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An intact yolk surrounded by egg white

An egg yolk is a part of an egg which feeds the developing embryo. The egg yolk is suspended in the egg white (known alternatively as albumen or glair/glaire) by one or two spiral bands of tissue called the chalazae. Prior to fertilization, the yolk together with the germinal disc is a single cell, one of the few single cells that can be seen by the naked eye.

As a food, yolks are a major source of vitamins and minerals. They contain all of the egg's fat and cholesterol, and about one-fifth of the protein. If left intact while cooking fried eggs, the yellow yolk surrounded by a flat blob of whites creates the distinctive sunny-side up form of the food. Mixing the two components together before frying results in the pale yellow form found in omelettes and scrambled eggs.

Uses[link]

• It is sometimes separated from the egg whites and used in cooking (for mayonnaise, custard, hollandaise sauce, crème brûlée, avgolemono, and ovos moles).
• It is used in painting as a component of traditional egg-tempera.
• It is used in the production of egg-yolk agar plate medium, useful in testing for the presence of Clostridium perfringens.
• Egg yolks also contains an antibody called antiglobulin (IgY). The antibody transfers from the laying hen to the egg yolk by passive immunity to protect both embryo and hatchling from microorganism invasion.
• Egg yolk can be used to make liqueurs such as Advocaat or eggnog.
• Egg yolks are used to extract Egg Oil which has various cosmetic, nutitional and medicinal uses.
• The developing embryo inside the egg uses the yolk as sustenance.

Composition of chicken egg yolk[link]

The yolk makes up about 33% of the liquid weight of the egg; it contains approximately 60 calories, three times the caloric content of the egg white.

The yolk of one large egg (50 g total, 17 g yolk) contains approximately: 2.7 g protein, 210 mg cholesterol, 0.61 g carbohydrates, and 4.51 g total fat. (USDA National Nutrient Database)

All of the fat-soluble vitamins (A, D, E, and K) are found in the egg yolk. Egg yolk is one of the few foods naturally containing vitamin D.

The composition (by weight) of the most prevalent fatty acids in egg yolk is typically as follows:^[1]

• Unsaturated fatty acids:
  • Oleic acid, 47%
  • Linoleic acid, 16%
  • Palmitoleic acid, 5%
  • Linolenic acid, 2%
• Saturated fatty acids:
  • Palmitic acid, 23%
  • Stearic acid, 4%
  • Myristic acid, 1%

Egg yolk is a source of lecithin as well as Egg Oil for cosmetic and pharmaeutical applications.

The yellow color is due to lutein and zeaxanthin, which are yellow or orange carotenoids known as xanthophylls.

Yolk proteins[link]

The different yolk proteins have distinct roles. Phosvitins are important in sequestering calcium, iron and other cations for the developing embryo. Phosvitins are one of the most phosphorylated (10%) proteins in nature, the high concentration of phosphate groups providing efficient metal-binding sites in clusters.^[2][3] Lipovitellins are involved in lipid and metal storage, and contain a heterogeneous mixture of about 16% (w/w) noncovalently bound lipid, most being phospholipid. Lipovitellin-1 contains two chains, LV1N and LV1C.^[4][5]

Double-yolk eggs[link]

Double-yolk eggs occur when ovulation occurs too rapidly, or when one yolk becomes joined with another yolk. These eggs may be the result of a young hen's reproductive cycle not yet being synchronized.^[6] Some hybrid breeds of hens also produce double-yolk eggs by default. Such eggs are produced in India. Eastern states known for that are West Bengal and Andhra Pradesh.

Some hens will rarely lay double-yolked eggs as the result of unsynchronized production cycles. Although heredity causes some hens to have a higher propensity to lay double-yolked eggs, these occur more frequently as occasional abnormalities in young hens beginning to lay.^{[citation needed]} Usually, a double-yolked egg will be longer and thinner than an ordinary single-yolk egg. Double-yolked eggs usually only lead to observed successful hatchlings under human intervention, as the chickens interfere with each other's hatching process and die.^[7]

Rarely, higher-order yolks occur.

Yolkless eggs[link]

Eggs without yolk are called "dwarf" or "wind" eggs.^[8] Such an egg is most often a pullet's first effort, produced before her laying mechanism is fully ready. In a mature hen, a wind egg is unlikely, but can occur if a bit of reproductive tissue breaks away, stimulating the egg producing glands to treat it like a yolk and wrap it in albumen, membranes and a shell as it travels through the egg tube. This will occur if, instead of a yolk, the egg contains a small particle of grayish tissue. An archaic term for a no yolk egg is a "cock" egg.^[9] Since they contained no yolk and therefore can't hatch, it was traditionally believed that these eggs were laid by roosters.^{[citation needed]} This type of egg occurs in many varieties of fowl. They have been found in chickens, both standard and bantams, guineas and coturnix quail. See Cock egg.

Gallery[link]

Other abnormal eggs:

• 

  A chicken egg frying with an extremely thick red yolk.

References[link]

External links[link]

Wikibooks Cookbook has a recipe/module on Eggs

Look up yolk in Wiktionary, the free dictionary.

• Anatomy of an Egg from the Exploratorium
• Making egg tempera from the Society of Tempera Painters
• Odd eggs info
• Egg Yolk Nutritional information and facts