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Egg yolks

Chicken egg, yolk, raw, fresh
Nutritional value per 100 g (3.5 oz)
Energy 1,325 kJ (317 kcal)
3.59 g
26.54 g
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 0.595 g
Glycine 0.488 g
Proline 0.545 g
Serine 1.326 g
Vitamins
Vitamin A equiv.
(48%)
381 μg
Thiamine (B1)
(15%)
0.176 mg
Riboflavin (B2)
(44%)
0.528 mg
Pantothenic acid (B5)
(60%)
2.990 mg
Folate (B9)
(37%)
146 μg
Choline
(167%)
820.2 mg
Vitamin D
(36%)
218 IU
Minerals
Calcium
(13%)
129 mg
Iron
(21%)
2.73 mg
Magnesium
(1%)
5 mg
Phosphorus
(56%)
390 mg
Potassium
(2%)
109 mg
Zinc
(24%)
2.30 mg
Other constituents
Water 52.31 g
Cholesterol 1085 mg

One large egg contains 17 grams of yolk.
Percentages are roughly approximated using US recommendations for adults.
Source: USDA Nutrient Database

The yolk is the portion of an egg with the primary function to supply food for the development of the animal embryo. Some kinds of eggs contain no yolk, for example because they are laid in situations where the food supply is adequate (such as in the body of the host of a parasitoid) or because the embryo develops in the parent's body, which supplies the food, sometimes through a placenta. Reproductive systems in which the mother's body supplies the embryo directly are said to be matrotrophic; those in which the embryo is supplied by yolk are said to be lecithotrophic. In many species, such as all birds, and most reptiles and insects, the yolk takes the form of a special storage organ constructed in the reproductive tract of the mother. In many other animals, especially very small species such as some fishes and invertebrates, the yolk material is not in a special organ, but inside the ovum.

Yolks, being mainly stored food, tend to be very concentrated, and in particular tend to be rich in nutrients such as vitamins, minerals, lipids and proteins. The proteins function partly as food in their own right, and partly in controlling the storage and supply of the other nutrients. For example, in some species the amount of yolk in an egg cell affects the developmental processes that follow fertilization. Yolk is not living cell material like protoplasm, but largely passive material, that is to say deutoplasm, in this case functioning mainly as food for the embryo. The food material, plus associated control structures, is supplied by the maternal body during the process called oogenesis. Some of the material is stored more or less in the form in which the maternal body supplied it, partly as processed by dedicated non-germ tissues in the egg, while part of the biosynthetic processing into its final form happens in the oocyte itself.


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