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
Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
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,
lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids in ...
s and
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s. 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 plant
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants t ...
s, 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
Egg white is the clear liquid (also called the albumen or the glair/glaire) contained within an egg. In chickens it is formed from the layers of secretions of the anterior section of the hen's oviduct during the passage of the egg. It forms a ...
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
frog
A frog is any member of a diverse and largely Carnivore, carnivorous group of short-bodied, tailless amphibians composing the order (biology), order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-f ...
s), 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
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
.
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
omelet
In cuisine, an omelette (also spelled omelet) is a dish made from beaten eggs, fried with butter or oil in a frying pan (without stirring as in scrambled egg). It is quite common for the omelette to be folded around fillings such as chives, ...
s 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
Hollandaise sauce ( or ; ), also called Dutch sauce, is a mixture of egg yolk, melted butter, and lemon juice (or a white wine or vinegar reduction). It is usually seasoned with salt, and either white pepper or cayenne pepper.
It is well kn ...
, ''
crème brûlée'', ''
avgolemono'' and ''
ovos moles
Ovos moles de Aveiro (literally, "soft eggs from Aveiro")—sometimes written as ovos-moles de Aveiro—are a local pastry delicacy from Aveiro District, Portugal, made of egg yolks and sugar, and sometimes chocolate. This mixture is then put in ...
''.
* 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
An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of t ...
called antiglobulin (
IgY
Immunoglobulin Y (abbreviated as IgY) is a type of immunoglobulin which is the major antibody in bird, reptile, and lungfish blood. It is also found in high concentrations in chicken egg yolk. As with the other immunoglobulins, IgY is a class ...
). The antibody transfers from the laying hen to the egg yolk by
passive immunity Passive immunity is the transfer of active humoral immunity of ready-made antibodies. Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta, and it can also be induced artificially, when ...
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
Linolenic acid is a type of naturally-occurring fatty acid. It can refer to either of two octadecatrienoic acids (i.e. with an 18-carbon chain and three double bonds, which are found in the '' cis'' configuration), or a mixture of the two. Linol ...
, 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.
The yellow color is due to
lutein and
zeaxanthin, which are yellow or orange
carotenoids known as
xanthophylls.
Yolk proteins
The different yolk's proteins have distinct roles.
Phosvitins are important in sequestering
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
,
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
, 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.
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.
Yolk vitamins and minerals
Yolks hold more than 90% of the
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
,
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
,
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
Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...
, according to the USDA.
[U.S. Department of Agriculture, Agricultural Research Service, 2010. USDA National Nutrient Database for Standard Reference, Release 23, Nutrient Data Laboratory Home Page: http://www.ars.usda.gov/nutrientdata]
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.
The average probability of finding this is 1 in 1200.
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.
Higher-order yolks are rare, although heavier poultry breeds such as the
Buff Orpington
The Orpington is a British breed of chicken. It was bred in the late nineteenth century by William Cook of Orpington, Kent, in south-east England. It was intended to be a dual-purpose breed, to be reared both for eggs and for meat, but soon bec ...
have been known to lay triple-yolk eggs occasionally.
File:Egg and maxi egg 1.jpg,
File:Egg and maxi egg 2.jpg,
File:Egg with two yellows.jpg,
Yolkless eggs
Eggs without yolks are known as "dwarf" or "wind" eggs,
or the archaic term "cock egg".
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. This type of egg occurs in many varieties of fowl and has been found in chickens, both standard and
bantams,
guineas, and
coturnix quail.
Yolk color
The color of an egg yolk is directly influenced by the makeup of the chicken feed. 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
The cochineal ( , ; ''Dactylopius coccus'') is a scale insect in the suborder Sternorrhyncha, from which the natural dye carmine is derived. A primarily sessile parasite native to tropical and subtropical South America through North America ...
to eggs in cooking.
In fish
All
bony fish, some
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s and
ray
Ray may refer to:
Fish
* Ray (fish), any cartilaginous fish of the superorder Batoidea
* Ray (fish fin anatomy), a bony or horny spine on a fin
Science and mathematics
* Ray (geometry), half of a line proceeding from an initial point
* Ray (gr ...
s have
yolk sac
The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...
s at some stage of development, with all
oviparous fish retaining the sac after hatching.
Lamniform sharks are
ovoviviparous, in that their eggs hatch ''in utero''; in addition to eating unfertilized eggs, unborn sharks participate in intrauterine-
cannibalism: stronger pups consume their weaker womb-mates.
[Leonard J. V. Compagno (1984). Sharks of the World: An annotated and illustrated catalogue of shark species known to date. Food and Agriculture Organization of the United Nations. . OCLC 156157504.]
See also
*
Vitellogenesis
References
External links
Anatomy of an Eggfrom the
Exploratorium
Making egg temperafrom the
Society of Tempera Painters
{{DEFAULTSORT:Egg Yolk
Eggs