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Century EggPreserved Duck Egg(皮蛋)

 

Century egg, 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, lime, and rice hulls for several weeks to several months, depending on the method of processing. Through the process, 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. 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.

Some eggs have patterns near the surface of the egg white that are likened to pine branches.

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 during the Ming dynasty 600 years ago in Hunan, when a home owner discovered duck eggs in a shallow pool of slaked lime that was used for mortar during construction of his home 2 months before. Upon tasting the eggs he set out to produce more, this time with the addition of salt to improve this taste, thus resulting in the present recipe of the century egg.

The traditional method for producing century eggs is a 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 the pH and sodium content of the clay mixture. This addition of natural alkaline compounds improved the odds of creating century eggs instead of spoilage and also increased the speed of the process.

Century EggA 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 together into a smooth paste. Each egg is then individually covered by hand, with gloves being up worn to prevent the corrosive action of the lime on skin. Each egg is 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. In about three years, the mud slowly dries and hardens into a crust, 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 ageing while wrapped in plastic is said to achieve the same effect as the traditional method. This is true to the extent that egg curing in both new and traditional methods is accomplished by introducing alkali hydroxide ions and sodium into the egg.

Although poisonous, lead (II) oxide is also 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.[4] Although zinc is an essential micronutrient, one needs to bear in mind that it can lead to a copper deficiency, so the zinc content needs to be checked for safety.

 

 

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