Eggshells in a Nutshell - The Science of Eggshells - Blue and Green Eggs
Blue eggs are blue because of biliverdin-IX, a pigment that’s produced in cells lining a hen’s oviduct.
Biliverdin-IX, like protoporphyrin-IX, the pigment in brown eggs, is made from an iron-containing chemical called heme that comes from broken-down red blood cells.
Biliverdin-IX is added to the hard testa layer of the egg shell during egg formation, so a blue egg is blue all the way through and is blue on the inside of the shell.
Chickens that lay green eggs incorporate both blue biliverdin-IX and brown protoporphyrin-IX into their eggshells. Since most or all of the brown pigment is in the bloom, the inside of a green eggshell is blue.
The hue of green eggs will vary depending on how much brown pigment is present—they can run the gradient from light blue-green to a dark olive.
While blue eggs are not uncommon in birds (think of robin’s eggs), they are a little unusual in chickens. Most of the chicken breeds that lay blue eggs originated in or contain breeding stock from South America.
This post was originally published March 5, 2017.
