Century egg

Century eggs (

Some eggs have patterns near the surface of the egg white 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.^{[citation needed]}

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.^[3] 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

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".^[6]

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.^[7]

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

The extremely toxic compound lead(II) oxide speeds up the reactions that create century eggs, leading to its use by some producers,^[3] whereas zinc oxide is now the recommended alternative.^[8] 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.

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 with

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".

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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 μg per 100 g (3.5 oz) of vitamin B12, which is notably higher than the 0.9 μg per 100 grams found in traditional chicken eggs.

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.^[10] 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 [kʰàj jîaw máː] (RTGS: khai yiao ma)
• Lao: ໄຂ່ຍ່ຽວມ້າ [kʰāj ɲīaw mâː]

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

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  With chili oil
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  With tofu
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  With congee
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  Pink century eggs in Laos