The Difference Between Orange And Yellow Egg Yolks Is Easily Explained By Science

Popular for their high-quality bioavailable protein and other dietary components, eggs come in a range of colors and sizes. Even the insides can be different colors, displaying yolks that vary from orange to yellow once you crack open the shells. Just like the difference between brown and white eggs can easily be explained by science, so too can the difference in the yolk color. It all comes down to how much and what type of pigment the yolk contains.

More specifically, the color of egg yolks are greatly affected by the xanthophyll pigments that they contain. Papers published in Advances in Nutrition and Poultry Science explain that xanthophylls are a class of oxygen-containing carotenoids, which are pigments important for photosynthesis and naturally occur in fruits and vegetables. University of Florida, Institute of Food and Agricultural Sciences Extension Orange County family and consumer sciences agent Hillary Ayers tells Food & Wine, "Not all plant material is equal when it comes to containing xanthophyll — some have higher concentrations and different types."

In fact, there are several hundred xanthophyll varieties. The natural pigments that egg yolks generally contain include capsanthin and capsorubin for red, and lutein and zeaxanthin for yellow. How much and what type of xanthophyll is in the egg yolks depends on what the chickens eat.

Carotenoids are an important part of human health, and for the most part, we get them from fruits and vegetables. Since chickens are among the animals that eat plants, researchers have been studying the deposition of carotenoids in animal foods, such as eggs (per a paper published in Food Chemistry: X). 

It's also possible to determine a hen's diet based on the color of its egg yolk. A pale yellow yolk, for instance, indicates that the chicken has a low-quality diet with a wheat base and low amount of carotenoids. That's because lutein — the xanthophyll pigment responsible for yellow — is the most common carotenoid in wheat, although the lutein concentration differs among the types of wheat. UC Davis Cooperative Extension researcher Richard Blatchford, told Food & Wine, "Hens that make grocery store eggs are usually fed a corn-based diet," so the yolks are yellow but not bright since corn is also a source of lutein. On the other hand, a darker orange yolk suggests that the chickens eat a high-quality diet. That could mean that the birds are pasture-raised and forage for food or that red-pigmented foods are added to their feed, such as marigold petals and red pepper.

It's important to keep in mind, though, that many poultry farmers keep track of yolk color with the DSM-Firmenich YolkFan, a 16-scale color index. And, in order to achieve a more consistent, aesthetically pleasing color value of around 7, synthetic carotenoids can be added to commercial hen feed, such as canthaxanthin for orange-red, and apo-ester for yellow.