The yolk of achickeneggDiagram of afish egg; the yolk is the area which is marked 'C'
Among animals which produceeggs, theyolk (/ˈjoʊk/; also known as thevitellus) is thenutrient-bearing portion of the egg whose primary function is to supplyfood for the development of theembryo. 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 thehost of aparasitoid) or because the embryo develops in the parent's body, which supplies the food, usually through aplacenta. Reproductive systems in which the mother's body supplies the embryo directly are said to bematrotrophic; those in which the embryo is supplied by yolk are said to belecithotrophic. In many species, such as allbirds, and mostreptiles andinsects, the yolk takes the form of a special storage organ constructed in thereproductive tract of the mother. In many other animals, especially very small species such as somefish andinvertebrates, the yolk material is not in a special organ, but inside theegg cell.
As stored food, yolks are often rich invitamins,minerals,lipids andproteins. 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 followfertilization.
The yolk is not livingcell material likeprotoplasm, but largely passive material, that is to saydeutoplasm. The food material and associated control structures are supplied duringoogenesis. Some of the material is stored more or less in the form in which the maternal body supplied it, partly as processed by dedicatednon-germ tissues in the egg, while part of thebiosynthetic processing into its final form happens in theoocyte itself.[1]
Apart from animals, other organisms, likealgae, especially in theoogamous, can also accumulate resources in their female gametes. Ingymnosperms, the remains of the femalegametophyte serve also as food supply, and inflowering plants, theendosperm.
In avian eggs, the yolk usually is a hue of yellow in color. It is spherical and is suspended in theegg white (known alternatively asalbumen or glair/glaire) by one or two spiral bands of tissue called thechalazae.
The yolk mass, together with the ovumproper (after fertilization, theembryo) are enclosed by thevitelline membrane, whose structure is different from acell membrane.[2][3] The yolk is mostly extracellular to theoolemma, being not accumulated inside thecytoplasm of the egg cell (as occurs infrogs),[4] contrary to the claim that the avian ovum (in strict sense) and its yolk are a single giant cell.[5][6]
After the fertilization, thecleavage of the embryo leads to the formation of thegerminal disc.
Asfood, thechicken egg yolk is a major source of vitamins and minerals. It contains all of the egg'sfat andcholesterol, and nearly half of theprotein.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 inomelets andscrambled eggs.
Egg yolk contains anantibody called antiglobulin (IgY). The antibody transfers from the laying hen to the egg yolk bypassive immunity to protect both embryo and hatchling from microorganism invasion.
Egg yolk can be used to make liqueurs such asAdvocaat oreggnog.
Egg yolk is used to extractegg oil which has various cosmetic, nutritional, and medicinal uses.
The yolk makes up about 33% of the liquid weight of the egg; it contains about 60 kilocalories (250 kJ), three times the energy content of the egg white, mostly due to its fat content.[clarification needed]
All of the fat-soluble vitamins (A,D,E andK) 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:[9]
Egg yolk is a source oflecithin, as well as egg oil, for cosmetic and pharmaceutical applications. Based on weight, egg yolk contains about 9% lecithin.[10]
The different yolk's proteins have distinct roles.Phosvitins are important in sequesteringcalcium,iron, and other cations for the developing embryo. Phosvitins are one of the mostphosphorylated (10%) proteins in nature; the high concentration of phosphate groups provides efficient metal-binding sites in clusters.[11][12] 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.[13][14]
A single yolk from a large egg contains roughly 22 mg of calcium, 66 mg of phosphorus, 9.5 micrograms ofselenium, and 19 mg ofpotassium, according to the USDA.[15]
Three similarly sized eggs in a hot frying pan. Each of the two yolks in the double-yolked eggs are smaller than typical for that size of egg.
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.[16]
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.[17]
Higher-order yolks are rare, though hens are known to occasionally lay even triple-yolk eggs.[18]
Comparison of an egg and an egg with a double-yolk (closed)
Comparison of an egg and an egg with a double-yolk (opened)
Eggs without yolks are known as "dwarf" or "wind" eggs,[19] or the archaic term "cock egg".[20] 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.[21] This type of egg occurs in many varieties of fowl and has been found in chickens, both standard andbantams,guineas, andcoturnix quail.
A chicken egg frying with an extremely thick red yolk. A normal-coloured yolk can also be seen, having been accidentally burst during the frying process.
The color of an egg yolk is directly influenced by the makeup of the chicken feed.[22] Egg yolk color is generally more yellow when given a feed containing a large component of yellow, fat-soluble pigments, such as thecarotenes in dark green plant material, for examplealfalfa. 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 arexanthophylls without much nutritional value, rather than thecarotenoids that act asprovitamin 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 ofcapsicum peppers, for example, tends to result in red or deep orange yolks. This has nothing to do with adding colors such ascochineal to eggs in cooking.[23]
Allbony fish, somesharks andrays haveyolk sacs at some stage of development, with alloviparous fish retaining the sac after hatching.Lamniform sharks areovoviviparous, in that their eggs hatchin utero; in addition to eating unfertilized eggs, unborn sharks participate in intrauterine-cannibalism: stronger pups consume their weaker womb-mates.[24][25][26]
The yolk in crustacean eggs is essential for embryonic development, serving as a nutrient reservoir. In decapod crustaceans, the primary yolk precursor protein is apolipocrustacein (apoCr), which differs from the traditional vitellogenins (Vtgs) found in most oviparous animals. ApoCr shares greater structural and evolutionary similarity with insect apolipophorin II/I (apoLp-II/I) and vertebrate apolipoprotein B (apoB), distinguishing it from other members of the large lipid transfer protein (LLTP) superfamily.[28]
ApoCr is a large glycolipoprotein, approximately 2,600 amino acids long, with conserved structural domains characteristic of LLTPs. These domains include an N-terminal lipid transfer module, a DUF1081 domain exclusive to apoLp-II/I and apoB, and a von Willebrand factor type D domain at the C-terminal. Additionally, it features a subtilisin-like cleavage site, a trait shared with apoLp-II/I. Evolutionary analyses reveal that apoCr is phylogenetically closer to apoLp-II/I than to Vtg proteins, indicating a distinct lineage for crustacean yolk proteins. In decapods, apoCr is typically expressed in both the ovary and hepatopancreas, supporting its dual roles in lipid metabolism and yolk formation. In some species, gene duplication events have resulted in multiple apoCr variants with tissue-specific functions.[28]
^Bellairs, Ruth; Osmond, Mark (2005).Atlas of Chick Development (2 ed.). Academic Press. pp. 1–4.link.
^Bellairs, R., Harkness, M. & Harkness, R. D. (1963). The vitelline membrane of the hen's egg: a chemical and electron microscopical study.Journal of Ultrastructure Research, 8, 339–59.
^Landecker, Hannah (2007).Culturing life: how cells became technologies. Cambridge, MA: Harvard University Press. p. 49.link.
^National Research Council, 1976,Fat Content and Composition of Animal Products, Printing and Publishing Office, National Academy of Science, Washington, D.C.,ISBN0-309-02440-4; p. 203,online edition
^Chris Clarke (2004).The science of ice cream. Cambridge, England: Royal Society of Chemistry. p. 49.ISBN0-85404-629-1. Retrieved2013-03-20.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):1021–9.doi:10.2108/zsj.23.1021.PMID17189915.S2CID7695406.
^Goulas A, Triplett EL, Taborsky G (1996). "Oligophosphopeptides of varied structural complexity derived from the egg phosphoprotein, phosvitin".J. Protein Chem.15 (1):1–9.doi:10.1007/BF01886805.PMID8838584.S2CID30240374.
^Banaszak LJ, Thompson JR (2002). "Lipid-protein interactions in lipovitellin".Biochemistry.41 (30):9398–9409.doi:10.1021/bi025674w.PMID12135361.
^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
^Peter Scott: Livebearing Fishes, p. 13. Tetra Press 1997.ISBN1-56465-193-2
^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.ISBN92-5-104543-7. OCLC 156157504.
^Klann, Marleen; Scholtz, Gerhard (March 2014). "Early embryonic development of the freshwater shrimp Caridina multidentata (Crustacea, Decapoda, Atyidae)".Zoomorphology.133 (3):295–306.doi:10.1007/s00435-014-0224-9.
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