A Rainbow of Eggs: What Determines the Shell Color in Chicken Eggs

Eggs, Livestock / Thursday, February 15th, 2018

A while back, a potential egg customer stopped by the farm on a sunny Sunday afternoon while we were outside working. I stopped my task at hand to greet him and show him where the self-serve porch mini refrigerator is set up. He promptly bought the last 5 dozen eggs that we had that day. As I helped him load the eggs into his cooler, we chatted casually about our hens and how hard it is to find local eggs in the nearby large city. As we were talking, he mentioned that he used to buy eggs from another farm but stopped because they were “dying their eggs brown”. While I was a bit skeptical, I didn’t press further to find out more about why this person thought that may be happening. I have never known a chicken farmer who I thought had the free time to purposely deceive customers like this, nor the inclination to do so.

Fast-forward a few months to another Sunday when I’m cooking breakfast in my kitchen. I cracked open a half-dozen eggs and scramble them up, when it dawned on me, the inside of a brown eggshell is actually white. Light-bulb moment! He thought they were dying the eggs, when in reality, it’s just how brown eggs are.

So how do we get all of those different egg colors?

As the egg-shell is formed in the final stages of the 26 hour egg-laying process, several layers are deposited which make up the entire shell. All egg shells are made mostly from calcium carbonate, which is white. All eggs start out as a white egg

Then how do we get brown eggs?

Chicken breeds that lay brown eggs carry genes which cause a pigmented layer to be applied to the outer layer of the shell. This pigment is called protoporphyrin. Chicken breeds that lay white eggs do not carry the genes for making any of the outer layers of pigment, but brown egg layers do. Because the brown pigment is applied toward the end of the shell-making process, the pigment doesn’t sink in to color the entire thickness of the shell. That’s where my new friend made his erroneous assumption.

Blue eggs work a little differently

A few chicken breeds lay a blue egg, the Auracana, Ameraucana and the Cream Legbar are the most notable ones found in the US. These breeds carry the genes to produce a  blue shell pigment called oocyanin. This pigment is produced from bile and is applied much earlier in the shell-making process, which means that the color seeps into all layers of the calcium carbonate and makes the entire shell appear blue.

And green eggs? Not just for Dr. Seuss anymore

Green eggs are created from hybrid chickens, by crossing a brown egg breed with a blue egg breed. The offspring of this cross will have the genes that produce both oocyanin and protoporphryn, meaning that this hybrid hen will make a blue shell with a brown coating, which gives an olive-green to khaki appearance when combined.egg collage


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