Yolk

• The developing embryo inside the egg uses the yolk as sustenance.
• It is at times separated from the egg white for cooking, and is frequently employed as an emulsifier, and is used in 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 yolk 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 yolk is used to extract egg oil which has various cosmetic, nutritional, and medicinal uses.

The yolk makes up about 33% of the liquid weight of the egg; it contains about 60 kilocalories (250 kJ), three times the energy content of the egg white, mostly due to its fat content.

The yolk of one large egg (56.7 g total, 17.6 g yolk .[7]

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 typically is:[8]

• 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.[9]

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

The different yolk's 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 provides efficient metal-binding sites in clusters.[10][11] 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.[12][13]

Yolks hold more than 90% of the calcium, iron, phosphorus, zinc, thiamine, vitamin B₆, folate, vitamin B₁₂, and pantothenic acid of the egg. In addition, yolks cover all of the fat-soluble vitamins: A, D, E, and K in the egg, as well as all of the essential fatty acids.

A single yolk from a large egg contains roughly 22 mg of calcium, 66 mg of phosphorus, 9.5 micrograms of selenium, and 19 mg of potassium, according to the USDA.[7]

Three similarly sized eggs in a hot frying pan. Each of the two yolks in the double-yolked eggs are smaller than typical for that size of egg.

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.[14]

Double-yolked eggs seldom lead to successful hatchlings without human intervention, as the chicks interfere with each other's hatching process and do not survive.[15]

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

• 
  Comparison of an egg and an egg with a double-yolk (closed)
• 
  Comparison of an egg and an egg with a double-yolk (opened)
• 
  Double-yolk egg - Opened

Eggs without yolks are known as "dwarf" or "wind" eggs,[16] or the archaic term "cock egg".[17] Such an egg is most often a pullet's first effort, produced before her laying mechanism is fully ready. Mature hens rarely lay a yolkless egg, but sometimes a piece 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 causes an egg to contain a small particle of grayish tissue instead of a yolk.

Since these eggs contain no yolk, and therefore cannot 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.

A chicken egg frying with an extremely thick red yolk. A normal-coloured yolk can also be seen, having been accidentally burst during the frying process.

The color of an egg yolk is directly influenced by the makeup of the chicken feed.[19] Egg yolk color is generally more yellow when given 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.

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, or even olive green, depending on the pigments in their feed. Feeding fowl 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.[20]