皮蛋

Pidan joins 皮 (pí), meaning skin or rind or outer layer, with 蛋 (dàn), egg. The compound names a duck, chicken, or quail egg preserved in a mixture of clay, ash, salt, quicklime, and rice husks for weeks or months. The alkaline environment raises the pH of the egg to around 9 or higher, setting the white into a translucent dark gel and transforming the yolk into a creamy greenish-grey sphere. The result looks, to an uninitiated eye, like something gone terribly wrong; it has not.

Chinese records describing preserved eggs appear in Ming dynasty texts from the fifteenth century. The Compendium of Materia Medica (本草綱目, Běncǎo Gāngmù), completed by Li Shizhen (李時珍) in 1578, describes a preserved egg preparation and its medicinal properties. The city of Yiyang in Hunan Province is traditionally credited as the origin point, where duck farmers reportedly discovered eggs preserving naturally in alkaline lime water around 600 years ago. The chemistry was understood empirically long before it was named.

The name 皮蛋 references the texture of the preserved white, which has the firm, slightly elastic quality of cured skin or leather. In Cantonese the same food is called pei daan; in Shanghainese and Wu dialects it goes by variations of the same characters. English speakers coined their own names in the nineteenth century: century egg, hundred-year egg, and thousand-year egg are all hyperbolic names for a preservation that takes two weeks to three months. These English names survive in common use even as pidan gains ground as the preferred term in food writing.

Pidan crossed into English food media primarily through Cantonese dim sum culture in the mid-twentieth century. The dish of congee with pidan and salted pork (皮蛋瘦肉粥, pídàn shòu ròu zhōu) became a standard comfort food reference in English-language writing about Chinese cuisine by the 1980s. Food scientists at Cornell University studied pidan's alkaline protein chemistry in the 1990s, and today the process is taught in food chemistry courses as a model of controlled denaturation. The word pidan appears in Alan Davidson's Oxford Companion to Food (1999) and has been stable in English culinary literature since.