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An egg can be described as spheroid or ovoid shaped cell laid by females of many different species, including birds, reptiles, amphibians and fish. Eggs have been eaten by mankind for millennia.
Bird eggs including chicken and turkey eggs consist of a protective eggshell, albumen (egg white) and vitellus (egg yolk), contained within various thin membranes. Every part of an egg is safe to eat, though the eggshell is generally discarded. Eggs are considered an excellent source of protein and choline. Because of this, the egg falls in the Meats category under the Food Guide Pyramid.
Bird eggs are a common food and one of the most adaptable ingredients utilised in cooking. They're important in lots of branches of the modern food industry. The most commonly used bird eggs are those from the chicken. Duck and goose eggs, and smaller eggs such as quail eggs are occasionally used as a gourmet ingredient, as are the biggest bird eggs, from ostriches. Gull eggs are considered a delicacy in England, as well as in some Scandinavian countries, particularly in Norway. In some African countries, guinea fowl eggs are frequently seen in marketplaces, especially in the spring of each year. Pheasant eggs and emu eggs are perfectly edible but less widely available. Sometimes they are obtainable from farmers, poulterers or luxury grocery stores. Most wild bird’s eggs are protected by laws in many countries, which prohibit collecting or selling them, or permit these only during specific periods of the year.
White is the common name for the clear liquid (also called the albumen or the glair/glaire) contained within an egg. In chickens it is formed from the layers of secretions of the anterior section of the hen's oviduct during the passage of the egg. It forms around either fertilized or unfertilized yolks. The primary natural purpose of egg white is to protect the yolk and provide additional nutrition for the growth of the embryo. Egg white consists primarily of about 90% water into which is dissolved 10% 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 1%. Egg white has many uses in food, and many others, including the preparation of vaccines such as those for influenza.
The yolk in a newly laid egg is round and firm. As the yolk ages, it absorbs water from the albumen, which increases its size and causes it to stretch and weaken the vitelline membrane (the clear casing enclosing the yolk). The resulting effect is a flattened and enlarged yolk shape. Yolk colour is dependent on the diet of the hen; if the diet contains yellow/orange plant pigments known as xanthophylls, then they are deposited in the yolk, colouring it. Lutein is the most abundant pigment in egg yolk. A colourless diet can produce an almost colourless yolk. Yolk colour is, for example, enhanced if the diet includes products such as yellow corn and marigold petals. In the US, the use of artificial colour additives is forbidden.
Quail eggs are considered a delicacy in many countries. They are used raw or cooked as tamago in sushi. In Colombia, quail eggs are considered less exotic than in other countries and an individual hard-boiled quail egg is a common topping on hot dogs and hamburgers. If a boiled egg is overcooked, a greenish ring sometimes appears around egg yolk. This is a manifestation of the iron and sulphur compounds in the egg. It can also occur when there's 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 couple of minutes in cold water until the egg is totally cooled prevents the greenish "ring" from forming on the surface of the yolk).