Eggshells in a Nutshell - The Science of Eggshells - White Eggs
Eggshells have three layers: That thing that some people call the "skin" on the inside of an eggshell? Its real name is the mammillary layer and it's a thin protein membrane.
The brittle middle layer, the testa, makes up the bulk of the shell and is made up of columns of calcite crystals held together by a protein matrix (imagine tiny crayons or pencils stacked together with their tips pointing out around the egg) – the spaces between the columns form pores. Moisture and gases can go through these pores—so an eggshell is porous like cloth, not airtight like plastic wrap. This middle layer provides the form and structure to the eggshell.
Calcite is a form of calcium carbonate – it’s the same stuff that’s in the shells of oysters and other marine animals.
The outer layer, a very thin layer of fat and protein, is called the bloom or cuticle. Think of it as paint or varnish that seals the pores of the testa to keep the stuff in the egg from evaporating and to keep bacteria out. When an egg is washed, the bloom is washed off, which is why a washed egg spoils faster than an unwashed egg.
As an egg passes through a hen’s oviduct, the inner mammillary layer is applied first, then the testa, and finally the bloom is applied right before the egg is laid. The bloom is still wet when the egg emerges from the hen.
It typically takes a hen twenty five hours to make an egg. Twenty of those hours are used for making the shell.
Calcite is white; thus, eggs are white. Eggs that are not white contain pigments that give them their characteristic color.
Learn about brown eggs here!
Learn about blue and green eggs here!
White egg courtesy of Jennifer
This post was originally published February 28, 2017.
