Century egg

Lore & Background

The method for creating century eggs likely arose from the need to preserve eggs in times of plenty by coating them in alkaline clay, similar to methods in some Western cultures. The clay hardens around the egg and results in curing rather than spoilage. The century egg has been known in Chinese cuisine since at least the 17th century CE, and its origins are the subject of several folk tales. One tale from Hunan during the Ming Dynasty tells of a homeowner who discovered duck eggs in a shallow pool of slaked lime used for mortar two months earlier; after tasting them, he added salt to improve flavor, leading to the present recipe. Another story involves a young duck farmer named Shuige leaving eggs in a woman's garden as a courting gesture; they were found half a month later in an ash pit, transformed into century eggs, and named 'pine-patterned eggs' in her honor.

Reader's Guide

Century eggs are significant as a traditional Chinese preserved food that demonstrates early understanding of alkaline preservation. They are eaten without further preparation beyond peeling and rinsing, or used in various dishes such as sliced with black vinegar in central China, wrapped with pickled ginger as a Cantonese hors d'œuvre, mixed with chilled tofu in Shanghai, or served over cold tofu with katsuobushi in Taiwan. They are also used in 'old-and-fresh eggs' omelettes and stir-fried with vegetables. A common dish is century egg and lean pork congee, often served in dim sum restaurants. At special events, quartered century eggs appear on cold platters. The eggs contain about 13% protein, 11% fat, and high sodium levels. Modern production has simplified the process using a solution of table salt, calcium hydroxide, and sodium carbonate. Some producers have used toxic lead(II) oxide to speed reactions, but zinc oxide is now recommended. In 2013, three factories in Jiangxi were found using industrial copper sulphate contaminated with heavy metals, leading to shutdowns and arrests. Despite misconceptions about preparation in horse urine, the eggs are generally safe when traditionally made.

Did You Know?

• The transforming agent in century eggs is an alkaline salt that raises the egg's pH to around 9–12 during curing.
• Some century eggs have patterns near the surface likened to pine branches; these are called Songhua eggs and are considered higher quality.
• In Thai and Lao, the common word for century egg translates to 'horse urine egg' due to its distinctive odor.
• A 17th-century writer Fang Yizhi described preserved eggs made with five kinds of tree ash and charcoal lime.

The Alchemical Transformation

At its core, the century egg is a feat of controlled chemistry. An alkaline salt, introduced through the curing mixture, gradually elevates the egg's pH from its natural state to somewhere between nine and twelve over the course of weeks or months. This sustained alkaline environment does something remarkable to the egg's internal structure: it dismantles the large, relatively flavorless proteins and fats that make up a raw egg and reassembles them into a constellation of smaller, intensely flavorful compounds. The yolk shifts to a deep greenish-grey, taking on a smooth, almost custard-like consistency, while the hydrogen sulfide and ammonia generated during the process lend it a pungent, assertive character. The white, meanwhile, transforms into a dark brown, translucent gel with a jelly-like texture and a deeply savory, salty-umami taste. Among the most prized results are the so-called Songhua eggs, whose whites display delicate crystalline patterns resembling pine branches near the surface. These patterned specimens are traditionally regarded as superior in quality to their unpatterned counterparts, and their name—literally 'pine flower' or 'pine-patterned'—has become a mark of distinction in Chinese food culture.

Origins Wrapped in Folklore

The precise origin of the century egg is lost to time, but the technique likely emerged from a practical need: preserving surplus eggs during periods of abundance by coating them in alkaline clay, a practice not unlike egg-preservation methods found in some Western traditions. The clay would harden around the shell, and rather than spoiling, the eggs would cure into something entirely new. By the seventeenth century, the dish was firmly established in Chinese cooking, known during the Ming Dynasty by the name 'Chaoszi.' The scholar Fang Yizhi recorded in his work 'Little Knowledge of Physics' that eggs from Chizhou were salted using five varieties of tree ash, with buckwheat grain ash lending a yellowish-white hue and charcoal lime producing a green, firmer result. Two beloved folk tales trace the dish's birth to Hunan. In one, a homeowner discovered duck eggs submerged in a shallow pool of slaked lime left over from home construction two months earlier; after tasting them, he added salt to improve the flavor and thus perfected the recipe. In another, a young duck farmer named Shuige left eggs in the ash pit of a woman called Songmei as a romantic gesture. When she cleaned the pit half a month later, the eggs had transformed, and in her honor he named their crystalline surface patterns 'pine-patterned.'

From Hand-Plastered Mud to Measured Solutions

The traditional production of century eggs is a slow, tactile craft. A paste is prepared by infusing roughly three pounds of tea in boiling water, then combining it with about three pounds of calcium oxide (doubled in winter), nine pounds of sea salt, and seven pounds of ash from burned oak. Each raw egg is individually enshrouded in this mixture by hand—gloves are essential to prevent chemical burns—and then rolled in rice chaff so the coated eggs do not stick together. They are stored in cloth-covered jars or tightly woven baskets, where the mud slowly dries and hardens into a protective crust over the course of several months. Modern food science has since revealed that the essential chemistry is simply the introduction of hydroxide and sodium ions into the egg, prompting a wave of simplifications. Today, a raw egg can be soaked in a solution of table salt, calcium hydroxide, and sodium carbonate for ten days, then aged in an airtight container for several weeks to achieve a comparable result. One persistent concern involves the use of lead(II) oxide, which accelerates the curing reactions but is extremely toxic; zinc oxide is now the recommended substitute, though producers must still monitor zinc levels, since excessive zinc intake can trigger copper deficiency in the consumer.

A Versatile Star Across the Chinese Table

Once cured, the century egg is remarkably adaptable across regional Chinese cuisines. In central China, it is simply sliced and drizzled with black vinegar as a standalone side dish. Cantonese cooks wrap chunks of the egg in thin slices of pickled ginger root, a preparation so popular it is sometimes sold on a stick as street food. In Shanghai, chopped century eggs are folded into chilled tofu for a cool, contrasting bite. Taiwanese home cooking favors sliced century eggs laid over cold tofu, finished with katsuobushi, soy sauce, and sesame oil—a presentation closely echoing the Japanese dish hiyayakko. Northern Chinese variations layer the egg over silken tofu with generous shreds of young ginger and chopped spring onions, then drizzle light soy sauce and sesame oil. The dish 'old-and-fresh eggs' pairs chopped century eggs with a freshly cooked omelette, juxtaposing the aged and the raw. In Taiwanese restaurants, chunks of century egg are stir-fried alongside vegetables. Perhaps the most ubiquitous preparation is the beloved 'century egg and lean pork congee,' where peeled eggs are quartered or eighths and simmered into rice porridge with sliced pork, a staple of dim sum restaurants and home kitchens alike.