Naturally green-shelled chicken eggs exist. The striking color is not a dye or a trick of the light, but a direct result of specialized biochemistry within the hen. This article explores the biological and genetic mechanisms that produce these unique green eggs.
The Science of Eggshell Coloration
The color of a chicken eggshell is determined by pigments deposited in the final hours before the egg is laid. This pigmentation occurs in the shell gland (uterus), the last stop in the hen’s oviduct. All egg colors originate from two primary classes of pigments: porphyrins and biliverdin/zinc chelate.
Porphyrins, specifically Protoporphyrin IX, are responsible for the reddish-brown colors seen in eggs laid by breeds like the Rhode Island Red. This pigment is typically concentrated on the outermost layer of the shell. In contrast, the blue-green pigments are deposited differently, setting the stage for the unique hue of a green egg.
The Mechanism Behind Green Eggs
The pigment responsible for blue and green coloration is Biliverdin, a breakdown product of heme (the iron-containing component of hemoglobin). Biliverdin is a blue-green pigment, and its presence alone produces a blue eggshell, such as those from the Araucana breed. The concentration of Biliverdin deposited influences the intensity of the blue color.
The key difference in green eggs is that the Biliverdin pigment is incorporated into the shell matrix earlier in the shell formation process. This means the blue-green color penetrates and is distributed throughout all the calcified layers of the shell. A hen that lays green eggs possesses the genetic trait for both a blue-shelled egg and a brown-shelled egg.
The resulting green color is achieved when the blue shell (colored throughout by Biliverdin) is overlaid with the brown pigment, Protoporphyrin IX, on the surface. This combination of a blue base coat and a brown topcoat optically mixes to create the green hue. This explains why a green eggshell often appears blue on the interior surface when cracked open.
Poultry Breeds That Lay Green Eggs
The ability to lay blue or green eggs is controlled by a dominant genetic trait known as the Oocyan gene. Research has traced the origin of this trait to an ancient insertion of a retrovirus into the chicken’s genome, which activated a gene called SLCO1B3. This mutation causes the transport of Biliverdin to the shell gland, enabling the deposition of the blue-green pigment.
Breeds that lay blue eggs, such as the Araucana and Ameraucana, carry this Oocyan gene. Green eggs are primarily laid by hybrid chickens known as Olive Eggers. These hybrids result from crossing a blue-egg-laying breed with a hen that lays a dark brown egg. The offspring inherits the dominant blue-egg gene and the brown-egg trait, combining the pigments to produce various shades of green.
Addressing the Green Yolk and Albumen Myth
While the shell can be naturally green, the internal components of a fresh, uncooked egg—the yolk and albumen—are not. The notion of a naturally occurring green yolk or albumen is a myth, often perpetuated by fiction. The color of a healthy yolk ranges from pale yellow to deep orange, depending on the hen’s diet.
If a hard-boiled egg yolk shows a greenish-gray ring, this is a chemical reaction, not a natural color. It occurs when an egg is overcooked, causing the sulfur in the egg white to react with the iron in the yolk to form ferrous sulfide. This compound is harmless and safe to eat, but it is a sign of overcooking. Any other green discoloration in an uncooked egg’s interior is a sign of spoilage, such as bacterial contamination.