Eggs as food


Eggs are laid by female animals of many different species, including birds, reptiles, amphibians, a few mammals, and fish, and many of these have been eaten by humans for thousands of years.

Eggs are nutrient dense: minerals, folate, vitamins, and fat soluble vitamins, and are rich source of bioactive compounds such as lutein and zeaxanthin, and high-quality protein.

Nutrients and bioactive compounds in eggs may contribute to improving cardiovascular disease, however eggs are also high in cholesterol.

One large egg contains approximately 186 mg of cholesterol.

Bird and reptile eggs consist of a protective eggshell, albumen, egg white, and vitellus (egg yolk), contained within various thin membranes. 

The most commonly consumed eggs are chicken eggs. 

Other poultry eggs including those of duck and quail also are eaten. 

Fish eggs are called roe and caviar.

Egg yolks and whole eggs store significant amounts of protein and choline.

Egg yolks and whole eggs are widely used in cooking. 

Potential health issues arising from cholesterol content, salmonella contamination, and allergy to egg proteins.

Mass production of chicken eggs is a global industry. 

Egg yolks and whole eggs store significant amounts of protein and choline.

Potential health issues arising from cholesterol content, salmonella contamination, and allergy to egg proteins.

The most commonly used bird eggs are those from the chicken, duck, and goose eggs. 

Smaller eggs, such as quail eggs, are used occasionally.

Eggs usually are candled to check their quality.

The size of its air cell is determined, and the examination also reveals whether the egg was fertilized and thereby contains an embryo.

Eggs may be washed before being placed in egg boxes, although washing may shorten their length of freshness.

The shape of an egg resembles a prolate spheroid with one end larger than the other and has cylindrical symmetry along the long axis.

It is surrounded by a thin, hard shell. 

Thin membranes exist inside the shell. 

The egg yolk is suspended in the egg white by one or two spiral bands of tissue called the chalazae.

The larger end of the egg contains an 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. 

As the air cell increases in size due to air being drawn through pores in the shell as water is lost, 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 float in the water and should not be eaten.

The eggshell’s color is caused by pigment deposition during egg formation in the oviduct.

Eggshell color may vary according to species and breed, from the more common white or brown to pink or speckled blue-green. 

Generally, chicken breeds with white ear lobes lay white eggs, whereas chickens with red ear lobes lay brown eggs.

There is no significant link between shell color and nutritional value.

A cultural preference for one color over another exists.

Candeling is less effective with brown eggs, they have a significantly higher incidence of blood spots.

The eggshell membrane is a clear film lining the eggshell.

The eggshell membrane is visible when peeling a boiled egg. 

The eggshell membrane is composed of fibrous proteins such as collagen type I, and used commercially as a dietary supplement.

The common name for the clear liquid (albumen) contained within an egg is white.

It is colorless and transparent initially, upon cooking it turns white and opaque. 

The purpose of egg white is to protect the yolk and provide additional nutrition during the growth of the embryo.

Egg white is formed from the layers of secretions of the anterior section of the hen oviduct during the passage of the egg.

Egg white forms around both fertilized and unfertilized yolks.

Egg white consists primarily of approximately 90 percent water, and 

10 percent proteins including albumins, mucoproteins, and globulins.

Egg white contains almost no fat and the carbohydrate content is less than one percent.

Egg white has many uses in food and many other applications, including the preparation of vaccines, such as those for influenza.

The yolk is round and firm, but as it ages, it absorbs water from the albumen, which increases its size and causes it to stretch and weaken the vitelline membrane which is the clear casing enclosing the yolk, resulting in a flattened and enlarged yolk shape.

Yolk color is dependent on the diet of the hen. 

If the diet contains yellow or orange plant pigments, they are deposited in the yolk, coloring it. 

Lutein is the most abundant pigment in egg yolk.

A diet without colorful foods may result in an almost colorless yolk. 

Abnormalities in eggs include:

Double-yolk eggs, when an egg contains two or more yolks.

This occurs when ovulation occurs too rapidly, or when one yolk becomes joined with another yolk.

Yolkless eggs;  whites but no yolk, usually occurs during a pullet’s first effort, produced before her laying mechanism is fully ready.

Double-shelled eggs caused by a counter-peristalsis contraction and occurs when a second oocyte is released by the ovary before the first egg has completely traveled through the oviduct and been laid.

Shell-less or thin-shelled eggs may be caused by egg drop syndrome.

The protein in raw eggs is only 51 percent bioavailable, whereas that of a cooked egg is nearer 91 percent bioavailable, meaning the protein of cooked eggs is nearly twice as absorbable as the protein from raw eggs.

Egg yolks are an important emulsifier in the kitchen, and are also used as a thickener, as in custards.

The egg white contains protein, but little or no fat, and may be used in cooking separately from the yolk. 

The proteins in egg white allow it to form foams and aerated dishes: may be aerated or whipped to a light, fluffy consistency, and often are used in desserts such as meringues and mousse.

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

Every part of an egg is edible.

The eggshell is generally discarded. 

Eggs proteins gel at different temperatures within the yolk and the white, and the temperature determines the gelling time. 

Egg yolk becomes a gel, or solidifies, between 61 and 70 °C (142 and 158 °F). 

Egg white gels at different temperatures: 60 to 73 °C (140 to 163 °F). 

The white contains exterior albumen which sets at the highest temperature. 

Generally, the white gels first because it is exposed to higher temperatures for longer.

To avoid the issue of salmonella, eggs may be pasteurized in-shell at 57 °C (135 °F) for an hour and 15 minutes. 

Sometimes If a boiled egg is overcooked, a greenish ring appears around egg yolk due to changes to the iron and sulfur compounds in the egg. 

Chilling an overcooked egg for a few minutes in cold water until it is completely cooled may prevent the greenish ring from forming on the surface of the yolk.

Overcooking harms the quality of the egg’s protein.

The  age of the egg and the conditions of its storage influence the flavor of the egg.

The bird’s diet affects the flavor of the egg as well: chickens eating rapeseed or soy meals, results in its intestinal microbes metabolizing  them into fishy-smelling triethylamine, which ends up in the egg.

The unpredictable diet of free-range hens will produce likewise, unpredictable egg flavors.

Handled egg may contain elevated levels of Salmonella bacteria that may cause severe food poisoning. 

The USDA thus recommends refrigerating eggs to prevent the growth of Salmonella, and to preserves the taste and texture.

Intact eggs, unwashed and unbroken, eggs may be left unrefrigerated for several months without spoiling.

In Europe, eggs are not usually washed, and the shells are dirtier, however the cuticle is undamaged, and they do not require refrigeration.

In the UK in particular, hens are immunized against salmonella and generally, their eggs are safe for 21 days.

The simplest method to preserve an egg is to treat it with salt.

Egg substitutes are made from just the white of the egg and usually have added vitamins and minerals, as well as vegetable-based emulsifiers and thickeners, such as xanthan gum or guar gum: 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

Vitamins Quantity %DV†

Vitamin A equiv. 19% 149 μg

Thiamine (B1) 6% 0.066 mg

Riboflavin (B2) 42% 0.5 mg

Niacin (B3) 0% 0.064 mg

Pantothenic acid (B5) 28% 1.4 mg

Vitamin B6 9% 0.121 mg

Folate (B9) 11% 44 μg

Vitamin B12 46% 1.11 μg

Choline 60% 294 mg

Vitamin D 15% 87 IU

Vitamin E 7% 1.03 mg

Vitamin K 0% 0.3 μg

Minerals Quantity %DV†

Calcium 5% 50 mg

Iron 9% 1.2 mg

Magnesium 3% 10 mg

Phosphorus 25% 172 mg

Potassium 3% 126 mg

Sodium 8% 124 mg

Zinc 11% 1.0 mg

Other constituents Quantity

Water 75 g

Cholesterol 373 mg

Refuse: 12% (shell).

An egg just large enough to be classified as large in the U.S. yields 50 grams of egg without shell.

A 50-gram (1.8 oz) medium/large chicken egg provides approximately 70 kilocalories (290 kJ) of food energy and 6 grams of protein.

Eggs supply the Daily Value (DV), including vitamin A (19 percent DV), riboflavin (42 percent DV), pantothenic acid (28 percent DV), vitamin B12 (46 percent DV), choline (60 percent DV), phosphorus (25 percent DV), zinc (11 percent DV) and vitamin D (15 percent DV). 

Cooking methods affect the nutritional values of eggs.

Chicken eggs that are especially high in omega-3 fatty acids are produced by feeding hens a diet containing polyunsaturated fats from sources such as fish oil, chia seeds, or flaxseeds.

Pasture-raised free-range hens, also produce eggs that are relatively enriched in omega-3 fatty acids when compared to those of cage-raised chickens.

A USDA study found no significant differences of macronutrients in various chicken eggs.

Cooked eggs have a lowered risk of salmonellosis.

Cooked eggs are easier to digest than raw eggs.

Health effects on egg consumption and human health has been conflicting: reviews have been on observational studies.

More than half the calories found in eggs come from the fat in the yolk.

50 grams of a large chicken egg contains approximately five grams of fat. 

Saturated fat (palmitic, stearic, and myristic acids) makes up 27 percent of the fat in egg.

The egg white consists primarily of water (around 90 percent) and protein (around 10 percent) and contains no cholesterol and little, if any, fat.

Some research suggests dietary cholesterol increases the ratio of total to HDL cholesterol and, therefore, adversely affects the body’s cholesterol.

 Other studies show that moderate consumption of eggs, up to one a day, does not appear to increase heart disease risk, while other research links egg consumption with increased cardiovascular risk.

Several studies link consuming additional half an egg per day increases insulin cardiovascular disease and all cause mortality.

A prospective observational study found that egg consumption was strongly associated with carotid artery plaque generation.

Meta-analyses found that consumption of one egg per day was not associated with cardiovascular disease risk.

In a study of 23 prospective studies with a median follow-up of 12. 28 years a total of 1,415,839 individuals with a total of 123,660 cases and 157,324 cardiovascular disease events included found: that higher consumption of eggs of more than one egg per day was not associated with increased risk of cardiovascular disease, but was associated with a significant reduction in risk of coronary artery disease (Krittanawong C).

in the above analysis consumption of more than one egg per day was not associated with an increased risk of stroke.

Test results are mixed between the relationship for egg/choline consumption and TMAO.

Daily dietary free choline supplementation significantly raises TMAO levels in plasma and urine, and increases platelet aggregation.

The daily consumption of eggs containing an equivalent total choline  content to the free choline, however, failed to increase TMAO or platelet aggregation.

There is conflicting results about a possible connection between egg consumption and type 2 diabetes.

A meta-analysis found a high egg consumption of 5 a week with increased risk of breast cancer compared to no egg consumption.

A 2021 review did not find a significant association between egg consumption and breast cancer.

Egg consumption may increase ovarian cancer risk, and risk of upper aero-digestive tract cancers in hospital-based case-control studies.

Eggs are one of the largest sources of phosphatidylcholine (lecithin) in the human diet, which is digested by bacteria in the gut and eventually converted into the compound TMAO, a compound linked with increased heart disease.

Another study found that type 2 diabetes mellitus and kidney disease also increase TMAO levels and that evidence for a link between TMAO and cardiovascular diseases may be due to confounding or reverse causality.

A 2013 meta-analysis found no association between egg consumption and heart disease or stroke.

A 2013 systematic review and meta-analysis found no association between egg consumption and cardiovascular disease or cardiovascular disease mortality, but did find egg consumption of more than once daily increased cardiovascular disease risk 1.69-fold in those with type 2 diabetes mellitus when compared to type 2 diabetics who ate less than one egg per week.

Another meta-analysis found that eating four eggs per week increased the risk of cardiovascular disease by six percent.

A meta-analysis of randomized clinical trials found that consumption of eggs increases total cholesterol (TC), LDL-C and HDL-C compared to no egg-consumption but not to low-egg control diets.

Two other meta-analyses found that moderate egg consumption (up to one egg a day) is not associated with an increased cardiovascular disease risk.

A 2020 umbrella review concluded that increased egg consumption is not associated with cardiovascular disease risk in the general population.

Another review found no association between egg consumption and cardiovascular disorders.

A 2021 review suggested that higher consumption of eggs (more than 1 a day) is not associated with increased risk of cardiovascular disease but is associated with a significant reduction in coronary artery disease risk.

A 2016 meta-analysis found that consumption of up to one egg a day may contribute to a decreased risk of total stroke.

Two recent meta-analyses found no association between egg intake and risk of stroke.

Contamination of eggs with other members of the genus Salmonella while exiting a female bird via the cloaca may occur, 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.

It is advised to refrigerate washed eggs, use them within two weeks, cook them thoroughly, and never consume raw eggs.

Containers and surfaces that have been used to process raw eggs should not come in contact with ready-to-eat food.

Of the eggs produced annually less than one in every 30,000 eggs show Salmonella, so infection is quite rarely induced by eggs.

Egg shells act as seals against bacteria entering the egg, but this seal can be broken through improper handling or if laid by unhealthy chickens. 

Egg allergy is one of the most common food allergies in infants.

Infants usually grow out of this allergy during childhood, if egg exposure is minimized.

Allergic reactions against egg white are more common than reactions against egg yolks.

Some people experience a food intolerance to egg whites.

Most commercially farmed chicken eggs intended for human consumption are unfertilized, since the laying hens are kept without roosters. 

Fertile eggs may be eaten, with little nutritional difference when compared to the unfertilized. 

Fertile eggs will not contain a developed embryo, as refrigeration temperatures inhibit cellular growth.

The U.S. Department of Agriculture grades eggs by the interior quality of the egg and the appearance and condition of the egg shell. 

Eggs of any quality grade may differ in weight/size.

U.S. Grade AA eggs have whites that are thick and firm; have yolks that are high, round, and practically free from defects; and have 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 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. 

Grade B egg shells must be unbroken, but may show slight stains.

Grade B eggs are seldom found in retail stores because usually they are used to make liquid, frozen, and dried egg products.

Eggshell color is a largely cosmetic issue, with no effect on egg quality or taste.

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