Eggs appear in a range of colors, from common whites and browns to blues and greens. The eggshell color is determined by the hen’s genetic makeup and the precise timing of pigment application as the egg travels through the reproductive tract. This variety is a direct outcome of biological mechanisms occurring before the egg is laid.
The Primary Role of Genetics
The single most important factor determining eggshell color is the breed and genetics of the hen laying the egg. A hen’s DNA dictates whether her reproductive system is programmed to deposit pigment onto the shell or not. For example, breeds like the Leghorn chicken are genetically predisposed to lay white eggs, while the Rhode Island Red is known for producing brown eggs.
A specific genetic trait, known as the oocyan gene, controls the ability to produce blue or green eggs, as seen in breeds like the Araucana. This trait is fixed for the hen’s entire lifetime, meaning a hen that lays a blue egg will never start laying a brown one.
The Chemistry of Eggshell Coloration
The coloration process takes place in the shell gland, also known as the uterus, during the last few hours before the egg is laid. The shell itself is primarily white calcium carbonate, so any color comes from pigments deposited onto or into this white base. The two main compounds responsible for the entire spectrum of egg colors are protoporphyrin and biliverdin.
Protoporphyrin is responsible for the brown and reddish hues seen in eggs. This pigment is deposited late in the process, only about 90 minutes before the egg is laid, and concentrates in the outermost layer and the cuticle. This late application explains why a brown egg is brown on the outside but remains white on the inside of the shell.
In contrast, biliverdin, which is a breakdown product of heme, creates the blue and green colors. For blue eggs, this pigment is applied much earlier, infusing the color throughout the entire thickness of the shell structure. Green eggs are produced when a hen that is genetically programmed to lay a blue egg also deposits a layer of brown protoporphyrin on top, resulting in an olive or green shade.
Factors Affecting Color Intensity
While genetics determines the color, several variables can affect the intensity or shade of that color. The age of the hen is a common factor, as older hens tend to lay eggs that are noticeably lighter in color. This lightening occurs because the hen produces larger eggs over time, but the total amount of pigment available remains the same, effectively diluting the color across a larger surface area.
Stress and environmental disturbances can also temporarily disrupt the pigment application process. High stress levels can lead to the premature release of the egg, resulting in a paler shell because the full amount of pigment has not been deposited in the final hours. Additionally, although diet does not change a brown egg to a blue one, a hen’s nutritional status can influence the richness or paleness of the genetically determined color.
Shell Color Versus Nutritional Content
The color of an eggshell has no relationship to the egg’s flavor, nutritional value, or internal quality. The nutritional content—including protein, vitamins, minerals, and fat—is virtually identical across all shell colors of the same size. This is because the shell color is determined by pigments deposited on the exterior, not by the composition of the yolk or albumen inside.
The factors that influence an egg’s nutritional profile are the hen’s diet and living conditions. For example, a hen with a diet rich in omega-3 fatty acids or one that is pasture-raised will lay eggs with higher levels of certain beneficial nutrients. Consumers should look at the hen’s environment and feed, not the shell color, when assessing an egg’s quality.