The eggΓÇÖs shell is made of hard but slightly porous calcium carbonate and lined on the inside and outside with protective membranes. Most eggs are washed before coming to market, which removes the outside membrane or cuticle; a light coating of mineral oil is often applied to replace it. The color of the shell is an indication of the breed of the hen and has no connection with the quality of the egg or of its flavor. Brown shells are preferred by some cooks, particularly in the NortheastΓÇöand certain species of chickens lay yellow or even light pink, green, or blue eggs.
A single egg white from a large egg weighs about 1 ounce, provides about 17 calories, is almost 90 percent water, and is otherwise made up mostly of protein, with only trace amounts of vitamins and minerals. The yolk of the same egg, although smaller in size (weighing about 1/2 ounce), is far denser and is richer in calories (about 60), nutrients, and flavor. Besides providing fat, cholesterol, vitamins, and minerals, the yolk offers a bit of protein and a measure of lecithinΓÇöthe compound with the ability to make sauces, like mayonnaise, thick and smooth. A whole egg contains all of the essential amino acids, an essential fatty acid called linoleic acid, 6 grams of protein, 4.5 grams of fat (1.5 grams of which are saturated), 213 milligrams of cholesterol, 1 gram of carbohydrate, 60 milligrams of potassium, and 65 milligrams of sodium, as well as 13 vitaminsΓÇöalmost all except C and niacin.
Besides shell, yolk, and white, an egg has several other parts: the chalazae, or chalazae cords, the twisted, slightly elastic strands of egg white that anchor the yolk to the ends of the shell so that it stays neatly in the middle (most prominent in very fresh eggs, these do not interfere with the taste or cooking of eggs but are strained out of custards before baking so they do not form tiny lumps; the germ cell, or germinal spotΓÇöthe tiny, pinhead-sized white spot that sits on top of a fresh egg yolk and carries the chickenΓÇÖs genetic informationΓÇöwhich has no effect on the quality of an egg; and the air cell, or slight space between the egg membrane and the eggshell at the wide end of every egg (the larger the air pocket, the older the egg; a very old egg will actually shrivel up inside its shell). The dark spots you sometimes see in egg yolks are nothing more than blood from capillaries that burst when the egg was formed. These spots are harmless and can easily be removed with the tip of a knife.
