The yolk of a chicken egg
Diagram of a fish egg; the yolk is the area marked 'D'

Among animals which produce eggs, the yolk (/ˈjoʊk/; 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.[1]

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.[2][3] The yolk is mostly extracellular to the oolemma, being not accumulated inside the cytoplasm of the egg cell (as occurs in frogs),[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, 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

Composition of chicken egg yolk

Chicken egg, yolk, raw, fresh
Nutritional value per 100 g (3.5 oz)
Energy1,325 kJ (317 kcal)
3.59 g
26.54 g
15.86 g
Tryptophan0.177 g
Threonine0.687 g
Isoleucine0.866 g
Leucine1.399 g
Lysine1.217 g
Methionine0.378 g
Cystine0.264 g
Phenylalanine0.681 g
Tyrosine0.678 g
Valine0.949 g
Arginine1.099 g
Histidine0.416 g
Alanine0.836 g
Aspartic acid1.550 g
Glutamic acid0.595 g
Glycine0.488 g
Proline0.545 g
Serine1.326 g
VitaminsQuantity
%DV†
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
MineralsQuantity
%DV†
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 constituentsQuantity
Water52.31 g
Cholesterol1085 mg
One large egg contains 17 grams of yolk.
†Percentages are roughly approximated using US recommendations for adults.
Source: USDA FoodData Central

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.

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:[7]

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.[8]

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, iron, 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.[9][10] 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.[11][12]

Yolk vitamins and minerals

Yolks hold more than 90% of the calcium, iron, 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, according to the USDA.[13]

Double-yolk eggs

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.[14]

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.[15]

Higher-order yolks are rare, though hens are known to occasionally lay even triple-yolk eggs.[16]

Yolkless eggs

Eggs without yolks are known as "dwarf" or "wind" eggs,[17] or the archaic term "cock egg".[18] 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.[19] 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

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.[20] 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 to eggs in cooking.[21]

In fish

All bony fish, some sharks and rays have yolk sacs 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.[22][23][24]

See also

References

  1. ↑ Barnes, Richard Stephen Kent (2001). The Invertebrates: A Synthesis. Wiley-Blackwell, p. 347. ISBN 978-0-632-04761-1.
  2. ↑ Bellairs, Ruth; Osmond, Mark (2005). Atlas of Chick Development (2 ed.). Academic Press. pp. 1–4. link.
  3. ↑ 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.
  4. ↑ Landecker, Hannah (2007). Culturing life: how cells became technologies. Cambridge, MA: Harvard University Press. p. 49. link.
  5. ↑ Patten, B. M. (1951). Early Embryology of the Chick, 4th edition. McGraw-Hill, New York, p. 17.
  6. ↑ Callebaut, M. (2008) Historical evolution of preformistic versus neoformistic (epigenetic) thinking in embryology, Belgian Journal of Zoology, vol. 138 (1), pp. 20–35, 2008
  7. ↑ 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
  8. ↑ Chris Clarke (2004). The science of ice cream. Cambridge, Eng: Royal Society of Chemistry. p. 49. ISBN 978-0-85404-629-4. Retrieved 2013-03-20. 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.
  9. ↑ 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. PMID 17189915. S2CID 7695406.
  10. ↑ 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. PMID 8838584. S2CID 30240374.
  11. ↑ Banaszak LJ, Thompson JR (2002). "Lipid-protein interactions in lipovitellin". Biochemistry. 41 (30): 9398–9409. doi:10.1021/bi025674w. PMID 12135361.
  12. ↑ Banaszak LJ, Anderson TA, Levitt DG (1998). "The structural basis of lipid interactions in lipovitellin, a soluble lipoprotein". Structure. 6 (7): 895–909. doi:10.1016/S0969-2126(98)00091-4. PMID 9687371.
  13. ↑ 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
  14. ↑ "Odd Eggs, Double Yolks, No Yolks, etc". poultryhelp.com. 2005-03-04. Retrieved 2008-10-25.
  15. ↑ Kruszelnicki, Karl S. (2003). "Double-yolked eggs and chicken development". Australian Broadcasting Corporation. Retrieved 2007-12-09.
  16. ↑ Clark, Thomas B. (May 1, 1932). "A Triple-yolk Egg*". Poultry Science. 11 (3): 176. doi:10.3382/ps.0110176. ISSN 0032-5791. Retrieved February 7, 2023 – via Elsevier.
  17. ↑ Rookledge, K. A; Heald, P. J (1996-06-25). "Dwarf Eggs and the Timing of Ovulation in the Domestic Fowl". Nature. 210 (5043): 1371. doi:10.1038/2101371a0. S2CID 4280287.
  18. ↑ "Cock's egg". Retrieved 2010-09-02.
  19. ↑ "OEDILF – Word Lookup". www.oedilf.com.
  20. ↑ Poultry Science by richard page 216
  21. ↑ Mathew Attokaran (13 January 2011). Natural Food Flavors and Colorants. John Wiley & Sons. pp. 1–. ISBN 978-0-470-95911-4. Retrieved 1 September 2013.
  22. ↑ Meisner A, Burns J (1997). "Viviparity in the Halfbeak Genera Dermogenys and Nomorhamphus (Teleostei: Hemiramphidae)". Journal of Morphology. 234 (3): 295–317. doi:10.1002/(SICI)1097-4687(199712)234:3<295::AID-JMOR7>3.0.CO;2-8. PMID 29852651. S2CID 46922423.
  23. ↑ Peter Scott: Livebearing Fishes, p. 13. Tetra Press 1997. ISBN 1-56465-193-2
  24. ↑ 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. ISBN 92-5-104543-7. OCLC 156157504.
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