|Century egg sliced open|
|Literal meaning||leather/skin egg|
|Alternative Chinese name|
|Literal meaning||pine-patterned egg|
|Vietnamese||trứng bách thảo|
(Khai Yiao Ma)
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 cuisine ingredient 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.
Through the process, the yolk becomes a dark green to grey colour, with a creamy consistency and an odor of sulphur and ammonia, while the white becomes a dark brown, translucent jelly with little flavour. 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 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 their flavor -- resulting in the present recipe of the century egg.
The traditional method for producing century eggs is the development and improvement from the aforementioned primitive process. Instead of using just clay, a mixture of wood ash, quicklime, and salt is included in the plastering mixture, thereby increasing its pH and sodium content. This addition of natural alkaline compounds improved the odds of creating century eggs instead of spoilage and also increased the speed of the aforementioned primitive process. A recipe for creating century eggs through this process starts with the infusion of three pounds of tea in boiling water. To the tea, three pounds of quicklime (or seven pounds when the operation is performed in winter), nine pounds of sea salt, and seven pounds of wood ash from burned oak is mixed into a smooth paste. While wearing gloves to prevent the lime corroding the skin, each egg is individually covered by hand, then rolled in a mass of rice chaff to keep the eggs from adhering to one another before they are placed in cloth-covered jars or tightly woven baskets. The mud slowly dries and hardens into a crust over several months, and then the eggs are ready for consumption.
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.
The poisonous compound lead(II) oxide increases the curing speed of century eggs, which has led to some unscrupulous producers in the past adding 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.
|This section does not cite any references or sources. (July 2011)|
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, soy sauce, and sesame oil in a style similar to Japanese hiyayakko. 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.
Some Chinese households cut them up into small chunks and cook them with rice porridge to create "century egg and lean pork congee" (Chinese: 皮蛋瘦肉粥; pinyin: pídàn shòuròu zhōu). This is sometimes served in dim sum restaurants. Rice congee, lean pork, and century egg are the main ingredients. Peeled century eggs are cut into quarters or eighths and simmered with the seasoned marinated lean slivers of pork until both ingredients are cooked into the rice congee. Fried dough sticks known as youtiao are commonly eaten with century egg congee. Another common variation of this dish is the addition of salted duck eggs into the congee mixture.
At special events like wedding banquets or birthday parties, a first course platter of sliced barbecued pork, pickled baby leeks, sliced abalone, pickled julienned carrots, pickled julienned daikon radish, seasoned julienned jellyfish, sliced pork, brawn and the quartered century eggs is served. This is called a lahng-poon in Cantonese, which simply means "cold dish".
Misconception and etymology 
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. 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 (both of which can reach a pH of at least 12).
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:
See also 
- Moskvitch, Katia (29 March 2013). "Black eggs and ripe guava lead Taiwan's tech revolution". BBC News. Retrieved 29 March 2013.
- McGee, Harold (2004). On Food and Cooking: The Science and Lore of the Kitchen. Scribner. p. 117. ISBN 0-684-80001-2.
- Hou, Xiangchuan (1981). "Hunger and technology: Egg preservation in China". Food and Nutrition Bulletin (The United Nations University Press) 3 (2). ISBN 92-808-0254-2.
- 益阳市政府网 (2008-07-31). [http://zyq.yiyang.hunancom.gov.cn/gqxx/44473.htm[dead link] "益阳名优特产：松花皮蛋"].
- Chen, JrWei; Su, HouPin (2004). "A new process for preparing spots-free pidan". Journal of the Chinese Society of Animal Science 33 (1): 79–88.
- by Billy. "Three Emperor Egg". Atablefortwo.com.au. Retrieved 2012-05-24.
- Helmenstine, Anne Marie. "Are Century Eggs Soaked in Horse Urine?" About.com. Thursday October 16, 2008. Retrieved on October 20, 2009.
- Wood, T.; Weckman, T. J.; Henry, P. A.; Chang, S.-L.; Blake, J. W.; Tobin, T. (1990). "Equine urine pH: Normal population distributions and methods of acidification". Equine Veterinary Journal 22 (2): 118–21. doi:10.1111/j.2042-3306.1990.tb04222.x. PMID 2318175.
- Cook, Janine D; Strauss, Kathy A.; Caplan, Yale H.; LoDico, Charles; Bush, Donna M. (2007). "Urine pH: the Effects of Time and Temperature after Collection". Journal of Analytical Toxicology 31 (8): 486–96. PMID 17988463.
- Ho, Mabel (1988). Chemistry Potpourri : Unlocking Chemistry through Investigations. Singapore: Toppan. ISBN 978-981-00-0454-5.
- Taiwan Livestock Research Institute and Philippine Council for Agriculture (2001) Enhancing the Value of Eggs: How to Make Balut and Century Eggs Retrieved March 24, 2007.
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