The yolk is a part of an egg (or just of the egg cell in non-egg-laying animals) that feeds the developing embryo in animals.

Also known as deutoplasm, this food material is accumulated during oogenesis, together with RNA molecules and other substances. These may be synthesized by the oocyte itself or by the cytoplasm of other non-germ cells.^[1] The amount of yolk in an egg cell affects the developmental processes that follow fertilization, like the cleavage.

Apart from animals, other organisms, like algae, specially in the oogamous, can also accumulate resources in their female gametes. In gymnosperms, the remains of the female gametophyte serve also as food supply, and in flowering plants, the endosperm.

In the avian egg, the 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 is a single cell, the ovum or egg cell, one of the few single cells that can be seen by the naked eye.^[2] This fact was discovered by Hoyer in 1858.^[3]

After the fertilization, the cleavage leads to the formation of the germinal disc.

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

• 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, nutritional and medicinal uses.
• The developing embryo inside the egg uses the yolk as sustenance.

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.^[6]

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:^[7]

• 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 pharmaceutical applications. Based on weight, egg yolk contains about 9% lecithin.^[8]

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

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.^[9][10] 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.^[11][12]

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.^[13]

Double-yolked eggs usually lead to observed successful hatchlings only under human intervention, as the chickens interfere with each other's hatching process and die.^[14]

Higher-order yolks are rare, although heavier poultry breeds such as the Buff Orpington have been known to lay triple-yolk eggs occasionally.

A cock en egg is an archaic term for a yolkless egg.^[15] Eggs without yolk are called "dwarf" or "wind" eggs.^[16] Such an egg is most often a pullet's first effort, produced before her laying mechanism is fully ready. Mature hens rarely lay a wind egg, but it sometimes happens that a bit of reproductive tissue breaks away and passes down the tube. Such a scrap of tissue may stimulate the egg-producing glands to react as though it were a yolk and wrap it in albumen, membranes and a shell as it travels through the egg tube. This is usually what caused an egg to contain a small particle of grayish tissue instead of a yolk.

An archaic term for a yolkless egg is a "cock egg".^[17] Since these eggs contain no yolk and therefore can not hatch, they were traditionally believed to have been laid by roosters.^[18] This type of egg occurs in many varieties of fowl and has been found in chickens, both standard and bantams, guineas and coturnix quail.

The color of an egg yolk is directly influenced by the makeup of the chicken feed.^[19] Egg yolk color is generally improved with a feed containing a large component of yellow, fat-soluble pigments, such as the carotenes in dark green plant material, for example alfalfa. Although much emphasis is put onto the color of the egg yolk, it does not reliably reflect the nutritional value of an egg. For example, some of the natural pigments that produce a rich yolk color are xanthophylls without much nutritional value, rather than the carotenoids that act as provitamin A in the body. Also, a diet rich in vitamin A itself, but without A-provitamins or xanthophylls, can produce practically colourless yolks that are just as nutritious as any richly colored yolks. Since unhealthy chickens produce fewer and smaller eggs, farmers ensure that whatever the source of their feed, the quality is adequate, so there is not likely to be much difference in the nutritional quality of the eggs.^[20]

In fact, yolks, particularly from free-range eggs, can be of a wide range of colors, ranging from nearly white, through yellow and orange to practically red, but even olive green, depending on the pigments in their food. Feeding fowls large amounts of capsicum peppers for example, tends to result in red or deep orange yolks. This has nothing to do with adding colors such as cochineal to eggs in cooking.^[21]

Other abnormal eggs:

• 
  A chicken egg frying with an extremely thick red yolk.
• 
  A double yolk egg.

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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