An egg is a round or oval body laid by the female of many animals, consisting of an ovum surrounded by layers of membranes and an outer casing, which acts to nourish and protect a developing embryo and its nutrient reserves.
Most edible eggs, including bird eggs and turtle eggs, consist of a protective, oval eggshell, the albumen (egg white), the vitellus (egg yolk), and various thin membranes.
Every part is edible, although the eggshell is generally discarded.
Nutritionally, eggs are considered a good source of protein and choline.
Bird eggs are a common food and one of the most versatile ingredients used in cooking. They are highly important in many branches of the modern food industry.
Types of Egg
When most people refer to an "egg," they mean a chicken's egg. But other eggs are sometimes used in the kitchen:
Bantam egg: The egg from a breed of small chicken; it is about half the size of a regular chicken egg and has the same characteristics.
Duck egg: It has an off-white shell and a rich flavour and higher fat content than a chicken's egg; when boiled, the white turns bluish and the yolk turns red-orange.
Goose egg: White-shelled, it is four to five times larger than a chicken egg and has a somewhat richer flavour.
Guinea fowl egg: An egg with an ivory shell flecked with brown; it has a more delicate flavour than that of a chicken egg.
Gull egg: Its shell is covered with light to dark brown blotches and comes in various small sizes; the egg has a slightly fishy flavour.
Ostrich egg: twenty times as large as a chicken egg its shell is thick and ivory-coloured.
Partridge egg: A small egg with a white, buff or olive shell and a mild flavour
Quail egg: A richly flavoured egg, it is small with a speckled brown shell.
Turkey egg: A large egg with a brown shell and a delicate flavour
Turtle egg: A reptile's egg with a mild, rich flavour; its soft shell is buff or speckled.
The most commonly used bird eggs are those from the chicken. Duck and goose eggs, and smaller eggs such as quail eggs are occasionally used as a gourmet ingredient, as are the largest bird eggs, from ostriches.
Pheasant eggs and Emu eggs are perfectly edible but less widely available. Sometime they are obtainable from farmers, poulterers, or luxury grocery stores. Most wild bird's eggs are protected by laws in many countries, which prohibit collecting or selling them, or only permit these during specific periods of the year.
Most commercially produced chicken eggs intended for human consumption are unfertilized, since the laying hens are kept without any roosters. Fertile eggs can be purchased and eaten as well, with little nutritional difference. Fertile eggs will not contain a developed embryo, as refrigeration prohibits cellular growth for an extended amount of time.
Chicken eggs are widely used in many types of dishes, both sweet and savoury.
Eggs can be pickled, hard-boiled, scrambled, fried and refrigerated. They can also be eaten raw, though this is not recommended for people who may be especially susceptible to salmonella, such as the elderly, the infirm, or pregnant women. In addition, the protein in raw eggs is only 51% bio-available, whereas a cooked egg is nearer 91% bio-available, meaning the protein of cooked eggs is nearly twice as absorbable as the protein from raw eggs. As an ingredient egg yolks are important emulsifier in the kitchen, and the proteins in eggs white makes all kinds of foams and aerated dishes possible.
Quail eggs are considered a delicacy in many countries. They are used raw or cooked as tamago in sushi. In Colombia, quail eggs are considered less exotic than in other countries, and a single hard-boiled quail egg is a common topping on hot dogs and hamburgers, often fixed into place with a toothpick.
A boiled egg can be distinguished from a raw egg without breaking the shell by spinning it. A hard-boiled egg's contents are solid due to the denaturisation of the protein, allowing it to spin freely, while viscous dissipation in the liquid contents of a raw egg causes it to stop spinning within approximately three rotations.
The albumen, or egg white contains protein but little or no fat. It is used in cooking separately from the yolk, and can be aerated or whipped to a light, fluffy consistency. The albumen is the healthiest bit of the egg. Beaten egg whites are used in desserts such as meringues and mousse.
If a boiled egg is overcooked, a greenish ring sometimes appears around egg yolk. This is a manifestation of the iron and sulphur compounds in the egg. It can also occur when there is an abundance of iron in the cooking water. The green ring does not affect the egg's taste; overcooking, however, harms the quality of the protein (chilling the egg for a few minutes in cold water until the egg is completely cooled prevents the greenish "ring” from forming on the surface of the yolk).
Quality Classification of Eggs
Under European law there are two classes of egg quality: A & B.
Grade A eggs are the highest grade. They are naturally clean, fresh eggs, internally perfect with shells intact and the air sac not exceeding 6mm in depth. The yolk must not move away from the centre of the egg on rotation. Grade A eggs are sold as shell eggs.
Grade B eggs are broken out and pasteurised.
In addition, there is another class of eggs called industrial eggs which are for non-food use only and are used in products such as shampoo and soap.
European Hens Egg Size Classification
Duck eggs are widely used as an alternative to hens eggs. Omelettes are especially good, sponges are incredibly light and pavlovas rarely stick . People often ask what the contents of a duck egg looks like; someone even wondered if the egg white was in fact green. As you can see from the photo it is actually very similar to any other egg but the white is very clear and the yolk is quite large.
Although duck eggs are predominantly white, some ducks lay blue eggs in various different shades. The reason for this is supposedly genetic; Khaki Campbell ducks were originally bred from Indian Runner ducks, which do lay blue eggs as a rule.
Cooking eggs safely
Some eggs contain salmonella bacteria, which can cause food poisoning. Cooking eggs until both the white and yolk are solid will kill any bacteria.
Eating raw eggs, or eggs with runny yolks, can cause food poisoning. So can eating foods made from:
raw egg, such as home-made mayonnaise
lightly cooked egg, such as hollandaise sauce
This is because cooking has not killed the bacteria in the eggs. If you're cooking a dish containing eggs, make sure you cook it until the food is piping hot all the way through.
Food poisoning can be particularly serious for:
babies and toddlers,
pregnant women, and
anyone who is already unwell.
This is because food poisoning is likely to affect these people more severely. So, if you're cooking eggs for any of the above, make sure both the white and yolk are solid.
Storing poultry and eggs safely
It's important to store poultry and eggs safely, to prevent bacteria spreading and avoid food poisoning.
Storing poultry safely:
Store raw poultry in a sealed container at the bottom of the fridge, so it can't touch or drip on to other food.
Follow any instructions on the label, for example, about storage, freezing, defrosting or cooking.
Keep raw poultry separate from cooked poultry.
Cool cooked poultry as quickly as possible.
Store it in a sealed container in the fridge.
Don't use poultry after its 'use by' date.
Storing eggs safely:
Store eggs away from other foods in a cool, dry place, ideally in the fridge. Use your fridge's egg tray, if it has one.
Keep eggs away from other foods after you've cracked them.
Serve and eat dishes containing eggs as soon as you've cooked them. If you're going to use it later, cool it quickly and put it in the fridge.
Don't use eggs after their 'best before' date.
Don't use eggs with damaged shells - dirt or bacteria can get inside if the shell is cracked.
When eggs are heated, the protein in the white and yolk starts to coagulate. This means that the liquid egg becomes firmer. As heating continues the egg eventually becomes solid. Boiled, poached, fried and scrambled eggs all demonstrate this process. The coagulation of eggs makes them suitable for a variety of functions.
A sauce or custard can be thickened by adding egg and heating. This also enriches the sauce by adding extra nutrients.
Whole raw egg adds moisture to a mixture and holds the ingredients together, e.g. rissoles. As the food is heated, the egg coagulates and keeps the product whole.
Egg enables coating such as breadcrumbs to stick to the surface of a product. The egg helps the coating to adhere to the product’s surface and forms a protective barrier during cooking.
Egg can be brushed over the surface of a baker item prior to baking to give a glossy golden brown finish.
Egg yolk contains lecithin which acts as an emulsifier. It can be used to stabilise emulsions. Adding egg yolk to a mixture of oil and water prevents the two liquids from separating. An example of this is mayonnaise which is an emulsion of oil and vinegar, held together by egg yolk. Industrially manufactured mayonnaise uses pasteurised egg (which has been heat treated to destroy potentially harmful micro-organisms), rather than raw egg, to ensure safety.
Whisking egg white incorporates air and produces a foam - a relatively stable mass of bubbles. If left to stand, the foam will gradually collapse, but when heated the foam becomes permanent, e.g. meringue. Whole egg incorporates air less well, but gives sponge cakes a light texture.
Industry can bulk purchase whole egg, egg white or egg yolk either in liquid, dried or frozen states. This will have been pasteurised to destroy micro-organisms, such as salmonella, which could multiply during storage and cause food poisoning.