Eggs Eggsamined

I started this article thinking it would be a simple table of egg grades and quality factors. But as I started researching the matter, I found that there is a lot of interesting information regarding eggs. I examine the structure of the egg, the effects of freshness, actual grading, and even dispel some common misconceptions regarding free-range and organic eggs and brown verses white eggs. So, which is a better egg, Grade A or Grade AA, brown or white, free-range, organic or neither? Let us find out.

Basic Egg Facts

Composition of the egg

Inside the egg

Inside on an eggDiagram courtesy of Alberta Egg Producers

Air Cell

The air cell can be the first indicator of egg quality. Air cells of no greater than 1/8 inch in height are of grade AA quality. Larger air cells up to 3/16 of an inch may be classified as grade A, air cells greater than 3/16 of one inch can only be grade B.


Albume is better known as the egg white. There are two distinct albume: a thick and a thin. 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.


Chalaza are strands of egg white that anchor the yolk in place in the center of the albume. They are not imperfections or beginning embryos.

Germinal Disc

The germinal disc is 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.


The shell is the egg’s outer covering which is largely composed of calcium carbonate.  It accounts for about 9 to 12% of its total weight depending on egg size.

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. Shell thickness is also related to egg size and the hen’s age. Older hens, for example, produce larger eggs with thinner shells.

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 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. A characteristic worth noting is that it is responsible for the egg’s emulsifying properties.

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.


Egg shell and yolk color may vary but color has nothing to do with egg quality, flavor, nutritive value, cooking characteristics or shell thickness.


The color comes from pigments in the outer layer of the shell and may range from white to deep brown. The hens breed determines the shell color. In general hens 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. 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, their eggs are generally more expensive.


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 indicates a very fresh egg.


Yolk color depends on the diet of the hen. If she gets plenty of yellow-orange plant pigments, 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.

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.

Sometimes a large batch of scrambled eggs may turn green. Although not pretty, the color change is harmless. Dr. Suess suggests serving eggs of this nature with ham. The color change occurs when eggs are cooked at too high a temperature, held for too long or both. 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 heat source.

Free-Range or Organic?:


The nutrient content of eggs is not affected by whether hens are raised free-range, in floor or cage operations, but rather by the breed and diet of the hens laying the eggs. Free-range eggs are, as suggested, those produced by hens raised outdoors or that have daily access to the outdoors. Few hens are actually raised outdoors due to seasonal conditions. Some egg farms are indoor floor operations and are sometimes wrongly referred to as free-range. Free-range eggs are generally more expensive because of higher production costs and lower volume.


The nutrient content of eggs is not affected by whether or not the ration is organic. Organic eggs are classified as eggs coming from hens fed rations having ingredients that were grown without pesticides, fungicides, herbicides or commercial fertilizers. Organic eggs are more expensive because of higher production costs and lower volume.


Obviously, when an egg is laid has a bearing on its freshness. This is only one of several factors. The temperature at which it is held, the humidity and the handling all play a part. The ideal conditions are at temperatures of 40°F. (4°C.) or lower and a humidity of approximately 75%. 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.

Proper handling means prompt gathering, washing and oiling of the eggs within a few hours after laying.

As an egg ages, the appearance changes with the white becoming thinner and the yolk becoming flatter. These changes do not have any great effect on the nutritional quality of the egg or its functional cooking properties in recipes. 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.

The three most important points to remember for egg freshness are:

  • The air cell becomes larger
  • The yolk becomes flatter, larger and breaks more easily
  • The thick white becomes thin and watery

Carton Dates:

Egg cartons from USDA-inspected plants must display a Julian date when 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.

Julian Dates:

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.


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. The weight of the bird is another.  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.

Egg sizes are Jumbo, Extra Large, Large, Medium, Small and Peewee.  Medium, Large and Extra Large are the sizes most commonly available. They are classified according to minimum net weight and 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 (2 ounces). To substitute another size, use the following chart.

Size Equivalents
Large Jumbo X-Large Medium Small
1 1 1 1 1
2 2 2 2 3
3 2 3 3 4
4 3 4 5 5
5 4 4 6 7
6 5 5 7 8
To Make 1 Cup
Egg Size Whole Whites Yolks
Jumbo 4 5 11
X-Large 4 6 12
Large 5 7 14
5 8 16
Small 6 9 18
Weight classes of U.S. Consumer Grades for Shell Eggs.
Size or weight class Minimum net weight per dozen (ounces) Minimum net weight 30 per dozen (pounds) Minimum net weight for individual eggs at rate per dozen (ounces)
Jumbo 30 56 29
Extra large 27 50 1/2 26
Large 24 45 23
Medium 21 39 1/2 20
Small 18 34 17
Peewee 15 28

Grading and Quality:

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.

Below is a simple table that differentiates the grades of eggs.

Grade AA Grade A Grade B
Break Out Appearance Covers a small area. Covers a moderate area. Covers a wide area.
Albumen Appearance 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 Appearance Yolk is firm, round and high. Yolk is firm and stands fairly high. Yolk is somewhat flattened and enlarged.
Shell Appearance 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.
Usage 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.
  • 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: 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: Egg spreads out more. The yolk is flattened and there is about as much (or more) thin white as thick white.
Egg Quality Decline

Egg Quality Decline

For more information see: USDA Egg-Grading Manual, a 56 page (yes 56 page) PDF that contains much more detailed information.

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  • By Robyn, 2009/08/12 @ 5:51 PM

    Good information, wasn’t aware of a few things
    gosh alot of work went into this thank you.

  • By doc22, 2009/08/13 @ 12:04 PM

    Any egg recipes?!

  • By doc22, 2009/08/15 @ 11:29 AM

    I must admit there was a lot of stuff I didn’t know about eggs and hens. Thanks for sharing the info!!!

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