Egg yolk

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Not to be confused with yoke.

The yolk is a part of an egg (or just of the egg cell in non-egg-laying animals) that feeds the developing embryo in animals. In whole eggs, 10% of the protein comes from the yolk.

Also known as deutoplasm, this food material is accumulated during oogenesis, together with RNA molecules and other substances. These may be synthesized by the oocyte itself or by the cytoplasm of other non-germ cells.[1] The amount of yolk in an egg cell affects the developmental processes that follow fertilization, like the cleavage.

Apart from animals, other organisms, like algae, specially 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[edit]

An intact yolk surrounded by egg white

In the avian egg, the yolk is suspended in the egg white (known alternatively as albumen or glair/glaire) by one or two spiral bands of tissue called the chalazae. Prior to fertilization, the yolk is a single cell, the ovum or egg cell, one of the few single cells that can be seen by the naked eye.[2] This fact was discovered by Hoyer in 1858.[3]

After the fertilization, the cleavage leads to the formation of the germinal disc.

As a food, chicken egg yolks are a major source of vitamins and minerals. They contain all of the egg's fat and cholesterol, and about one-half of the protein. If left intact while cooking fried eggs, the yellow yolk surrounded by a flat blob of whites creates a distinctive sunny-side up form. Mixing the two components together before frying results in a pale yellow mass, as in omelettes and scrambled eggs.

Uses[edit]

  • Egg yolk can be used to make liqueurs such as Advocaat or eggnog.
  • Egg yolks are used to extract egg oil which has various cosmetic, nutritional and medicinal uses.
  • The developing embryo inside the egg uses the yolk as sustenance.

Composition of chicken egg yolk[edit]

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
(60%)
2.990 mg
Folate (B9)
(37%)
146 μg
Choline
(167%)
820.2 mg
Vitamin D
(36%)
218 IU
Trace metals
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 1240 mg

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

The yolk makes up about 33% of the liquid weight of the egg; it contains approximately 60 calories, three times the caloric content of the egg white.

The yolk of one large egg (50 g total, 17 g yolk) contains approximately: 2.7 g protein, 210 mg cholesterol, 0.61 g carbohydrates, and 4.51 g total fat.[4]

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 is typically as follows:[5]

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.[6]

The yellow color is due to lutein and zeaxanthin, which are yellow or orange carotenoids known as xanthophylls.

Yolk proteins[edit]

The different yolk 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 providing efficient metal-binding sites in clusters.[7][8] 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.[9][10]

Double-yolk eggs[edit]

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.[11]

Double-yolked eggs usually lead to observed successful hatchlings only under human intervention, as the chickens interfere with each other's hatching process and die.[12]

Higher-order yolks are rare, although heavier poultry breeds such as the Buff Orpington have been known to lay triple-yolk eggs occasionally.

Yolkless eggs[edit]

Main article: Cock egg

A cock egg is an archaic term for a yolkless egg.[13] Eggs without yolk are called "dwarf" or "wind" eggs.[14] Such an egg is most often a pullet's first effort, produced before her laying mechanism is fully ready. Mature hens rarely lay a wind egg, but it sometimes happens that a bit 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 caused an egg to contain a small particle of grayish tissue instead of a yolk.

Since these eggs contain no yolk and therefore can not hatch, they were traditionally believed to have been laid by roosters.[15] 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[edit]

The color of an egg yolk is directly influenced by the makeup of the chicken feed.[16] Egg yolk color is generally improved with 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. Since unhealthy chickens produce fewer and smaller eggs, farmers ensure that whatever the source of their feed, the quality is adequate, so there is not likely to be much difference in the nutritional quality of the eggs.[17]

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, but even olive green, depending on the pigments in their food. Feeding fowls 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.[18]

Gallery[edit]

Other abnormal eggs:

References[edit]

  1. ^ Barnes, Richard Stephen Kent (2001). The Invertebrates: A Synthesis. Wiley-Blackwell, p. 347. ISBN 978-0-632-04761-1.
  2. ^ Patten, B. M. (1951). Early Embryology of the Chick, 4th edition. McGraw-Hill, New York, p. 17.
  3. ^ Callebaut, M. Historical evolution of preformistic versus neoformistic (epigenetic) thinking in embryology.
  4. ^ 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
  5. ^ 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
  6. ^ Chris Clarke (2004). The science of ice cream. Cambridge, Eng: Royal Society of Chemistry. p. 49. ISBN 0-85404-629-1. 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." 
  7. ^ 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. 
  8. ^ 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. 
  9. ^ Banaszak LJ, Thompson JR (2002). "Lipid-protein interactions in lipovitellin". Biochemistry 41 (30): 9398–9409. doi:10.1021/bi025674w. PMID 12135361. 
  10. ^ 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. 
  11. ^ "Odd Eggs, Double Yolks, No Yolks, etc.". poultryhelp.com. 2005-03-04. Retrieved 2008-10-25. 
  12. ^ Kruszelnicki, Karl S. (2003). "Double-yolked eggs and chicken development". Australian Broadcasting Corporation. Retrieved 2007-12-09. 
  13. ^ "Cock's egg". Retrieved 2010-09-02. 
  14. ^ "Dwarf Eggs and the Timing of Ovulation in the Domestic Fowl". Nature Publishing Group. 1996-06-25. Retrieved 2008-10-25. 
  15. ^ http://www.oedilf.com/db/Lim.php?Word=cock%27s%20egg
  16. ^ Poultry Science by richard page 216[full citation needed]
  17. ^ Donald D. Bell; William Daniel Weaver (January 2002). Commercial Chicken Meat and Egg Production. Springer. ISBN 978-0-7923-7200-4. Retrieved 1 September 2013. 
  18. ^ Mathew Attokaran, PhD (13 January 2011). Natural Food Flavors and Colorants. John Wiley & Sons. pp. 1–. ISBN 978-0-470-95911-4. Retrieved 1 September 2013. 

External links[edit]