Eggs are laid by females of many different species, including birds, reptiles, amphibians, and fish, and have probably been eaten by mankind for millennia. Bird and reptile eggs consist of a protective eggshell, albumen (egg white), and vitellus (egg yolk), contained within various thin membranes. Popular choices for egg consumption are chicken, duck, quail, roe, and caviar, but the egg most often consumed by humans is the chicken egg, by a wide margin.
Egg yolks and whole eggs store significant amounts of protein and choline,[1][2] and are widely used in cookery. Due to their protein content, the United States Department of Agriculture (USDA) categorizes eggs as Meats within the Food Guide Pyramid.[1] Despite the nutritional value of eggs, there are some potential health issues arising from egg quality, storage, and individual allergies.
Chickens and other egg-laying creatures are widely kept throughout the world, and mass production of chicken eggs is a global industry. There are issues of regional variation in demand and expectation, as well as current debates concerning methods of mass production, with the European Union planning to ban battery farming of chickens from 2012.
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Bird eggs have been valuable foodstuffs since prehistory, in both hunting societies and more recent cultures where birds were domesticated. The chicken was probably domesticated for its eggs from jungle fowl native to tropical and subtropical Southeast Asia and India before 7500 BCE. Chickens were brought to Sumer and Egypt by 1500 BCE, and arrived in Greece around 800 BCE, where the quail had been the primary source of eggs.[3] In Thebes, Egypt, the tomb of Haremhab, built about 1420 BCE, shows a depiction of a man carrying bowls of ostrich eggs and other large eggs, presumably those of the pelican, as offerings.[4] In ancient Rome, eggs were preserved using a number of methods, and meals often started with an egg course.[4] The Romans crushed the shells in their plates to prevent evil spirits from hiding there.[5] In the Middle Ages, eggs were forbidden during Lent because of their richness.[5] The word mayonnaise possibly was derived from moyeu, the medieval French word for the yolk, meaning center or hub.[5]
Egg scrambled with acidic fruit juices were popular in France in the 17th century; this may have been the origin of lemon curd.[6]
The dried egg industry developed in the 19th century, before the rise of the frozen egg industry.[7] 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.[7] The production of dried eggs significantly expanded during World War II, for use by the United States Armed Forces and its allies.[7]
In 1911, 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.[8]
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.[5] 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,[9] 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.[10] Pheasant eggs and emu eggs are perfectly edible, but less widely available.[9] 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 permit these only during specific periods of the year.[9]
See also fish eggs.
Egg white coagulates, or solidifies, when it reaches temperatures between 144 °F and 149 °F (62.2 °C-65 °C).[11] Egg yolk coagulates at slightly higher temperatures, between 149 °F and 158 °F (65 °C-70 °C).[11]
If a boiled egg is overcooked, a greenish ring sometimes appears around egg yolk due to the iron and sulfur 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 it is completely cooled prevents the greenish "ring” from forming on the surface of the yolk.
Cooking also increases the risk of atherosclerosis due to increased oxidization of the cholesterol contained in the egg yolk.[12]
Chicken eggs are widely used in many types of dishes, both sweet and savory, including many baked goods. Some of the most common preparataion methods include scrambled, fried, hard-boiled, soft-boiled, and pickled. They can also be eaten raw, though this is not recommended for people who may be especially susceptible to salmonellosis, such as the elderly, the infirm, or pregnant women. In addition, the protein in raw eggs is only 51% bioavailable, whereas that of a cooked egg is nearer 91% bioavailable, meaning the protein of cooked eggs is nearly twice as absorbable as the protein from raw eggs.[13] As an ingredient, egg yolks are an important emulsifier in the kitchen, and the proteins in egg white allow it to form foams and aerated dishes.
The albumen, or egg white, contains protein but little or no fat, and can be used in cooking separately from the yolk. Egg whites may be aerated or whipped to a light, fluffy consistency, and are often used in desserts such as meringues and mousse. Ground egg shells are sometimes used as a food additive to deliver calcium.[14] Every part of an egg is edible, although the eggshell is generally discarded.
Although the age of the egg and the conditions of its storage have a greater influence, the bird's diet does affect the flavor of the egg.[6] For example, when a brown-egg chicken breed eats rapeseed or soy meals, its intestinal microbes metabolize them into fishy-smelling triethylamine, which ends up in the egg.[6] The unpredictable diet of free-range hens will produce unpredictable eggs.[6] Duck eggs tend to have a flavor distinct from, but still resembling, chicken eggs.
Careful preservation of edible eggs is extremely important, as an improperly handled egg can contain elevated levels of Salmonella bacteria that can cause severe food poisoning. The USDA recommends refrigerating eggs in order to to prevent the growth of Salmonella. Refrigeration also preserves the taste and texture. However uncracked eggs can be left unrefrigerated for several months without spoiling[3].
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.[15] 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 osmotic equilibrium.[15] Their yolks take on an orange-red color and solidify, but the white remains liquid. They are boiled before consumption, and are 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, such as ginger or allspice. Frequently, beetroot juice is added to impart a red color to the eggs.[16] 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.[16] If marinated for several days or more, the red color will reach the yolk.[16] If the eggs are marinated in the mixture for several weeks or more, the vinegar 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.[15] Pickled eggs made this way will generally keep for a year or more without refrigeration.[15]
A century egg or hundred-year-old egg is preserved by coating 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.[17] This chemical process breaks down some of the complex, flavorless proteins and fats of the yolk into simpler, flavorful ones, which in some way may be thought of as an "inorganic" version of fermentation.
For those who do not consume eggs, alternatives used in baking include other rising agents or binding materials, such as ground flax seeds or potato starch flour. Tofu can also act as a partial binding agent, since it is high in lecithin due to its soy content. Applesauce can be used, as well as arrowroot and banana. Extracted soybean lecithin, in turn, is often used in packaged foods as an inexpensive substitute for egg-derived lecithin.
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 xanthan gum or guar gum. These allow the product to maintain the nutrition and several culinary properties of real eggs, making possible foods such as Hollandaise sauce, custard, mayonnaise, and most baked goods with these substitutes.
| Nutritional value per 100 g (3.5 oz) | |
|---|---|
| Energy | 647 kJ (155 kcal) |
| Carbohydrates | 1.12 g |
| Fat | 10.6 g |
| Protein | 12.6 g |
| - Tryptophan | 0.153 g |
| - Threonine | 0.604 g |
| - Isoleucine | 0.686 g |
| - Leucine | 1.075 g |
| - Lysine | 0.904 g |
| - Methionine | 0.392 g |
| - Cystine | 0.292 g |
| - Phenylalanine | 0.668 g |
| - Tyrosine | 0.513 g |
| - Valine | 0.767 g |
| - Arginine | 0.755 g |
| - Histidine | 0.298 g |
| - Alanine | 0.700 g |
| - Aspartic acid | 1.264 g |
| - Glutamic acid | 1.644 g |
| - Glycine | 0.423 g |
| - Proline | 0.501 g |
| - Serine | 0.936 g |
| Water | 75 g |
| Vitamin A equiv. | 140 μg (18%) |
| Thiamine (vit. B1) | 0.066 mg (6%) |
| Riboflavin (vit. B2) | 0.5 mg (42%) |
| Pantothenic acid (B5) | 1.4 mg (28%) |
| Folate (vit. B9) | 44 μg (11%) |
| Vitamin B12 | 1.11 μg (46%) |
| Choline | 225 mg (46%) |
| Vitamin D | 87 IU (15%) |
| Calcium | 50 mg (5%) |
| Iron | 1.2 mg (9%) |
| Magnesium | 10 mg (3%) |
| Phosphorus | 172 mg (25%) |
| Potassium | 126 mg (3%) |
| Zinc | 1.0 mg (11%) |
| Cholesterol | 424 mg |
| For edible portion only. Refuse: 12% (shell). One large egg is 50 grams. Percentages are relative to US recommendations for adults. Source: USDA Nutrient Database |
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Eggs add protein to a person's diet, as well as various other nutrients.
Chicken eggs are the most commonly eaten eggs. They supply all essential amino acids for humans,[18] and provide several vitamins and minerals, including retinol[19] (vitamin A), riboflavin (vitamin B2), folic acid (vitamin B9), vitamin B6, vitamin B12, choline, iron, calcium, phosphorus and potassium. They are also a single-food source of protein.
All of the egg's vitamin A, D, and E are in the egg yolk. The egg is one of the few foods to 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 the human body may not absorb much cholesterol from eggs.[20]) The yolk makes up about 33% of the liquid weight of the egg. It contains all of the fat, slightly less than half of the protein, and most of the other 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.[21]
The diet of the laying hens can greatly affect the nutritional quality of the eggs. For instance, chicken eggs that are especially high in omega 3 fatty acids are produced by feeding laying hens a diet containing polyunsaturated fats and kelp meal. Pasture-raised free-range hens which forage largely for their own food also tend to produce eggs with higher nutritional quality in having less cholesterol and fats while being several times higher in vitamins and omega 3 fatty acids than standard factory eggs [22] Focusing on the protein and crude fat content, a 2010 USDA study determined there were no significant differences of these two macronutrients in consumer chicken eggs.[23]
Cooked eggs are easier to digest,[24] as well as having a lower risk of salmonellosis.[25]
The shape of an egg resembles a prolate spheroid 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.)
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.[3]
Egg shell color is caused by pigment deposition during egg formation in the oviduct and can vary according to species and breed, from the more common white or brown to pink or speckled blue-green. In general, chicken breeds with white ear lobes lay white eggs, whereas chickens with red ear lobes lay brown eggs.[26] Although there is no significant link between shell color and nutritional value, there is often a cultural preference for one color over another (see 'Production issues', below).
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. Lutein is the most abundant pigment in egg yolk.[27] A colorless diet can produce an almost colorless yolk. Yolk color is, for example, enhanced if the diet includes products such as yellow corn and marigold petals.[28] In the US, the use of artificial color additives is forbidden.[28]
See Double-yolk eggs and Yolkless eggs.
More than half the calories found in eggs come from the fat in the yolk; a large (50 gram) chicken egg contains approximately 5 grams of fat. People on a low-cholesterol diet may need to reduce egg consumption; however, only 27% of the fat in egg is saturated fat (palmitic, stearic and myristic acids[29]) that contains LDL cholesterol. The egg white consists primarily of water (87%) and protein (13%) and contains no cholesterol and little, if any, fat.
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;[30] whereas other studies show that moderate consumption of eggs, up to one a day, does not appear to increase heart disease risk in healthy individuals.[31] Harold McGee argues that the cholesterol in the yolk is not what causes a problem, because fat (in particular, saturated) is much more likely to raise cholesterol levels than the actual consumption of cholesterol.[3] A 2007 study of nearly 10,000 adults demonstrated no correlation between moderate (six per week) egg consumption and cardiovascular disease or strokes, except in the subpopulation of diabetic patients who presented an increased risk of coronary heart disease.[32] Other research supports the idea that a high egg intake increases cardiovascular risk in diabetic patients.[33] A 2009 prospective cohort study of over 21,000 individuals suggests that "egg consumption up to 6/week has no major effect on the risk of CVD and mortality and that consumption of 7+/week is associated with a modest increased risk of total mortality" in males, whereas among males with diabetes, "any egg consumption is associated with an increased risk of all-cause mortality and there was suggestive evidence for an increased risk of MI and stroke".[34]
Consumption of eggs has been linked to an increased risk of type 2 diabetes in both men and women. A 2008 study using data on over 50,000 individuals collected by the Physicians' Health Study I (1982–2007) and the Women's Health Study (1992–2007) determined the “data suggest that high levels of egg consumption (daily) are associated with an increased risk of type 2 diabetes.”[35] However, a study published in 2010 found no link between egg consumption and type 2 diabetes.[36]
A health issue associated with eggs is contamination by pathogenic bacteria, such as Salmonella enteritidis. Contamination of eggs exiting a female bird via the cloaca may also occur with other members of the Salmonella genus, so care must be taken to prevent the egg shell from becoming contaminated with fecal matter. In commercial practice, eggs are quickly washed with a sanitizing solution within minutes of being laid. The risk of infection from raw or undercooked eggs is dependent in part upon the sanitary conditions under which the hens are kept.
Health experts advise people to refrigerate eggs, use them within two weeks, cook them thoroughly, and never consume raw eggs.[25] 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 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.[37][38][39] Egg shells act as hermetic seals that 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. In the UK, the British Egg Industry Council award the lions stamp to eggs that, among other things, come from hens that have been vaccinated against Salmonella.[40][41][42]
One of the most common food allergies in infants is eggs.[43] Infants usually have the opportunity to grow out of this allergy during childhood, if exposure is minimized.[44] Allergic reactions against egg white are more common than reactions against egg yolks.[45]
In addition to true allergic reactions, some people experience a food intolerance to egg whites.[45]
Food labeling practices in most developed countries 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.
Information obtained by the Canadian Integrated Program for Antimicrobial Resistance (CIPARS) “strongly indicates that cephalosporin resistance in humans is moving in lockstep with use of the drug in poultry production.” According to the Canadian Medical Association Journal, the unapproved antibiotic ceftiofur is routinely injected into eggs in Quebec and Ontario to discourage infection of hatchlings. Although the data are contested by the industry, antibiotic resistance in humans appears to be directly related to the antibiotic's use in eggs.[46]
Most commercially farmed chicken eggs intended for human consumption are unfertilized, since the laying hens are kept without roosters. Fertile eggs can be eaten, with little nutritional difference to the unfertilized. Fertile eggs will not contain a developed embryo, as refrigeration temperatures inhibit cellular growth for an extended time. Sometimes an embryo is allowed to develop but eaten before hatching as with balut.
The US Department of Agriculture grades eggs by the interior quality of the egg (see Haugh unit) and the appearance and condition of the egg shell. Eggs of any quality grade may differ in weight (size).
In Australia[47] and the European Union, eggs are graded by the hen farming method, free range, battery cageed, etc.
Chicken eggs are also graded by size for the purpose of sales.
Although egg color is a largely cosmetic issue, with no effect on egg quality or taste, it is a major issue in production due to regional and national preferences for specific colors, and the results of such preferences on demand. For example, in most regions of the United States, chicken eggs are generally white. In some parts of the northeast of that country, particularly New England, where a television jingle for years proclaimed "brown eggs are local eggs, and local eggs are fresh!", brown eggs are more common. Local chicken breeds, including the Rhode Island Red, lay brown eggs. Brown eggs are also preferred in countries as diverse as Costa Rica, Ireland, and the United Kingdom. In Brazil and Poland, white chicken eggs are generally regarded as industrial, and brown or reddish ones are preferred. Small farms and smallholdings, particularly in economically advanced nations, may sell eggs of widely varying colors and sizes, with combinations of white, brown, speckled (red), green, and blue eggs in the same box or carton, while the supermarkets at the same time sell mostly eggs from the larger producers, of the color preferred in that nation or region.
These cultural trends have been observed for many years. The New York Times reported during the Second World War that housewives in Boston preferred brown eggs and those in New York preferred white eggs.[48] In February 1976, the British New Scientist magazine, in discussing issues of chicken egg color, stated "Housewives are particularly fussy about the colour of their eggs, preferring even to pay more for brown eggs although white eggs are just as good".[49] As a result of these trends, brown eggs are usually more expensive to purchase in regions where white eggs are considered 'normal', due to lower production.[50] In the United Kingdom it is very difficult to buy white eggs, with most supermarkets supplying only the more popular brown eggs. By direct contrast, in Egypt it is very hard to source brown eggs, as demand is almost entirely for white ones.
Research conducted in France in the 1970s demonstrated blue chicken eggs (as laid by certain breeds, including araucanas,[51] heritage skyline, and cream legbar) can be stronger and more resilient to breakage, yet an article in New Scientist magazine (contemporary with that research) stated there was little to no demand for blue-colored eggs from housewives, despite the clear advantages.[49]
Research at Nihon University, Japan in 1990 revealed a number of different issues were important to Japanese housewives when deciding which eggs to buy; however, color was a distinct factor, with most Japanese housewives preferring the white color.[52]
Egg producers carefully consider cultural issues, as well as commercial ones, when selecting the breed or breeds of chicken used for production, as egg color varies between breeds.[53] Among producers and breeders, brown eggs are often referred to as 'tinted', while the speckled eggs preferred by some consumers are often referred to as being 'red' in color.[54]
Commercial factory farming operations often involve raising the hens in small, crowded cages, preventing the chickens from engaging in natural behaviors, such as wing-flapping, dust-bathing, scratching, pecking, perching and nest-building. Such restrictions can lead to pacing and escape behavior.[55]
Many hens confined to battery cages, and some raised in cage-free conditions, are debeaked 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.[56] Molting can be induced by extended feed withdrawal, water withdrawal or controlled lighting programs.
Laying hens are often slaughtered between 100 and 130 weeks of age, when their egg productivity starts to decline.[57] 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 en mass after hatching.[58]
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 in the United States of America, as there is no legal definition or regulations for eggs labeled as free-range in that country.[59]
In the United States, increased public concern for animal welfare has pushed various egg producers to promote eggs under a variety of standards. The most widespread standard in use is determined by United Egg Producers through their voluntary program of certification.[60] The United Egg Producers program includes guidelines regarding housing, food, water, air, living space, beak trimming, molting, handling, and transportation, however, opponents such as The Humane Society have alleged UEP Certification is misleading and allows a significant amount of unchecked animal cruelty.[61] 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 and so on, and "Certified Organic", which requires hens to have outdoor access and be fed only organic vegetarian feed and so on, are the most stringent.[62][63]
The European Union will shortly introduce a ban of conventional battery cages for egg-laying hens. This ban is expected to come into effect from 1 January 2012, as outlined in EU Directive 1999/74/EC.[64] The EU will permit the use of "enriched" cages that must meet certain space and amenity requirements. Egg producers in many member states have objected to the new quality standards.
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.
The tradition of a dancing egg is held during the feast of Corpus Christi in Barcelona and other Catalan cities since the 16th century. It consists of an emptied egg, positioned over the water jet from a fountain, which starts turning without falling.
Although a food item, eggs are sometimes thrown at houses, cars, or people. 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.[65] 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 very messy when broken.
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