Pidan (century egg / thousand-year egg) — duck eggs preserved in a mixture of clay, ash, salt, quicklime, and rice hulls for 2–3 months — is one of the most distinctive preserved foods in the world. The alkaline environment (pH 9–12) denatures and gels the egg white to a dark, translucent black-green and the yolk to a creamy, dark grey-green with a specific sulphurous, mineral, intensely savoury flavour. The process is the extreme end of the alkaline food transformation spectrum that includes nixtamalisation (MX-04) and pretzel-making.
- **The alkaline chemistry:** The quicklime (calcium oxide) and ash produce a strongly alkaline environment. Sodium hydroxide-equivalent pH 9–12 denatures the egg proteins without heat — the white sets to a firm jelly; the yolk becomes creamy. - **The colour:** The dark colour of the white comes from a Maillard-adjacent browning that occurs at alkaline pH — the proteins and sugars interact differently at high pH than under neutral conditions. - **Serving:** Peeled, cut into wedges — with pickled ginger (to moderate the sulphurous compounds), drizzled with sesame oil and soy. - **In congee (CC-05):** The classic pidan application — the egg is cut and placed in the hot congee, its rich, mineral complexity contrasting the neutral rice.
China: The Cookbook