Eggshell

An eggshell is the outer covering of a hard-shelled

Worm eggs

Nematode eggs present a two layered structure: an external vitellin layer made of chitin that confers mechanical resistance and an internal lipid-rich layer that makes the egg chamber impermeable.^[1]

Insect eggs

Insects and other arthropods lay a large variety of styles and shapes of eggs. Some of them have gelatinous or skin-like coverings, others have hard eggshells. Softer shells are mostly protein. It may be fibrous or quite liquid. Some arthropod eggs do not actually have shells, rather, their outer covering is actually the outermost embryonic membrane, the choroid, which protects inner layers. This can be a complex structure, and it may have different layers, including an outermost layer called an exochorion. Eggs which must survive in dry conditions usually have hard eggshells, made mostly of dehydrated or mineralized proteins with pore systems to allow respiration. Arthropod eggs can have extensive ornamentation on their outer surfaces.^{[citation needed]}

Fish, amphibian and reptile eggs

Fish and amphibians generally lay eggs which are surrounded by the extraembryonic membranes but do not develop a shell, hard or soft, around these membranes. Some fish and amphibian eggs have thick, leathery coats, especially if they must withstand physical force or desiccation. These types of eggs can also be very small and fragile.^{[citation needed]}

While many reptiles lay eggs with flexible, calcified eggshells, there are some that lay hard eggs. Eggs laid by snakes generally have leathery shells which often adhere to one another. Depending on the species, turtles and tortoises lay hard or soft eggs. Several species lay eggs which are nearly indistinguishable from bird eggs.^{[citation needed]}

Bird eggs

The

The fibrous chicken

While the bulk of eggshell is made of calcium carbonate, it is now thought that the protein matrix has an important role to play in eggshell strength.[13] These proteins affect crystallization, which in turn affects the eggshell structure. The concentration of eggshell proteins decreases over the life of the laying hen, as does eggshell strength.^{[citation needed]}

In an average laying hen, the process of shell formation takes around 20 hours. Pigmentation is added to the shell by papillae lining the oviduct, coloring it any of a variety of colors and patterns depending on species. Since eggs are usually laid blunt end first, that end is subjected to most pressure during its passage and consequently shows the most color.^{[citation needed]}

As they contain mainly calcium carbonate, bird eggshells dissolve in various acids, including the vinegar used in cooking. While dissolving, the calcium carbonate in an eggshell reacts with the acid to form carbon dioxide.^[14]

Environmental issues

The US food industry generates 150,000 tons of shell waste per year.

Monotremes, egg-laying mammals, lay soft-shelled eggs similar to those of reptiles. The shell is deposited on the egg in layers within the uterus. The egg can take up fluids and grow in size during this process, and the final, most rigid layer is not added until the egg is full-size.^{[citation needed]}

Egg teeth

Hatching birds, amphibian and egg-laying reptiles have an egg-tooth used to start an exit hole in the hard eggshell.^[18][19]

Uses

Eggshell waste is fundamentally composed of calcium carbonate, and has the potential to be used as raw material in the production of lime.^[20]

Pharmaceuticals

The rich

Eggshells have been incorporated into fertilizers as a soil conditioner.^[16][23] They have also been used as a supplement to animal feed.^[16][23] More recently the egg calcium carbonate particles have been used as coating pigments for ink-jet printing.^[23] Powdered eggshells are also used in making paper pulp.^[15] Recently eggshell waste has been used as a low cost catalyst for biodiesel production.^[22] Chicken eggshells have been additionally incorporated as a calcium precursor into the synthesis of calcium-based metal-organic frameworks (MOFs).^[24]

Recently, researchers have utilized chicken eggshells as a biofiller with a conducting polymer to enhance its sensing properties. Typically, eggshells were used as biofiller in polyaniline matrix to detect ammonia gas. The optimum ratio between eggshells and polyaniline could enhance this sensor measurement.^[25]

Ostrich eggshells have been used by

• Eggshell skull rule, in tort law
• Walk on eggshells, an idiom in the English language
• Eggshell membrane, a dietary supplement

References