Eggs, Basic Facts & Nutrient Content
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
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.
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 souffles, meringues, puffy omelets,
and angel food and sponge cakes.
meat spots. Occasionally found on
an egg yolk. Contrary to popular opinion, these tiny spots do not indicate a
fertilized egg. Rather, they are caused by the rupture of a blood vessel on the
yolk surface during formation of the egg or by a similar accident in the wall
of the oviduct. Less than 1% of all eggs produced have blood spots.
Mass candling methods reveal
most eggs with blood spots and those eggs are removed but, even with electronic
spotters, it is impossible to catch all of them. As an egg ages, the yolk takes
up water from the albumen to dilute the blood spot so, in actuality, a blood
spot indicates that the egg is fresh. Both chemically and nutritionally, these
eggs are fit to eat. The spot can be removed with the tip of a knife, if you
Egg cartons from
USDA-inspected plants must display a Julian date--the date the eggs were
packed. Although not required, they may also carry an expiration date beyond
which the eggs should not be sold. In USDA-inspected plants, this date cannot
exceed 30 days after the pack date. It may be less through choice of the packer
or quantity purchaser such as your local supermarket chain. Plants not under
USDA inspection are governed by laws of their states.
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.
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.
Egg shell and yolk color may
vary, but color has nothing to do with egg quality, flavor,
nutritive value, cooking characteristics or shell
* Shell: The color comes from
pigments in the outer layer of the shell and may range in various breeds from
white to deep brown. The breed of hen determines the color of the shell. Breeds
with white feathers and ear lobes lay white eggs; breeds with red feathers and
ear lobes lay brown eggs. White eggs are most in demand among American buyers.
In some parts of the country, however, particularly in New England, brown shells are preferred. The Rhode Island Red, New Hampshire and Plymouth Rock are breeds that lay brown eggs.
Since brown-egg layers are slightly larger birds and require more food, brown
eggs are usually more expensive than white.
* White: Egg albumen in raw
eggs is opalescent and does not appear white until it is beaten or cooked. A
yellow or greenish cast in raw white may indicate the presence of riboflavin.
Cloudiness of the raw white is due to the presence of carbon dioxide which has
not had time to escape through the shell and thus indicates a very fresh egg.
On very rare occasions, a
hard-cooked egg white may darken to a caramel shade due to a high amount of
iron in the cooking water or to a carbonylamine-type
reaction. Using fresh eggs and cooling them quickly after cooking helps to
prevent this darkening.
* Yolk: color depends on the
diet of the hen. If she gets plenty of yellow-orange plant pigments known as
xanthophylls, they will be deposited in the yolk. Hens fed mashes containing
yellow corn and alfalfa meal lay eggs with medium
yellow yolks, while those eating wheat or barley yield lighter-colored yolks. A colorless diet, such as white cornmeal
produces almost colorless yolks. Natural yellow-orange substances such as
marigold petals may be added to light-colored feeds
to enhance yolk color. Artificial color additives are not permitted. Gold or
lemon-colored yolks are preferred by most buyers in
this country. Yolk pigments are relatively stable and are not lost or changed
Sometimes there is a greenish
ring around hard-cooked egg yolks. It is the result of sulfur
and iron compounds in the egg reacting at the surface of the yolk. It may occur
when eggs are overcooked or when there is a high amount of iron in the cooking
water. Although the color may be a bit unappealing, the eggs are still
wholesome and nutritious and their flavor is
unaffected. Greenish yolks can best be
avoided by using the proper cooking time and temperature and by rapidly cooling
the cooked eggs.
concentric green rings may be seen in hard-cooked egg yolks. A yolk develops
within the hen in rings. Iron in the hen's feed or water as the rings are
formed may cause this coloring. Sometimes
a large batch of scrambled eggs may turn green. Although not pretty, the color
change is harmless.
It is due to a chemical
change brought on by heat and occurs when eggs are
cooked at too high a temperature, held for too long or both. Using stainless
steel equipment and low cooking temperature, cooking in small batches and
serving as soon as possible after cooking will help to prevent this. If it is
necessary to hold scrambled eggs for a short time before serving, it helps to
avoid direct heat. Place a pan of hot water between the pan of eggs and the
Break Out Appearance
Covers a small area.
Covers a moderate area.
Covers a wide area.
White is thick and stands high; chalaza prominent.
White is reasonably thick, stands fairly high; chalaza prominent.
Small amount of thick white; chalaza small or absent. Appears weak and watery.
Yolk is firm, round and high.
Yolk is firm and stands fairly high.
Yolk is somewhat flattened and enlarged.
Approximates usual shape; generally clean,* unbroken; ridges/rough spots that do not affect the shell strength are permitted.
Abnormal shape; some slight stained areas permitted; unbroken; pronounced ridges/thin spots permitted.
Ideal for any use, but are especially desirable for poaching, frying and cooking in shell.
Ideal for any use, but are especially desirable for poaching, frying and cooking in shell.
Good for scrambling, baking, and as an ingredient in other foods.
*An egg may be considered clean if it has only very
small specks, stains or cage marks. Source: USDA
Grade AA: A 'Grade AA' egg will stand up tall. The yolk is firm and the area covered by the white is small. There is a large proportion of thick white to thin white.
Grade A: A 'Grade A' egg covers a relatively small area. The yolk is round and upstanding. The thick white is large in proportion to the thin white and stands fairly well around the yolk.
Grade B: A 'Grade B' egg spreads out more. The yolk is flattened and there is about as much (or more) thin white as thick white.
can be incubated and developed into chicks. Fertile eggs are not more nutritious than nonfertile eggs, do not keep as well as nonfertile
eggs and are more expensive to produce. Fertile eggs may contain a small amount
of male hormone, but there are no known advantages.
True free-range eggs are
those produced by hens raised outdoors or that have daily access to the
outdoors. Due to seasonal conditions, however, few hens are actually raised
outdoors. Some egg farms are indoor floor operations and these are sometimes
erroneously referred to as free-range operations. Due to higher production
costs and lower volume per farm, free-range eggs are generally more expensive.
The nutrient content of eggs is not affected by whether hens are raised
free-range or in floor or cage operations. [Bee's
note: this is not true. A chicken's
natural diet includes worms, insects, bugs, beetles, etc., like any other bird,
which are a very important source of oil soluble vitamins and fat. This definitely influences the nutrients
contained in their eggs.]
How recently an egg was laid
has a bearing on its freshness but is only one of many factors. The temperature
at which it is held, the humidity and the handling all play their part. These
variables are so important that an egg one week old, held under ideal
conditions, can be fresher than an egg left at room temperature for one day.
The ideal conditions are temperatures that don't go above 40°F. (4°C.) and a relative humidity of
70 to 80%.
Proper handling means prompt
gathering, washing and oiling of the eggs within a few hours after laying. Most commercially produced eggs reach supermarkets
within a few days of leaving the laying house. If the market and the buyer
handle them properly, they will still be fresh when they reach the table.
It is not true that freshness
can be judged by placing an egg in salt water. A carefully controlled brine
test is sometimes used to judge shell thickness of eggs for hatching purposes
but has no application to freshness of table eggs.
How important is
"freshness"? As an egg ages, the white becomes thinner and the yolk
becomes flatter. These changes do not have any great effect on the nutritional
quality of the egg or its functional cooking properties in recipes. Appearance
may be affected, though. When poached or fried, the fresher the egg, the more
it will hold its shape rather than spread out in the pan. On the other hand, if
you hard cook eggs that are at least a week old, you'll find them easier to
peel after cooking and cooling than fresher eggs.
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.
Shell Membranes: 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.
Classification determined by
interior and exterior quality and designated by letters — AA, A and B. In many
egg packing plants, the USDA provides a grading service for shell eggs. Its
official grade shield certifies that the eggs have been graded under federal
supervision according to USDA standards and regulations. The grading service is
not mandatory. Other eggs are packed under state regulations which must meet or
exceed federal standards.
In the grading process, eggs
are examined for both interior and exterior quality and are sorted according to
weight (size). Grade quality and size are not related to one another. In
descending order of quality, grades are AA, A and B.
There is no difference in
nutritive value between the different grades.
Because production and
marketing methods have become very efficient, eggs move so rapidly from laying
house to market that you will find very little difference in quality between
Grades AA and A.
Although grade B eggs are just as wholesome to eat, they rate
lower in appearance when broken out. Almost no Grade B's find
their way to the retail supermarket. Some go to institutional egg users
such as bakeries or foodservice operations, but most go to egg breakers for use
in egg products.
Starting with January 1 as
number 1 and ending with December 31 as 365, these numbers represent the
consecutive days of the year. This numbering system is sometimes used on egg
cartons to denote the day the eggs are packed. Fresh shell eggs can be stored
in their cartons in the refrigerator for 4 to 5 weeks beyond this date with
insignificant quality loss.
These are eggs from hens fed
rations having ingredients that were grown without pesticides, fungicides,
herbicides or commercial fertilizers. No commercial laying hen's rations ever
to higher production costs and lower volume per farm, organic eggs are more
expensive than eggs from hens fed conventional feed. The nutrient content of
eggs is not affected by whether or not the ration is organic [Bee's note:
However the nutrient content of eggs is affected by the foods eaten. Certified organic free-range eggs from
chickens eating their natural diet, which includes insects, bugs, worms, beetles
and other meats, provides the most nutritious eggs.]
outer covering, accounting for about 9 to l2% of its total weight depending on
egg size. The shell is the egg's
first line of defense against bacterial
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
Uses for eggshells vary from
the thrifty (compost) to the creative (decorating). [Bee's note: eggshells can
also be ground up and consumed for calcium and other minerals.]
Several factors influence the
size of an egg. The major factor is the age of the hen. As the hen ages, her
eggs increase in size.
The breed of hen from which
the egg comes is a second factor. Weight of the bird is another. Pullets significantly
underweight at sexual maturity will produce small eggs.
Environmental factors that
lower egg weights are heat, stress, overcrowding and poor nutrition.
All of these variables are of
great importance to the egg producer. Even a slight shift in egg weight
influences size classification and size is one of the factors considered when
eggs are priced. Careful flock management benefits both the hens and the
Egg sizes are Jumbo, Extra
Large, Large, Medium, Small and Peewee. Medium, Large and Extra Large are the
sizes most commonly available.
Sizes are classified
according to minimum net weight expressed in ounces per dozen.
Although any size egg may be
used for frying, scrambling, cooking in the shell or poaching, most recipes for
baked dishes such as custards and cakes are based on the use of Large eggs. To substitute another size, use the following
To Make 1 Cup
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.
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.
Basic Egg Facts