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 (egg white), and
vitellus (egg yolk), contained within various thin
membranes. Popular choices for egg consumption are
chicken, duck, quail, roe, and caviar, but the egg
most often consumed by humans is the chicken egg,
by a wide margin.
Egg yolks and whole eggs store significant amounts
of protein and choline, and are widely used in cookery.
Due to their protein content, the United States
Department of Agriculture (USDA) categorizes eggs
as Meats within the Food Guide Pyramid. Despite
the nutritional value of eggs, there are some potential
health issues arising from egg quality, storage,
and individual allergies.
Chickens and other egg-laying creatures are widely
kept throughout the world, and mass production of
chicken eggs is a global industry. There are issues
of regional variation in demand and expectation,
as well as current debates concerning methods of
mass production, with the European Union's ban on
battery farming of chickens.
Chicken eggs are widely used in many types of dishes,
both sweet and savory, including many baked goods.
Some of the most common preparation methods include
scrambled, fried, hard-boiled, soft-boiled, and
pickled. They can also be eaten raw, though this
is not recommended for people who may be especially
susceptible to salmonellosis, such as the elderly,
the infirm, or pregnant women. In addition, the
protein in raw eggs is only 51% bioavailable, whereas
that of a cooked egg is nearer 91% bioavailable,
meaning the protein of cooked eggs is nearly twice
as absorbable as the protein from raw eggs. As an
ingredient, egg yolks are an important emulsifier
in the kitchen, and the proteins in egg white allow
it to form foams and aerated dishes.
The albumen, or egg white, contains protein but
little or no fat, and can be used in cooking separately
from the yolk. Egg whites may be aerated or whipped
to a light, fluffy consistency, and are often used
in desserts such as meringues and mousse. Ground
egg shells are sometimes used as a food additive
to deliver calcium. Every part of an egg is edible,
although the eggshell is generally discarded.