# Century egg

Century eggs (Chinese: 皮蛋; pinyin: pídàn; Jyutping: pei4 daan2), also known as alkalized or preserved egg, are a Chinese egg-based culinary dish made by preserving duck, chicken, or quail eggs in a mixture of clay, ash, salt, quicklime, and rice hulls for several weeks to several months, depending on the processing method.
Through the process, the yolk becomes dark greenish grey in color, with a creamy consistency and strong flavor due to the hydrogen sulfide and ammonia present, while the white becomes dark brown in color, with a translucent jelly-like appearance and salty flavor. The transforming agent in the century egg is an alkaline salt, which gradually raises the pH of 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 to pine branches. These patterned eggs are regarded as having better quality than the normal century eggs and are called Songhua eggs (Chinese: 松花蛋), variously translated as pine flower eggs or pine-patterned eggs.

## History

The method for creating century eggs likely came about through the need to preserve eggs in times of plenty by coating them in alkaline clay, 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 in Hunan during the Ming Dynasty, when a homeowner discovered duck eggs in a shallow pool of slaked lime that 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 of Shuige (水哥, lit. water-brother), also from Hunan, leaving duck eggs in the garden of a woman by the name of Songmei (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 writer Fang 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".

## Methods
### Traditional

The traditional method for producing century eggs developed through improvement of the aforementioned primitive process. Instead of using only clay, a mixture of wood ash, calcium oxide, and salt is included in the plastering mixture, thereby increasing its pH and sodium content. 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) of tea in boiling water. A smooth paste is then formed by mixing 1.4 kg (3 lb) of calcium oxide (3.2 kg or 7 lb, if done in winter), 4.1 kg (9 lb) of sea salt, and 3.2 kg (7 lb) of ash from burned oak, before being added to the tea. Each egg is individually covered by hand, with gloves worn to protect the skin from chemical burns. It is then rolled in a mass of rice chaff, to keep the eggs from adhering to one another, before the eggs are placed in cloth-covered jars or tightly woven baskets. The mud slowly dries and hardens into a crust over several months. The eggs are then ready for consumption.

### Modern

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 a solution of table salt, calcium hydroxide and sodium carbonate for 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 introducing hydroxide and sodium ions into the egg, regardless of the method used.
The extremely toxic compound lead(II) oxide speeds up the reactions that create century eggs, leading to its use by some producers, whereas zinc oxide is now the recommended alternative. Although zinc is essential for life, excessive zinc consumption can lead to copper deficiency, and the finished product should have its zinc level assessed for safety.

### Chemistry behind the transformation

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.

## Uses

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 with black vinegar and served as a side dish. As an hors d'œuvre, the Cantonese wrap chunks of this egg with slices of pickled ginger root (sometimes sold on a stick as street food). A Shanghainese recipe mixes chopped century eggs with chilled tofu. In Taiwan, it is popular to eat sliced century eggs placed 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 (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) an omelette made with fresh eggs. The century eggs may also be cut into chunks and stir fried with vegetables, which is most commonly found in Taiwanese cuisine.
Some Chinese households cut them up into small chunks and cook them with rice porridge to create "century egg and lean pork congee" (Chinese: 皮蛋瘦肉粥; pinyin: pídàn shòuròu zhōu). This 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 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. Another common variation of this dish is the addition of salted duck eggs into the congee mixture.
At special events like wedding banquets or birthday parties, a first-course platter of sliced barbecued pork, pickled baby leeks, sliced abalone, pickled julienned carrots, pickled julienned daikon radish, seasoned julienned jellyfish, sliced pork, head cheese and the quartered century eggs is served. This is called a lahng-poon in Cantonese, which simply means "cold dish".

## Nutritional facts

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.

## Misconception and etymology

Century eggs are sometimes avoided due to the belief that they are prepared by soaking eggs in horse urine for 100 years, but there is no valid evidence to support this. In Thai and Lao, the common word for century egg translates to "horse urine egg", due to the distinctive urine-like odor of the food:
* Thai: ไข่เยี่ยวม้า (RTGS: khai yiao ma)
* Lao: ໄຂ່ຍ່ຽວມ້າ

## Safety

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 in Jiangxi province were found to be using industrial copper sulphate in 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 broadcaster CCTV, 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.

## Gallery

* Century egg coated in a caustic mixture of mud and rice husk
* Century egg showing snow-flake/pine-branch (松花, sōnghuā) patterns. These patterns are dendrites of various salts.
* Arranged century egg on a plate
* Century egg with congee
* Century eggs for sale in Hong Kong
* Pink century eggs in Laos