Egg (food)
Overview
Eggs are laid by females of many different species, including birds
Bird egg
Bird eggs are laid by females and incubated for a time that varies according to the species; a single young hatches from each egg. Average clutch sizes range from one to about 17...

, reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s, amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s, and fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, and have probably been eaten by mankind for millennia. Bird and reptile eggs consist of a protective eggshell
Eggshell
An eggshell is the outer covering of a hard-shelled egg and of some forms of eggs with soft outer coats.- Insect eggs :Insects and other arthropods lay a variety of styles and shapes of eggs. Some have gelatinous or skin-like coverings, others have hard eggshells. Softer shells are mostly protein....

, albumen (egg white
Egg white
Egg white is the common name for the clear liquid 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 around either fertilized or unfertilized egg yolks...

), and vitellus (egg yolk
Egg yolk
An egg yolk is a part of an egg which feeds the developing embryo. The egg yolk is suspended in the egg white by one or two spiral bands of tissue called the chalazae...

), contained within various thin membranes. Popular choices for egg consumption are chicken
Chicken
The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...

, duck
Duck
Duck is the common name for a large number of species in the Anatidae family of birds, which also includes swans and geese. The ducks are divided among several subfamilies in the Anatidae family; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered...

, roe
Roe
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 and sea urchins...

, and caviar
Caviar
Caviar, sometimes called black caviar, is a luxury delicacy, consisting of processed, salted, non-fertilized sturgeon roe. The roe can be "fresh" or pasteurized, the latter having much less culinary and economic value....

, but the egg most often consumed by humans is the chicken egg, by a wide margin.

Egg yolks and whole eggs store significant amounts of protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 and choline
Choline
Choline is a water-soluble essential nutrient. It is usually grouped within the B-complex vitamins. Choline generally refers to the various quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation....

, and are widely used in cookery.
Discussions
Encyclopedia
Eggs are laid by females of many different species, including birds
Bird egg
Bird eggs are laid by females and incubated for a time that varies according to the species; a single young hatches from each egg. Average clutch sizes range from one to about 17...

, reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s, amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s, and fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, and have probably been eaten by mankind for millennia. Bird and reptile eggs consist of a protective eggshell
Eggshell
An eggshell is the outer covering of a hard-shelled egg and of some forms of eggs with soft outer coats.- Insect eggs :Insects and other arthropods lay a variety of styles and shapes of eggs. Some have gelatinous or skin-like coverings, others have hard eggshells. Softer shells are mostly protein....

, albumen (egg white
Egg white
Egg white is the common name for the clear liquid 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 around either fertilized or unfertilized egg yolks...

), and vitellus (egg yolk
Egg yolk
An egg yolk is a part of an egg which feeds the developing embryo. The egg yolk is suspended in the egg white by one or two spiral bands of tissue called the chalazae...

), contained within various thin membranes. Popular choices for egg consumption are chicken
Chicken
The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...

, duck
Duck
Duck is the common name for a large number of species in the Anatidae family of birds, which also includes swans and geese. The ducks are divided among several subfamilies in the Anatidae family; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered...

, roe
Roe
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 and sea urchins...

, and caviar
Caviar
Caviar, sometimes called black caviar, is a luxury delicacy, consisting of processed, salted, non-fertilized sturgeon roe. The roe can be "fresh" or pasteurized, the latter having much less culinary and economic value....

, but the egg most often consumed by humans is the chicken egg, by a wide margin.

Egg yolks and whole eggs store significant amounts of protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 and choline
Choline
Choline is a water-soluble essential nutrient. It is usually grouped within the B-complex vitamins. Choline generally refers to the various quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation....

, and are widely used in cookery. Due to their protein content, the United States Department of Agriculture (USDA) categorizes eggs as Meats within the Food Guide Pyramid
Food guide pyramid
A food guide pyramid is a triangular or pyramid-shaped nutrition guide divided into sections to show the recommended intake for each food group. The first food pyramid was published in Sweden in 1974. The most widely known food pyramid was introduced by the United States Department of Agriculture...

. Despite the nutritional value of eggs, there are some potential health issues arising from egg quality, storage, and individual allergies.

Chickens and other egg-laying creatures are widely kept throughout the world, and mass production of chicken eggs is a global industry. There are issues of regional variation in demand and expectation, as well as current debates concerning methods of mass production, with the European Union
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...

 planning to ban battery farming
Battery cage
In poultry farming, battery cages are an industrial agricultural confinement system used primarily for egg-laying hens...

 of chickens from 2012.

History

Bird eggs have been valuable foodstuffs since prehistory
Prehistory
Prehistory is the span of time before recorded history. Prehistory can refer to the period of human existence before the availability of those written records with which recorded history begins. More broadly, it refers to all the time preceding human existence and the invention of writing...

, in both hunting societies and more recent cultures where birds were domesticated. The chicken was probably domesticated for its eggs from jungle fowl native to tropical and subtropical Southeast Asia and India 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
Thebes, Egypt
Thebes is the Greek name for a city in Ancient Egypt located about 800 km south of the Mediterranean, on the east bank of the river Nile within the modern city of Luxor. The Theban Necropolis is situated nearby on the west bank of the Nile.-History:...

, the tomb of Haremhab, built about 1420 BCE, shows a depiction of a man carrying bowls of ostrich eggs and other large eggs, presumably those of the pelican
Pelican
A pelican, derived from the Greek word πελεκυς pelekys is a large water bird with a large throat pouch, belonging to the bird family Pelecanidae....

, as offerings. In ancient Rome
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

, eggs were preserved using a number of methods, and meals often started with an egg course. The Romans
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

 crushed the shells in their plates to prevent evil spirits from hiding there. In the Middle Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

, eggs were forbidden during Lent
Lent
In the Christian tradition, Lent is the period of the liturgical year from Ash Wednesday to Easter. The traditional purpose of Lent is the preparation of the believer – through prayer, repentance, almsgiving and self-denial – for the annual commemoration during Holy Week of the Death and...

 because of their richness. The word mayonnaise
Mayonnaise
Mayonnaise, , often abbreviated as mayo, is a sauce. It is a stable emulsion of oil, egg yolk and either vinegar or lemon juice, with many options for embellishment with other herbs and spices. Lecithin in the egg yolk is the emulsifier. Mayonnaise varies in color but is often white, cream, or pale...

 possibly was derived from moyeu, the medieval French word for the yolk, meaning center or hub.

Egg scrambled with acidic fruit juices were popular in France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 in the 17th century; this may have been the origin of lemon curd
Lemon curd
Fruit curd is a dessert spread and topping usually made with lemon, lime, orange or raspberry. The basic ingredients are beaten egg yolks, sugar, fruit juice and zest which are gently cooked together until thick and then allowed to cool, forming a soft, smooth, intensely-flavored spread...

.

The dried egg industry developed in the 19th century, before the rise of the frozen egg industry. In 1878, a company in St. Louis, Missouri
St. Louis, Missouri
St. Louis is an independent city on the eastern border of Missouri, United States. With a population of 319,294, it was the 58th-largest U.S. city at the 2010 U.S. Census. The Greater St...

 started to transform egg yolk and white into a light-brown, meal-like substance by using a drying process. The production of dried eggs significantly expanded during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, for use by the United States Armed Forces and its allies.

In 1911, the egg carton
Egg carton
An egg carton is a carton designed for carrying and transporting whole eggs. These cartons have a dimpled form in which each dimple accommodates an individual egg and isolates that egg from eggs in adjacent dimples...

 was invented by Joseph Coyle in Smithers, British Columbia
Smithers, British Columbia
Smithers is a town located in northwestern British Columbia, Canada, approximately halfway between Prince George and Prince Rupert. Smithers is located in the Regional District of Bulkley-Nechako....

, to solve a dispute about broken eggs between a farmer in Bulkley Valley
Bulkley Valley
The Bulkley Valley is located in the northwest Central Interior of British Columbia, Canada.-Geography:The 257 km long Bulkley River runs through the valley which is bounded on the west by the Hudson Bay Mountain range and on the east by the Babine Mountains...

 and the owner of the Aldermere Hotel. Early egg cartons were made of paper.

Varieties

Bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

 eggs are a common food
Food
Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals...

 and one of the most versatile ingredients used in cooking. They are important in many branches of the modern food industry
Food industry
The food production is a complex, global collective of diverse businesses that together supply much of the food energy consumed by the world population...

. The most commonly used bird eggs are those from the chicken
Chicken
The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...

. Duck
Duck
Duck is the common name for a large number of species in the Anatidae family of birds, which also includes swans and geese. The ducks are divided among several subfamilies in the Anatidae family; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered...

 and goose
Goose
The word goose is the English name for a group of waterfowl, belonging to the family Anatidae. This family also includes swans, most of which are larger than true geese, and ducks, which are smaller....

 eggs, and smaller eggs, such as quail eggs, are occasionally used as a gourmet ingredient, as are the largest bird eggs, from ostrich
Ostrich
The Ostrich is one or two species of large flightless birds native to Africa, the only living member of the genus Struthio. Some analyses indicate that the Somali Ostrich may be better considered a full species apart from the Common Ostrich, but most taxonomists consider it to be a...

es. Gull
Gull
Gulls are birds in the family Laridae. They are most closely related to the terns and only distantly related to auks, skimmers, and more distantly to the waders...

 eggs are considered a delicacy in England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

, as well as in some Scandinavian countries, particularly in Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

. In some African countries, guineafowl
Guineafowl
The guineafowl are a family of birds in the Galliformes order, although some authorities include the guineafowl as a subfamily, Numidinae, of the family Phasianidae...

 eggs are commonly seen in marketplaces, especially in the spring of each year. Pheasant
Pheasant
Pheasants refer to some members of the Phasianinae subfamily of Phasianidae in the order Galliformes.Pheasants are characterised by strong sexual dimorphism, males being highly ornate with bright colours and adornments such as wattles and long tails. Males are usually larger than females and have...

 eggs and emu
Emu
The Emu Dromaius novaehollandiae) is the largest bird native to Australia and the only extant member of the genus Dromaius. It is the second-largest extant bird in the world by height, after its ratite relative, the ostrich. There are three subspecies of Emus in Australia...

 eggs are perfectly edible, but less widely available. Sometimes they are obtainable from farmers, poulterers, or luxury grocery stores. Most wild birds’ eggs are protected by laws in many countries, which prohibit collecting or selling them, or permit these only during specific periods of the year.

See also fish eggs.

Cooking issues

Egg white coagulates, or solidifies, when it reaches temperatures between 144 °F and 149 °F (62.2 °C-65 °C). Egg yolk coagulates at slightly higher temperatures, between 149 °F and 158 °F (65 °C-70 °C).

If a boiled egg is overcooked, a greenish ring sometimes appears around egg yolk due to the iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 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 Chilling the egg for a few minutes in cold water until it is completely cooled prevents the greenish "ring” from forming on the surface of the yolk.

Cooking also increases the risk of atherosclerosis
Atherosclerosis
Atherosclerosis is a condition in which an artery wall thickens as a result of the accumulation of fatty materials such as cholesterol...

 due to increased oxidization
Beta oxidation
Beta oxidation is the process by which fatty acids, in the form of Acyl-CoA molecules, are broken down in mitochondria and/or in peroxisomes to generate Acetyl-CoA, the entry molecule for the Citric Acid cycle....

 of the cholesterol
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...

 contained in the egg yolk
Egg yolk
An egg yolk is a part of an egg which feeds the developing embryo. The egg yolk is suspended in the egg white by one or two spiral bands of tissue called the chalazae...

.

Types of dishes

Chicken eggs are widely used in many types of dishes, both sweet and savory, including many baked goods
Baking
Baking is the technique of prolonged cooking of food by dry heat acting by convection, and not by radiation, normally in an oven, but also in hot ashes, or on hot stones. It is primarily used for the preparation of bread, cakes, pastries and pies, tarts, quiches, cookies and crackers. Such items...

. Some of the most common preparataion methods include scrambled, fried
Fried egg
A fried egg is a food made from an egg fried whole with minimal accompaniment. They are traditionally eaten for breakfast in western countries, but may be eaten at other times of the day.-China:...

, hard-boiled, soft-boiled, and pickled
Pickled egg
Pickled eggs are hard boiled eggs preserved by pickling.After the eggs are hard boiled the shell is removed and they are submerged in a solution of vinegar, salt, spices, and other seasonings. Recipes vary from the traditional brine solution used for pickles to other solutions which can impart a...

. They can also be eaten raw, though this is not recommended for people who may be especially susceptible to salmonellosis
Salmonellosis
Salmonellosis is an infection with Salmonella bacteria. Most people infected with Salmonella develop diarrhea, fever, vomiting, and abdominal cramps 12 to 72 hours after infection. In most cases, the illness lasts four to seven days, and most people recover without treatment...

, such as the elderly, the infirm, or pregnant women. In addition, the protein in raw eggs is only 51% bioavailable, whereas that of a cooked egg is nearer 91% bioavailable, meaning the protein of cooked eggs is nearly twice as absorbable as the protein from raw eggs. As an ingredient, egg yolks are an important emulsifier in the kitchen, and the proteins in egg white allow it to form foams and aerated dishes.

The albumen, or egg white, contains protein but little or no fat
Fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...

, and can be used in cooking separately from the yolk. Egg whites may be aerated or whipped to a light, fluffy consistency, and are often used in desserts such as meringue
Meringue
Meringue is a type of dessert made from whipped egg whites and sugar, occasionally some recipes may call for adding an acid such as cream of tartar or a small amount of vinegar and a binding agent such as cornstarch found in icing sugar which may be added in addition to the corn starch which...

s and mousse
Mousse
Mousse is derived from the French word mousse which means "lather" or "foam". A mousse is a prepared food that incorporates air bubbles to give it a light and airy texture...

. Ground egg shells are sometimes used as a food additive
Food additive
Food additives are substances added to food to preserve flavor or enhance its taste and appearance.Some additives have been used for centuries; for example, preserving food by pickling , salting, as with bacon, preserving sweets or using sulfur dioxide as in some wines...

 to deliver calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

. Every part of an egg is edible, although the eggshell is generally discarded.

Flavor variations

Although the age of the egg and the conditions of its storage have a greater influence, the bird's diet does affect the flavor of the egg. For example, when a brown-egg chicken breed eats rapeseed
Rapeseed
Rapeseed , also known as rape, oilseed rape, rapa, rappi, rapaseed is a bright yellow flowering member of the family Brassicaceae...

 or soy meals, its intestinal microbes metabolize them into fishy-smelling triethylamine
Triethylamine
Triethylamine is the chemical compound with the formula N3, commonly abbreviated Et3N. It is also abbreviated TEA, yet this abbreviation must be used carefully to avoid confusion with triethanolamine, for which TEA is also a common abbreviation....

, which ends up in the egg. The unpredictable diet of free-range hens will produce unpredictable eggs. Duck eggs tend to have a flavor distinct from, but still resembling, chicken eggs.

Preservation

Careful preservation of edible eggs is extremely important, as an improperly handled egg can contain elevated levels of Salmonella
Salmonella
Salmonella is a genus of rod-shaped, Gram-negative, non-spore-forming, predominantly motile enterobacteria with diameters around 0.7 to 1.5 µm, lengths from 2 to 5 µm, and flagella which grade in all directions . They are chemoorganotrophs, obtaining their energy from oxidation and reduction...

bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 that can cause severe food poisoning. The USDA recommends refrigerating eggs in order to to prevent the growth of Salmonella. Refrigeration also preserves the taste and texture. However uncracked eggs can be left unrefrigerated for several months without spoilinghttp://www.motherearthnews.com/Sustainable-Farming/1977-11-01/Fresh-Eggs.aspx.

The simplest method to preserve an egg is to treat it with salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

. Salt draws water out of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and mold
Mold
Molds are fungi that grow in the form of multicellular filaments called hyphae. Molds are not considered to be microbes but microscopic fungi that grow as single cells called yeasts...

s, which prevents their growth. The Chinese salted duck egg
Salted duck egg
A salted duck egg is a Chinese preserved food product made by soaking duck eggs in brine, or packing each egg in damp, salted charcoal. In Asian supermarkets, these eggs are sometimes sold covered in a thick layer of salted charcoal paste. The eggs may also be sold with the salted paste removed,...

 is made by immersing duck eggs in brine
Brine
Brine is water, saturated or nearly saturated with salt .Brine is used to preserve vegetables, fruit, fish, and meat, in a process known as brining . Brine is also commonly used to age Halloumi and Feta cheeses, or for pickling foodstuffs, as a means of preserving them...

, or coating them individually with a paste of salt and mud
Mud
Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...

 or clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

. The eggs stop absorbing salt after about a month, having reached osmotic equilibrium. Their yolks take on an orange-red color and solidify, but the white remains liquid. They are boiled before consumption, and are often served with rice congee
Congee
Congee is a type of rice porridge popular in many Asian countries. It can be eaten alone or served with a side dish. Names for congee are as varied as the style of its preparation...

.
Another method is to make pickled egg
Pickled egg
Pickled eggs are hard boiled eggs preserved by pickling.After the eggs are hard boiled the shell is removed and they are submerged in a solution of vinegar, salt, spices, and other seasonings. Recipes vary from the traditional brine solution used for pickles to other solutions which can impart a...

s, by boiling them first and immersing them in a mixture of vinegar
Vinegar
Vinegar is a liquid substance consisting mainly of acetic acid and water, the acetic acid being produced through the fermentation of ethanol by acetic acid bacteria. Commercial vinegar is produced either by fast or slow fermentation processes. Slow methods generally are used with traditional...

, salt, and spices, such as ginger
Ginger
Ginger is the rhizome of the plant Zingiber officinale, consumed as a delicacy, medicine, or spice. It lends its name to its genus and family . Other notable members of this plant family are turmeric, cardamom, and galangal....

 or allspice
Allspice
Allspice, also called Jamaica pepper, pepper, myrtle pepper, pimenta, or newspice, is a spice that is the dried unripe fruit of Pimenta dioica , a mid-canopy tree native to the Greater Antilles, southern Mexico, and Central America, now cultivated in many warm parts of the world...

. Frequently, beetroot
Beetroot
The beetroot, also known as the table beet, garden beet, red beet or informally simply as beet, is one of the many cultivated varieties of beets and arguably the most commonly encountered variety in North America, Central America and Britain.-Consumption:The usually deep-red roots of beetroot are...

 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 can 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
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 and penetrate the egg, making it acidic enough to inhibit the growth of bacteria and molds. Pickled eggs made this way will generally keep for a year or more without refrigeration.
A century egg
Century egg
Century egg or pidan , also known as preserved egg, hundred-year egg, thousand-year egg, thousand-year-old egg, and millennium egg, is a Chinese cuisine ingredient made by preserving duck, chicken or quail eggs in a mixture of clay, ash, salt, lime, and rice hulls for several weeks to several...

 or hundred-year-old egg is preserved by coating an egg in a mixture of clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

, wood ash
Wood ash
Wood ash is the residue powder left after the combustion of wood. Main producers of wood ash are wood industries and power plants.-Composition:...

, salt, lime
Calcium oxide
Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....

, and rice
Rice
Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...

 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
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 of the egg from around 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 (food)
Fermentation in food processing typically is the conversion of carbohydrates to alcohols and carbon dioxide or organic acids using yeasts, bacteria, or a combination thereof, under anaerobic conditions. Fermentation in simple terms is the chemical conversion of sugars into ethanol...

.

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 flour. Tofu
Tofu
is a food made by coagulating soy milk and then pressing the resulting curds into soft white blocks. It is part of East Asian and Southeast Asian cuisine such as Chinese, Japanese, Korean, Indonesian, Vietnamese, and others. There are many different varieties of tofu, including fresh tofu and tofu...

 can also act as a partial binding agent, since it is high in lecithin due to its soy content. Applesauce
Applesauce
Apple sauce or applesauce is a purée made of apples. It can be made with peeled or unpeeled apples and a variety of spices . Fruit flavorings or sweeteners such as sugar, high-fructose corn syrup, or sucralose are also commonly added...

 can be used, as well as arrowroot
Arrowroot
Arrowroot, or obedience plant , Bermuda arrowroot, araru, ararao, is a large perennial herb found in rainforest habitats...

 and banana
Banana
Banana is the common name for herbaceous plants of the genus Musa and for the fruit they produce. Bananas come in a variety of sizes and colors when ripe, including yellow, purple, and red....

. Extracted soybean lecithin
Lecithin
Lecithin is a generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues, and in egg yolk, composed of phosphoric acid, choline, fatty acids, glycerol, glycolipids, triglycerides, and phospholipids .The word lecithin was originally coined in 1847 by...

, in turn, is often used in packaged foods as an inexpensive substitute for egg-derived lecithin.

Other egg substitutes are made from just the white of the egg for those who worry about the high cholesterol and fat content in eggs. These products usually have added vitamins and minerals, as well as vegetable-based emulsifiers and thickeners such as xanthan gum
Xanthan gum
Xanthan gum is a polysaccharide, derived from the bacterial coat of Xanthomonas campestris, used as a food additive and rheology modifier, commonly used as a food thickening agent and a stabilizer...

 or guar gum
Guar gum
Guar gum, also called guaran, is a galactomannan. It is primarily the ground endosperm of guar beans. The guar seeds are dehusked, milled and screened to obtain the guar gum. It is typically produced as a free-flowing, pale, off-white-colored, coarse to fine ground powder.-Production:Guar gum is an...

. These allow the product to maintain the nutrition and several culinary properties of real eggs, making possible foods such as Hollandaise sauce
Hollandaise sauce
Hollandaise sauce is an emulsion of egg yolk and butter, usually seasoned with lemon juice. In appearance it is light yellow and opaque, smooth and creamy. The flavor is rich and buttery, with a mild tang added by the lemon juice, yet not so strong as to overpower mildly-flavored foods.Hollandaise...

, custard
Custard
Custard is a variety of culinary preparations based on a cooked mixture of milk or cream and egg yolk. Depending on how much egg or thickener is used, custard may vary in consistency from a thin pouring sauce , to a thick pastry cream used to fill éclairs. The most common custards are used as...

, mayonnaise
Mayonnaise
Mayonnaise, , often abbreviated as mayo, is a sauce. It is a stable emulsion of oil, egg yolk and either vinegar or lemon juice, with many options for embellishment with other herbs and spices. Lecithin in the egg yolk is the emulsifier. Mayonnaise varies in color but is often white, cream, or pale...

, and most baked goods with these substitutes.

Nutritional value

Eggs add protein to a person's diet, as well as various other nutrients.

Chicken
Chicken
The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...

 eggs are the most commonly eaten eggs. They supply all essential amino acid
Essential amino acid
An essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized de novo by the organism , and therefore must be supplied in the diet.-Essentiality vs. conditional essentiality in humans:...

s for humans, and provide several vitamins and minerals, including retinol
Retinol
Retinol is one of the animal forms of vitamin A. It is a diterpenoid and an alcohol. It is convertible to other forms of vitamin A, and the retinyl ester derivative of the alcohol serves as the storage form of the vitamin in animals....

 (vitamin A), riboflavin
Riboflavin
Riboflavin, also known as vitamin B2 or additive E101, is an easily absorbed micronutrient with a key role in maintaining health in humans and animals. It is the central component of the cofactors FAD and FMN, and is therefore required by all flavoproteins. As such, vitamin B2 is required for a...

 (vitamin B2), folic acid
Folic acid
Folic acid and folate , as well as pteroyl-L-glutamic acid, pteroyl-L-glutamate, and pteroylmonoglutamic acid are forms of the water-soluble vitamin B9...

 (vitamin B9), vitamin B6
Vitamin B6
Vitamin B6 is a water-soluble vitamin and is part of the vitamin B complex group. Several forms of the vitamin are known, but pyridoxal phosphate is the active form and is a cofactor in many reactions of amino acid metabolism, including transamination, deamination, and decarboxylation...

, vitamin B12
Vitamin B12
Vitamin B12, vitamin B12 or vitamin B-12, also called cobalamin, is a water-soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood. It is one of the eight B vitamins...

, choline
Choline
Choline is a water-soluble essential nutrient. It is usually grouped within the B-complex vitamins. Choline generally refers to the various quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation....

, iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

, phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

 and potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

. They are also a single-food source of protein.

All of the egg's vitamin A, D
Vitamin D
Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....

, and E
Vitamin E
Vitamin E is used to refer to a group of fat-soluble compounds that include both tocopherols and tocotrienols. There are many different forms of vitamin E, of which γ-tocopherol is the most common in the North American diet. γ-Tocopherol can be found in corn oil, soybean oil, margarine and dressings...

 are in the egg yolk. The egg is one of the few foods to naturally contain vitamin D. A large egg yolk contains approximately 60 Calorie
Calorie
The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule...

s (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
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...

 (although one study indicates the human body may 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
Fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...

, slightly less than half of the protein, and most of the other nutrients. It also contains all of the choline, and one yolk contains approximately half of the recommended daily intake. Choline
Choline
Choline is a water-soluble essential nutrient. It is usually grouped within the B-complex vitamins. Choline generally refers to the various quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation....

 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
Neural development
Neural development comprises the processes that generate, shape, and reshape the nervous system, from the earliest stages of embryogenesis to the final years of life. The study of neural development aims to describe the cellular basis of brain development and to address the underlying mechanisms...

.

The diet of the laying hens can greatly affect the nutritional quality of the eggs. For instance, chicken eggs that are especially high in omega 3 fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...

s are produced by feeding laying hens a diet containing polyunsaturated fats and kelp
Kelp
Kelps are large seaweeds belonging to the brown algae in the order Laminariales. There are about 30 different genera....

 meal. Pasture-raised free-range hens
Pastured poultry
Pastured poultry is a sustainable agriculture technique that calls for the raising of laying chickens, meat chickens , and/or turkeys on pasture, as opposed to indoor confinement...

 which forage largely for their own food also tend to produce eggs with higher nutritional quality in having less cholesterol and fats while being several times higher in vitamins and omega 3 fatty acids than standard factory eggs Focusing on the protein and crude fat content, a 2010 USDA study determined there were no significant differences of these two macronutrients in consumer chicken eggs.

Cooked eggs are easier to digest, as well as having a lower risk of salmonellosis
Salmonellosis
Salmonellosis is an infection with Salmonella bacteria. Most people infected with Salmonella develop diarrhea, fever, vomiting, and abdominal cramps 12 to 72 hours after infection. In most cases, the illness lasts four to seven days, and most people recover without treatment...

.

Anatomy and characteristics

The shape of an egg resembles a prolate spheroid
Prolate spheroid
A prolate spheroid is a spheroid in which the polar axis is greater than the equatorial diameter. Prolate spheroids stand in contrast to oblate spheroids...

 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 chalaza
Chalaza
The chalaza is a structure inside bird and reptile eggs and plant ovules. It attaches or suspends the yolk or nucellus within the larger structure.-In animals:...

e (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 down and contract after it is laid. Chicken eggs are graded according to the size of this air cell, measured during candling
Candling
Candling is a method used in embryology to study the growth and development of an embryo inside an egg. The method uses a bright light source behind the egg to show details through the shell, and is so called because the original sources of light used were candles.The technique of using light to...

. 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, 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 actually float in the water and should not be eaten.

Shell

Egg shell color is caused by pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

 deposition during egg formation in the oviduct
Oviduct
In non-mammalian vertebrates, the passageway from the ovaries to the outside of the body is known as the oviduct. The eggs travel along the oviduct. These eggs will either be fertilized by sperm to become a zygote, or will degenerate in the body...

 and can vary according to species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 and breed
Breed
A breed is a group of domestic animals or plants with a homogeneous appearance, behavior, and other characteristics that distinguish it from other animals or plants of the same species. Despite the centrality of the idea of "breeds" to animal husbandry, there is no scientifically accepted...

, from the more common white or brown to pink or speckled blue-green. In general, 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, there is often a cultural preference for one color over another (see 'Production issues', below).

Yolk

The yolk
Egg yolk
An egg yolk is a part of an egg which feeds the developing embryo. The egg yolk is suspended in the egg white by one or two spiral bands of tissue called the chalazae...

 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
Vitelline membrane
The vitelline membrane is a structure directly adjacent to the outer surface of the plasma membrane of an ovum. It is composed mostly of protein fibers, with protein receptors needed for sperm binding which, in turn, are bound to sperm plasma membrane receptors...

 (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/orange plant pigments known as xanthophyll
Xanthophyll
Xanthophylls are yellow pigments that form one of two major divisions of the carotenoid group. The name is from Greek xanthos + phyllon , due to their formation of the yellow band seen in early chromatography of leaf pigments...

s, then they are deposited in the yolk, coloring it. Lutein
Lutein
Lutein is a xanthophyll and one of 600 known naturally occurring carotenoids. Lutein is synthesized only by plants and like other xanthophylls is found in high quantities in green leafy vegetables such as spinach and kale...

 is the most abundant pigment in egg yolk. A colorless diet can produce an almost colorless yolk. Yolk color is, for example, enhanced if the diet includes products such as yellow corn
Corn
Corn is the name used in the United States, Canada, and Australia for the grain maize.In much of the English-speaking world, the term "corn" is a generic term for cereal crops, such as* Barley* Oats* Wheat* Rye- Places :...

 and marigold
Tagetes
Tagetes is a genus of 56 species of annual and perennial mostly herbaceous plants in the sunflower family . The genus is native to North and South America, but some species have become naturalized around the world. One species, T...

 petals. In the US, the use of artificial color additives is forbidden.

Cholesterol and fat

More than half the calories found in eggs come from the fat in the yolk; a large (50 gram) chicken egg contains approximately 5 grams of fat. People on a low-cholesterol diet may need to reduce egg consumption; however, only 27% of the fat in egg is saturated fat
Saturated fat
Saturated fat is fat that consists of triglycerides containing only saturated fatty acids. Saturated fatty acids have no double bonds between the individual carbon atoms of the fatty acid chain. That is, the chain of carbon atoms is fully "saturated" with hydrogen atoms...

 (palmitic
Palmitic acid
Palmitic acid, or hexadecanoic acid in IUPAC nomenclature, is one of the most common saturated fatty acids found in animals and plants. Its molecular formula is CH314CO2H. As its name indicates, it is a major component of the oil from palm trees . Palmitate is a term for the salts and esters of...

, stearic
Stearic acid
Stearic acid is the saturated fatty acid with an 18 carbon chain and has the IUPAC name octadecanoic acid. It is a waxy solid, and its chemical formula is CH316CO2H. Its name comes from the Greek word στέαρ "stéatos", which means tallow. The salts and esters of stearic acid are called stearates...

 and myristic acid
Myristic acid
Myristic acid, also called tetradecanoic acid, is a common saturated fatty acid with the molecular formula CH312COOH. A myristate is a salt or ester of myristic acid....

s) that contains LDL cholesterol. The egg white
Egg white
Egg white is the common name for the clear liquid 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 around either fertilized or unfertilized egg yolks...

 consists primarily of water (87%) and protein (13%) 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
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...

 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
Heart disease
Heart disease, cardiac disease or cardiopathy is an umbrella term for a variety of diseases affecting the heart. , it is the leading cause of death in the United States, England, Canada and Wales, accounting for 25.4% of the total deaths in the United States.-Types:-Coronary heart disease:Coronary...

 risk in healthy individuals. Harold McGee
Harold McGee
Harold McGee is an American author who writes about the chemistry, technique and history of food and cooking and has written two seminal books on kitchen science. His first book, On Food and Cooking: The Science and Lore of the Kitchen was initially published in 1984. A greatly revised second...

 argues that the cholesterol in the yolk is not what causes a problem, because fat (in particular, saturated) is much more likely to raise cholesterol levels than the actual consumption of cholesterol. A 2007 study of nearly 10,000 adults demonstrated no correlation between moderate (six per week) egg consumption and cardiovascular disease
Cardiovascular disease
Heart disease or cardiovascular disease are the class of diseases that involve the heart or blood vessels . While the term technically refers to any disease that affects the cardiovascular system , it is usually used to refer to those related to atherosclerosis...

 or stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

s, except in the subpopulation of diabetic patients who presented an increased risk of coronary heart disease
Coronary heart disease
Coronary artery disease is the end result of the accumulation of atheromatous plaques within the walls of the coronary arteries that supply the myocardium with oxygen and nutrients. It is sometimes also called coronary heart disease...

. Other research supports the idea that a high egg intake increases cardiovascular risk in diabetic patients. A 2009 prospective cohort study of over 21,000 individuals suggests that "egg consumption up to 6/week has no major effect on the risk of CVD
Cardiovascular disease
Heart disease or cardiovascular disease are the class of diseases that involve the heart or blood vessels . While the term technically refers to any disease that affects the cardiovascular system , it is usually used to refer to those related to atherosclerosis...

 and mortality and that consumption of 7+/week is associated with a modest increased risk of total mortality" in males, whereas among males with diabetes, "any egg consumption is associated with an increased risk of all-cause mortality and there was suggestive evidence for an increased risk of MI
Myocardial infarction
Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...

 and stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

".

Type 2 diabetes

Consumption of eggs has been linked to an increased risk of type 2 diabetes in both men and women. A 2008 study using data on over 50,000 individuals collected by the Physicians' Health Study I (1982–2007) and the Women's Health Study (1992–2007) determined the “data suggest that high levels of egg consumption (daily) are associated with an increased risk of type 2 diabetes.” However, a study published in 2010 found no link between egg consumption and type 2 diabetes.

Contamination

A health issue associated with eggs is contamination by pathogenic bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, such as Salmonella enteritidis. Contamination of eggs exiting a female bird via the cloaca
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...

 may also occur with other members of the Salmonella genus, so care must be taken to prevent the egg shell from becoming contaminated with fecal matter. In commercial practice, 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 eggs, use them within two weeks, cook them thoroughly, and never consume raw eggs. As with meat
Meat
Meat is animal flesh that is used as food. Most often, this means the skeletal muscle and associated fat and other tissues, but it may also describe other edible tissues such as organs and offal...

, 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 as prevalent 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. However, this has not been the case in other countries, where Salmonella enteritidis and Salmonella typhimurium infections due to egg consumptions are major concerns.
Egg shells act as hermetic seal
Hermetic seal
A hermetic seal is the quality of being airtight. In common usage, the term often implies being impervious to air or gas. When used technically, it is stated in conjunction with a specific test method and conditions of usage.-Etymology :...

s 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
British Egg Industry Council
The British Egg Industry Council is an organisation set up in 1986 to represent the British egg industry. It currently has 11 member organisations including the British Egg Products Association, the National Farmers Union and the British Free Range Egg Producers' Association...

 award the lions stamp to eggs that, among other things, come from hens that have been vaccinated against Salmonella
Salmonella
Salmonella is a genus of rod-shaped, Gram-negative, non-spore-forming, predominantly motile enterobacteria with diameters around 0.7 to 1.5 µm, lengths from 2 to 5 µm, and flagella which grade in all directions . They are chemoorganotrophs, obtaining their energy from oxidation and reduction...

.

Food allergy

One of the most common food allergies
Food allergy
A food allergy is an adverse immune response to a food protein. They are distinct from other adverse responses to food, such as food intolerance, pharmacological reactions, and toxin-mediated reactions....

 in infants is eggs. Infants usually have the opportunity to grow out of this allergy during childhood, if exposure is minimized. Allergic reaction
Allergy
An Allergy is a hypersensitivity disorder of the immune system. Allergic reactions occur when a person's immune system reacts to normally harmless substances in the environment. A substance that causes a reaction is called an allergen. These reactions are acquired, predictable, and rapid...

s against egg white are more common than reactions against egg yolks.

In addition to true allergic reactions, some people experience a food intolerance
Food intolerance
Food intolerance or non-allergic food hypersensitivity is a term used widely for varied physiological responses associated with a particular food, or compound found in a range of foods....

 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.

Antibiotic resistance

Information obtained by the Canadian Integrated Program for Antimicrobial Resistance (CIPARS) “strongly indicates that cephalosporin resistance in humans is moving in lockstep with use of the drug in poultry production.” According to the Canadian Medical Association Journal, the unapproved antibiotic ceftiofur
Ceftiofur
Ceftiofur is an antibiotic of the cephalosporin type , licensed for use in veterinary medicine. It was first described in 1987. It is marketed by pharmaceutical company Pfizer as Excede....

 is routinely injected into eggs in Quebec and Ontario to discourage infection of hatchlings. Although the data are contested by the industry, antibiotic resistance in humans appears to be directly related to the antibiotic's use in eggs.

Farming issues

Most commercially farmed chicken eggs intended for human consumption are unfertilized, since the laying hens are kept without rooster
Rooster
A rooster, also known as a cockerel, cock or chanticleer, is a male chicken with the female being called a hen. Immature male chickens of less than a year's age are called cockerels...

s. Fertile eggs can be eaten, with little nutritional difference to the unfertilized. Fertile eggs will not contain a developed embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

, as refrigeration temperatures inhibit cellular growth for an extended time. Sometimes an embryo is allowed to develop but eaten before hatching as with balut.

Grading by quality and size

The US Department of Agriculture grades eggs by the interior quality of the egg (see Haugh unit
Haugh unit
The Haugh unit is a measure of egg protein quality based on the height of its egg white . 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 and strength....

) 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; yolks that are high, round, and practically free from defects; and 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 they are usually used to make liquid, frozen, and dried egg products, as well as other egg-containing products.


In Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 and the European Union
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...

, eggs are graded by the hen farming method, free range
Free range
thumb|250px|Free-range chickens being fed outdoors.Free range is a term which outside of the United States denotes a method of farming husbandry where the animals are allowed to roam freely instead of being contained in any manner. In the United States, USDA regulations apply only to poultry and...

, battery cage
Battery cage
In poultry farming, battery cages are an industrial agricultural confinement system used primarily for egg-laying hens...

ed, etc.

Chicken eggs are also graded by size
Chicken egg sizes
Chicken eggs are graded by size, for the purpose of sales. The United States Department of Agriculture sizing is based by weight per dozen. The most common US size of chicken egg is 'Large' and is the egg size commonly referred to for recipes...

 for the purpose of sales.

Color of eggshell

Although egg 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
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, chicken eggs are generally white. In some parts of the northeast
Northeastern United States
The Northeastern United States is a region of the United States as defined by the United States Census Bureau.-Composition:The region comprises nine states: the New England states of Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island and Vermont; and the Mid-Atlantic states of New...

 of that country, particularly New England
New England
New England is a region in the northeastern corner of the United States consisting of the six states of Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut...

, where a television jingle for years proclaimed "brown eggs are local eggs, and local eggs are fresh!", brown eggs are more common. Local chicken breeds, including the Rhode Island Red
Rhode Island Red
The Rhode Island Red is a breed of chicken . They are a utility bird, raised for meat and eggs, and also as show birds. They are a popular choice for backyard flocks because of their egg laying abilities and hardiness. Non-industrial strains of the Rhode Island Red are listed as recovering by the...

, lay brown eggs. Brown eggs are also preferred in countries as diverse as Costa Rica
Costa Rica
Costa Rica , officially the Republic of Costa Rica is a multilingual, multiethnic and multicultural country in Central America, bordered by Nicaragua to the north, Panama to the southeast, the Pacific Ocean to the west and the Caribbean Sea to the east....

, Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...

, and the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

. In Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

 and Poland
Poland
Poland , officially the Republic of Poland , is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north...

, white chicken eggs are generally regarded as industrial, and brown or reddish ones are preferred. Small farms and smallholdings, particularly in economically advanced nations, may sell eggs of widely varying colors and sizes, with combinations of white, brown, speckled (red), green, and blue 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 British New Scientist
New Scientist
New Scientist is a weekly non-peer-reviewed English-language international science magazine, which since 1996 has also run a website, covering recent developments in science and technology for a general audience. Founded in 1956, it is published by Reed Business Information Ltd, a subsidiary of...

 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 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
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...

 it is very hard to source brown eggs, as demand is almost entirely for white ones.

Research conducted in France in the 1970s demonstrated blue chicken eggs (as laid by certain breeds, including araucanas, heritage skyline, and cream legbar) can be stronger and more resilient to breakage, yet an article in New Scientist magazine (contemporary with that research) stated there was little to no demand for blue-colored eggs from housewives, despite the clear advantages.

Research at Nihon University
Nihon University
Nihon University is the largest university in Japan. Akiyoshi Yamada, the minister of justice, founded Nihon Law School in October 1889....

, Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

 in 1990 revealed a number of different issues were important to Japanese housewives when deciding which eggs to buy; however, 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 chicken used for production, as egg color varies between breeds. Among producers and breeders, brown eggs are often referred to as 'tinted', while the speckled eggs preferred by some consumers are often referred to as being 'red' in color.

Living conditions of birds

Commercial factory farming
Factory farming
Factory farming is a term referring to the process of raising livestock in confinement at high stocking density, where a farm operates as a factory — a practice typical in industrial farming by agribusinesses. The main products of this industry are meat, milk and eggs for human consumption...

 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 can lead to pacing and escape behavior
Escape response
Escape response, escape reaction, or escape behaviour is a possible reaction in response to stimuli indicative of danger, in particular, it initiates an escape motion of an animal...

.

Many hens confined to battery cages, and some raised in cage-free conditions, are debeaked
Debeaking
Debeaking, also called beak-trimming is the partial removal of the beak of poultry, especially layer hens and turkeys although it may also be performed on quail and ducks. Most commonly, the beak is shortened permanently, although regrowth can occur. The trimmed lower beak is somewhat longer than...

 to prevent harming each other and cannibalism
Cannibalism
Cannibalism is the act or practice of humans eating the flesh of other human beings. It is also called anthropophagy...

. 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 force molted
Forced molting
Induced molting is the practice by the commercial egg industry of artificially provoking a complete flock of hens to molt simultaneously. During the molting period, the hens go out of production for a period of at least two weeks. This has the effect of allowing the hen's reproductive tracts to...

 to increase egg quality and production level after the molting. Molting can be induced by extended feed withdrawal, water withdrawal or controlled lighting programs.

Laying hens are often slaughtered between 100 and 130 weeks of age, when their egg productivity starts to decline. Due to modern selective breeding
Selective breeding
Selective breeding is the process of breeding plants and animals for particular genetic traits. Typically, strains that are selectively bred are domesticated, and the breeding is sometimes done by a professional breeder. Bred animals are known as breeds, while bred plants are known as varieties,...

, laying hen strain
Strain (biology)
In biology, a strain is a low-level taxonomic rank used in three related ways.-Microbiology and virology:A strain is a genetic variant or subtype of a micro-organism . For example, a "flu strain" is a certain biological form of the influenza or "flu" virus...

s 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 culled en mass after hatching.

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 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
Animal welfare is the physical and psychological well-being of animals.The term animal welfare can also mean human concern for animal welfare or a position in a debate on animal ethics and animal rights...

 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
United Egg Producers
United Egg Producers is an agricultural cooperative in the Midwestern United States which represents the interests of regional egg producers.-History:The group was formed in 1968 as an umbrella organization encompassing five regional egg cooperatives...

 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 be fed only organic vegetarian feed and so on, are the most stringent.

The European Union
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...

 will shortly introduce a ban of conventional battery cages for egg-laying hens. This ban is expected to come into effect from 1 January 2012, as outlined in EU Directive 1999/74/EC. The EU will permit the use of "enriched" cages that must meet certain space and amenity requirements. Egg producers in many member states have objected to the new quality standards.

Cultural significance

A popular Easter
Easter
Easter is the central feast in the Christian liturgical year. According to the Canonical gospels, Jesus rose from the dead on the third day after his crucifixion. His resurrection is celebrated on Easter Day or Easter Sunday...

 tradition in some parts of the world is the decoration of hard-boiled eggs (usually by dyeing, but often by spray-painting). Adults often hide the eggs for children to find, an activity known as an Easter egg hunt. A similar tradition of egg painting
Egg decorating
Egg decorating is the art or craft of decorating eggs. It is quite a popular art/craft form because of the attractive, smooth, oval shape of the egg...

 exists in areas of the world influenced by the culture of Persia. Before the spring equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

 in the Persian New Year tradition (called Norouz
Norouz
Nowrūz is the name of the Iranian New Year in Iranian calendars and the corresponding traditional celebrations. Nowruz is also widely referred to as the Persian New Year....

), each family member decorates a hard-boiled egg and sets them together in a bowl.

The tradition of a dancing egg
Dancing egg
The dancing egg is an old tradition that takes place in several towns in Catalonia during the feast of Corpus Christi. The tradition probably started at the Cathedral of Barcelona....

 is held during the feast of Corpus Christi
Corpus Christi (feast)
Corpus Christi is a Latin Rite solemnity, now designated the solemnity of The Most Holy Body and Blood of Christ . It is also celebrated in some Anglican, Lutheran and Old Catholic Churches. Like Trinity Sunday and the Solemnity of Christ the King, it does not commemorate a particular event in...

 in Barcelona and other Catalan
Catalonia
Catalonia is an autonomous community in northeastern Spain, with the official status of a "nationality" of Spain. Catalonia comprises four provinces: Barcelona, Girona, Lleida, and Tarragona. Its capital and largest city is Barcelona. Catalonia covers an area of 32,114 km² and has an...

 cities since the 16th century. It consists of an emptied egg, positioned over the water jet from a fountain, which starts turning without falling.

Although a food item, eggs are sometimes thrown at houses, cars, or people. This act, known commonly as "egging
Egging
Egging refers to the act of throwing eggs at houses, cars, or people. Egging is a criminal offence in most areas. The eggs are usually raw, but are sometimes hard-boiled or expired. This can be considered vandalism....

" in the various English-speaking countries, is a minor form of vandalism
Vandalism
Vandalism is the behaviour attributed originally to the Vandals, by the Romans, in respect of culture: ruthless destruction or spoiling of anything beautiful or venerable...

 and, therefore, usually a criminal
Crime
Crime is the breach of rules or laws for which some governing authority can ultimately prescribe a conviction...

 offense and is capable of damaging property (egg whites can degrade certain types of vehicle paint) as well as causing serious eye injury. On Halloween
Halloween
Hallowe'en , also known as Halloween or All Hallows' Eve, is a yearly holiday observed around the world on October 31, the night before All Saints' Day...

, for example, trick or treat
Trick or Treat
Trick or Treat is a 1952 American animated short film produced by Walt Disney Productions and released by RKO Radio Pictures. The cartoon, which takes place on Halloween night, follows a series of pranks between Donald Duck and his nephews Huey, Dewey, and Louie who are aided by Witch Hazel...

ers have been known to throw eggs (and sometimes flour) at property or people from whom they received nothing. Eggs are also often thrown in protests, as they are inexpensive and nonlethal, yet very messy when broken.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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