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Anatomy of an Egg

Martha Stewart Living Television

Eggs are as commonplace in the kitchen as milk or butter, important both as recipe ingredients and a complete food in themselves. But how familiar are you with this versatile food? 

Knowing the basic anatomy of an egg is the first step in learning how to prepare one perfectly -- whether whipped into a souffle or simply fried.

Renowned chef, author, and cooking instructor Madeleine Kamman explains the various parts of an egg and offers tips for working with these "miracles in a shell."

Parts of an Egg
The yolk makes up approximately one third of the weight of an egg and contains all of the fat and just under half the protein. It is extremely vitamin-rich, containing every vitamin except C; it's also one of the few natural food sources of vitamin D. The yolk acts as an emulsifier in recipes, allowing combinations of ingredients -- such as oil and water -- that normally don't mix. A fresh yolk will be firm and will stand tall when the egg is broken. The color of the yolk is largely determined by what the hen is fed.

Albumen (egg white)
The albumen makes up the remaining two-thirds of an egg's liquid weight, and is about 87 percent water. It contains more than half of the protein of the egg, is fat-free, and rich in niacin, riboflavin, chlorine, magnesium, potassium, sodium, and sulfur. The albumen directly surrounding the yolk is called the "thick white," and is firmer and thicker than the outer layer. This is especially true in a very fresh egg; as an egg ages, the entire white thins and becomes more watery. A cloudy egg white -- often seen in very fresh eggs -- is caused by the harmless presence of carbon dioxide, which escapes as the egg ages.

These small, white, stringy pieces act as an anchor between the yolk and the thick white, and are most apparent in fresh eggs. They are harmless and needn't be removed, though they can be strained away for aesthetic purposes if desired.

Air cell
A pocket of air that forms between the wide end of the eggshell and the albumen and increases in size as an egg ages. A very fresh egg will have little or no air cell.

USDA Grades
The United States Department of Agriculture (USDA) has established a grading system for eggs sold in the shell. It is not mandatory that eggs be graded by this system to be sold, but only those that meet the standards will carry the USDA grade shield on the package. In order of quality, USDA grades are AA, A, and B. Grades are not determined by size or nutritional value, but rather by age, shape and appearance of the shell, yolk, and albumen, and size and location of the air cell. An egg graded AA will have a firm yolk and a thick albumen, a small air cell, and will generally not be more than 10 days old. Grade A eggs can be ten days to several weeks old, and will have a fairly firm, upstanding yolk and a good proportion of thick white to thin. Both AA and A must have oval-shaped shells with one larger end; a misshapen egg will automatically be grade B. A grade B egg is more watery than the higher grades, and will have a larger air cell. B eggs are rarely sold in grocery stores and generally go straight to manufacturers of egg products.

Commercial eggs vary widely in size, from extra-jumbo to peewee. The most commonly sold size is large -- this is the size called for in most recipes. A large egg should weigh no less than 2 ounces, whereas a jumbo will weigh about 2 3/4 ounces, and a small about 1 1/2 ounces. Peewee eggs are produced by hens that are not fully mature -- usually less than a year old. As a hen ages, the eggs she produces increase in size.

According to the USDA, eggs must be marked with an expiration date of no later than 30 days after they are packed for shipment. If refrigerated, eggs will keep for several weeks. For best results, store eggs in the container they are sold in, on a refrigerator shelf rather than in an egg compartment on the door, which is subject to frequent temperature changes as the door opens and closes. To test an egg for freshness, use Madeleine's simple trick: Place a whole, raw egg in the shell in a bowl of water. If it sinks, it is fresh; an egg that floats is filled with air and has gone bad.

The freshest eggs you can find are the best for frying and poaching, as they are the firmest and will hold their shape well. Eggs that are 10 days old and older are actually better for certain purposes, such as whipping and beating for recipes, and for hard-boiling, since a slightly older egg is easier to peel once it is cooked.

Learn more about Madeleine Kamman and her cookbook, "The New Making of a Cook."

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