Century Egg

Pickled egg

Century egg (‘pidan,’ also known as preserved egg and millennium egg) is a Chinese cuisine ingredient made by preserving fowl eggs in a mixture of clay, ash, salt, quicklime (calcium oxide), 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 gray color  with a creamy consistency and an odor of sulfur 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 fungal 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 around 600 years ago in Hunan during the Ming Dynasty, 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, resulting in the present recipe of the century egg.

Even though the traditional method is still widely practiced, modern understanding of the chemistry behind the formation of century eggs has led to many simplifications in the recipe. For instance, soaking the eggs in a brine of salt, calcium hydroxide, and sodium carbonate for 10 days followed by several weeks of aging while wrapped in plastic is said to achieve the same effect as the traditional method. This is because egg-curing in both the new and traditional methods is accomplished by introducing hydroxide ions and sodium into the egg. Although poisonous, lead(II) oxide is known to increase the curing speed of century eggs, leading some less-scrupulous century egg producers in the past to add it to their curing mixtures. However, zinc oxide is now used as a safer alternative. Although zinc is an essential micronutrient, excessive zinc consumption can lead to copper deficiency, so the zinc content needs to be checked for safety.

Century eggs can be eaten without further preparation, on their own or as a side dish. As an hors d’œuvre, the Cantonese wrap chunks of this egg with slices of pickled ginger root (sometimes sold on a stick as street food). A Shanghainese recipe mixes chopped century eggs with chilled tofu. In Taiwan, it is popular to eat century eggs on top of cold tofu with katsuobushi (bonito flakes), soy sauce, and sesame oil in a style similar to Japanese hiyayakko (chilled tofu and toppings). A variation of this recipe common in northern China is to slice century eggs over chilled silken (soft) tofu, adding liberal quantities of shredded young ginger and chopped spring onions as a topping, and then drizzling light soy sauce and sesame oil over the dish, to taste. They are also used in a dish called ‘old-and-fresh eggs,’ where chopped century eggs are combined with (or used to top) an omelet made with fresh eggs.

According to a persistent misconception, century eggs are, or were once, prepared by soaking eggs in horse urine. The myth may arise from the pungent odor of ammonia (a side-product of protein breakdown) 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. In Thai and Lao, the common word for this type of preserved egg literally means ‘horse urine eggs,’ due to this myth and the distinctive ammonia odor of century eggs.

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