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Century egg sliced open |
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Century egg, also known as preserved egg, hundred-year egg,
thousand-year egg, and thousand-year-old egg, is a Chinese cuisine
ingredient made by preserving duck, chicken or quail eggs in a mixture
of clay, ash, salt, lime, and rice hull for several weeks to several
months, depending on the method of processing. |
After the process is completed, 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 or taste. 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 or snow flakes (see
photo gallery below).
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According to some the century egg has over 5 centuries of
history behind its production. 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. |
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Left: Century egg
coated in a caustic curing mixture. |
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.
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 together into a smooth paste. Each
egg is then individually covered by hand, with gloves being 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 months, 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.
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Century eggs can be eaten without further
preparation, on their own 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. |
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Left: Arranged
century egg on a plate. |
In Taiwan it is popular to eat century eggs on top of cold tofu with
katsuobushi (dried skipjack tuna flakes), 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.
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Century egg showing snow-flake/pine-branch
patterns. |
