Farm eggs display a range of colors, from white and brown to blue and green, contrasting with the uniform appearance of supermarket eggs. These distinct shell colors are a biological outcome determined by the hen’s genetic makeup, which dictates the type and amount of pigments deposited during egg formation.
The Science of Shell Color
Eggshell colors arise from pigment deposition as the egg develops within the hen. All eggshells begin as white, composed primarily of calcium carbonate. As the egg travels through the hen’s oviduct, reaching the shell gland, pigments are applied during the final hours of shell formation before laying.
The two main pigments are protoporphyrin-IX and biliverdin. Protoporphyrin-IX, a reddish-brown pigment, creates brown and tinted eggshells, deposited on the outer layers, making brown eggs white inside. Biliverdin produces blue and blue-green hues, deposited throughout the entire shell, making blue eggs consistently colored inside and out. Green eggs result from a combination of these pigments, with brown pigment laid over a blue shell.
Breed Determines Egg Color
A hen’s genetic background primarily dictates egg color. Specific chicken breeds are known for producing particular eggshell colors. Leghorns, Anconas, and Andalusians, for example, lay white eggs because they lack the genetic ability to deposit pigments.
Brown eggs come from breeds like the Rhode Island Red, Plymouth Rock, Sussex, and Barnevelder. The French Marans breed lays dark brown eggs due to a high concentration of protoporphyrin-IX. Blue eggs are a trait of breeds like the Araucana and Ameraucana, which carry a dominant gene for biliverdin deposition. Green eggs often come from crossbred chickens, such as Olive Eggers, resulting from breeding a blue egg-laying hen with a brown egg-laying rooster.
Debunking Egg Color Myths
Eggshell color holds no influence over nutritional value, taste, or overall quality. These characteristics are determined by the hen’s diet, health, and egg freshness. For example, a hen’s age can affect shell strength, with older hens sometimes laying eggs with thinner shells. Environmental factors like stress or disease can also impact egg quality, including shell integrity.
Supermarket eggs typically exhibit a uniform white or brown color because commercial farming focuses on high-producing breeds that consistently lay those colors. This uniformity results from selective breeding and commercial practices, not differences in egg quality based on shell color. Consumers may associate certain colors with specific qualities, but these perceptions are not scientifically supported.
Shell Color Versus Yolk Color
While eggshell color is a genetic trait, yolk color is primarily influenced by the hen’s diet. Yolk coloration comes from carotenoids, which are yellow, orange, and red plant pigments consumed by the hen. Xanthophylls, a type of carotenoid, are key contributors like lutein and zeaxanthin.
Feed ingredients rich in these pigments, such as yellow corn, alfalfa meal, marigold petals, and paprika, lead to more vibrant, darker orange yolks. Hens that forage outdoors and consume a varied diet often produce richer colored yolks due to diverse carotenoid intake. Conversely, a diet primarily of wheat or barley, lower in carotenoids, results in paler yellow yolks. A darker yolk color does not necessarily indicate superior nutritional content, although it is often aesthetically preferred by consumers.