The eggs you find in the supermarket have a hazel/pink shell of varying intensity. In contrast, eggs from the small country farms of yesteryear were (and are) more often white. In some communities, countries (in France for example), people who see eggs with a colour other than hazelnut/eggshell say they are not real hen’s egg. There are also many people who claim that the colour of the eggshell depends on how the hen is fed.
M.A.C. therefore believe it is useful to provide some information to dispel fantasies and legends while also introducing some technical terms to respect their scientific nature.
The colour of the hen’s egg shell is related to the genetic make-up of the laying hen, thus to her breed, and does not depend on how she feeds. How she feeds only influences the colour of the yolk.
In the picture below you can see that the egg is born after a long process.
In particular, the shell only forms in the last 10 cm of the hen’s genital apparatus, which we can call the uterus, which is actually where the mineralisation of the shell takes place.
The egg is composed of many layers:
- After this first ‘layer’ we find the spongiosa and mammillary layers, which together form the shell proper, consisting of calcite crystals;
- Even further inwards we find the testaceous membranes, which delimit the air chamber These membranes adhere to the shell and overlap each other, separating only at the egg’s obtuse pole to form the so-called air chamber, which is not present in the newly laid egg and increases rapidly in volume as the egg ages after laying. The height of the inner tube is one of the most widely used indices of the egg’s freshness.
- The shell, together with the inner membranes and cuticle, forms a barrier to the entry of microorganisms from the outer surface into the interior of the egg. It is a mineralised, rigid and fragile structure that gives the egg its characteristic ‘ovoid’ shape. Its porous structure, permeable to gases and water vapour, consists mainly of calcium carbonate and small amounts of magnesium carbonate and tricalcium phosphate.
The colour of the egg shell comes from substances secreted into the spongy tissue at the level of the uterus, including protoporphyrin IX (a relative of haemoglobin). The deposition of protoporphyrin IX creates the pinkish to brown colouration depending on its intensity (including the presence of some darker or lighter speckles due to the various sex-related genes present).
But why then are there also blue shells?
Among the breeds that lay blue eggs are the Araucana and the Schijndelaar. The blue colour of their eggs comes from biliverdin and zinc chelated biliverdin, substances that derive from a form of degradation of the same substances that otherwise colour from pink to brown, but belong to the so-called dominant ‘olive gene’. However, there is a difference: the blue colour (in various shades) is also found on the inside of the shell and it is curious to know that by crossing hens that lay blue eggs with others that lay brown eggs, hens are born that produce green eggs.
And the white eggs?
The white colour of the shell seems to depend on a low amount of protoporphyrin IX. Crossing breeds that produce white eggs with breeds that produce pink or otherwise brown eggs results in hens that produce eggs of a colour intermediate to that of the ‘parents’.
It has been discovered that there is also a gene that inhibits protoporphyrin to such an extent that the shell appears so white that it has given rise to the term ‘chalky white eggs‘. White eggs are generally produced by Mediterranean breeds (e.g. Leghorn).
Do eggs with different coloured shells taste different?
No. The organoleptic qualities of the egg (i.e. the -subjective- properties that can be perceived and evaluated by the sense organs, such as smell, taste, colour) are influenced by the hen’s diet and are in general not at all related to the colour of the shell. However, current white-shelled breeds have higher lipid levels than brown-shelled ones. We can confirm that private consumption is influenced by historical factors and each country has its own preference for white or brown eggs. The industry, on the other hand, is based on objective data and therefore prefers white-shelled eggs precisely because of their higher lipid content.
For a more in-depth look at how the hen’s egg feeds us go to this link How does the hen’s egg feed us?
The editorial staff of M.A.C.
