Egg yolk
Overview
An egg yolk is a part of an egg
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...

 which feeds the developing embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

. The egg yolk is suspended in the egg white
Egg white
Egg white is the common name for the clear liquid 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 around either fertilized or unfertilized egg yolks...

 (known alternatively as albumen or glair/glaire) by one or two spiral bands of tissue called the chalaza
Chalaza
The chalaza is a structure inside bird and reptile eggs and plant ovules. It attaches or suspends the yolk or nucellus within the larger structure.-In animals:...

e. Prior to fertilization, the yolk together with the germinal disc
Germinal disc
-Birds:The germinal disc, also called the blastodisc, is a small, circular, white spot on the surface of the yellow yolk of a bird's egg.-Mammals:...

 is a single cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

, one of the few single cells that can be seen by the naked eye.

As a food
Egg (food)
Eggs are laid by females of many different species, including birds, reptiles, amphibians, and fish, and have probably been eaten by mankind for millennia. Bird and reptile eggs consist of a protective eggshell, albumen , and vitellus , contained within various thin membranes...

, yolks are a major source of vitamin
Vitamin
A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on...

s and minerals.
Encyclopedia
An egg yolk is a part of an egg
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...

 which feeds the developing embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

. The egg yolk is suspended in the egg white
Egg white
Egg white is the common name for the clear liquid 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 around either fertilized or unfertilized egg yolks...

 (known alternatively as albumen or glair/glaire) by one or two spiral bands of tissue called the chalaza
Chalaza
The chalaza is a structure inside bird and reptile eggs and plant ovules. It attaches or suspends the yolk or nucellus within the larger structure.-In animals:...

e. Prior to fertilization, the yolk together with the germinal disc
Germinal disc
-Birds:The germinal disc, also called the blastodisc, is a small, circular, white spot on the surface of the yellow yolk of a bird's egg.-Mammals:...

 is a single cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

, one of the few single cells that can be seen by the naked eye.

As a food
Egg (food)
Eggs are laid by females of many different species, including birds, reptiles, amphibians, and fish, and have probably been eaten by mankind for millennia. Bird and reptile eggs consist of a protective eggshell, albumen , and vitellus , contained within various thin membranes...

, yolks are a major source of vitamin
Vitamin
A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on...

s and minerals. They contain all of the egg's fat
Fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...

 and cholesterol
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...

, and about one-fifth of the protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

.
If left intact while cooking fried eggs, the yellow yolk surrounded by a flat blob of whites creates the distinctive sunny-side up
Fried egg
A fried egg is a food made from an egg fried whole with minimal accompaniment. They are traditionally eaten for breakfast in western countries, but may be eaten at other times of the day.-China:...

 form of the food. Mixing the two components together before frying results in the pale yellow form found in omelette
Omelette
In cuisine, an omelette or omelet is a dish made from beaten eggs quickly cooked with butter or oil in a frying pan, sometimes folded around a filling such as cheese, vegetables, meat , or some combination of the above...

s and scrambled eggs
Scrambled eggs
Scrambled eggs is a dish made from beaten whites and yolks of eggs . Beaten eggs are put into a hot pot or pan and stirred frequently, forming curds as they coagulate.-Sample preparation:...

.

Uses

  • It is sometimes separated
    Separating eggs
    Separating eggs is a process usually used in cooking, in which one removes the egg yolk from the egg white. This is used to allow one part of the egg to be used without the other part — for instance, many recipes require frothing egg whites to make a foam, which will not work if the yolk is...

     from the egg whites and used in cooking (for mayonnaise
    Mayonnaise
    Mayonnaise, , often abbreviated as mayo, is a sauce. It is a stable emulsion of oil, egg yolk and either vinegar or lemon juice, with many options for embellishment with other herbs and spices. Lecithin in the egg yolk is the emulsifier. Mayonnaise varies in color but is often white, cream, or pale...

    , custard
    Custard
    Custard is a variety of culinary preparations based on a cooked mixture of milk or cream and egg yolk. Depending on how much egg or thickener is used, custard may vary in consistency from a thin pouring sauce , to a thick pastry cream used to fill éclairs. The most common custards are used as...

    , hollandaise sauce
    Hollandaise sauce
    Hollandaise sauce is an emulsion of egg yolk and butter, usually seasoned with lemon juice. In appearance it is light yellow and opaque, smooth and creamy. The flavor is rich and buttery, with a mild tang added by the lemon juice, yet not so strong as to overpower mildly-flavored foods.Hollandaise...

    , crème brûlée
    Crème brûlée
    Crème brûlée , also known as burnt cream, crema catalana, or Trinity cream is a dessert consisting of a rich custard base topped with a contrasting layer of hard caramel...

    , avgolemono
    Avgolemono
    Avgolemono is a family of Mediterranean sauces and soups made with egg and lemon juice mixed with broth, heated until they thicken. Avgolémono 'egg-lemon' is the Greek name; in Arabic, it is called tarbiya or beida bi-lemoune 'egg with lemon'; and in Turkish terbiye...

    , and ovos moles).
  • It is used in painting as a component of traditional egg-tempera
    Tempera
    Tempera, also known as egg tempera, is a permanent fast-drying painting medium consisting of colored pigment mixed with a water-soluble binder medium . Tempera also refers to the paintings done in this medium. Tempera paintings are very long lasting, and examples from the 1st centuries AD still exist...

    .
  • It is used in the production of egg-yolk agar plate
    Agar plate
    An agar plate is a Petri dish that contains a growth medium used to culture microorganisms or small plants like the moss Physcomitrella patens.Selective growth compounds may also be added to the media, such as antibiotics....

     medium, useful in testing for the presence of Clostridium perfringens
    Clostridium perfringens
    Clostridium perfringens is a Gram-positive, rod-shaped, anaerobic, spore-forming bacterium of the genus Clostridium. C. perfringens is ever present in nature and can be found as a normal component of decaying vegetation, marine sediment, the intestinal tract of humans and other vertebrates,...

    .
  • Egg yolks also contains an antibody
    Antibody
    An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...

     called antiglobulin (IgY). The antibody transfers from the laying hen to the egg yolk by passive immunity
    Passive immunity
    Passive immunity is the transfer of active humoral immunity in the form of readymade antibodies, from one individual to another. Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta, and can also be induced artificially, when high levels...

     to protect both embryo and hatchling from microorganism invasion.
  • Egg yolk can be used to make liqueurs such as Advocaat
    Advocaat
    Advocaat is a rich and creamy liqueur made from eggs, sugar and brandy. It has a smooth, custard-like flavor and is similar to eggnog. In English-speaking countries it generally contains 15% alcohol , but in Continental Europe the typical alcohol content differs from country to country and is...

     or eggnog
    Eggnog
    Eggnog, or egg nog, is a sweetened dairy-based beverage traditionally made with milk and/or cream, sugar, beaten eggs , and liquor...

    .
  • Egg yolks are used to extract Egg Oil
    Egg Oil
    Egg Oil , also known as egg yolk oil , is derived from the yolk of chicken eggs consisting mainly of triglycerides with traces of lecithin and is usually free of egg proteins...

     which has various cosmetic, nutitional and medicinal uses.

Composition of chicken egg yolk

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. (USDA National Nutrient Database)

All of the fat-soluble vitamins (A
Vitamin A
Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...

, D
Vitamin D
Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....

, E
Vitamin E
Vitamin E is used to refer to a group of fat-soluble compounds that include both tocopherols and tocotrienols. There are many different forms of vitamin E, of which γ-tocopherol is the most common in the North American diet. γ-Tocopherol can be found in corn oil, soybean oil, margarine and dressings...

, and K
Vitamin K
Vitamin K is a group of structurally similar, fat soluble vitamins that are needed for the posttranslational modification of certain proteins required for blood coagulation and in metabolic pathways in bone and other tissue. They are 2-methyl-1,4-naphthoquinone derivatives...

) 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:
  • Unsaturated fatty acids:
    • Oleic acid
      Oleic acid
      Oleic acid is a monounsaturated omega-9 fatty acid found in various animal and vegetable fats. It has the formula CH37CH=CH7COOH. It is an odorless, colourless oil, although commercial samples may be yellowish. The trans isomer of oleic acid is called elaidic acid...

      , 47%
    • Linoleic acid
      Linoleic acid
      Linoleic acid is an unsaturated n-6 fatty acid. It is a colorless liquid at room temperature. In physiological literature, it has a lipid number of 18:2...

      , 16%
    • Palmitoleic acid
      Palmitoleic acid
      Palmitoleic acid, or -9-hexadecenoic acid, is an omega-7 monounsaturated fatty acid with the formula CH35CH=CH7COOH that is a common constituent of the glycerides of human adipose tissue. It is present in all tissues, but generally found in higher concentrations in the liver...

      , 5%
    • Linolenic acid, 2%
  • Saturated fatty acids:
    • Palmitic acid
      Palmitic acid
      Palmitic acid, or hexadecanoic acid in IUPAC nomenclature, is one of the most common saturated fatty acids found in animals and plants. Its molecular formula is CH314CO2H. As its name indicates, it is a major component of the oil from palm trees . Palmitate is a term for the salts and esters of...

      , 23%
    • Stearic acid
      Stearic acid
      Stearic acid is the saturated fatty acid with an 18 carbon chain and has the IUPAC name octadecanoic acid. It is a waxy solid, and its chemical formula is CH316CO2H. Its name comes from the Greek word στέαρ "stéatos", which means tallow. The salts and esters of stearic acid are called stearates...

      , 4%
    • Myristic acid
      Myristic acid
      Myristic acid, also called tetradecanoic acid, is a common saturated fatty acid with the molecular formula CH312COOH. A myristate is a salt or ester of myristic acid....

      , 1%


Egg yolk is a source of lecithin
Lecithin
Lecithin is a generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues, and in egg yolk, composed of phosphoric acid, choline, fatty acids, glycerol, glycolipids, triglycerides, and phospholipids .The word lecithin was originally coined in 1847 by...

.

The yellow color is due to lutein
Lutein
Lutein is a xanthophyll and one of 600 known naturally occurring carotenoids. Lutein is synthesized only by plants and like other xanthophylls is found in high quantities in green leafy vegetables such as spinach and kale...

 and zeaxanthin
Zeaxanthin
Zeaxanthin is one of the most common carotenoid alcohols found in nature. It is important in the xanthophyll cycle. Synthesized in plants & some micro-organisms, it is the pigment that gives paprika , corn, saffron, and many other plants & microbes their characteristic color.The name is derived...

, which are yellow or orange carotenoids known as xanthophyll
Xanthophyll
Xanthophylls are yellow pigments that form one of two major divisions of the carotenoid group. The name is from Greek xanthos + phyllon , due to their formation of the yellow band seen in early chromatography of leaf pigments...

s.

Yolk proteins

The different yolk proteins have distinct roles. Phosvitins are important in sequestering calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

, iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 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. 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.

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. Some hybrid breeds of hens also produce double yolk eggs by default. Such eggs are produced in West Bengal, India and in particular by Arambagh Hatcheries in Arambagh.

Some hens will rarely lay double-yolked eggs as the result of unsynchronized production cycles. Although heredity
Heredity
Heredity is the passing of traits to offspring . This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve...

 causes some hens to have a higher propensity to lay double-yolked eggs, these occur more frequently as occasional abnormalities in young hens beginning to lay. Usually a double-yolked egg will be longer and thinner than an ordinary single-yolk egg. Double-yolked eggs usually only lead to observed successful hatchlings under human intervention, as the chickens interfere with each other's hatching process and die.

Rarely, higher-order yolks occur.

Yolkless eggs

Eggs without yolk are called "dwarf" or "wind" eggs. Such an egg is most often a pullet's first effort, produced before her laying mechanism is fully ready. In a mature hen, a wind egg is unlikely, but can occur if a bit of reproductive tissue breaks away, stimulating the egg producing glands to treat it like a yolk and wrap it in albumen, membranes and a shell as it travels through the egg tube. This has occurred if, instead of a yolk, the egg contains a small particle of grayish tissue. An archaic term for a no yolk egg is a "cock" egg. Since they contained no yolk and therefore can't hatch, it was traditionally believed that these eggs were laid by roosters. This type of egg occurs in many varieties of fowl. They have been found in chickens, both standard and bantams, guineas
Guineafowl
The guineafowl are a family of birds in the Galliformes order, although some authorities include the guineafowl as a subfamily, Numidinae, of the family Phasianidae...

 and coturnix
Coturnix
Coturnix is a genus of Old World quail in Phasianidae.- Species list :*Blue Quail, Coturnix adansonii*King Quail, Coturnix chinensis*Rain Quail, Coturnix coromandelica*Harlequin Quail, Coturnix delegorguei*Common Quail, Coturnix coturnix...

 quail. See Cock egg
Cock egg
A cock egg is an archaic term for a yolkless egg . Since they contained no yolk and therefore cannot hatch, these eggs were traditionally believed to be laid by roosters. This gave rise to the myth that when a cock's egg was hatched, it would produce a cockatrice, a fearsome serpent which could...

.

External links

  • Anatomy of an Egg from the Exploratorium
    Exploratorium
    The Exploratorium is a museum in San Francisco with over 475 participatory exhibits, all of them made onsite, that mix science and art. It also aims to promote museums as informal education centers....

  • Making egg tempera from the Society of Tempera Painters
    Society of Painters in Tempera
    The Society of Painters in Tempera was founded in 1901 by Christiana Herringham and a group of British painters who were interested in reviving the art of tempera painting. Lady Herringham was an expert copyist of the Italian Old Masters and had translated Il Libro dell' Arte o Trattato della...

  • Double Yolked Egg - External Size Comparison from Parker Pond Farm
  • Odd eggs info
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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