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From Wikipedia, the free encyclopedia

An egg yolk surrounded by the 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 more formally as albumen or ovalbumin) 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 almost half 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.



Composition of chicken egg yolk

Chicken egg, yolk, raw, fresh
Nutritional value per 100 g (3.5 oz)
Energy 1,325 kJ (317 kcal)
Carbohydrates 3.59 g
Fat 26.54 g
Protein 15.86 g
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 1.970 g
Glycine 0.488 g
Proline 0.646 g
Serine 1.326 g
Water 52.31 g
Vitamin A equiv. 381 μg (42%)
Thiamine (Vit. B1) 0.176 mg (14%)
Riboflavin (Vit. B2) 0.528 mg (35%)
Pantothenic acid (B5) 2.990 mg (60%)
Folate (Vit. B9) 146 μg (37%)
Calcium 129 mg (13%)
Iron 2.73 mg (22%)
Magnesium 5 mg (1%)
Phosphorus 390 mg (56%)
Potassium 109 mg (2%)
Zinc 2.30 mg (23%)
Choline 682.3 mg
Cholesterol 1234 mg
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.

One large egg (50 grams in weight, 17 gram yolk) contains approximately: 2.7g protein, 210 mg cholesterol, 0.61g carbohydrates and 4.51g total fat. (USDA National Nutrient Database)

All of the fat soluble vitamins, (A, D, E and K) are found in the egg yolk. Egg yolks are 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]

Egg yolk is a source of lecithin, an emulsifier and surfactant.

The yellow color is caused by lutein and zeaxanthin, which are yellow or orange carotenoids known as xanthophylls

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 productive cycle not yet being synchronized.[2]

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 unborn chickens would otherwise fight each other and die.[3]

Yolkless eggs

Eggs without yolk are called "dwarf" or "wind" eggs.[4] 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 has occurred 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.[5] 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.


  1. ^ 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
  2. ^ "Odd Eggs, Double Yolks, No Yolks, etc.". 2005-03-04. Retrieved 2008-10-25. 
  3. ^ Kruszelnicki, Karl S. (2003). "Double-yolked eggs and chicken development". Australian Broadcasting Corporation. Retrieved 2007-12-09. 
  4. ^ "Dwarf Eggs and the Timing of Ovulation in the Domestic Fowl". Nature Publishing Group. 1996-06-25. Retrieved 2008-10-25. 
  5. ^ "FAQ about Eggs". 2007-02-06. Retrieved 2008-10-25. 

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