Food

Why are century eggs black? The science behind their color

Clarissa WeiJun 03, 2020

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.

(Read more: Century egg myths: No, they’re not made of horse urine or a hundred years old)

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.

Century eggs are made by coating fresh eggs in mud mixed with salt, soda ash, and quicklime. / Photo: Patrick Wong

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.

(Read more: ‘We took it off the menu because it was a little weird’: Junzi Kitchen head chef on the challenges of selling Chinese food in New York)

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.

Century eggs are often served in rice porridge. / Photo: Shutterstock

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.

Producer: Clarissa Wei

Videographer: Patrick Wong

Editor: Nicholas Ko

Animation: Ray Ngan

Mastering: Victor Peña

Clarissa Wei
Clarissa Wei is a senior reporter at Goldthread. She spent the bulk of her career as a freelance journalist and has written for outlets like VICE, CNN, Eater, among others. Clarissa has backpacked to over a dozen provinces in China and was once a volcano hiking guide in Nicaragua.
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