Century egg, also known as preserved egg, hundred-year egg, thousand-year egg, and thousand-year-old egg, is a Chinese cuisine ingredient made by preserving duck, chicken or quail eggs in a mixture of clay, ash, salt, lime, and rice hull for several weeks to several months, depending on the method of processing. After the process is completed, 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 or taste. The transforming agent in the century egg is its alkaline material, which gradually raises the pH of the egg from around 9 to 12 or more. This chemical process breaks down some of the complex, flavorless proteins and fats, which produces a variety of smaller flavourful compounds.
I actually had this in Hong Kong. Doesn’t look very appetizing, but turned out to be really good!