Century egg

Century eggs(Chinese:皮蛋;pinyin:pídàn;Jyutping:pei4 daan2), also known asalkalizedorpreserved egg, are aChineseegg-based culinary dish made by preservingduck,chicken, orquail eggsin a mixture ofclay,ash,salt,quicklime, andrice hullsfor several weeks to several months, depending on the processing method.
Through the process, theyolkbecomes dark greenish grey in color, with a creamy consistency and strong flavor due to thehydrogen sulfideandammoniapresent, while thewhitebecomes dark brown in color, with a translucent jelly-like appearance and salty flavor. The transforming agent in the century egg is analkalinesalt, which gradually raises thepHof the egg to around 9–12 during the curing process.This chemical process breaks down some of the complex, flavorless proteins and fats, which produces a variety of smaller flavorsome compounds.
Some eggs have patterns near the surface of the egg white which are likened topinebranches. These patterned eggs are regarded as having better quality than the normal century eggs and are calledSonghua eggs(Chinese:松花蛋), variously translated aspine flower eggsorpine-patterned eggs.
The method for creating century eggs likely came about through the need to preserve eggs in times of plenty by coating them inalkalineclay, which is similar to methods of egg preservation in some Western cultures.The clay hardens around the egg and results in the curing and creation of century eggs instead of spoiled eggs.
The century egg has at least four centuries of history behind its production. Its discovery, though not verifiable, was said to have occurred around 600 years ago inHunanduring theMing Dynasty, when a homeowner discovered duck eggs in a shallow pool ofslaked limethat was used for mortar during construction of his home two months before. Upon tasting the eggs, he set out to produce more – this time with the addition of salt to improve their flavor – resulting in the present recipe of the century egg.An alternate story involves a young duck farmer by the name ofShuige(水哥,lit.water-brother), also from Hunan, leaving duck eggs in the garden of a woman by the name ofSongmei(松妹,lit.pine-sister) as a courting gesture. The eggs were not discovered until the woman cleaned out the ash pit half a month later where they had turned into century eggs. In her honour, the farmer named the transformed eggs with their delicate crystalline patterns on their surfaces "pine-patterned eggs".
Preserved eggs were called "Chaoszi" in the Ming Dynasty. The 17th century writerFang Yizhi's "Little Knowledge of Physics(物理小識) states: "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".
The traditional method for producing century eggs developed through improvement of the aforementioned primitive process. Instead of using onlyclay, a mixture ofwood ash,calcium oxide, andsaltis included in the plastering mixture, thereby increasing itspHandsodiumcontent. The addition of calcium oxide and wood ash to the mixture lowers the risk of spoilage and also increases the speed of the process. A recipe for creating century eggs starts with the infusion of 1.4 kg (3 lb) ofteain boiling water. A smooth paste is then formed by mixing 1.4 kg (3 lb) ofcalcium oxide(3.2 kg or 7 lb, if done in winter), 4.1 kg (9 lb) ofsea salt, and 3.2 kg (7 lb) of ash from burnedoak, before being added to the tea. Each egg is individually covered by hand, with gloves worn to protect the skin fromchemical burns. It is then rolled in a mass of ricechaff, to keep the eggs from adhering to one another, before the eggs are placed in cloth-coveredjarsor tightly wovenbaskets. The mud slowly dries and hardens into a crust over several months. The eggs are then ready for consumption.
Even though the traditional method is still widely practised, modern understanding of the chemistry behind the formation of century eggs has led to many simplifications in the recipe. Today, soaking raw eggs in asolutionoftable salt,calcium hydroxideandsodium carbonatefor 10 days, followed by several weeks of aging in an airtight container, can achieve a similar effect to the traditional method. This is because the chemical reaction needed to produce century eggs is accomplished by introducinghydroxideandsodiumionsinto the egg, regardless of the method used.
The extremely toxic compoundlead(II) oxidespeeds up the reactions that create century eggs, leading to its use by some unscrupulousproducers,whereaszinc oxideis now the recommended alternative.Although zinc is essential for life, excessive zinc consumption can lead tocopper deficiency, and the finished product should have its zinc level assessed for safety.
The production of century eggs involves a complex physico-chemical transformation, wherein eggs are preserved at room temperature in an alkaline pickling solution. This process converts the egg white into a transparent, yellow gel that exhibits distinctive optical and mechanical properties. Facilitated by a high-pH environment, typically using a mixture of sodium hydroxide and salt, this method involves immersing raw eggs in a concentrated alkaline solution. The high pH causes the egg white proteins, primarily ovalbumin, to denature and then reassemble into a globular network of fine strands. This network forms through a combination of non-specific interactions, including long-range electrostatic repulsion and short-range attractions, resulting in the creation of a stable, elastic gel. Remarkably, this gel retains its structure even when boiled. Meanwhile, the egg yolk undergoes minimal changes during this process. This technique illustrates a non-specific aggregation pathway common among globular proteins, effectively utilized to preserve the eggs and significantly alter their texture.
Century eggs can be eaten without further preparation other than peeling and rinsing them – on their own, or as a side dish. In central China, they are sliced into pieces and drizzled withblack vinegarand served as a side dish. As anhors d'œuvre, the Cantonese wrap chunks of this egg with slices of pickledginger root(sometimes sold on a stick asstreet food). A Shanghainese recipe mixes chopped century eggs with chilledtofu. InTaiwan, it is popular to eat sliced century eggs placed on top of coldtofuwithkatsuobushi,soy sauce, andsesame oil, in a style similar toJapanesehiyayakko. A variation of this recipe common in northern China is to slice century eggs over chilled silken (soft)tofu, add liberal quantities of shredded young ginger and chopped spring onions as a topping, and then drizzle 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) anomelettemade with fresh eggs.The century eggs may also be cut into chunks and stir fried with vegetables, which is most commonly found inTaiwanese cuisine.
Some Chinese households cut them up into small chunks and cook them with rice porridge to create "century egg and lean porkcongee" (Chinese:皮蛋瘦肉粥;pinyin:pídàn shòuròu zhōu). This is sometimes served indim sumrestaurants. Rice congee, lean pork, and century egg are the main ingredients. Peeled century eggs are cut into quarters or eighths and simmered with the seasoned marinated lean slivers of pork until both ingredients are cooked into the rice congee. Fried dough sticks known asyoutiaoare commonly eaten with century egg congee. Another common variation of this dish is the addition ofsalted duck eggsinto the congee mixture.
At special events likewedding banquetsor birthday parties, a first-course platter of slicedbarbecued pork,pickledbaby leeks, slicedabalone,pickledjuliennedcarrots, pickled julienneddaikon radish,seasoned julienned jellyfish, slicedpork,head cheeseand the quartered century eggs is served. This is called alahng-pooninCantonese, which simply means "cold dish".
Research indicates that century eggs are an excellent source of Vitamin B12, which is crucial for metabolism and DNA synthesis. The yolk of century eggs typically contains about 1.9 ± 0.6 to 0.8 ± 0.3 μg of vitamin B12, which is notably higher than the 0.9 μg per 100 grams found in traditional chicken eggs, concentrated primarily in the yolk. Despite undergoing an alkaline preservation process, which could potentially impact their nutrient content, century eggs maintain a substantial amount of Vitamin B12. Furthermore, the conditions of this preservation do not compromise the B12 content, underscoring the nutritional resilience of these eggs.
A key feature of century eggs is the bioavailability of Vitamin B12 during digestion. Studies demonstrate that during in vitro gastric digestion, a significant amount of the Vitamin B12 in the yolk of century eggs is released in a free form, enhancing its digestibility compared to that in regular cooked eggs. For instance, approximately 52% of the B12 from century egg yolk converts to a form that is easily absorbed, far exceeding the conversion rates in standard boiled or fried chicken eggs. This improved bioavailability is particularly significant as absorption of Vitamin B12 from dietary sources can be generally inefficient, especially from plant-based foods.
Century eggs are sometimes avoided due to the belief that they are prepared by soaking eggs inhorseurinefor 100 years, but there is no valid evidence to support this.InThaiandLao, the common word for century egg translates to "horse urine egg", due to the distinctive urine-like odor of the food:
Century eggs prepared in traditional ways are generally safe to consume. However, there have been incidents of malpractice in century egg production that causes eggs to be contaminated. In 2013, three factories inJiangxi provincewere found to be using industrialcopper sulphatein century egg production to shorten the production time. The industrial copper sulphate was contaminated with heavy metals and toxic chemicals.The incident was exposed on China's national broadcasterCCTV, causing local officials to shut down 30 factories for inspection. The police arrested three people involved in the case and seized four businesses involved in the case.