Century Egg

01

A preserved egg, not an old one

A century egg is cooked by chemistry rather than heat, in weeks rather than a century.

Peel a century egg and the white is no longer white. It has set into a firm, springy jelly, brown and see-through like amber or dark tea, sometimes with fine white crystals across the surface in the shape of pine needles. The yolk is dark green, shading lighter toward the middle, and it can be firm, or soft and creamy at the center. It smells faintly of sulfur when first cut. The first taste is sharp and a little astringent; as you chew, it turns long, savory and rich.

Chinese has three common names for it. 皮蛋, “skin egg,” because the peeled white is springy like a skin. 松花蛋, “pine-flower egg,” for the feathery crystal pattern on a well-made egg. 变蛋, “changed egg,” because a fresh egg turns into something else. English calls it century egg, hundred-year egg, thousand-year egg or simply preserved egg; Hong Kong’s food safety authority uses “lime-preserved egg.” The dramatic names are a joke that stuck: Fuchsia Dunlop notes that it is called the thousand-year-old egg “in the West,” and Chinese writers point out that the egg itself has existed for well under a thousand years.

Most century eggs are duck eggs. Chicken eggs are cured the same way; having thinner shells, they often come out with golden rather than dark-green yolks.

Sources [1][3][9]

02

What happens inside the shell

The whole process depends on alkali seeping through the shell, and stopping it at the right moment.

Traditionally, fresh duck eggs were caked in a paste of mud, strong tea, salt and alkaline ingredients such as wood ash, soda, lye and quicklime, rolled in rice husks so they did not stick together, and packed in jars. Dunlop gives about three months as the traditional curing time.

Alkali passes through the tiny pores in the shell. The egg white first turns liquid and then sets again as an elastic gel; when the alkali reaches the yolk, it breaks down proteins and fats into the savory compounds that give the egg its taste, including glutamates and the hydrogen sulfide that gives the sulfurous smell. The difficulty is timing: if the alkali keeps flowing in after the white has set, the gel liquefies again before the yolk is ready. The answer was to block the pores. Old recipes added huangdan powder, a lead oxide, which seals the shell; lead also builds up in the egg. Modern eggs use copper or zinc compounds to seal the pores instead, which is what packaging calls “lead-free.”

The white pine-flower crystals are mainly magnesium hydroxide. A soft-centered yolk comes from stopping the cure early, while the yolk is still creamy; golden yolks come from letting the hydrogen sulfide escape, through thinner shells or a crack, and a stronger alkali.

Sources [1][2][3]

03

Five hundred years of written recipes

Nobody knows who first made a century egg, but the recipes go back to the Ming.

The earliest clear recipe is in Zhuyu Shanfang Zabu, compiled by Song Xu and Song Gongwang, father and son, of Huating, today’s Songjiang in Shanghai, and finished in 1504. It calls the eggs 混沌子 and tells you to mix a dou of burnt charcoal ash and a sheng of lime with brine, bring it to the boil, let it cool, coat the eggs and wait five to seven days until “yolk and white are mixed in one.” A late-Ming book, Yangyu Yueling, describes 牛皮鸭子, “ox-hide duck eggs”: a hundred eggs with ten liang of salt, five sheng of chestnut-wood charcoal ash and a sheng of lime, turned every three days three times and sealed for a month. The scholar Fang Yizhi, writing around the Ming–Qing transition, says Chizhou in Anhui produced “changed eggs” cured with ash from five kinds of trees, and already distinguishes two results: buckwheat-straw ash gave a yolk and white mixed together, while adding charcoal and lime made them green and firm.

Where it was invented is unknown. The first written recipe comes from near Shanghai and the first named place is in Anhui. Local stories of duck eggs found, by luck, after lying in lime or ash cannot be traced either. What is certain is that by the Qing dynasty the egg was eaten from Beijing to Guangdong and admired, not merely tolerated. Yuan Mei, while ill in Guangzhou, recorded a poem by a Guangdong county magistrate, Wu Shixian, who wrote that inside the egg were “the colors of rosy clouds” and beyond its taste “the smoke of pine and bamboo.” Xu Ke’s Qing Bai Lei Chao explains that in Beijing people avoided the word 蛋, “egg,” because it was used in insults, so a century egg was simply called 松花, “pine flower”; the book also lists century eggs among the four cold plates of a Shanghai tavern set meal.

Outsiders were less sure. In colonial Hong Kong, the physician and lawyer Ho Kai, who opposed the food bylaws of the 1887 Public Health Ordinance, argued that words like “tainted” and “unwholesome” were culturally relative, pointing out that preserved eggs, which Europeans found distasteful, were delicacies to Chinese people.

Sources [1][4][5][6]

04

How Guangdong eats it

In Cantonese cooking the century egg is a seasoning as much as a dish.

Century egg and pork congee, 皮蛋瘦肉粥, is the way most people first meet the egg. A restaurant version starts with rice simmered for about two hours at a lively boil, with a little oil so it is silky rather than thin, until the grains break down. Diced century egg and shredded ginger go in about five minutes before the end and are stirred until the yolk dissolves into the congee and seasons it. Thin slices of pork, marinated and massaged with a little water and starch so they stay tender, go in last for two or three minutes, then salt, white pepper and spring onions. Heating also drives off much of the egg’s sulfurous smell.

As a cold starter, century eggs are cut into wedges and served with slices of pickled young ginger, 皮蛋酸姜, or with a ginger-and-vinegar dip; the acid takes the edge off the alkaline taste. In soup, diced century egg, often with salted duck egg, is fried in a little lard until lightly browned, then simmered with ginger, garlic and stock until the broth turns milky, and poured over blanched greens such as amaranth or bok choy: 上汤苋菜, amaranth in superior stock, is the best-known version. Cantonese bakers have long used the egg in pastry too: preserved-egg puff, 皮蛋酥, is a traditional wedding pastry, and preserved-egg mooncakes appeared in Guangzhou tea houses in the Republican period.

Sources [1][2][3][7][10][13]

05

Tofu, peppers and other ways across China

Most other regions eat century eggs cold, with something sour, salty or hot.

Dunlop describes a home cook who sets out century eggs cut into “rainbow segments” to keep guests going while she finishes cooking. In Beijing banquets it is a standard cold dish. With silken tofu it becomes the simplest dish in the repertoire: the tofu is turned out whole, the eggs are chopped on top, and a dressing of soy sauce, vinegar, sesame oil and garlic or chili is poured over; many people mash the two together and eat it with rice.

The Hunan version is the boldest. Green chilies are charred over a flame or charcoal, peeled, and pounded in a mortar, seasoned simply with salt, soy sauce and a little vinegar; the century eggs are added in pieces and pounded gently with a circular motion, so that the creamy yolks coat the peppers. A Yunnan restaurant in Beijing pairs golden century eggs with sour tree tomatoes.

Sources [1][3][11][12]

06

If you are trying it for the first time

Many people dislike century eggs at first; plenty of them change their minds.

Fuchsia Dunlop, who went on to become one of the best-known English writers on Chinese food, describes her first century egg, in a Hong Kong restaurant in 1992, as something she could barely swallow. Later in the same book she writes that preserved duck eggs, like fermented tofu, are prized in China for their strong and complex umami.

The smell is the main hurdle, and it fades. Cut eggs left to stand for a while lose some of their hydrogen sulfide, heat drives off more, and an acid such as vinegar or pickled ginger tempers the alkaline taste. So start with congee, where the egg is cooked and diluted, or with century egg and tofu, then try a plain wedge with ginger.

Sources [1][2]

07

Lead, safety and buying

Lead was a real problem in older century eggs; current standards deal with it.

Lead oxide was a traditional additive, and eggs made with it were high in lead. China’s current standard for century eggs, GB/T 9694-2014, in force since December 2015, applies the general contaminant limits of GB 2762, which cap lead at 0.5 milligrams per kilogram. “Lead-free” on the box means within that limit, not zero. In 2013 there were reports of industrial-grade copper sulphate being used to speed up production on the Mainland; Hong Kong’s Centre for Food Safety noted that copper sulphate is a permitted processing aid there, used in place of lead oxide, and that its own tests for lead and other heavy metals in preserved eggs had all been satisfactory. Dunlop describes visiting an unlicensed workshop near Chengdu whose eggs were ready in seven to ten days, a sign, her companion said, of shortcuts with additives. Buy eggs with a brand, a production date and a standard number on the box, from shops you trust.

A good century egg has a clean, intact shell; once peeled, the white is a translucent dark brown and springy. If the white is light green, soft and falling apart, throw the egg away, since it may be contaminated; if you are unsure about an egg, steaming it thoroughly makes it safer. To peel, crack the broad end, where the air pocket is, and roll the egg under your palm. A knife dipped in cold boiled water, or a length of thread pulled tight, cuts the sticky yolk cleanly; steaming the eggs for a few minutes first firms them for dicing.

Sources [1][2][8][9][12]

08

Century egg and pork congee at home: the slow version

Sources [10][14]

09

More to try