Carton Dates

Egg cartons from USDA-inspected plants are required to display a Julian date—the date the eggs were packed. The numbers represent the consecutive days of the year, starting with January 1 as number 1 and ending with December 31 as 365. This numbering system is used to denote the day the eggs are packed.

Fresh shell eggs can be stored in their cartons in the refrigerator for 4-5 weeks beyond this date without significant quality loss.

Shell

The egg’s outer covering accounts for 9 to l2% of its total weight, and is the egg’s first line of defense against contamination from bacteria. The shell is comprised of calcium carbonate (about 94%) with small amounts of magnesium carbonate, calcium phosphate and other organic matter (including protein).

The minerals and vitamins in the hen’s diet greatly influence the strength of an egg shell, 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 even an egg with no shell at all.

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.

A single egg has between 7,000 and 17,000 tiny pores distributed over the shell surface. 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 contamination.

Eggshells can be used for other things as well, ranging from the creative (decorating) to the practical (compost for flower beds and gardens).

Yolk

The yolk (yellow portion) makes up about 33% of the liquid weight of the egg. It contains all of the egg’s fat and almost 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 all of the zinc. The yolk of a large egg contains about 59 calories.

Double-yolked eggs usually come from young hens whose egg production cycles are not fully synchronized or from hens old enough to produce extra large eggs. Genetics are also a factor: 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.

The yolk is responsible for the egg’s emulsifying properties, and in fertilized eggs, the yolk is the site of embryo formation.

Color

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

Shell Color

The shell’s color is determined by the breed of the hen. Breeds with white feathers and ear lobes lay white eggs; breeds with red feathers and ear lobes lay brown eggs. The color comes from pigments in the outer layer of the shell and may range from white to deep brown.

White eggs are most in demand among American buyers. In some parts of the country and particularly in New England, though, brown shells are preferred. Breeds like the Rhode Island Red, New Hampshire and Plymouth Rock lay brown eggs. Since brown-egg layers are slightly larger birds and require more food, brown eggs are usually more expensive than white.

Egg White

Egg albumen in raw eggs does not appear white until it is beaten or cooked. When the raw egg white is cloudy, it indicates the presence of carbon dioxide that has not had time to escape through the egg shell. This means the egg is very fresh. A yellow or greenish cast in raw white may indicate the presence of riboflavin.

Very rarely, a hard-cooked egg white may darken to a caramel shade. This is caused by 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.

Egg Yolk

The hen’s diet determines the color of the yolk. Consuming a lot of yellow-orange plant pigments (known as xanthophylls) results in a brighter yolk. Hens fed mashes containing yellow corn and alfalfa meal lay eggs with medium yellow yolks. Those eating wheat or barley yield lighter-colored yolks. A colorless diet, such as white cornmeal produces almost colorless yolks. Natural yellow-orange substances (like marigold petals) may be added to light-colored feeds to enhance yolk color (but artificial color additives are not permitted). Most consumers in the U.S. prefer gold or lemon-colored yolks. Yolk pigments are relatively stable and do not change during cooking.

Sometimes hard-cooked egg yolks have a greenish ring. This occurs when eggs are overcooked or when the cooking water has a high amount of iron, and is the result of sulfur and iron compounds in the egg reacting at the surface of the yolk. The eggs are still wholesome and nutritious, and their flavor is unaffected. Not overcooking the eggs and rapid cooling will help avoid greenish yolks.

Yolks develop in rings, and the iron in a hen’s feed or water may occasionally cause several concentric green rings. These can be seen in hard-cooked egg yolks.

Scrambled eggs may also occasionally turn green (remember Dr. Seuss’ Green Eggs and Ham?). The change in color occurs when eggs are cooked at too high a temperature or held too long. Although not pretty, the color change is harmless.

Using stainless steel equipment, low cooking temperatures, cooking in small batches and serving as soon as possible after cooking will help to prevent color changes. When scrambled eggs must be held 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.

Air Cell

The air cell is the empty space between the white and shell at the large end of the egg. The air cell is easiest to see when peeling a hard-cooked egg.

When eggs are laid, they are warm. As they cool, the contents contract and the inner shell membrane separates from the outer shell membrane to form the air cell. 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.

The size of the air cell is one of the criteria used to grade eggs. In Grade AA eggs, the air cell may not exceed 1/8” in depth and is about the size of a dime. The air cell of Grade A eggs may exceed 3/16” in depth. For Grade B eggs, there is no limit on air cell size.

Although the air cell usually forms in the large end of the egg, it occasionally moves as the egg is rotated. It is then called a free or floating air cell. If parts of the air cell break into smaller air bubbles, the egg has what is called a bubbly air cell.

Albumen (commonly known as egg white)

Albumen accounts for 2/3’s of an egg’s liquid weight and 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 1) the inner thick (or chalaziferous) white, 2) the inner thin white, 3) the outer thick white and 4) 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.

When egg albumen is beaten vigorously, it foams and increases in volume 6-8 times. Egg foams are essential for making soufflés, meringues, puffy omelets, angel food cakes and sponge cakes.

Blood Spots (sometimes called meat spots)

Less than 1% of eggs have blood spots, which are caused by a ruptured blood vessel on the yolk surface during formation of the egg or by a similar accident in the wall of the oviduct. Most eggs with blood spots are removed but even with electronic spotters it is impossible to catch them all. Contrary to popular opinion, these tiny spots on the yolk do not indicate a fertilized egg.

In actuality, a blood spot indicates that the egg is fresh. Because the yolk absorbs water from the egg white as it ages, the blood spot dilutes with time.

Eggs with blood spots are fit to eat, both nutritionally and chemically. If desired, however, the spot can be removed with the tip of a knife.

Chalaza

Chalazae are ropey strands of egg white that hold the yolk in place in the center of the thick white; the more prominent the chalazae, the fresher the egg. They are neither imperfections nor beginning embryos, and do not interfere with the cooking or beating of the white. Although some cooks like to strain them from stirred custard, they do not need to be removed.

Germinal Disc

The germinal disc is barely noticeable as a slight depression on the surface of the yolk. It is the entrance of the latebra, the channel leading to the center of the yolk. When the egg is fertilized, sperm enter the germinal disc, travel to the center and a chick embryo begins to form.

Membranes

Shell Membranes

Every egg has two shell membranes, an inner and an 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

The vitelline membrane covers the yolk and protects it from breaking. This membrane is weakest at the germinal disc, and tends to become more fragile as the egg ages.

Fertile Eggs

Fertile eggs are those which can be incubated and developed into chicks. Fertile eggs are not more nutritious than non-fertile eggs, do not keep as well as non-fertile eggs and are more expensive to produce.

Cage-Free Eggs

True free-range eggs are produced by hens raised outdoors or hens with daily access to the outdoors. Due to seasonal conditions, however, few hens are actually raised outdoors. Some egg farms have indoor floor operations that are sometimes erroneously referred to as free-range operations.

The nutrient content of eggs is not affected by whether hens are raised free-range or in floor or cage operations. Due to higher production costs and lower volume per farm, free-range eggs are generally more expensive.

Organic Eggs

Organic eggs come from uncaged hens fed a diet of ingredients grown without pesticides, fungicides, herbicides or commercial fertilizers. No commercial laying hen rations ever contain hormones.

Because of higher production costs and lower volumes, organic eggs are more expensive than eggs from hens fed conventional feed. The eggs nutrient content is the same regardless of type of diet.

Freshness

When an egg was laid has a bearing on its freshness, but is only one of many factors. Temperature at which it is held, humidity and handling are all parts of the equation. These variables are so important that an egg one week old that is held under ideal conditions, can be fresher than an egg left at room temperature for one day. 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 of being laid. When handled properly, most commercially produced eggs reach supermarkets within a few days of leaving the laying house.

A common myth is that freshness can be judged by placing an egg in salt water. A controlled brine test is sometimes used to judge shell thickness for hatching purposes, but has no application to freshness of table eggs.

As an egg ages, the white becomes thinner and the yolk becomes flatter. These changes do not have any great effect on nutritional quality or functional cooking properties.

Appearance may be affected, though. The fresher the egg, the more it will hold its shape when poached or fried (doesn’t spread out so much in the pan). On the other hand, hard- cooked eggs at least a week old are much easier to peel than fresher eggs.