Cookbook:Egg

Egg
Egg
Category	Basic foodstuffs

Bird and fish eggs are common food sources. For fish eggs, see caviar.

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 pieces connecting the yolk to the shell

An egg yolk is the part of an egg which serves as the food source for the developing embryo inside. Prior to fertilization the yolk together with the germinal disc is a single cell. Mammalian embryos live off their yolk until they implant on the wall of the uterus. The egg yolk is suspended in the egg white (known more formally as albumen or ovalbumin) by one or two spiral bands of tissue called the chalazae.

Egg white is the common name for the clear liquid (also called albumen or glair/glaire) contained within an egg. It is the cytoplasm of the egg, which until fertilization is a single cell. It consists mainly of about 10% proteins dissolved in water. Its primary purpose is to protect the egg yolk and also to provide additional nutrition for the growth of the embryo, as it is rich in proteins and is of high nutritional value. Unlike the egg yolk, it contains little fat.

Characteristics

Yolk

White

The main constituent of egg white is water at 83%. Ovalbumin is the main protein constituent and is structurally a serpin (a class of proteins), although it does not have a known function in inhibiting other proteins. 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%

Other protein components include, flavoprotein (.8%), ovoglycoprotein (.5%), ovomacroglobulin (.5%), ovoinhibitor (.1%) and avidin (.05%).

Denaturation

All proteins, including those in egg white, are made of long chains of amino acids which, similar to beads on a string. In a raw egg, these strings are raveled up in a tangled compact mass. Chemical bonds and interactions between the amino acids within each protein hold this mass in a specific shape and stop it from unraveling. As an egg cooks, 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 these amino acids and the amino acids of neighboring proteins, causing the texture change in a cooked egg.

Types

Chicken eggs are standard, and the term "egg" without any other qualifiers when used in a recipe most likely refers to chicken eggs. Duck, goose, quail, and ostrich eggs (the largest edible bird eggs in the world) are occasionally used as gourmet ingredients. Reptile eggs, particularly turtle eggs, are sometimes eaten as well.

Chicken eggs of different colors
Smallest to largest: quail, jumbo chicken, and duck eggs
Clockwise from top: turkey, goose, and duck eggs
Smallest to largest: quail, chicken, and ostrich eggs
An ostrich egg

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.

SIZE	PER EGG	PER-100 MIN
XL (very large)	73 g and more	7.3 kg
L (large)	63 to 73 g	6.4 kg
M (medium)	53 to 63 g	5.4 kg
S (small)	under 53 g	4.5 kg

USDA weight standard

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

SIZE	MIN NET PER DOZEN	CONVERTED TO MIN GRAMS/EGG
Jumbo	30 ounces	70.9 g
Extra Large	27 ounces	63.8 g
Large	24 ounces	56.7 g
Medium	21 ounces	49.6 g
Small	18 ounces	42.5 g
Peewee	15 ounces	35.4 g

Selection and storage

Eggs are most often procured whole, in their shells. 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 stored at cool room temperatures for up to several weeks. However, some regions such as the United States have most commercially-available eggs undergo a washing process that strips the protective cuticle from the egg, thereby requiring storage in the refrigerator to prevent spoilage. Make sure to check the recommended storage conditions for your eggs in the region you purchase them in.

Eggs may also be sold as "egg products", which refers to eggs that are removed from their shells for processing. The processing of egg products includes breaking eggs, filtering, mixing, stabilizing, blending, pasteurizing, cooling, freezing or drying, and/or packaging. Egg products include whole eggs, whites, yolks and various blends with or without non-egg ingredients that are processed and pasteurized. They may be available in liquid, frozen, or dried forms.

Freezing

For use in baking, frozen eggs can be used in place of fresh ones if used as soon as thawed soft. Drop them into boiling water that has been removed from the heat source, letting them remain until the water is cold. They will be soft all through and beat up as well as fresh eggs do.

Safety

Food-borne illness

Raw eggs may carry Salmonella bacteria contamination, and they should be avoided by those with weak or undeveloped immune systems, such as the elderly, infirm, or pregnant. The bacteria can be killed by fully cooking eggs.

Spoilage

Fresh eggs will eventually spoil, though the timing depends on their degree of processing and storage conditions. When eggs become rotten, the yolk will turn green and the egg will emit a sulphurous smell when broken.

It is possible to test an egg for freshness, prior to breaking. To do so, drop the egg into a cup of water. A fresh egg will sink to the bottom of the cup, while an older or rotten egg will float towards the surface due to the buildup of gases within the shell. For extra caution, eggs can be broken into a cup one at a time before adding to a mixture—this allows for the bad ones to be easily detected and rejected without ruining the rest of the batch.

Although deemed offensive by most Western palates, fermented eggs are considered a delicacy by some in China, when prepared using a special method which includes letting them sit for three months to age. However they can be considered dangerous.

Explosion

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

Techniques

Peeling eggs

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. This is a common problem with an easy solution. Simply add a couple tablespoons of salt to the water when you boil your eggs and your eggs will peel easily. Alternatively, place the eggs in cold water for a few minutes after you're done boiling them.

Separating eggs

The traditional way to separate an egg (into yolk and white) 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. A simpler way is to form a slack fist with one hand, crack the egg into it, and let the white run through your fingers into a bowl. It's a bit unnerving at first, but you soon develop a feel for the yolk. And if you are using the yolk for mayonnaise for example, the heat from your hand will helpfully warm it. Don't forget to wash your hands first!

Foaming

The protein partially unravels and forms a good leavening agent. A foam is formed by the protein forming a stable film around the included air. Studies show that the best foam forms when the unraveling of the protein is only partial. Over beating egg whites destabilizes the foam by fully unraveling the protein molecules. The protein is elastic, so when the egg white is cooked, and the air expands, the white stretches then sets in the expanded position.

Many cooks recommend using a copper bowl to beat egg whites. This has the effect of making it take longer to form the foam, but leads to a much more stable foam. This is probably because the copper atoms form a complex with the conalbumin protein which makes it difficult to unravel, which means that it takes around twice the time to get a good foam, but the foam is much more difficult to over beat. (The danger of overbeating may be the reason many cooks recommend beating by hand rather than using an electric whisk).

The one possible drawback of using a copper bowl is the toxicity of copper. However, the amount incorporated into the egg whites is likely to be far too small to cause ill effect.

Cream of tartar (potassium bitartrate) is an acidic salt that can be used to change the pH of the egg white to an acidic range of around 3-4. This has the effect of stabilizing the foam, and is therefore a good alternative to using a copper bowl. 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 (acids react with metals, in general), thus potentially increasing the toxic effect.

Use

Eggs are frequently used to bind other ingredients together, trap air in the food, or create an emulsion. Depending on the intended product, only the egg yolk, only the egg white, or both may be used for preparing a food. In most recipes, a whole egg may be replaced with two egg whites to make a dish lower in fat and cholesterol.

The primary cooking techniques for eggs are:

Egg wash is also often used in baking and battering.

Egg white is often separated and used for cooking (for glairs, meringues, soufflés, and some omelettes), whence it derives its name: when albumen is beaten or cooked it turns white.

Egg white is used to remove sediments from champagne and beer and to clarify broths.

Substitution

Some people can't eat eggs because of allergies or because of ethical convictions about egg production. People who have allergies are usually allergic only to the egg whites, due to the much higher protein content of the white as opposed to the yolk (the white is nearly pure protein, while the yolk is mostly fat.) Often people are only allergic as children and later grow out of it. People with ethical convictions against eggs include vegans and some vegetarians.

All these people must use egg substitutes if they wish to eat anything like an egg. No egg substitutes are perfect replacements, and most are very application-specific, but in many recipes an acceptable finished product can be achieved. Common substitutes from scratch include cornstarch (1 Tablespoon dissolved in 3 Tbsp. of warm water per egg) or soy flour (1 Tbsp. + 2 Tbsp. water). Many use ready-made substitutes such as Ener‑G brand egg replacer, which is largely made from potato starch. Tofu plus seasonings are often used as a substitute for scrambled eggs.

For baking, shredded flax or chia seeds (8 g/1 Tbsp. stirred into 45 ml/3 Tbsp. of water and allowed to stand for 20 minutes) can be used in doughs and batters that are dark enough that the dark-colored seeds won't be visible. For light-colored doughs and batters, 45 ml/3 Tbsp. of aquafaba or 1 Tbsp. cornstarch dissolved in 3 Tbsp. warm water is recommended.

Other options for sweet or fruity doughs and batters include one mashed ripe banana or 60 ml/¼ cup applesauce per egg.

Aquafaba can also be whipped to stiff peaks, just like egg whites. Adding ¼ teaspoon cream of tartar per 4 egg equivalents will help the aquafaba whip stiff.