Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example because they are laid in situations where the food supply is sufficient (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, usually 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 fish and invertebrates, the yolk material is not in a special organ, but inside the egg cell.
As stored food, yolks are often rich in vitamins, minerals, lipids and proteins. The proteins function partly as food in their own right, and partly in regulating 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.
The yolk is not living cell material like protoplasm, but largely passive material, that is to say deutoplasm. The food material and associated control structures are supplied during 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.
Apart from animals, other organisms, like algae, especially 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.
Chicken egg yolk
In avian eggs, the yolk usually is a hue of yellow in color. It is spherical and is suspended in the egg white (known alternatively as albumen or glair/glaire) by one or two spiral bands of tissue called the chalazae.
The yolk mass, together with the ovum proper (after fertilization, the embryo) are enclosed by the vitelline membrane, whose structure is different from a cell membrane. The yolk is mostly extracellular to the oolemma, being not accumulated inside the cytoplasm of the egg cell (as occurs in frogs), contrary to the claim that the avian ovum (in strict sense) and its yolk are a single giant cell.
After the fertilization, the cleavage of the embryo leads to the formation of the germinal disc.
As food, the chicken egg yolk is a major source of vitamins and minerals. It contains all of the egg's fat and cholesterol, and nearly half of the protein. If left intact when an egg is fried, the yellow yolk surrounded by a flat blob of egg white creates a distinctive "sunny-side up" form. Mixing the two components together before cooking results in a yellow (from pale yellow to almost orange, depending on the breed of hen) mass, as in omelets and scrambled eggs.
Uses
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.
Composition of chicken egg yolk
The yolk makes up about 33% of the liquid weight of the egg; it contains about , three times the energy content of the egg white, mostly due to its fat content.
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: 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%