Overview

Despite the name, century eggs (pidan) are not fermented for a century — traditional production takes roughly two to four months, and modern accelerated methods can take just weeks. Duck, chicken, or quail eggs are coated in a paste of clay, ash, salt, and quicklime (sometimes with rice hulls), which raises the internal pH of the egg to around 9–12. This alkaline environment breaks down proteins and fats without bacterial fermentation in the traditional sense — it is more accurately a chemical curing process.

What happens inside the shell

The alkaline conditions denature egg proteins, producing a translucent, amber, jelly-like white and a creamy, greenish-black yolk. Hydrogen sulfide generated during the process reacts with iron in the yolk to help create its dark color, and ammonia compounds contribute to the food’s characteristic strong aroma. The result is intensely savory (umami), with a texture and flavor that is very much an acquired taste outside the cuisines where it originated.

Varieties

Standard pidan (duck egg) is the most common; some versions develop a “pine-branch” crackle pattern from mineral deposits in the white. Chicken and quail versions are milder and smaller than the traditional duck version.

Culinary use

Century eggs are typically sliced and served as a cold appetizer with ginger, scallion, soy sauce, and sesame oil; shredded into rice congee; paired with silken tofu; or used as a filling in mooncakes. They are eaten in small quantities as a flavor accent rather than a primary protein source.

Safety notes

Lead: some traditional and lower-quality production methods historically used lead compounds to help seal and cure the egg, which posed a real lead-exposure risk. Regulated commercial production today largely uses lead-free curing methods, but this is the single most important safety consideration for this food — buy from reputable, regulated producers and avoid unbranded or informally imported products of unknown provenance.
Century eggs are high in sodium and dietary cholesterol; they are traditionally eaten in small amounts rather than as a regular staple.
As with any egg product, proper sourcing and storage matter for general food safety.

Sources

USDA FoodData Central — fdc.nal.usda.gov
U.S. FDA import alerts on lead-contaminated preserved eggs (general food-safety guidance on sourcing) — fda.gov

Clinically reviewed by Dr. Shilpa Thakur. Last reviewed 2026-07-20. Educational information, not medical advice.