The dish arose in China as a novel way to preserve fresh eggs.
Century eggs, sometimes also called thousand-year-old eggs, are one of the most misunderstood dishes in Chinese cuisine.
With their black-brown exterior, gelatin-like texture, and runny greenish-black yolk, the preserved eggs are often a sight gag on late-night comedy shows. James Corden, host of The Late Late Show, frequently uses them in his truth-or-dare segment with celebrities.
Common misconceptions about century eggs revolve around why they’re black, including speculation of unsavory ingredients like horse urine, toxic lead, and artificial food coloring.
But the real reason for the eggs’ unique color is a simple chemical reaction, the same one that gives cooked bacon and fresh toast their brown color.
The science behind century eggs
Century eggs are essentially preserved eggs. Their production in China likely arose to preserve excess supplies of fresh eggs. Nowadays, duck eggs are often used to make them.
The eggs are covered in mud mixed with salt, soda ash, and quicklime. The soda ash and quicklime react with water to form sodium hydroxide.
Through osmosis, the sodium hydroxide breaks down protein in the egg and hardens the albumen. That’s why when you open a century egg, the inside is gelatin.
The eggs are left to sit for up to three months. The mud creates a vacuum seal that keeps out bacteria and other microorganisms, which means century eggs are ready to eat and don’t need to be cooked.
But why are they black?
The century egg’s distinct hue comes from the Maillard reaction, a natural browning effect that’s accelerated in a highly alkaline environment.
Sodium hydroxide is alkaline. As it breaks down the egg’s protein into glucose and amino acids, the two chemicals react. This reaction, called the Maillard reaction, produces the distinct color of century eggs.
It’s also what causes browning when you fry bacon on a pan or roast meat in an oven.