to have outdoor access and to be fed only organic vegetarian feed and so on, are the most stringent. Effective 1 January 2012, the European Union banned conventional battery cages for egg-laying hens, as outlined in EU Directive 1999/74/EC. The EU permits the use of "enriched" furnished cages that must meet certain space and amenity requirements. Egg producers in many member states have objected to the new quality standards while in some countries, even furnished cages and family cages are subject to be banned as well. The production standard of the eggs is visible on a mandatory
egg marking Egg marking is a form of egg labelling that includes an ''egg code'' stamped on the egg itself. In the EU there is a producer code regulated by law since 2004. It allows consumers to distinguish free range eggs and organic farming eggs from the i ...
categorization where the EU egg code begins with 3 for caged chicken to 1 for free-range eggs and 0 for organic egg production.


Killing of male chicks

In battery cage and free-range egg production, unwanted male chicks are killed at birth during the process of securing a further generation of egg-laying hens. In Germany in June 2019, a court decided that culling chicks "violates the country's laws against killing animals without a justifiable reason". However, the court permitted an exemption for hatcheries until a viable alternative is possible.


Cultural significance

A popular Easter tradition in some parts of the world is a decoration of hard-boiled eggs (usually by dyeing, but often by spray-painting). A similar tradition of egg painting exists in areas of the world influenced by the culture of Persia. Before the spring equinox in the Persian New Year tradition (called ''
Norouz Nowruz ( fa, نوروز, ; ), zh, 诺鲁孜节, ug, نەۋروز, ka, ნოვრუზ, ku, Newroz, he, נורוז, kk, Наурыз, ky, Нооруз, mn, Наурыз, ur, نوروز, tg, Наврӯз, tr, Nevruz, tk, Nowruz, ...
''), each family member decorates a hard-boiled egg and they set them together in a bowl. In Northern Europe and North America, Easter eggs may be hidden by adults for children to find in an Easter egg hunt. They may be rolled in some traditions. In Eastern and Central Europe, and parts of England, Easter eggs may be tapped against each other to see whose egg breaks first. Since the sixteenth century, the tradition of a dancing egg is held during the feast of Corpus Christi in Barcelona and other
Catalan Catalan may refer to: Catalonia From, or related to Catalonia: * Catalan language, a Romance language * Catalans, an ethnic group formed by the people from, or with origins in, Northern or southern Catalonia Places * 13178 Catalan, asteroid #1 ...
cities. It consists of a hollow eggshell, positioned over the water jet from a fountain, which causes the eggshell to revolve without falling.


See also

*
Fish eggs Roe ( ) or hard roe is the fully ripe internal egg masses in the ovaries, or the released external egg masses, of fish and certain marine animals such as shrimp, scallop, sea urchins and squid. As a seafood, roe is used both as a cooking, coo ...
* Cock egg * Ham and eggs * List of foods


References


External links


British Egg Industry and the Lion Mark


* ttps://www.fda.gov/food/guidanceregulation/guidancedocumentsregulatoryinformation/eggs/default.htm Egg InformationU.S. Food and Drug Administration (2011)
Egg Basics for the Consumer: Packaging, Storage, and Nutritional Information
. (2007) University of California Agriculture and Natural Resources. Accessed 23 May 2014. {{DEFAULTSORT:Egg (Food) Egg (food) Types of food Articles containing video clips