Why Are Some Eggs White

The colour of an egg’s shell is determined by the breed of the hen that lays it rather than by diet or farming method. As chickens evolved and were selectively bred for egg production, different breeds developed distinct genetics that influence pigment deposition during shell formation. Two primary pigments account for the vast majority of eggshell hues: protoporphyrin IX, which produces brown tones, and biliverdin, which imparts blue or green shades. In breeds lacking these pigments, the shell remains white, reflecting the colour of calcium carbonate with no additional colouring.

How Pigments Reach the Shell
Eggs form within the hen’s oviduct over roughly twenty‑four hours. After the yolk and whites assemble, the shell membranes and calcification phase begin, depositing layers of calcium carbonate. During the final hours of shell development, the oviduct secretes pigments onto the surface. In breeds such as the Leghorn, which lay white eggs, this pigmentation step is effectively bypassed. Without pigment infusion, the shell remains its natural white. In contrast, breeds like the Rhode Island Red lay brown eggs by overlaying protoporphyrin on the shell’s surface, while Araucanas and Ameraucanas introduce biliverdin to produce blue or green eggs.

Breed Examples and Egg Colours
White‑egg layers typically include commercial strains descended from the Mediterranean‑region Leghorn chicken. Their prolific egg‑laying abilities and efficient feed conversion made them popular worldwide for white egg production. By contrast, heritage and dual‑purpose breeds often deliver brown, blue or speckled eggs, reflecting pigment diversity. Although white, brown, blue and green remain the most common shades, some rarer breeds yield eggs in hues ranging from deep olive to pale pink.

Myths About Nutrition and Quality
A persistent misconception holds that brown eggs are more nutritious or flavourful than white eggs. In reality shell colour has no bearing on nutritional content, egg size, shell strength or taste. All eggs contain comparable levels of protein, vitamins and minerals within their yolks and whites. Variations in flavour stem more from hen diet, freshness and cooking method than shell pigmentation. Consumers may also perceive slight differences in mouthfeel if one egg spends more time at room temperature than another, but the shell colour itself remains neutral in culinary performance.

Practical Implications for Consumers
Supermarkets often display white and brown eggs side by side, sometimes at different prices. Brown eggs may carry a premium purely for marketing reasons linked to perceptions of natural or free-range farming. However, both white‑ and brown‑shell eggs can be produced under identical farming conditions from intensive care systems to organic pasture‑based flocks. When choosing eggs, shoppers should prioritise production labels such as free range, organic or RSPCA assured rather than shell colour as an indicator of animal welfare or sustainability.

Conclusion
Eggshell colour arises from the genetic makeup of the hen and the presence or absence of specific pigments deposited during shell formation. White eggs result when no pigment is added, reflecting the pure calcium carbonate shell. Brown, blue and other hues come from the controlled application of pigments by the hen’s oviduct. Shell colour does not influence nutrition, taste or quality; it simply reflects the fascinating diversity of chicken breeds shaped by centuries of agricultural breeding.