What Are Century Eggs?

Century eggs, also known as preserved eggs, hundred-year eggs, thousand-year eggs, thousand-year-old eggs, century-old eggs, millennium eggs, skin eggs, black eggs or old eggs, 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 method of processing.

Through the process, the yolk becomes a dark green to grey color, with a creamy consistency and strong flavor due to the hydrogen sulfide and ammonia present, while the white becomes a dark brown, translucent jelly with a 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, also known as pine flower eggs or pine-patterned eggs. In 2014, 3 million tons of Songhua eggs were consumed in China.

Methods

Traditional

The traditional method for producing century eggs developed through improvement of the aforementioned primitive process. Instead of using just 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. To the tea, 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 is mixed into a smooth paste. 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 while wrapped in plastic, is said to achieve the same effect as 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 which create century eggs, leading to its use by some unscrupulous 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.

Credit: Photo by Jo del Corro