Cookbook:Egg

Bird and fish eggs are common food sources. This chapter discusses bird eggs only—for fish eggs, see roe.

Composition
For the purposes of cooking, chicken and other bird eggs consist of the following major parts; all but the shell are edible:


 * Shell: hard outer covering consisting largely of calcium carbonate
 * Membrane: thin, flexible layer between the shell and the white
 * White: clear, viscous fluid consisting primarily of albumen protein and water
 * Yolk: yellow center, consisting of fats and proteins
 * Chalaza: fibrous white cord connecting the yolk to the shell



Shell
The shell is the hard outer layer of a poultry egg, made primarily of calcium carbonate. Its primary function is to keep the egg safe, preventing foreign material and contaminants from getting inside. It does, however, contain minuscule pores that allow gases and other tiny molecules to permeate. The most common colors are white and brown, though specialty chicken eggs may be blue or have speckles. The color of the eggshell has no bearing on nutritional content or other properties of the egg.

White
After the shell, the next major component of the egg is the white, also called albumen, which consists primarily of water and protein. Unlike the egg yolk, it contains little fat. The white makes up about 67% of the de-shelled chicken egg, and it is further subdivided into the thin white and the thick white, as well as the chalazae. The thin white is thinner and more watery, while the thick white surrounds the yolk and is firmer. The chalazae are harmless strands that hold the yolk in place and degrade with age.

Of the many proteins in the white, ovalbumin is the main constituent. The major proteins found in egg white (> 1% of the total protein content) are listed below:


 * Ovalbumin 54%
 * Conalbumin 13%
 * Ovomucoid 11%
 * Lysozyme 3.5%
 * Globulins (G2, G3) 8.0% (?)
 * Ovomucin 1.5%

Yolk
In the center of the egg white is the round, yellow yolk, which is about 33% of the de-shelled chicken egg. Like the white, it has a substantial protein content (~15–16%). However, unlike the white, it contains a significant proportion of fats (~32–35%). The intensity of the yolk color and flavor depend primarily on the diet of the laying bird —foods like marigold, alfalfa, and yellow corn contain more yellow-orange pigments and will therefore give a more intense color to the yolk. This color is generally not reflective of egg quality, though some may prefer a more intense coloration. The yolk may sometimes have a spot of blood on it—this is harmless.

One important constituent of the yolk is lecithin, an emulsifier that helps blend together fats and water —this emulsifying quality contributes to eggs some of their most useful properties. Heating egg yolk for an extended period can cause it to develop a greenish-grey color as a result of the reaction between iron in the yolk and sulfur in the white. This is harmless but sometimes unappealing. Sulfur also contributes a distinctly "eggy" flavor, which gets stronger with egg age and heating.

Coagulation
All proteins, including those in egg white, are made of long chains of amino acids which. In a raw egg, these strings are raveled up in a tangled compact mass. As an egg is heated, the heat causes the bonds within the proteins to break, a process called denaturation. As these proteins strings unfold and entangle with other proteins, new bonds form between them, causing the coagulation and set texture in a cooked egg. Overheating the egg for too long a period causes the protein network to overly contract, which ends up squeezing out moisture instead of trapping it—thus, overcooked eggs are dry and rubbery. Additionally, it's important to note that when heated too quickly, the egg proteins don't properly unravel, so they make a weaker thickener. Gentle and slow heating thereby improves the thickening capacity of the egg.

Because of their different components, the egg white and yolk set at different temperatures. The following table shows the temperatures at which different egg components denature and coagulate: By combining the eggs with other ingredients, it becomes possible to modulate the temperature of coagulation. Simple dilution raises the required temperature by physically separating the egg proteins. Sugar, starch, and fat also interfere with the proteins' ability to unravel and reconnect, so adding these also raises the coagulation temperature. Conversely, adding acid to the mixture tends to speed up the protein denaturation, which lowers the temperature of coagulation.

The texture of cooked egg can also be modulated by physical disturbance. Regular stirring of an egg mixture while heating breaks up the mixture somewhat, which prevents it from solidifying into one large mass. This is why stirring a custard or an omelet gives it a softer texture than letting it cook undisturbed.

Aging
As a whole egg ages in its shell, a number of changes take place, most of which are caused by the pores in the shell. First, over time, carbon dioxide dissolved in the egg diffuses out through the shell; this has the effect of making the egg more alkaline over time, and the egg becomes thinner and flatter. In addition to carbon dioxide, moisture also escapes through the pores as the egg ages, and the incoming air expands the air pocket inside the egg. Thus, older eggs will increasingly float in water.

Species
All birds lay eggs, and several domesticated birds regularly lay unfertilized eggs, which we use as food. For example, duck, goose, quail, turkey, and ostrich eggs (the largest edible bird eggs in the world) are all eaten, each with their own particular characteristics. However, the most commonly used egg by far is the chicken egg, and the term "egg" without any other qualifiers will refer to chicken eggs in this Cookbook and elsewhere.

Processing
While whole, fresh eggs are the standard, a variety of processed "egg products" are available for purchase. In-shell heat-treated eggs are available to reduce the risk from eating undercooked eggs, though these are not suitable for foaming. Raw eggs can also be pre-shelled and sold as liquid or frozen eggs, yolks, or whites. They can also be dried and powdered, after which stabilizers may be added to help them whip better.

Sizing
Many countries have a standardized sizing system for chicken eggs, though the terminology used by each system is different and may not agree with the others. So, for normal recipes, it is best to avoid specifying egg size. In general, most recipes use "standard" eggs unless otherwise specified, which correspond to USDA size large and EU size M; these standard eggs tend to weigh about 50–60 g each out of shell.

EU weight standard
EU eggs are specified as per-egg weight ranges, plus a per-100 minimum.

USDA weight standard
USDA eggs are specified as the minimum weight of a dozen.

Selection and storage
Eggs are most often procured whole, in their shells. When purchasing, inspect the eggs for any cracks in the shells. Do not purchase or use any cracked eggs, as they are easily contaminated.

Keeping
Eggs can "go bad" in two primary ways. The first is spoilage caused by microbial contamination, which results in a bad odor and can cause food poisoning. You can easily detect this kind of spoilage by smell. The second way is a side effect of simple aging, where the egg simply degrades in quality and dries out —this does not necessarily pose any danger, but it is reflective of lower quality. A simple method for testing if eggs have gone bad involves submerging them in a deep container of water, such as a glass or large bowl. Eggs that touch the bottom of the container are usually good, while eggs that float entirely are no longer good to eat.

Refrigeration
Whole eggs may be stored at room temperature or in the fridge, depending on how they are processed. Chicken eggs are covered in a protective layer called the cuticle, which helps prevent bacterial contamination and subsequent spoilage. Eggs in the European Union and many other places leave this layer intact, and these eggs can be safely stored at cool room temperatures for up to a few weeks. However, some regions such as the United States have most commercially-available eggs undergo a washing process that removes external contaminants but strips the protective cuticle from the egg; this slightly increases the risk of microbial contamination from minuscule cracks, thereby necessitating storage in the refrigerator to prevent spoilage. It has, however, been noted that refrigeration does extend the shelf life of eggs, regardless of whether or not they have been washed. Whole eggs kept in the back of the fridge (not the door, where it is warmer) will remain good for many weeks. Cracked eggs will last for up to a few days.

Freezing
Egg whites can be easily frozen as is, though they will not foam as well after thawing. Yolks, however, thicken irreversibly if frozen without treatment. To reduce this risk, mix the yolks with ⅛ tsp salt or 1½ tsp corn syrup per 4 standard yolks. Thaw frozen eggs in the fridge or cool water.

Food-borne illness
Though relatively rare, raw eggs may be contaminated with Salmonella bacteria, which can cause food poisoning. They should therefore be avoided by those with weak or undeveloped immune systems, such as the elderly, infirm, or pregnant. Note that while proper storage reduces the risk of eggs going bad, it does not eliminate the risk of salmonella poisoning. Unwashed eggs may pick up salmonella on the outside of the shell, which can then contaminate foods, while in rare cases salmonella may enter the egg before it is laid. The bacteria can only be killed by heating (5 minutes at 140°F/60°C or 1 minute at 190°F/88°C). 

Explosion
Due to the formation of steam inside the shell, whole eggs can explode if cooked above boiling temperatures. Never microwave or bake a whole egg.

Separating eggs
Separating eggs is when the yolk and the white are physically separated from each other. One traditional way to separate an egg is to crack the egg into two roughly equal parts, and pass the yolk from one half of the shell to the other, letting the white fall into a bowl below; however, this runs the risk of breaking the yolk with the shell and contaminating the white with it. A simpler way is to crack the egg into your clean hand and let the white run through your fingers. Similarly, you can crack the entire egg into a bowl and gently scoop the yolk out with your hands. Generally, it is easier to separate eggs when cold, since the yolks are slightly firmer and will puncture less easily.

Foaming
Eggs are excellent foaming agents, as the egg proteins unravel and form a stable network around the air bubbles when whipped. The protein is elastic, so when the egg white is cooked, and the air expands, the white stretches and sets in the expanded position. Egg whites in particular can be whipped to a much greater degree (8 times their liquid volume), since the fat present in the yolks disrupts the protein network stabilizing the bubbles. Even a tiny amount of residual fat from other sources will inhibit the foaming of egg whites.

A few things will improve the stability of egg white foams. The first is to use fresh egg whites, which will foam slower but more stably than older whites. The second is to add a little cream of tartar or another acid, which helps the proteins denature and stick together to form a more stable foam. Sugar also stabilizes the foam, which is why a meringue is more stable than a simple whipped white mixture. Finally, whipping the whites in a copper bowl creates a more stable foam that is more resistant to overbeating. Note that cream of tartar should not be used if a copper bowl is used, because it can react with the copper and force more of it into solution, thus potentially increasing the toxic effect.

Note that when using egg whites as a leavening agent in batters, it is better to whip them to medium rather than stiff peaks. This is because stiff peaks will be harder to incorporate into the batter and risk deflating during the process, and because stiff whites can over-expand and collapse when heated in the oven.

Tempering
Eggs must always be tempered when incorporating them into hot liquids. Otherwise, you risk curdling the eggs before they can be properly incorporated.

Peeling
The membrane and shell of fresh chicken eggs cling to the egg white when it is hard-boiled, making it nearly impossible to de-shell/peel—older eggs are better for this purpose. Alternatively, shock the eggs in cold water for a few minutes after you're done boiling them.

Use
There are numerous ways to prepare eggs as the focus of a dish, such as baking, boiling, frying (straight or scrambled), and poaching.

Eggs are also widely incorporated into dishes, both sweet and savory, where they play a variety of roles. Owing to the coagulation ability of their constituent proteins, eggs frequently contribute binding capacity, structure, and stability to cooked preparations. This is particularly notable in goods like cakes and custards. The fat and water content in eggs help prevent starch retrogradation and slow staling in baked goods. Egg yolks help emulsify various sauces, and egg whites are used to help remove sediments from liquids like wine and stock in order to clarify them. Through their foaming capacity, eggs are often an important leavening or lightening agent. Examples include mousses, soufflés, and cakes. Eggs also contribute richness of flavor and golden color to foods, and egg washes are often applied to baked goods to enhance their appearance.

When incorporating eggs into mixtures, it is easiest to use room temperature eggs. In particular, you must not incorporate cold eggs into a mixture with a solid fat, since the cold can cause the fat to curdle and not properly incorporate. Letting cold eggs sit in warm water is a great way to quickly warm them up for use.

Avoid using aluminum tools when working with eggs, since they will discolor the eggs and impart a grey color.

Substitution
In some cases, eggs from different birds can be substituted for chicken eggs. If doing so, you'll want to adjust the number of the non-chicken eggs to get the same weight equivalent of chicken egg. However, it is important to note that non-chicken eggs can have a different ratio of yolk to white, which will impact texture, and they may have a slightly different flavor as well. Generally speaking, it's safe to use non-chicken eggs in stovetop recipes that focus primarily on the egg, like omelets.

For those who do not eat yolks, replacing the yolk with extra egg white in whole-egg recipes can work. Many whole-egg substitutes like Egg Beaters are based on egg whites. Do note that leaving out the yolks will remove some flavoring and emulsification power, and the mixture will set and dry a little earlier.

For those who do not eat any eggs at all, there are a few substitution options depending on the dish. No egg substitutes are perfect replacements, and most are very application-specific, but in many recipes an acceptable finished product can be achieved. Tofu plus seasonings are often used as a substitute for scrambled eggs. Cornstarch (1 tbsp dissolved in 3 tbsp warm water per egg) or soy flour (1 tbsp + 2 tbsp water) can be used when egg is required for thickening. For baking, ground flax or chia seeds (8 g&#x200B;/&#x200B;1 Tbsp gelled in 45 ml&#x200B;/&#x200B;3 tbsp water) can be used in doughs and batters that are dark enough for the seeds to be unnoticeable. For light-colored doughs and batters, 45 ml&#x200B;/&#x200B;3 Tbsp aquafaba or 1 tbsp cornstarch in 3 tbsp warm water is recommended. Other options for sweet or fruity doughs and batters include one mashed ripe banana or 60 ml&#x200B;/&#x200B;¼ cup applesauce per egg. If you want the foaming properties of egg whites, aquafaba is a reasonable substitute. Adding ¼ teaspoon cream of tartar per 4 egg equivalents will help the aquafaba whip stiff.