 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.
Roe and caviar are edible eggs produced by fish.
Culinary use
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.
Flavor
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. 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. The unpredictable diet of free-range hens will produce unpredictable eggs.
Problems with cooking
If a boiled egg is overcooked, a greenish ring sometimes appears around egg yolk. This is a manifestation of 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 the egg 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.
Substitutes
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 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 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
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.
Nutritional value
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.
Health issues of eating chicken eggs
Cholesterol and fat
More than half the calories found in eggs come from the fat in the yolk; a 100 gram chicken egg contains approximately 10 grams of fat. People on a low-cholesterol diet may need to reduce egg consumption, although only 27% of the fat in egg is saturated fat (Palmitic, Stearic and Myristic acids) 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; 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.
Type 2 diabetes
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 that the “data suggests that high levels of egg consumption (daily) are associated with an increased risk of type 2 diabetes.”
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. 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. 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.
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.
|
Chocolate Cake
