Composition
of the Egg
COMPOSTION OF THE EGG
SHELL
AIR CELL
YOLK
SHELL
MEMBRANE
LATEBRA
GERMINAL
DISC
CHALAZAE
VITELLINE
(VOLK)
MEMBRANE
CHALAZAE
THIN
ALBUMEN
THICK
ALBUMEN
AIR CELL
The empty space between the white and shell at the large end of the
egg.
When an egg is first laid, it is warm. As it cools, the contents contract and the
inner shell membrane separates from the outer shell membrane to form the air
cell.
The candler uses the size of the air cell as one basis for determining grade. In
Grade AA eggs, the air cell may not exceed 1/8-inch in depth and is about the
size of a dime. The air cell of Grade A eggs may exceed 3/16-inch in depth. For
Grade B eggs, there is no limit on air cell size.
As the egg ages, moisture and carbon dioxide leave through the pores of the
shell, air enters to replace them and the air cell becomes larger.
Although the air cell usually forms in the large end of the egg, it occasionally
moves freely toward the uppermost point of the egg as the egg is rotated.
It is then called a free or floating air cell. If the main air cell ruptures, resulting in
one or more small separate air bubbles floating beneath the main air cell, it is
known as a bubbly air cell.
You can see the air cell in the flattened end of a peeled, hard-cooked
egg.
ALBUMEN
Also known as egg white. Albumen accounts for most of an egg's liquid weight,
about 67%. It contains more than half the egg's total protein, niacin, riboflavin,
chlorine, magnesium, potassium, sodium and sulfur. The albumen consists
of 4 alternating layers of thick and thin consistencies. From the yolk outward,
they are designated as the inner thick or chalaziferous white, the inner thin white, the
outer thick white and the outer thin white. Egg white tends to thin out as an egg
ages because its protein changes in character. That's why fresh eggs sit up tall
and firm in the pan while older ones tend to spread out.
Albumen is more opalescent than truly white. The cloudy appearance comes
from carbon dioxide. As the egg ages, carbon dioxide escapes, so the
albumen of older eggs is more transparent than that of fresher eggs.
When egg albumen is beaten vigorously, it foams and increases in volume 6 to
8 times. Egg foams are essential for making soufflés, meringues, puffy
omelets, and angel food and sponge cakes.
CHALAZA
Ropey strands of egg white which anchor the yolk in place in the center of the
thick white. They are neither imperfections nor beginning embryos.
The more prominent the chalazae, the fresher the egg. Chalazae do not interfere
with the cooking or beating of the white and need not be removed, although
some cooks like to strain them from stirred custard.
GERMINAL DISC
The entrance of the latebra, the channel leading to the center of the yolk. The germinal
disc is barely noticeable as a slight depression on the surface of the yolk. When
the egg is fertilized, sperm enter by way of the germinal disc, travel to the
center and a chick embryo starts to form.
MEMBRANES
Shell Membranes
SHELL
Just inside the shell are two shell membranes, inner and outer. After the egg is laid and
it begins to cool, an air cell forms between these two layers at the large end of
the egg.
Vitelline Membrane:
This is the covering of the yolk. Its strength protects the yolk from
breaking. The vitelline membrane is weakest at the germinal disc and
tends to become more fragile as the egg ages.
The egg's outer covering, accounting for about 9 to 12% of its total weight
depending on egg size. The shell is the egg's first line of defense against
bacterial contamination.
The shell is largely composed of calcium carbonate (about 94%) with small
amounts of magnesium carbonate, calcium phosphate and other organic matter
including protein.
Shell strength is greatly influenced by the minerals and vitamins in the hen's diet,
particularly calcium, phosphorus, manganese and Vitamin D. If the diet is
deficient in calcium, for instance, the hen will produce a thin or soft-shelled egg
or possibly an egg with no shell at all. Occasionally an egg may be prematurely
expelled from the uterus due to injury or excitement. In this case, the shell has
not had time to be completely formed. Shell thickness is also related to egg size
which, in turn, is related to the hen's age. As the hen ages, egg size increases.
The same amount of shell material which covers a smaller egg must be
"stretched" to cover a larger one, hence the shell is thinner.
Seven to 17 thousand tiny pores are distributed over the shell surface, a
greater number at the large end. As the egg ages, these tiny holes permit
moisture and
carbon dioxide to move out and air to move in to form the air cell. The shell is covered with a
protective coating called the cuticle or bloom. By blocking the pores, the cuticle
helps to preserve freshness and prevent microbial contamination of the
contents.
Uses for eggshells vary from the thrifty (compost) to the creative (decorating).
YOLK
The yolk or yellow portion makes up about 33% of the liquid weight of the egg.
It contains all of the fat in the egg and a little less than half of the protein.
With the exception of riboflavin and niacin, the yolk contains a higher proportion
of the egg's vitamins than the white. All of the egg's vitamins A, D and E are in
the yolk. Egg yolks are one of the few foods naturally containing vitamin D.
The yolk also contains more phosphorus, manganese, iron, iodine, copper, and
calcium than the white, and it contains all of the zinc. The yolk of a Large egg
contains about 59 calories.
Double-yolked eggs are often produced by young hens whose egg production
cycles are not yet completely synchronized. They're often produced, too, by
hens who are old enough to produce Extra Large eggs. Genetics is a factor,
also. Occasionally a hen will produce double-yolked eggs throughout her
egg-laying career. It is rare, but not unusual, for a young hen to produce an egg
with no yolk at all.
In fertilized eggs, the yolk is the site of embryo formation.
It is the yolk which is responsible for the egg's emulsifying
properties.
COMPOSITON OF THE EGG
Fill in the blanks using the following words. Some words may be used more
than
once.
Air Cell
Chalazae
Latebra
Shell
Thick Albumen
Yolk
Thin Albumen
Germinal Disc
Shell Membrane
Vitelline Yolk Membrane
