Century egg

• 1. Introduction

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  Century eggs, called pídàn in Mandarin, are also sold as preserved eggs and hundred-year eggs, yet a batch is ready in weeks or months, not a century. Crack the shell and the first surprise is color. The white has turned a dark brown and become translucent and jelly-like. The yolk at its center is a dark greenish-grey. Cooks make these eggs from ducks, chickens or quail, and the process starts with a coating of clay, ash, salt, quicklime and rice hulls. Why would anyone wrap a fresh egg in something so caustic on purpose? What does that coating do to the egg inside? And how did a food that looks like this become a fixture of Chinese tables?

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• 2. Alkaline Alchemy

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  A pH of around 9-12 is the number that explains a century egg. The egg reaches it gradually during curing, driven by an alkaline salt that acts as the transforming agent. That alkalinity breaks down complex proteins and fats that have almost no flavor of their own. What remains is a set of smaller compounds, and those carry the taste. The yolk ends up smooth and strong, with hydrogen sulfide and ammonia behind its sharp character. The white turns gelatinous, salty and savory, with the umami depth that fans of the egg describe. Some eggs also show patterns near the surface of the white, which people liken to pine branches, and those eggs are regarded as better quality than plain ones.

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• 3. Tales of Origin

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  The 17th century is the earliest period from which the century egg is known to have been part of Chinese cuisine. Its true beginnings are unclear. One likely explanation is practical. In times of plenty, people needed to save surplus eggs, and coating them in alkaline clay was a preserving method with parallels in some Western cultures. The clay hardens around the shell, and the egg cures instead of spoiling.

  Folk tales offer livelier explanations. In one, a homeowner in Hunan during the Ming Dynasty found duck eggs in a shallow pool of slaked lime left from mixing mortar for his house, two months after he built it. He tasted them, liked them, and tried again, this time adding salt for flavor. That, the story says, produced the recipe used today.

  A second tale involves a young Hunan duck farmer named Shuige, whose name means water-brother. He left duck eggs in the garden of a woman named Songmei, meaning pine-sister, as a courting gesture. She found them only when she cleaned out her ash pit, half a month later, and by then they had become century eggs. In her honour, the farmer called the eggs, with their delicate crystalline surface patterns, pine-patterned eggs. In Chinese they are Songhua eggs, also translated as pine flower eggs.

  Written evidence comes from the Ming Dynasty, when preserved eggs went by the name Chaoszi. The 17th-century writer Fang Yizhi described them in his book Little Knowledge of Physics: "The eggs produced in Chizhou are salted with five kinds of tree ash. Buckwheat grain ash is mixed to make it yellow and white, and charcoal lime is added to make it green and tough". His account names Chizhou as one place where such eggs were made.

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• 4. Tea, Ash and Lime

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  Three pounds of tea, steeped in boiling water, begin one traditional recipe for century eggs. Into that tea goes a smooth paste of three pounds of calcium oxide, nine pounds of sea salt and seven pounds of ash from burned oak. In winter the recipe calls for seven pounds of calcium oxide instead of three. This mixture grew out of the older, simpler practice of coating eggs with clay alone. Adding wood ash, calcium oxide and salt raised both the alkalinity and the sodium content, and it lowered the risk of spoilage while speeding the process.

  Each egg is coated by hand, and the worker wears gloves to guard against chemical burns. A roll in rice chaff keeps the eggs from sticking together. They then go into cloth-covered jars or tightly woven baskets, where the mud dries into a hard crust over several months.

  The chemistry has since been simplified. Soaking raw eggs for 10 days in a solution of table salt, calcium hydroxide and sodium carbonate, then aging them several weeks in an airtight container, gives a similar result. Whatever the method, the reaction only needs hydroxide and sodium ions to enter the egg. Even so, the traditional method is still widely practised.

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• 5. The Poisoned Shortcut

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  Lead(II) oxide is an extremely toxic compound, yet some producers have used it because it speeds up the reactions that make century eggs. Zinc oxide is now the recommended alternative. It is not harmless either. Zinc is essential for life, but too much can lead to copper deficiency, so a finished product should be tested for its zinc level.

  In 2013, a scandal in Jiangxi province showed the risk of cutting corners. Three factories were found using industrial copper sulphate to shorten production time. That copper sulphate was contaminated with heavy metals and toxic chemicals. China's national broadcaster, CCTV, exposed the practice, and local officials shut down 30 factories for inspection. Police arrested three people and seized four businesses connected to the case. Eggs made in the traditional ways are generally considered safe to eat.

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• 6. From Congee to Banquet

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  Peeling and rinsing are all a century egg needs before it reaches the table. In central China, cooks slice it and drizzle it with black vinegar as a side dish. The Cantonese wrap chunks of it in slices of pickled ginger root, and sometimes sell the result on a stick as street food. A Shanghainese recipe chops the egg into chilled tofu.

  Taiwan has its own favorite, with sliced eggs set over cold tofu and topped with katsuobushi, soy sauce and sesame oil. It resembles the Japanese dish hiyayakko. Northern China has a variation over chilled silken tofu, with plenty of shredded young ginger, chopped spring onions, light soy sauce and sesame oil. Taiwanese cooking also stir-fries chunks of the egg with vegetables. Another dish, called old-and-fresh eggs, combines chopped century eggs with a fresh-egg omelette, either folded inside or laid on top.

  Century egg and lean pork congee, called pídàn shòuròu zhōu, is a household staple that dim sum restaurants sometimes serve. Quarters or eighths of peeled egg simmer with marinated slivers of lean pork until everything melts into the rice. Fried dough sticks called youtiao are the usual companion, and some cooks stir salted duck eggs into the pot as well.

  At wedding banquets and birthday parties, the egg takes a place in a first-course platter called a lahng-poon in Cantonese, meaning cold dish. The platter holds sliced barbecued pork, pickled baby leeks, sliced abalone, jellyfish and head cheese, with quartered century eggs among them.

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• 7. Salt and Suspicion

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  Sodium is the nutritional headline of a century egg. The recommended daily allowance is 1500 mg. A 100-gram portion of the white contains 2250 mg, or 150 percent of that allowance. The same weight of yolk holds 1516 mg, about 100 percent. In composition, the egg is roughly 13 percent protein, 11 percent fat, 2 percent carbohydrate and 2 percent ash. Alkaline curing also reshapes its amino acids. Leucine, asparagine and glutamine are synthesized in the process, while lysine, serine and threonine are reduced.

  A persistent belief holds that these eggs are soaked in horse urine for a hundred years. No valid evidence supports it, though it explains why some people avoid the food. Part of the reason may be the smell, since the Thai and Lao words for century egg translate to horse urine egg. The Thai term is khai yiao ma, and the Lao is written ໄຂ່ຍ່ຽວມ້າ. Both names refer to the urine-like odor of the egg, not to its ingredients, so the food has acquired both a distinctive smell and a rumor to match.

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