Daily Archives: July 7, 2016
iPhoneOgraphy – 07 Jul 2016 (Day 189/366)
The yolk in an egg is material of which the primary function is to supply food for the development of the animal embryo. Some kinds of eggs contain no yolk, for example because they are laid in situations where the food supply is adequate (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, sometimes 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 fishes and invertebrates, the yolk material is not in a special organ,but inside the ovum.
Yolks, being mainly stored food, tend to be very concentrated, and in particular tend to be rich in nutrients such as vitamins, minerals, lipids and proteins. The proteins function partly as food in their own right, and partly in controlling 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. Yolk is not living cell material like protoplasm, but largely passive material, that is to say deutoplasm, in this case functioning mainly as food for the embryo. The food material, plus associated control structures, is supplied by the maternal body during the process called 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.
In the avian egg, the yolk usually is some shade of yellow in colour. 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. 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. This fact was discovered by Hoyer in 1858.
After the fertilization, the cleavage 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 frying results in a pale yellow mass, as in omelettes and scrambled eggs.
The color of an egg yolk is directly influenced by the makeup of the chicken feed. 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.
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 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.