Century eggs – also called preserved eggs, thousand-year eggs or pídàn – are duck eggs (sometimes chicken or quail) cured for a few weeks in a strongly alkaline mixture. They’re often described as “fermented duck eggs”, but they aren’t fermented: no bacteria or yeasts do the work. The alkali itself transforms the egg, turning the white into a firm, translucent amber-brown jelly and the yolk into a creamy, dark green-gray center with a rich, savory, slightly sulfurous flavor.
How Century Eggs Are Made
Traditionally, eggs were coated in a paste of quicklime, wood ash, salt and tea mixed with clay, then rolled in rice hulls and left for weeks. Commercial producers usually soak eggs in a solution of sodium hydroxide (lye) and salt, often with small amounts of metal salts such as zinc or copper compounds that help control how fast the alkali penetrates. Commercial pidan is typically pickled for around 25 days and then ripened (Hou et al., 2023).
What happens inside is chemistry, not fermentation. The alkali denatures the egg proteins, which then re-form into a gel held together mainly by ionic and disulfide bonds; the gel structure becomes more compact over the first few weeks of pickling (Zhao et al., 2016). Browning reactions color the white, and sulfur compounds give the yolk its color and aroma.
The Lead Question
Lead oxide was historically used in some preserved egg recipes to speed curing, and lead contamination is why many packs are labeled “lead-free” (無鉛). In a study of 25,213 adults in Tianjin, researchers described preserved egg as a high-lead-containing food and found that people who ate it more often were more likely to report depressive symptoms – an association, not proof of cause (Yu et al., 2017). Food scientists are developing lead-free and metal-free methods, for example using eggshell ash or controlled lye alone (Hou et al., 2023; Geng et al., 2026).
What this means for you:
- Buy eggs labeled lead-free from established brands and shops.
- Treat them as an occasional food, not a daily one – they’re also high in sodium.
- Be extra careful with how often you serve them to children and during pregnancy, when lead exposure matters most.
- Don’t make them at home. Curing uses caustic chemicals, and there’s no simple home check for contamination.
What Century Eggs Taste Like
The white is firm and jelly-like with little flavor of its own; the yolk is creamy and intensely savory, often compared to a strong cheese, with a sulfur and slight ammonia note that’s strongest right after peeling. Letting a peeled egg sit for a few minutes, or rinsing it, softens the smell.
How to Buy and Prepare Them
- Where: Chinese and other Asian grocery stores, usually sold in packs, sometimes still in their clay-and-husk coating.
- What to look for: uncracked shells and a “lead-free” label.
- Preparing: remove any coating, crack and peel the shell, rinse, then slice with a wet knife or a piece of thread so the yolk doesn’t smear.
How to Eat Century Eggs
- Congee: diced into rice porridge with pork (the recipe card above).
- With silken tofu: chilled tofu topped with diced century egg, soy sauce, sesame oil, scallions and fried shallots.
- Taiwanese-style salad: quartered eggs with ginger and a black vinegar, soy and sesame dressing.
- Stir-fried greens: tossed with spinach or water spinach and garlic.
For foods that really are fermented, see the world’s best fermented foods, Japan’s natto or the Philippine balao-balao.
