Bird eggs are a common food source. The most commonly used bird eggs are those from the chicken, duck, and goose, but smaller eggs such as quail eggs are occasionally used as a gourmet ingredient, as are the largest bird eggs, from ostriches. 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.

Chicken eggs are widely used in many types of cooking. Dishes that use eggs range from both sweet to savoury dishes. Eggs may be pickled; hard-boiled, scrambled, fried and refrigerated; or eaten raw, though the latter is not recommended for people who may be susceptible to salmonella, such as the elderly, the infirm, or pregnant women. In addition, the protein in raw eggs are 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.

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 denaturation of the protein, allowing it to spin freely, while the inertia of the liquid contents of a raw egg causes it to stop spinning within approximately three rotations.

The Eggshell
Egg shell color is caused by pigment deposition during egg formation in the oviduct and can vary according to breed, from the more common white or brown to pink or speckled blue-green. Although there is no significant link between shell color and nutritional value, there is often a cultural preference for one color over another. For example, in most regions of the United States, eggs are generally white; while in the northeast of that country and in the United Kingdom, eggs are generally light-brown. In Brazil and Poland, white eggs are generally regarded as industrial, and brown or reddish eggs are preferred. Regarding chicken eggs, the color of the egg depends on the breed of the bird. In general, chicken breeds with white ear lobes lay white eggs, whereas chickens with red ear lobes lay brown eggs.

Ground egg shells are sometimes used as a food additive to deliver calcium.

Egg Whites
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. Beaten egg whites are used in desserts such as meringues and mousse.

The Yolk
The yolk in a newly laid egg is round and firm. As the yolk ages it absorbs water from the albumen which increases its size and causes it to stretch and weaken the vitelline membrane (the clear casing enclosing the yolk). The resulting effect is a flattened and enlarged yolk shape.

Yolk color is dependent on the diet of the hen; if the diet contains yellow/orange plant pigments known as xanthophylls, then they are deposited in the yolk, coloring it. A colorless diet can produce an almost colorless yolk. Farmers may enhance yolk color with artificial pigments, or with natural supplements rich in lutein (marigold petals are a popular choice), but in most locations, this activity is forbidden.

Egg characteristics
The shape of an egg is an oval with one end larger than the other end. The egg has cylindrical symmetry along the long axis.

An egg is surrounded by a thin, hard shell. Inside, the egg yolk is suspended in the egg white by one or two spiral bands of tissue called the chalazae (from the Greek word khalazi, meaning hailstone or hard lump.)

Air cell
The larger end of the egg contains the air cell that forms when the contents of the egg cool down and contract after it is laid. Chicken eggs are graded according to the size of this air cell, measured during candling. A very fresh egg has a small air cell and receives a grade of AA. As the size of the air cell increases, and the quality of the egg decreases, the grade moves from AA to A to B. This provides a way of testing the age of an egg: as the air cell increases in size, the egg becomes less dense and the larger end of the egg will rise to increasingly shallower depths when the egg is placed in a bowl of water. A very old egg will actually float in the water and should not be eaten.

Nutritional value

Eggs provide a significant amount of protein to one's diet, as well as various nutrients. Chicken eggs are the most commonly eaten eggs, and are highly nutritious. They supply a large amount of complete, high-quality protein (which contains all essential amino acids for humans), and provide significant amounts of several vitamins and minerals, including vitamin A, riboflavin, folic acid, vitamin B6, vitamin B12, choline, iron, calcium, phosphorus and potassium. They are also one of the least expensive single-food sources of complete protein. One large chicken egg contains approximately 7 grams of protein.

It also contains all of the choline, and one yolk contains approximately half of the recommended daily intake. Choline is an important nutrient for development of the brain, and is said to be important for pregnant and nursing women to ensure healthy fetal brain development.

Recently, chicken eggs that are especially high in Omega 3 fatty acids have come on the market. These eggs are made by feeding laying hens a diet containing polyunsaturated fats and kelp meal. Nutrition information on the packaging is different for each of the brands.

Eggs may have different nutritional content depending on the feed and living conditions of the chickens who lay them. Mother Earth News compared eggs from "battery" chickens and eggs from pastured chickens, and found that when compared to the battery eggs, the pastured eggs contained, on average, four times as many omega-3 fatty acids, twice as much vitamin E, half the cholestrol and between two and six times as much beta carotene.

Health issues of eating chicken eggs

Cholesterol and fat
About 60% of the calories in an egg come from fat; Chicken egg yolks contain about 10 grams of fat. People on a low-cholesterol diet may need to cut down on egg consumption, although most of the fat in egg is unsaturated fat and may not be harmful. The egg white consists primarily of water (87%) and protein (13%) and contains no cholesterol and little, if any, fat.

Some people try to avoid eggs in their diet because they are high in cholesterol, which is concentrated in the yolk. This issue is sometimes addressed by eating only some or none of the yolk. People sometimes remove the yolk themselves, or may use prepared egg substitutes such as Egg Beaters.

There is debate over whether egg yolk presents a health risk. Some research suggests it may lower total Low density lipoprotein ("bad" cholesterol) while raising High density lipoprotein ("good" cholesterol).[citation needed] Some advocate the eating of raw eggs and egg yolks for this reason, as cholesterol in the yolk is healthier when uncooked. However issues of salmonella contamination remain for raw eggs. Food scientist Harold McGee argues that the cholesterol in the yolk is not what causes a problem as fat (particularly saturated) is much more likely to raise cholesterol levels than the actual consumption of cholesterol.

Contamination
A health issue associated with eggs is contamination by pathogenic bacteria like Salmonella enteritidis. Contamination of eggs exiting a female bird via the cloaca may also occur with other members of the Salmonella group, so care must be taken to avoid the egg shell becoming contaminated with fecal matter. In commercial practice, eggs are quickly washed with a sanitizing solution within minutes of being laid.

Most health experts advise people to cook their eggs thoroughly before eating them, as the heat is necessary to kill any infectious micro-organisms that may be present. Raw and undercooked eggs have been associated with salmonella infection. As with meat, containers and surfaces that have been used to process raw eggs should not come in contact with ready-to-eat food.

The risk of infection from raw or undercooked eggs is dependent in part upon the sanitary conditions under which the hens are kept. Some smaller egg producers make a point of keeping their hens in cleaner (and, in some people's view, more humane) conditions, and observe few or no cases of salmonella in the birds themselves.

Recent evidence suggests the problem is not as prevalent as once thought. A study by the U.S. Department of Agriculture in 2002 (Risk Analysis April 2002 22(2):203-18) showed that of the 69 billion eggs produced annually, only 2.3 million of them are contaminated with salmonella - equivalent to just one in every 30,000 eggs - thus showing that salmonella infection is quite rarely induced by eggs.

Egg shells act as hermetic seals which guard against bacteria entering, but this seal can be broken through improper handling or if laid by unhealthy chickens. Most forms of contamination enter through such weaknesses in the shell. Fresher eggs often have a more prominent chalazae. eggs are bad


Food allergy
One of the most common food allergies in infants is eggs. Infants usually have the opportunity to grow out of this allergy during childhood, if exposure is minimized. Generally, physicians will recommend feeding only the yolks to infants because of the higher risk of allergic reaction to the egg white.

The egg allergy is prevalent enough in the United States that food labeling practices now include eggs, egg products and the processing of foods on equipment that also process foods containing eggs in a special allergen alert section of the ingredients on the labels.

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