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 placing eggs in mud made from alkaline clay and water in order to preserve eggs in times of plenty. The clay hardens around the egg and likely resulted in the curing and creation of century eggs instead of spoiled eggs.
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 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 , nine pounds of sea-salt, and seven pounds of wood ash from burning 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. The recipe makes around 100 to 150 century eggs.
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 , 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 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 slightly poisonous, lead oxide is also known to increase the curing speed of century eggs and is thus added to the curing mixture by some century egg producers in China, which is the world's largest producer of century eggs. Zinc oxide may be used as a replacement Although zinc is an essential micronutrient it can lead to a copper deficiency, so the zinc content should be checked for safety.
Century eggs can be eaten without further preparation, on their own as a side dish. As an hors d'?uvre, the Cantonese wrap chunks of this egg with slices of pickled . 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 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 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'' , and 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 together 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.
On special events, like wedding banquets or birthday parties, a first course platter of sliced , pickled , sliced abalone, pickled 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 , which simply means cold dish.
A popular street food in Hong Kong consists of whole century eggs coated in fish meal, breaded, and deepfried, producing a snack analogous to the ubiquitous Scotch egg in the United Kingdom.
According to a persistent myth, century eggs are or once were prepared by soaking eggs in horse urine. However, this is not plausible since urine is usually acidic or very weakly alkaline, and would not actually preserve the eggs. The myth may arise from the ammonia smell created during some production processes.
* Mabel Ho ''Chemistry Potpourri : Unlocking Chemistry through Investigations'', Singapore Science Centre
* H.C. Hou , , ''The United Nations University Press Food and Nutrition Bulletin'' Chapter 3, 3, ISBN 92-808-0254-2
* Taiwan Livestock Research Institute and Philippine Council for Agriculture Retrieved March 24 2007.