Century Egg (Alkaline-Cured Duck Egg)

Variations

• Songhua dan (松花蛋), "pine-flower egg": named for the white needle-shaped crystals that come out on the surface of a finished white. Two months at high pH breaks the protein down as far as free amino acids, and some of them pass their solubility and crystallise. It is the sign that it worked, not a fault.
• The traditional coating: a stiff paste of clay, ash, rice husk, salt and lime patted around each egg, then rolled in husk. Identical chemistry, and the husk is there to stop the eggs sticking to each other. That is the "흙과 재, 소금, 석회, 쌀겨" ko.wikipedia describes.
• Hen eggs instead of duck: duck yolks carry more fat and set creamier in the middle; hen eggs come out a little firmer. Quail eggs work too and take less time.
• Salted duck egg (鹹鴨蛋): the same egg cured in salt rather than alkali. Instead of going dark and creamy the yolk turns orange and hard. Same material, different cure, opposite result.
• Pidan doufu: what this page finishes with — quartered over chilled silken tofu with nothing but soy and sesame oil.
• Century egg congee: the Cantonese bowl, simmered with lean pork. As jeonbokjuk explains, a congee is a dish about bursting the grain; the century egg dissolves into it and thickens the liquid.
• Buying them: every Chinese grocer sells them. The reason to make a batch is to find out what is actually happening.
• On lead: some producers used to add lead oxide to speed the cure and sharpen the pattern. It is banned and commercial eggs are lead-free, and nothing in this method involves it.

Tips

• It cooks with no heat at all. Eggs stands on two numbers — white near 62°C, yolk near 68°C — and every egg dish here is a story about not overshooting them. No heat is used in a century egg at any point. Above about pH 12 the proteins unfold and link up again exactly as heat makes them. What differs is how coarse the network is: the one alkali builds scatters far less light, which is why the white sets clear and amber rather than opaque and white.
• Four eggs, four answers to one problem. Onsen tamago removes the gradient between surface and centre so that only the yolk sets; scotch egg staggers two centres a centimetre apart in time; tea eggs use the shell as a mask. This one removes the heat.
• On this site alkali has always been a colour device. Bagels say so outright — "the alkali is a colour device" — and pretzels push it as far as lye. Lanzhou lamian uses kansui to make a dough extensible. Across eighteen pages the alkali touches a dough or a bath and the food is then cooked. Here the alkali is the cooking.
• The causality is the reverse of hongeo-hoe. In skate, ammonia released as the flesh's own urea breaks down is what pushes the pH up towards 9. Here alkali poured in from outside raises the pH, and that high pH takes the protein apart, which is where the ammonia smell comes from. Same smell, cause and effect swapped.
• Ammonia is normal; sulphur is not. A sharp ammoniacal smell means the protein is being cut apart. A rotten-egg sulphur smell, or a white still running like water, means throw it out.
• Safety comes first. Slaked lime and soda ash are strong alkalis. Gloves and eye protection, glass or ceramic only, no aluminium, and nothing that is not labelled food grade. Keep it away from children and animals.
• Do not shorten it. Too short and the middle of the white stays liquid; far too long and it turns bitter and rubbery. Opening one at five weeks is the only reliable test.
• Cut them with thread. A blade presses the creamy centre of the yolk along the cut face. A length of thread pulled through gives a clean one.