Pi dan (皮蛋, literally skin egg — in English commonly called century egg, despite the process taking 1-3 months rather than 100 years) is one of the most distinctive and misunderstood preservation techniques in Chinese cooking. Duck eggs (or occasionally chicken or quail eggs) are preserved in a strongly alkaline medium — traditionally a paste of quicklime (slaked lime), clay, salt, ash, and rice husks applied to the shell and aged for 1-3 months. The alkalinity penetrates the egg shell, raising the pH of the egg white and yolk to 9-12, which denatures the proteins in a way that heat cannot reproduce, producing: a translucent, deep grey-green, gelatinous egg white with a complex, almost ammonia-edged savoury flavour, and a creamy, olive-green yolk with a rich, deeply savoury, slightly sulphurous flavour. The finished egg has no offensive flavour when fresh and properly made — its reputation for extreme flavour comes primarily from poorly made examples.
The chemistry: The alkaline solution (calcium hydroxide + sodium hydroxide) penetrates the shell through pores, raising the interior pH from approximately 7 to 9-12. At this pH range, the proteins in the egg white and yolk undergo alkaline hydrolysis — they denature and gel in a way distinct from heat-induced denaturation. The resulting texture and flavour cannot be achieved by any other preservation method. Modern pi dan are made with sodium hydroxide (NaOH, lye) solutions rather than the traditional quicklime-clay paste, which reduces the production time from months to weeks. How to eat pi dan: Peel and cut into wedges. Serve with a dipping sauce of finely sliced ginger and Chinkiang vinegar — the acid of the vinegar counterbalances the alkaline flavour of the egg. The classic Cantonese preparation: pi dan tofu (皮蛋豆腐, pi dan doufu) — sliced pi dan and silken tofu, dressed with light soy, sesame oil, garlic, and chilli oil.
Judging by smell before peeling: A properly preserved pi dan has a distinctive but not offensive aroma. An egg that smells of strong ammonia has been over-aged.
Fuchsia Dunlop, Invitation to a Banquet (2023); Eileen Yin-Fei Lo, Mastering the Art of Chinese Cooking (2009)