"Century egg or pidan (Chinese: 皮蛋; pinyin: pídàn), 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.[1]
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.[2] This chemical process breaks down some of the complex, flavorless proteins and fats, which produces a variety of smaller flavorful compounds."
https://en.m.wikipedia.org/wiki/Century_egg#