Century Eggs

Century Duck Egg

Century eggs, also known as, preserved egg, hundred-year egg, thousand-year egg, thousand-year-old egg, and millennium egg, is a Chinese delicacy. It is made by preserving the eggs of a duck, chicken or quail. The preservation process does not involve one drop of horse urine or indeed the urine of any other animal despite what the urban myth states. The preservation involves a little chemistry.

Why would anyone preserve an egg? Well, before agriculture became an industrialised process producing a guaranteed supply of food, foodstuffs were preserved during plentiful times for leaner times of the year. It’s a practice that is seen in many cultures around the world. One prime example is jams which came from excess fruit.

The century egg is thought to have originated during the Ming Dynasty in Hunan. A legend tells of a homeowner discovering duck eggs in a shallow pool of slaked lime he used to build his home. He decided to eat these eggs. They didn’t taste too bad so he set about producing more eggs but added salt to improve the flavour.

The alkaline clay coating the eggs cured them to produce century eggs instead of causing them to spoil. The traditional method that is used today by some manufacturers of century eggs uses a mixture of wood ash, quicklime and salt in a plaster mixture coating the egg. This mixture is then rolled in rice straw by hand and left to cure for several months as the plaster mixture dries. The typical time period is around three months. During this time, the pH of the egg rises to around 9 and higher if the curing stage is longer. The pH of horse urine ranges from 7.4 to 7.9 which is not high enough to raise the pH of an egg.

With the understanding of chemistry involved of the formation of century eggs, the modern recipe has been simplified. Soaking eggs in a brine solution containing salt, calcium hydroxide and sodium carbonate for 10 days and then wrapping individual eggs in plastic to age for several weeks produces the same results as the traditional method.

The result is typically a dark coloured egg. The yolk becomes dark green to grey in colour and takes on a creamy consistency. The white  of the egg turns into a transparent brown jelly. When it comes to the taste, it is incredibly subjective and it also depends on who made the century egg as the taste can vary. In my experience, the yolk while creamy can have a faint odour of sulphur and ammonia, (this is probably why the horse urine myth continues to survive). The white of the egg can taste like regular egg white or can taste as if it has been salted.

I have never eaten century eggs on their own. It has always been a side dish or cooked with other dishes. There are a plethora of Youtube videos of people eating just the egg alone. I suspect it’s more to do with bravado and oneupmanship than to actually inform people as to what this food oddity is. Let’s face it, it is a little bit odd to eat century eggs with other foodstuffs, let alone eating them as a sole dish.

I like to have century eggs with a savoury rice porridge. I am not fussy as to whether the savoury rice porridge is chicken or pork based. Either will do. Finished off with a small dash of soy sauce and sesame oil. It is the breakfast of champions and an extravagant start to the day.


Filed under chemistry365, Science

6 responses to “Century Eggs

  1. Lil

    I love century eggs and recently tried a cold dish of cubed century eggs and tofu marinated in sesame oil and sprinkle with spring onion – it’s delicious (imho).

  2. Pingback: Chemistry World blog » Easter Eggsperiments

  3. (Tassie_Gal)

    Dad and I had them at a funny little steam boat place in Singapore. That was a lovely night.

  4. niiiiiiiiiiiinja

    i eat century eggs on their own. yum. 😀

  5. Oh yeah, I eat them on their own too if I am in the mood for some protein-y stuff. But than hard boiled egg for me lol. A dash of vinegar (Asian black rice vinegar) makes it even better. Though most of the time I do throw them in noodle soup.

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