Eggs

Humans and human ancestors have scavenged and eaten animal eggs 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 ratites, are eaten regularly but much less commonly than those of chickens. People may also eat the eggs of reptiles, amphibians, and fish. Fish eggs consumed as food are known as roe or caviar.

Bird and reptile eggs consist of a protective eggshell, albumen ( egg white), and vitellus ( egg yolk), 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). Despite the nutritional value of eggs, there are some potential health issues arising from cholesterol content, salmonella contamination, and allergy to egg proteins.

Chickens and other egg-laying creatures are kept widely throughout the world and mass production of chicken eggs is a global industry. In 2009, an estimated 62.1 million metric tons of eggs were produced worldwide from a total laying flock of approximately 6.4 billion hens. 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, 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 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 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 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 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 and one of the most versatile ingredients used in cooking. They are important in many branches of the modern food industry.

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

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 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, 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 is a clear film lining the eggshell, visible when one peels a boiled egg. Primarily, it composed of fibrous proteins such as collagen type I. 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.

Egg white consists primarily of approximately 90 percent water into which is dissolved 10 percent proteins (including albumins, mucoproteins, 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