Egg-cellent little demo for spring

Six brown eggs glowing red in a carton. The carton is glowing blue under UV light.Did you know brown chicken eggs fluoresce red? We found this great photo of brown eggs under UV light at Photography Of The Invisible World1, a blog created by Dr Klaus Schmitt of Weinheim, Germany. Using special lenses and equipment Dr. Schmitt posts his film and photographic work experimenting with a vast array of different effects. His artwork captures the beauty of objects photographed under different light sources —  ultraviolet, visible and infrared. It is an outstanding site for those who love photography, science and fluorescence.

Protophorphyrin IX2 structure.

The reason for the brown colour of the egg and its red fluorescence emission is protophorphyrin IX2 — see structure at right — which is deposited as the shell’s outer cuticle layer is formed. The structure is similar to heme, the pigment in red blood cells; heme is protoporphyrin IX with Fe2+ complexed to the four N atoms. The egg shell itself is 97% calcium carbonate.3 Interestingly, one can rub the brown colour away immediately after the egg is laid and is still damp. Once the shell dries, no amount of rubbing can remove the colour.

It is the genetics of the hen that determine whether a chicken will lay a white or brown egg. Another curious piece of trivia is that one can predict the colour of a chicken’s egg by looking at the chicken’s earlobes. Hens with light-coloured lobes (e.g., White Leghorns) will likely lay white eggs. Hens with dark lobes, typically red, will likely lay brown eggs. The colour of the ear lobes are not genetically linked, but chickens with white lobes have been bred to have white eggs, and chickens with red lobes, brown.4 Although genetics determine egg colour, other factors — such as stress, age and hormones — can affect the shade (the amount of protoporphyrin IX).4,5

Protoporphyrin IX is synthesized in the epithelial cells of the shell gland.6 The pigment accumulates in the viscous, protein-rich cuticle that is deposited on the outermost layer of the egg. This happens in the last three to four hours of shell formation.5 It is this cuticle layer that can be rubbed off before it dries. The presence of the pigment, and the amount of it, does not affect the shell thickness, nor the quality, flavour, cooking characteristics or nutritional value of an egg.3,5

As for the demonstration, all you need is a UV light and a brown egg. You may want to place the brown egg on brown paper to contrast the colour of the egg under UV light.

You can take this demo one step further with a simple experiment:  Put the brown egg in 50 mL of ethyl acetate and add 0.10 mol/L hydrochloric acid — drop by drop.2 The trick is to watch the reaction under UV light. The acid reacts with the calcium carbonate, releasing the pigment and producing visible bubbles of carbon dioxide.

CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)

The less dense organic layer of ethyl acetate, which floats on the HCl(aq), dissolves the protoporphyrin IX as the reaction proceeds. This is an opportunity to discuss solvent extraction. One should be able to observe the gradual increase in fluorescence emission — in  this case, a purple colour should be seen.6 Another suggestion would be to remove the pigment with sandpaper and add these to ethyl acetate; this will avoid the use of HCl.

The Journal of Chemical Education has a lab that expands the demo to identify the extracted eggshell pigments by UV-Vis spectroscopy.7

As with everything around us, interesting chemistry can be found, even in a brown egg — so buy some eggs and enjoy the spring season.

Notes and references (all websites accessed February 2018)

  1.   (, From Photography of the Invisible World 1
  2. A. Nery, R. Liegel and C. Fernandez, Fluorescence and Chemiluminescence: Teaching Basic Principles by Simple Demonstration, Experiments, October, 2003. This Chemical Education Journal (CEJ) reference has information about protoporphyrin IX and other activities involving fluorescence. The expanded demonstration procedure presented was taken from this article.
  4. A Review of Egg Color in Chickens — Marans Chicken Club USA  
  6. Michael Jansen, Crescent School, Toronto tried parts of the lab with success. He was able to observe the ethyl acetate layer fluoresce. He did note that he ended up using 2 M HCl.  
  7. Michelle L. Dean, Tyson A. Miller, Christian Brückner, Journal of Chemical Education, 2011, 88 (6), pages 788–792. The article is called “Egg-Citing! Isolation of Protoporphyrin IX