Century Egg 皮蛋

Century egg ( 皮蛋 ), or alkalized 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.

When the Westerners first reached China, they were puzzled with this preserved egg, assumed that it took very long time for it to become dark coloured, thus named it ‘century egg’ or ‘thousand-year egg’.   It is also called ‘pine-branch egg’ ( 松花蛋 ) for the snow-flake pattern on its white.   

Through the process, the yoke becomes a dark grey colour, with a creamy consistency and an odor of sulphur and ammonia.  The egg white become a dark brown, translucent jelly with little flavour.  

Century egg was said to be in existence some 600 hundred years ago during Ming Dynasty in Hunan, when some duck eggs were discovered in a pool of slake lime.

《益阳县志》:“皮蛋于明朝初年在湖南省益阳县在一个偶然的机会下发现。当时有一家人所养的鸭在家里的一个灰卤里下蛋。这些蛋在两个月后被发现，录皮而看，蛋白蛋黄皆已凝固”。明代称之为‘混沌子’。

Traditionally duck eggs are coated in a mixture of alkaline clay ( 3 tea : 3 quick lime : 7 oak ash : 9 sea salt ), then covered in rice husk to avoid the eggs from adhering to one another.

方以智《物理小识》: “池州出变蛋，以五种树灰盐之，大约以荞麦谷灰则黄白杂糅，加炉炭石灰，则绿而坚韧”。

In modern production , duck eggs were soaked for 10 days if solution of calcium hydroxide, and sodium carbonate, then followed by several weeks of aging.

Lead oxide is also known to shortened the curing period.  But its highly poisonous, thus very unlikely is used.   On other side, zinc oxide is a safer alternative.  But excessive zinc consumption  can lead to copper deficiency.

There are some misconception that century eggs are soaked in horse urine, due to its pungent ammonia odour.  Horse urine, however, is only slightly basic, ranging from pH 7.5 – 7.9, not enough to raise the eggs’ pH to >9.