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 important in many branches of the modern food industry. 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. Gull eggs are considered a delicacy in England, as well as in some Scandinavian countries, particularly in Norway. In some African countries, guineafowl eggs are commonly seen in marketplaces, especially in the spring of each year. Pheasant eggs and emu eggs are perfectly edible but less widely available. Sometimes they are obtainable from farmers, poulterers, or luxury grocery stores. Most wild birds’ 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 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 savory. Eggs can be pickled, hard-boiled, soft-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 that of 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 an 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 denaturation 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 can 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. Ground egg shells are sometimes used as a food additive to deliver calcium. Boiled eggs that are difficult to peel are usually too fresh. Fresh eggs have a lower pH, and this does not allow the shell to separate easily from the underlying albumen. When put into vinegar the shell will dissolve slowly.
Other egg substitutes are made from just the white of the egg for those who worry about the high cholesterol and fat content in eggs. These products usually have added vitamins and minerals as well as vegetable-based emulsifiers and thickeners such as xantham gum or guar gum. These allow the product to maintain the nutrition found in an egg as well as several culinary properties of real eggs. This makes it possible for food like Hollandaise sauce, custard, mayonnaise, as well as most baked goods to be prepared using these substitutes.
Preservation of edible eggs is extremely important, as an improperly handled egg can contain salmonella, a bacteria that can cause severe food poisoning. The simplest method to preserve an egg is to treat it with salt. Salt draws water out of bacteria and molds, which prevents their growth. The Chinese salted duck egg is made by immersing duck eggs in brine, or coating them individually with a paste of salt and mud or clay. The eggs stop absorbing salt after about a month, having reached chemical equilibrium. Their yolks become an orange-red colored solid, but the white remains liquid. They are boiled before consumption and often served with rice congee.
Another method is to make pickled eggs, by boiling them first and immersing them in a mixture of vinegar, salt, and spices like ginger or allspice. Frequently, beetroot juice is added to impart a red color to the eggs. If the eggs are immersed in it for a few hours, the distinct red, white, and yellow colors can be seen when the eggs are sliced. If marinated for several days or more, the red color will reach to the yolk. If the eggs are marinated in the mixture for several weeks or more, vinegar's acetic acid will dissolve much of the shell's calcium carbonate and penetrate the egg, making it acidic enough to inhibit the growth of bacteria and molds. Pickled eggs made this way will generally keep for a year or more without refrigeration.
A century egg or hundred-year-old egg is preserved by fermenting an egg in a mixture of clay, wood ash, salt, lime, and rice straw for several weeks to several months, depending on the method of processing. After the process is completed, the yolk becomes a dark green, cream-like substance with a strong odor of sulfur and ammonia, while the white becomes a dark brown, transparent jelly with a comparatively mild, distinct flavor. The transforming agent in a century egg is its alkaline material, which gradually raises the pH of the egg from around 9 to 12 or more. This chemical process causes an "inorganic version" of fermentation, which breaks down some of the complex, flavorless proteins and fats of the yolk into simpler, flavorful ones.
Egg scrambled with acidic fruit juices were popular in France in the 17th century; this may have been the origin of lemon curd.
The dried egg industry developed in the 19th century, before the rise of the frozen egg industry. In 1878, a company in St. Louis, Missouri started to transform egg yolk and white into a light-brown, meal-like substance by using a drying process. The production of dried eggs significantly expanded during World War II, for use by the United States Armed Forces and its allies.
The egg carton was invented by Joseph Coyle in Smithers, British Columbia, to solve a dispute about broken eggs between a farmer in Bulkley Valley and the owner of the Aldermere Hotel. Early egg cartons were made of paper.
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.)
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.
Some hens will lay double-yolked eggs as the result of unsynchronized production cycles. Although heredity causes some hens to have a higher propensity to lay double-yolked eggs, these occur more frequently as occasional abnormalities in young hens beginning to lay. Usually a double-yolked egg will be longer and thinner than an ordinary single-yolk egg. Double-yolked eggs occur rarely, only leading to observed successful hatchings under human intervention, as the unborn chickens would otherwise fight each other and die.
It is also possible for a young hen to produce an egg with no yolk at all. Yolkless eggs are usually formed about a bit of tissue that is sloughed off the ovary or oviduct. This tissue stimulates the secreting glands of the oviduct and a yolkless egg results.
Eggs add protein to one's diet, as well as various nutrients.
Chicken eggs are the most commonly eaten eggs. They supply all essential amino acids for humans, and provide several vitamins and minerals, including vitamin A, riboflavin, folic acid, vitamin B6, vitamin B12, choline, iron, calcium, phosphorus and potassium. They are also an inexpensive single-food source of protein.
All of the egg's vitamin A, D and E is in the egg yolk. The egg is one of the few foods which naturally contain Vitamin D. A large egg yolk contains approximately 60 Calories (250 kilojoules); the egg white contains about 15 Calories (60 kilojoules). A large yolk contains more than two-thirds of the recommended daily intake of 300 mg of cholesterol (although one study indicates that the human body may not absorb much cholesterol from eggs). The yolk makes up about 33% of the liquid weight of the egg. It contains all of the fat in the egg and slightly less than half of the protein and much of the nutrients. 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.
There is debate over whether egg yolk presents a health risk. Some research suggests dietary cholesterol increases the ratio of total to HDL cholesterol and, therefore, adversely affects the body's cholesterol profile; whereas other studies show that moderate consumption of eggs, up to two per day, does not appear to increase heart disease risk in healthy individuals. 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. A 2007 study of nearly 10,000 adults demonstrated no correlation between moderate (6 per week) egg consumption and cardiovascular disease or strokes except in the sub-population of diabetic patients which presented an increased risk of coronary heart disease. Other research supports the idea that a high egg intake increases cardiovascular risk in diabetic patients. However, some "no correlation" findings have come under attack by independent observers for flawed methodology and financial ties to the egg industry.
Health experts advise people to refrigerate eggs, use them within two weeks, cook them thoroughly, and never consume raw eggs. As with meat, containers and surfaces that have been used to process raw eggs should not come in contact with ready-to-eat food.
A study by the U.S. Department of Agriculture in 2002 (Risk Analysis April 2002 22(2):203-18) suggests the problem is not as prevalent as once thought. It showed that of the 69 billion eggs produced annually only 2.3 million are contaminated with salmonella - equivalent to just one in every 30,000 eggs - thus showing that salmonella infection is quite rarely induced by eggs. However, this has not been the case in other countries where Salmonella enteritidis and Salmonella typhimurium infections due to egg consumptions are major concerns , , .
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.
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.
U.S. Grade AA eggs have whites that are thick and firm; yolks that are high, round, and practically free from defects; and clean, unbroken shells. Grade AA and Grade A eggs are best for frying and poaching where appearance is important. U.S. Grade A eggs have characteristics of Grade AA eggs except that the whites are "reasonably" firm. This is the quality most often sold in stores. U.S. Grade B eggs have whites that may be thinner and yolks that may be wider and flatter than eggs of higher grades. The shells must be unbroken, but may show slight stains. This quality is seldom found in retail stores because they are usually used to make liquid, frozen, and dried egg products.
|Size||Mass per egg||Cooking Yield (Volume)|
|Jumbo||Greater than 2.5 oz. or 71g|
|Very Large or Extra Large (XL)||Greater than 2.25 oz. or 64g||56 mL (4 tbsp)|
|Large (L)||Greater than 2 oz. or 57g||46 mL (3.25 tbsp)|
|Medium (M)||Greater than 1.75 oz. or 50g||43 mL (3 tbsp)|
|Small (S)||Greater than 1.5 oz. or 43g|
|Peewee||Greater than 1.25 oz. or 35g|
In Europe, modern egg sizes are defined as follows:
|Size||Mass per egg|
|Very Large||73g and over|
|Small||53g and under|
|Size||Mass per egg|
|Mega or XXXL||72g|
In New Zealand sizes are based on the minimum mass per egg:
|Size||Minimum mass per egg|
|Size 0||Greater than 75g|
|Size 7||less than 45g|
Many hens confined to battery cages, and some raised in cage-free conditions, are de-beaked to prevent harming each other and cannibalism. According to critics of the practice, this can cause hens severe pain to the point where some may refuse to eat and starve to death. Some hens may be force molted to increase egg quality and production level after the molting. Molting can be induced by extended feed withdrawal, water withdrawal or controlled lighting programs.
Laying hens are often slaughtered between 100 - 130 weeks of age when their egg productivity starts to decline. Due to modern selective breeding, laying hen strains differ from meat production strains. As male birds of the laying strain do not lay eggs and are not suitable for meat production, they are generally culled at the hatchery.
Free-range eggs are considered by some advocates to be an acceptable substitute to factory farmed eggs. Free range laying hens are given outdoor access instead of being contained in crowded cages. Questions on the actual living conditions of free range hens have been raised as there is no legal definition or regulations for eggs labeled as free range in the US.
In the US, increased public concern for animal welfare has pushed various egg producers to release eggs under a variety of different standards. The most widespread standard in use is used by United Egg Producers and is a volunteer program known as United Egg Producers Certified(UEP Certified). The program includes guidelines with regard to housing, feed, water, air, space allowance, beak trimming, molting, handling, and transportation; however, critics such as The Humane Society have alleged UEP Certification misleadingly allows for a significant amount of animal cruelty. Other standards include "Cage Free", "Natural", "Certified Humane", and "Certified Organic." Of these standards, "Certified Humane", which carries requirements for stocking density and cage-free keeping, among others, and "Certified Organic", which requires hens have outdoor access and are fed only organic, vegetarian feed, among other requirements, are the most stringent.
A popular Easter tradition in some parts of the world is the decoration of hard-boiled eggs (usually by dyeing but often by spray-painting). Adults often hide the eggs for children to find, an activity known as an Easter egg hunt. A similar tradition of egg painting exists in areas of the world influenced by the culture of Persia. Before the spring equinox in the Persian New Year tradition (called Norouz), each family member decorates a hard-boiled egg and sets them together in a bowl.
Although a food item, eggs are sometimes thrown at houses, cars, or people generally on Halloween. This act, known commonly as egging in the various English-speaking countries, is a minor form of vandalism and, therefore, usually a criminal offense and is capable of damaging property (egg whites can degrade certain types of vehicle paint) as well as causing serious eye injury. On Halloween, for example, trick or treaters have been known to throw eggs (and sometimes flour) at property or people from whom they received nothing. Eggs are also often thrown in protests, as they are inexpensive and nonlethal, yet at the same time very messy when broken.