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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-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.
Composition of chicken egg yolk 
|Nutritional value per 100 g (3.5 oz)|
|Energy||1,325 kJ (317 kcal)|
|- Tryptophan||0.177 g|
|- Threonine||0.687 g|
|- Isoleucine||0.866 g|
|- Leucine||1.399 g|
|- Lysine||1.217 g|
|- Methionine||0.378 g|
|- Cystine||0.264 g|
|- Phenylalanine||0.681 g|
|- Tyrosine||0.678 g|
|- Valine||0.949 g|
|- Arginine||1.099 g|
|- Histidine||0.416 g|
|- Alanine||0.836 g|
|- Aspartic acid||1.550 g|
|- Glutamic acid||0.595 g|
|- Glycine||0.488 g|
|- Proline||0.545 g|
|- Serine||1.326 g|
|Vitamin A equiv.||381 μg (48%)|
|Thiamine (vit. B1)||0.176 mg (15%)|
|Riboflavin (vit. B2)||0.528 mg (44%)|
|Pantothenic acid (B5)||2.990 mg (60%)|
|Folate (vit. B9)||146 μg (37%)|
|Calcium||129 mg (13%)|
|Iron||2.73 mg (21%)|
|Magnesium||5 mg (1%)|
|Phosphorus||390 mg (56%)|
|Potassium||109 mg (2%)|
|Zinc||2.30 mg (24%)|
|One large egg contains 17 grams of yolk.
Percentages are relative to
US recommendations for adults.
Source: USDA Nutrient Database
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)
The composition (by weight) of the most prevalent fatty acids in egg yolk is typically as follows:
- Unsaturated fatty acids:
- Saturated fatty acids:
Yolk proteins 
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. 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.
Double-yolk eggs 
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. 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. Usually, a double-yolked egg will be longer and thinner than an ordinary single-yolk egg. 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.
Rarely, higher-order yolks occur, although heavier poultry breeds such as the Buff Orpington have been known to lay triple-yolk eggs in their lifetimes.
Yolkless eggs 
Eggs without yolk are called "dwarf" or "wind" eggs. 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. Since these eggs contain no yolk and therefore can not hatch, they were traditionally believed to have been laid by roosters. This type of egg occurs in many varieties of fowl and has been found in chickens, both standard and bantams, guineas and coturnix quail. See Cock egg.
Yolk color 
The color of an egg yolk is directly influenced by the quality of the chicken feed. Egg Yolk color is generally improved with a higher quality feed and vice-versa. Although much emphasis is put onto the color of the egg yolk, it is not indicative of an egg's nutritional value.
Other abnormal eggs:
- National Research Council, 1976, Fat Content and Composition of Animal Products, Printing and Publishing Office, National Academy of Science, Washington, D.C., ISBN 0-309-02440-4; p. 203, online edition
- Chris Clarke (2004). The science of ice cream. Cambridge, Eng: Royal Society of Chemistry. p. 49. ISBN 0-85404-629-1. Retrieved 2013-03-20. "Egg yolk has the approximate composition (by weight) of 50% water, 16% protein, 9% lecithin, 23% other fat, 0.3% carbohydrate and 1.7% minerals."
- Matsubara T, Sawaguchi S, Ohkubo N (2006). "Identification of two forms of vitellogenin-derived phosvitin and elucidation of their fate and roles during oocyte maturation in the barfin flounder, Verasper moseri". Zool. Sci. 23 (11): –. PMID 17189915.
- Goulas A, Triplett EL, Taborsky G (1996). "Oligophosphopeptides of varied structural complexity derived from the egg phosphoprotein, phosvitin". J. Protein Chem. 15 (1): –. PMID 8838584.
- Banaszak LJ, Thompson JR (2002). "Lipid-protein interactions in lipovitellin". Biochemistry 41 (30): 9398–9409. doi:10.1021/bi025674w. PMID 12135361.
- Banaszak LJ, Anderson TA, Levitt DG (1998). "The structural basis of lipid interactions in lipovitellin, a soluble lipoprotein". Structure 6 (7): 895–909. doi:10.1016/S0969-2126(98)00091-4. PMID 9687371.
- "Odd Eggs, Double Yolks, No Yolks, etc.". poultryhelp.com. 2005-03-04. Retrieved 2008-10-25.
- Kruszelnicki, Karl S. (2003). "Double-yolked eggs and chicken development". Australian Broadcasting Corporation. Retrieved 2007-12-09.
- "Dwarf Eggs and the Timing of Ovulation in the Domestic Fowl". Nature Publishing Group. 1996-06-25. Retrieved 2008-10-25.
- "FAQ about Eggs". homesteadingtimes.com. 2007-02-06. Retrieved 2008-10-25.[dead link]
- Poultry Science by richard page 216
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