Century Egg

Century egg, also known as preserved egg, hundred-year egg, thousand-year egg, thousand-year-old egg, and millennium egg (or Pidan in Mandarin), 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, cream-like substance with a strong odor of sulphur and ammonia, while the white becomes a dark brown, transparent jelly with little flavor.[citation needed] 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. 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.

Myth

According to a persistent myth, century eggs are, or were once, prepared by soaking eggs in horse urine. The myth may arise from the pungent odor of ammonia given off by century eggs, which is reminiscent of urine. Horse urine itself, however, is only slightly basic, ranging in pH from 7.5 to 7.9. Though human urine left standing can reach a maximum pH of around 9.4—the same as a 1-molar solution of ammonia—it is still less than that produced by mixing quicklime (calcium oxide) or wood ash in water.

In Thai and Lao, the common word for this type of preserved egg literally means "horse urine eggs", either due to this myth or the distinctive ammonia odor of century eggs:

Thai: ไข่เยี่ยวม้า [kʰàj jâw máː] (RTGS: khai yiao ma)
Lao: ໄຂ່ຢ່ຽວມ້າ [kʰāj jāw mâː]

Another misconception many Westerners have is that these eggs really are hundreds of years old due to the misleading name. In reality, they are rarely older than several months.