An egg yolk is the central part of a bird egg, which is designed to provide nutrients to a developing chick before it hatches. Yolks are commonly very high in a number of vitamins and minerals, making them an enviable food source for humans as well as many other animals. The vast majority of the eggs that make it to human markets are unfertilized, which means that they are free of any embryos. In most cases, yolks are yellow or orange in color.
Place in Biology
All bird species reproduce through the laying of eggs, and the yolk plays a pivotal role in helping developing chicks to grow. The embryonic chick typically develops next to or on top of the yolk, and is able to extract nutrients and sustenance that will support it until it is strong enough to break out of the shell.
Nutritional Attributes
The specific nutritional benefits of egg yolks vary depending on their size and type, but all are packed with protein and vitamins. Most are high in vitamins A, D, and E, though the B vitamins are sometimes also present. Essential minerals like folate, calcium, and iron are usually a part of the mix, as well.
As a Food Source
The same nutrients that help baby birds develop can also help a range of other animals, and the rich yolk is what makes eggs a desirable food source. Foxes and snakes are well-known egg thieves in and around hen houses, and animals of all varieties eat bird eggs found in the wild.
Most human cultures also prize eggs. Western cooking typically focuses on chicken eggs, but nearly any sort of bird egg is edible. Ostrich, quail, and dove egg yolks are used as food sources and recipe additives in many parts of the world.
Possible Downsides
Despite the benefits of egg yolks, most health experts recommend consuming them only in moderation because of how high they tend to be in cholesterol. Cholesterol, a substance present in the blood, is not necessarily bad — but too much of it can cause a number of health problems, mostly related to the heart. Regular consumption of egg yolks is often linked to “high cholesterol,” a medical condition that often requires medication to correct.
Culinary Uses
There are a great many ways to cook egg yolks. Simply boiling eggs whole is usually the simplest method, though scrambling or frying are also popular alternatives. These preparations typically make use both of the egg yolk and the clear membranes that surround it, usually referred to as the egg white.
Interaction with Egg Whites
Egg whites contain few of the nutritive benefits that yolks do, but are nonetheless useful for many baking and cooking projects because of their smooth, oily consistency. Some recipes require the yolk to be separated from the whites, usually for density reasons. Custards, souffles, and thick sauces like hollandaise commonly use yolks in isolation. Some more complex cakes and pastries require the egg yolk to be separated from the white while assembling the ingredients, even if both are ultimately used.
Cosmetic and Scientific Uses
Egg yolks may also have a place outside of the kitchen, particularly when it comes to cosmetics. Rinsing hair with egg yolk is believed by many to improve shine and elasticity, for instance, and applying beaten yolks to the skin is a common home remedy for dry or irritated skin.
Egg yolks may also be used in the pharmaceutical industry or in other science research endeavors. The yolk’s even consistency makes it an ideal surface upon which to test different molecular interactions and growth patterns.