What is "century egg"

n. A Chinese delicacy made by preserving an egg in its shell in a mixture of charcoal and lime for around a hundred days.

Century egg or pidan , also known as preserved egg, hundred-year egg, thousand-year egg, thousand-year-old egg, and millennium egg, is a Chinese preserved food product and delicacy made by preserving duck, chicken or quail eggs in a mixture of clay, ash, salt, quicklime, and rice hulls for several weeks to several months, depending on the method of processing.

Through the process, the yolk becomes a dark green to grey color, with a creamy consistency and strong flavor due to the hydrogen sulfide and ammonia present, while the white becomes a dark brown, translucent jelly with a salty flavor. The transforming agent in the century egg is an alkaline salt, which gradually raises the pH of the egg to around 9–12, during the curing process. This chemical process breaks down some of the complex, flavorless proteins and fats, which produces a variety of smaller flavorful compounds.

Some eggs have patterns near the surface of the egg white that are likened to pine branches, and that gives rise to one of its Chinese names, the pine-patterned egg.