Exquisitely simple, yet enormously complex, the egg is one of nature's marvels. The eggs used in baking mainly come from hens.

Eggs' cooking properties are so varied, in fact, that eggs have been called "the cement that holds the castle of cuisine together." 

WHAT'S IN AN EGG?:

Whole Eggs: Contain a yolk and a white. These are encased in a shell.

Egg Yolks: It is the yolk or yellow portion which is responsible for the egg's emulsifying properties from the fat and lecithin contained in them (whites do not contain fat). Both contribute to the fine texture of baked goods and which bring the water and fat phases together in a recipe for a creamier, smoother texture. This is important when baking and making creamy sauces. The yolk also contains 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. The yolk of a large egg contains about 59 calories. The downside is it contains 213 mg of cholesterol, a significant percentage of the American Heart Association's suggested limit of 300 mg per day.

Egg Whites: Also known as albumen, it accounts for most of an egg's liquid weight, about 67%.

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 soufflés, meringues, puffy omelets, and angel food and sponge cakes.

Two dried egg white products, pasteurized dried egg white powder and meringue powder, can be used in recipes. They are both available at kitchenware and cake decorating shops, bakery suppliers, some supermarkets and by mail-order. Follow the directions below for reconstituting because it takes longer for the powder to dissolve than the label states.

Chalazae: the cord looking thing hanging from the egg is the chalazae. Now, the fresher the egg the stronger or pronounced the chalazae. Now, that's a Greek word that means hailstone. What's important is that it's essentially a rope of egg white that twists itself into being as the egg travels from the ovary to the nest. Its sole purpose is to keep the yolk centered in the egg. Remove it before using the egg. You may also have to strain a custard after baking to get rid of them or pieces .

SIZE DOES MATTER!!: The grades of eggs are defined by the USDA. They are AA, A, B, C. Most of the eggs on the market are Grade AA, or A. The B and C eggs go in for pet foods and other egg products.

Although any size egg may be used for frying, scrambling, cooking in the shell or poaching, most recipes for baking have been developed on the basis of a 48 (1.6931502 ounces) to 50 grams (1.7636981 ounces) egg (weight), or a large one, unless otherwise specified. I always use 50 gram weight (20 gram yolk and 30 gram white) for Grade A, large eggs in my baking recipes even if it indicates to use just "eggs".

USES OF EGGS: Eggs have many uses in baking and cooking. They can bind ingredients as in meatloaves or croquettes. They can also leaven such baked high rises as soufflés and sponge cakes. Their thickening talent is seen in custards and sauces. Eggs emulsify mayonnaise, salad dressings and Hollandaise sauce and are frequently used as a glaze to coat breads and cookies. They clarify soups and coffee. In boiled candies and frostings, eggs retard crystallization. They also enable coating. In general, eggs add color and flavor. As a finishing touch, they can be hard cooked and used as a garnish.

WAYS TO MAKE EGGS SAFE:

Although the overall risk of egg contamination is very small, the risk of foodborne illness from eggs is highest in raw and lightly cooked dishes. To eliminate risk and ensure food safety, replace all your recipes calling for raw or lightly cooked eggs with cooked egg recipes or use pasteurized eggs or egg products when you prepare them. To cook eggs for these recipes, use the following methods to adapt your recipes:

Cooking Whole Eggs for Use in Recipes – As a nutritious combination of egg whites and yolks, whole eggs should be fully cooked for assured safety in recipes that call for raw or lightly cooked eggs. The following method can be used with any number of eggs and works for a variety of recipes.

In a heavy saucepan, stir together the eggs and either sugar, water or other liquid from the recipe (at least 1/4 cup sugar, liquid or a combination per egg). Cook over low heat, stirring constantly, until the egg mixture coats a metal spoon with a thin film or reaches 160° F. Immediately place the saucepan in ice water and stir until the egg mixture is cool. Proceed with the recipe.
 

Cooking Egg Yolks for Use in Recipes – Because egg yolks are a fine growth medium for bacteria, cook them for use in mayonnaise, Hollandaise sauce, Caesar salad dressing, chilled souffles, chiffons, mousses and other recipes calling for raw egg yolks. The following method can be used with any number of yolks.

In a heavy saucepan, stir together the egg yolks and liquid from the recipe (at least 2 tablespoons liquid per yolk). Cook over very low heat, stirring constantly, until the yolk mixture coats a metal spoon with a thin film, bubbles at the edges or reaches 160° F. Immediately place the saucepan in ice water and stir until the yolk mixture is cool. Proceed with the recipe.

Cooking Egg Whites for Use in Recipes – Cooking egg whites before use in all recipes is recommended for full safety. The following method can be used with any number of whites and works for chilled desserts as well as Seven-Minute Frosting, Royal Icing and other frosting recipes calling for raw egg whites.

In a heavy saucepan, the top of a double boiler or a metal bowl placed over water in a saucepan, stir together the egg whites and sugar from the recipe (at least 1/4 cup per white), water (1 teaspoon per white) and cream of tartar (1/8 teaspoon per each 2 whites). Cook over low heat or simmering water, beating constantly with a portable mixer at low speed, until the whites reach 160° F. Pour into a large bowl. Beat on high speed until the whites stand in soft peaks. Proceed with the recipe.

Note that you must use sugar to keep the whites from coagulating too rapidly. Test with a thermometer as there is no visual clue to doneness. If you use an unlined aluminum saucepan, eliminate the cream of tartar or the two will react and create an unattractive gray meringue.

Making an Italian Meringue Buttercream by adding hot sugar syrup to egg whites while beating them does not bring the egg whites to much above 135 degrees F and does not fully pasteurize them. If, however, you bring the sugar syrup all the way to the hardball stage (250 to 266° F), the whites will reach a high enough temperature, used for Divinity and similar recipes.

Pasteurized dehydrated eggs and are already safe; that is, they have been heated sufficiently to kill bacteria and then made into a powdered form. Use them to prepare meringue, eggnog, homemade mayonnaise, or other dishes that require raw eggs.

Pasteurized raw eggs are now available from Davidson's Pasteurized Eggs Company: The egg industry has developed methods for pasteurizing eggs in the shell to destroy salmonella bacteria. Egg products made of plain whole eggs are pasteurized (heated to destroy bacteria), but not cooked, by bringing them to 140° F and keeping them at that temperature for 3 1/2 minutes. NOTE: Separating pasteurized eggs for a recipe is a little trickier and beating the egg whites takes several minutes longer than traditional ones. 

HOW TO BUY EGGS: 

1. Never buy eggs that haven't been refrigerated because they are potentially hazardous to consume. (There are also egg substitutes and dried egg products).

2. When buying fresh eggs, reach back in the refrigerator case to select the coldest dozen you can. Check on how the eggs look and the carton date. Choose another container if there are any problems. 

3. Then, check the dates on the outside of the carton to indicate whether they are fresh or not: 

4. Then open the egg container to make sure none are cracked, broken or dirty. The shells should be clean but slightly dull. Shiny shells are a sign of old eggs.

TEST FOR FRESHNESS: The air cell, or the empty space between the white and shell at the large end of the egg, gets larger as the egg ages. 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 cell. You can see the air cell in the flattened end of a peeled, hard-cooked egg.

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.

EGG SUBSTITUTES OR REPLACERS: For those who are allergic to eggs or wish to avoid cholesterol, alternatives are available. Refer to the egg substitutes page.  

DRIED EGG AND EGG WHITE PRODUCTS: Pasteurized egg products are Salmonella free, but they must be handled carefully to avoid contamination. 

Pasteurized Egg Powder: Dried egg products are available in the supermarket. Just reconstitute and use. It is spray dried egg albumen, which can be used in most recipes requiring fresh egg whites.

Use in uncooked foods such as marzipan and buttercream icing, or foods which are lightly cooked (pie meringues), without the worries associated with salmonella because it is heat treated to meet USDA egg safety standards. It produces an exceptionally high volume, stable egg white foam for use in angel food cakes, chiffon pies, meringues, and divinity.  

Eggology are pasteurized egg whites, sold in liquid form. They have been tested for salmonella and are packaged only 3 days after harvest. Most whole eggs in the supermarket are 6 weeks old when you buy them. The fresher the egg whites the lighter and fresher the texture and taste, they produce more volume and are more stable. Look for them soon in your supermarket. You can find out more about them by calling 1-818-610-2223.

Meringue powder: ONE LARGE EGG WHITE = 2 teaspoons of meringue powder plus 2 tablespoons warm water. Meringue powder is another pasteurized egg white product. This is dried egg whites with sugar, cream of tartar and cornstarch. The powder like dried egg whites, is pasteurized and completely safe from harmful bacteria. It is used for royal icing, meringue and boiled icing. Meringue powder can be used to make safe meringue for pies and other desserts. Meringue cookies are easy to make, as well. Find it in cake decorating stores or from: http://www.wilton.com. 

General Storage: Unopened dried whites should be stored in a cool and dry place. Once opened, they should be refrigerated.

STORAGE:  All eggs will begin to deteriorate upon storage. The question and problem is how to minimize

Always store eggs in their carton and leave the shells unwashed; they contain a natural protective coating. Each eggshell contains 6,000–8,000 microscopic pores, and eggs pick up odors if stored uncovered. Place them round side up to keep the egg yolk centered in the white and away from the air pocket. 

Left-overs:  In preparing recipes, you may have some left-over whole eggs, whites and/or yolks. 

When storing, always label covered containers in which you store the insides of these eggshells with the number they contain and the date stored. Use them within 2 to 3 days in a fully cooked dish or freeze them for later use. 

Store in Refrigerator:  Store whole eggs from their shells for 2 - 3 days, covered tightly.

If the eggs are separated, you can refrigerate raw whites for up to 4 days and unbroken raw yolks, covered with water, for up to 2 days in a tightly sealed container. If you can’t use the yolks quickly enough, hard cook them just as you would cook whole eggs in the shell, drain them well and refrigerate them in a tightly sealed container for up to 4 or 5 days. For longer storage, freeze raw whites, sugared or salted yolks and cooked yolks for up to 1 year.

Store in Freezer: It's best to freeze the yolks separately from the whites because they store better. But, if you store a whole egg, whisk it with a pinch of salt or a drop of corn syrup per egg to stabilize the egg proteins. 

If separating eggs, crack, separate and store in tightly closed, marked containers.

some information from aeb.org