An
egg yolk is a part of an
eggIn most birds and reptiles, an egg is the zygote, resulting from fertilization of the ovum. To enable incubation the egg is usually kept within a favourable temperature range as it nourishes and protects the growing embryo. When the embryo is adequately developed it breaks out of the egg in the...
which feeds the developing
embryoAn 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 whiteEgg white is the common name for the clear liquid contained within an egg. It is the cytoplasm of the egg, which until fertilization is a single cell . It consists mainly of about 15% proteins dissolved in water...
(known more formally as albumen or
ovalbuminOvalbumin is the main protein found in egg white, making up 60-65% of the total protein. While Ovalbumin displays sequence and three-dimensional homology to the serpin superfamily, it is a noninhibitory serpin...
) by one or two spiral bands of tissue called the
chalazaThe chalaza is a structure inside animal 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 discThe Germinal Disc is a small, circular, white spot on the surface of the yellow yolk of an egg that has been laid. The nucleus of the egg is in the blastodisc....
is a single
cellThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, 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
foodAn egg is a round or oval body laid by the female of any number of different species, consisting of an ovum surrounded by layers of membranes and an outer casing, which acts to nourish and protect a developing embryo and its nutrient reserves...
, yolks are a major source of
vitaminA vitamin is an organic compound required as a nutrient in tiny amounts by an organism. The term 'vitamin' first became popular in the early 1800's as a contraction of the words 'vital' and 'mineral', though the actual meaning of the word has developed somewhat since that time...
s and minerals.
An
egg yolk is a part of an
eggIn most birds and reptiles, an egg is the zygote, resulting from fertilization of the ovum. To enable incubation the egg is usually kept within a favourable temperature range as it nourishes and protects the growing embryo. When the embryo is adequately developed it breaks out of the egg in the...
which feeds the developing
embryoAn 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 whiteEgg white is the common name for the clear liquid contained within an egg. It is the cytoplasm of the egg, which until fertilization is a single cell . It consists mainly of about 15% proteins dissolved in water...
(known more formally as albumen or
ovalbuminOvalbumin is the main protein found in egg white, making up 60-65% of the total protein. While Ovalbumin displays sequence and three-dimensional homology to the serpin superfamily, it is a noninhibitory serpin...
) by one or two spiral bands of tissue called the
chalazaThe chalaza is a structure inside animal 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 discThe Germinal Disc is a small, circular, white spot on the surface of the yellow yolk of an egg that has been laid. The nucleus of the egg is in the blastodisc....
is a single
cellThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, 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
foodAn egg is a round or oval body laid by the female of any number of different species, consisting of an ovum surrounded by layers of membranes and an outer casing, which acts to nourish and protect a developing embryo and its nutrient reserves...
, yolks are a major source of
vitaminA vitamin is an organic compound required as a nutrient in tiny amounts by an organism. The term 'vitamin' first became popular in the early 1800's as a contraction of the words 'vital' and 'mineral', though the actual meaning of the word has developed somewhat since that time...
s and minerals. They contain all of the egg's
fatFats consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water. Chemically, fats are generally triesters of glycerol and fatty acids. Fats may be either solid or liquid at normal room temperature, depending on their structure and composition...
and
cholesterolCholesterol is a lipidic, waxy steroid found in the cell membranes and transported in the blood plasma of all animals. It is an essential component of mammalian cell membranes, where it is required to establish proper membrane permeability and fluidity...
, and almost half of the
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
.
If left intact while cooking fried eggs, the yellow yolk surrounded by a flat blob of egg white creates the distinctive
sunny-side upFried eggs are traditionally eaten for breakfast in English-speaking countries, but may be eaten at other times of the day in other cultures.A fried egg is constructed by taking an egg and frying it....
form of the food. Mixing the two components together before frying results in the pale yellow form found in
omeletteAn omelette or omelet is a preparation of beaten egg quickly cooked with butter or oil in a frying pan, usually folded around a filling such as cheese, vegetables, meat , or some combination of the above...
s and
scrambled eggsScrambled eggs is a dish made from beaten whites and yolks of eggs . Beaten eggs are put into a hot greased pan and stirred frequently, forming curds as they coagulate.-Sample preparation:...
.
Uses
- It is sometimes separated
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 white and used in cooking (for mayonnaiseMayonnaise is a thick condiment. White or yellowish-white in color, it is a stable emulsion of oil, salt and vinegar, or lemon juice which uses egg yolks as an emulsifier...
, custardCustard is a range of preparations based on milk and eggs. Most commonly, custard refers to a dessert or dessert sauce, but custard bases are also used for quiches and other savoury foods. As a dessert, it is made from a combination of milk or cream, egg yolks, sugar, and vanilla...
, hollandaise sauceHollandaise sauce is an emulsion of butter and lemon juice or vinegar using egg yolks as the emulsifying agent, usually seasoned with salt and a little black pepper or cayenne pepper. It is a French sauce, so named because it was believed to have mimicked a Dutch sauce. Hollandaise sauce is well...
, crème brûléeCrème brûlée , burnt cream, crema catalana, or Trinity cream is a dessert consisting of a rich custard base topped with a layer of hard caramel, created by caramelizing sugar under a broiler, with a propane or butane torch or other intense heat source, or by pouring sugar...
, avgolemonoAvgolemono is a Greek soup made with egg and lemon juice mixed with broth, heated until they thicken but before they boil and curdle. Avgolémono translates to "egg-lemon," in English....
, and ovos-molesOvos-moles is a local delicacy from Aveiro, Portugal, made of egg yolks and sugar.- Ingredients :Ovos Moles de Aveiro *1/4 cup short-grain rice*2 cups water*1 cup sugar...
).
- It is used in painting as a component of traditional egg-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 first centuries AD still...
.
- It is used in the production of egg-yolk agar plate
An agar plate is a sterile Petri dish that contains a growth medium used to culture microorganisms or small plants like the moss Physcomitrella patens....
medium, useful in testing for the presence of Clostridium perfringensClostridium perfringens is a Gram-positive, rod-shaped, anaerobic, spore-forming bacterium of the genus Clostridium. C. perfringens is ubiquitous in nature and can be found as a normal component of decaying vegetation, marine sediment, the intestinal tract of humans and other vertebrates, insects,...
.
- Egg yolk also contains an antibody
Antibodies are gamma globulin proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses...
called Immunoglobulin yolk (IgY). The antibody transfers from the laying hen to the egg yolk by passive immunityPassive 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 is a rich and creamy liqueur made from eggs, sugar and brandy. It has a smooth, custard-like flavor. In English-speaking countries it generally contains 15% alcohol , but in Continental Europe the typical alcohol content differs from country to country and is generally somewhere between...
.
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.
One large egg (50 grams in weight, 17 gram yolk) contains approximately: 2.7g protein, 210 mg cholesterol, 0.61g carbohydrates and 4.51g total fat. (USDA National Nutrient Database)
All of the fat soluble vitamins, (
AVitamin A is linked to a family of similarly shaped molecules, the retinoids, which complete the remainder of the vitamin sequence. Its important part is the retinyl group, which can be found in several forms. In foods of animal origin, the major form of vitamin A is an ester, primarily retinyl...
,
DVitamin D is a group of fat-soluble prohormones, the two major forms of which are vitamin D2 and vitamin D3 . Vitamin D obtained from sun exposure, food, and supplements, is biologically inert and must undergo two hydroxylation reactions to be activated in the body...
,
EVitamin E is a generic term for tocopherols and tocotrienols. Vitamin E is a family of α-, β-, γ-, and δ-tocopherols and corresponding four tocotrienols. Vitamin E is a fat-soluble antioxidant that stops the production of reactive oxygen species formed when fat undergoes oxidation...
and
KVitamin K denotes a group of lipophilic, hydrophobic vitamins that are needed for the posttranslational modification of certain proteins, mostly required for blood coagulation...
) are found in the egg yolk. Egg yolks are 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 is a mono-unsaturated omega-9 fatty acid found in various animal and vegetable sources. It has the formula CH37CH=CH7COOH)...
47 %
- Linoleic acid
Linoleic acid is an unsaturated omega-6 fatty acid. It is a colorless liquid. In physiological literature, it is called 18:2. Chemically, linoleic acid is a carboxylic acid with an 18-carbon chain and two cis double bonds; the first double bond is located at the sixth carbon from the omega...
16 %
- Palmitoleic acid
Palmitoleic acid, or -9-hexadecenoic acid, is an omega-7 monounsaturated fatty acid with the formula CH
35CH=CH
7COOH. that is a common constituent of the glycerides of human adipose tissue. It is present in all tissues, but generally found in higher concentrations...
5 %
- Linolenic acid
Linolenic acid can refer to either of two fatty acids:*α-Linolenic acid – an ω-3 fatty acid found in many vegetable oils. The unmodified term linolenic acid most commonly refers to this substance.*γ-Linolenic acid – an ω-6 fatty acid...
2 %
- Saturated fatty acids:
- Palmitic acid
Palmitic acid,CH
314COOH or hexadecanoic acid in IUPAC nomenclature, is one of the most common saturated fatty acids found in animals and plants. As its name indicates, it is a major component of the oil from palm trees . The word palmitic is from the French "palmitique", the...
23 %
- Stearic acid
Stearic acid or 18:0 is a saturated fatty acid. It is a waxy solid, and its chemical formula is C
18H
36O
2, or CH
316COOH. Its name comes from the Greek word stéar , which means tallow...
4 %
- Myristic acid
Myristic acid, also called tetradecanoic acid or 14:0 is a common saturated fatty acid with the molecular formula CH
312COOH. A myristate is a salt or ester of myristic acid....
1 %
Egg yolk is a source of
lecithinLecithin is any of a 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...
, an emulsifier and
surfactantSurfactants are wetting agents that lower the surface tension of a liquid, allowing easier spreading, and lower the interfacial tension between two liquids.- Etymology :The term surfactant is a blend of surface active agent...
.
The yellow color is caused by
luteinLutein is one of over 600 known naturally occurring carotenoids. Found in green leafy vegetables such as spinach and kale, lutein is employed by organisms as an antioxidant and for blue light absorption...
and
zeaxanthinZeaxanthin is one of the two carotenoids contained within the retina of the eye. Within the central macula, zeaxanthin is the dominant component, whereas in the peripheral retina, lutein predominates....
, which are yellow or orange carotenoids known as xanthophylls
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 productive cycle not yet being synchronized.
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 and Coturnix Quail.
External links
- Anatomy of an Egg from the Exploratorium
The Exploratorium is a museum in San Francisco full of hundreds of hands-on exhibits, most of them made onsite, that mix science and art. It is also a leader in the movement to promote museums as informal education centers. Founded in 1969 by the noted physicist and educator Dr...
- Eggs from the Agricultural Marketing Service
The Agricultural Marketing Service is a division of the United States Department of Agriculture, and has programs in six commodity areas: cotton, dairy, fruit and vegetable, livestock and seed, poultry, and tobacco...
of the U.S. Department of AgricultureThe United States Department of Agriculture is the United States federal executive department responsible for developing and executing U.S. federal government policy on farming, agriculture, and food...
- Making egg tempera from the Society of Tempera Painters
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...