Century egg (pinyin: pí dàn), also known as preserved egg, hundred-year egg, thousand-year egg, thousand-year-old egg, and millennium egg, is a Chinese cuisine ingredient made by preserving duck, chicken or quail eggs in a mixture of clay, ash, salt, lime, 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 colour, with a creamy consistency and an odor of sulphur and ammonia, while the white becomes a dark brown, translucent jelly with little flavor. The transforming agent in the century egg is its alkaline material, which gradually raises the pH of the egg to around 9, 12, or more 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.
The origin of the method for creating century eggs likely came about through the need to preserve eggs in times of plenty by coating them in alkaline clay, which is similar to methods of egg preservation in some Western cultures. The clay hardens around the egg and resulted in the curing and creation of century eggs instead of spoiled eggs.
According to some, the century egg has over five centuries of history behind its production. Its discovery, though not verifiable, was said to have occurred during the Ming Dynasty 600 years ago in Hunan, when a homeowner discovered duck eggs in a shallow pool of slaked lime that was used for mortar during construction of his home two months before. Upon tasting the eggs, he set out to produce more, this time with the addition of salt to improve the taste, thus resulting in the present recipe of the century egg.