Chicken eggs are convenient food item. It is easy to make a number of recipes. They can be scrambled, boiled, fried, or poached. Eggs are used in custards and cakes and so many other confectioneries. The versatility of eggs is a reflection of their intricate chemical makeup. Eggs contain a big list of chemicals including proteins, fats, vitamins, minerals, and water.
We wrongly think that eggshells are impervious. Eggs do have holes. A single eggshell is perforated by 9,000 pores, on average. The shell forms the egg’s container, protecting it and acting as a permeable membrane for air and moisture to pass through.
Eggshell is made mainly of calcium carbonate which is chemical insoluble in water and is also major constituent of limestone. Calcium carbonate content is about 95% and rest 5% is a mix of other minerals, such as calcium phosphate and magnesium carbonate, as well as soluble and insoluble proteins. These components strongly influence the strength of the shell. A hen on a diet low in calcium or vitamin D, for example, lays eggs having thin, soft shells, or no shells at all.
Generally the shell of egg is white or brown but chickens lay eggs of other colors, from pink to green to blue. The colors of eggs come from pigments that are secreted by the hen and deposited on the eggshell’s outer layers during formation in the chicken’s oviduct, the canal that eggs travel through from the ovaries to the outside world. Egg color is a genetic trait, so colors vary from breed to breed. Brown eggshells contain the pigment protoporphyrin, a breakdown product of hemoglobin. Found only on the shell’s surface, the brown pigment can be dissolved by vinegar or rubbed off with sandpaper. Blue and green hues are caused by the pigment oocyanin, a by-product of bile formation. White eggshells are devoid of these pigments.
Surrounded by the eggshell, the slimy, clear fluid of the egg is the albumen, or egg white—the egg’s cytoplasm. It consists of 90% water, seven major proteins, and no fat. Main protein present in the albumen is called ovalbumin and it accounts for 54% of the white.
In a fresh egg, the albumen contains carbon dioxide, which diffuses out of the egg as it ages. With the loss of CO2, the egg white becomes more alkaline and thins. Because of CO2 loss through the shell pores, an egg a few weeks old will be easier to peel after boiling than a fresh egg with a higher CO2 concentration, although the cause of this phenomenon isn’t completely understood.
Yolk makes up one-third of the egg’s weight, is the near-opposite of albumen in chemical composition. It contains all of the egg’s fat and cholesterol, half of its protein, and four times the calories of the white. A yolk’s golden yellow color is due to the diet of the hen. A diet rich in the yellow and orange plant pigments called xanthophylls leads to a yellow yolk. If the hen’s diet is low in these pigments, the yolk can be almost colorless.
Yolks contain all of the vitamin content in the egg, including six B vitamins, as well as vitamins A, D, and E. The yolk also contains the antioxidants lutein and zeaxanthin and trace amounts of β-carotene, phosphorus, iron, magnesium, and other metals.
The freshness of an egg can be determined from the appearance of its yolk. A fresh egg has a round, firm yolk and a tight surrounding membrane, called the vitelline membrane. As the egg ages, the yolk absorbs water from the albumen, which distends the membrane and results in a looser, flattened yolk.
The greenish gray ring that can form around the yolk of a boiled egg comes from overcooking. The iron and sulfur in the yolk form ferrous sulfides, creating the green ring at the yolk’s surface. Although the color is unappealing, the research proves that the ring does not affect an egg’s flavor and nutritional content.
Contributing to an egg’s normal odor are a number of volatile constituents, including hydrocarbons, phenols, indans, indoles, pyrroles, pyrazines, and sulfides, including hydrogen sulfide. Dimethyl sulfide and dimethyl trisulfide in small amounts contribute to the characteristic odor and flavor of eggs, even fresh ones.
A truly rotten egg is formed when bacteria penetrate the shell and produce foul-smelling hydrogen sulfide.
The complexity of eggs puts them in the spotlight of health debates. Although eggs are high in protein and vitamins and low in fat and sugar, they’re heavy on cholesterol, which could raise a person’s risk of heart disease. An egg can contain up to 250 mg of cholesterol, which is 83% of the U.S. recommended daily allowance of 300 mg.