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Eggs from China, also eaten in Taiwan and Southeast Asia
Fresh duck eggs are soaked in, or coated with, a strong alkaline mix of sodium hydroxide (or lime and ash or soda ash), salt and tea, for several weeks to several months depending on the method. The alkali soaks through the shell and turns the white into a translucent jelly and the yolk dark green. This is a chemical cure, not a microbial fermentation.
| Fresh, uncracked duck eggs | [gao2022] |
| Sodium hydroxide in the pickling solution4.5 % (m/v) | [gao2022] |
| Salt (NaCl)3.0 % (m/v) | [gao2022] |
| Copper sulfate0.4 % (m/v) | Copper or copper-zinc salts help seal the shell pores late in pickling; lead is prohibited[gao2022][zheng2023] |
| Tea (infused in the traditional coating paste) | [s1][s2] |
Total time across the recorded steps: 3.2 months
What changes: Starting material: fresh, uncracked duck eggs
Output: fresh, uncracked eggs.
Microbes at work in this step
Not documented for this step.
Measured measured in a studySet fixed by a specification or tested recipeReported one source reports itTypical a general value, not sourced for this fermentUnconfirmed not yet checked against a source
Fresh duck eggs are soaked in, or coated with, a strong alkaline mix of sodium hydroxide (or lime and ash or soda ash), salt and tea, for several weeks to several months depending on the method. The alkali soaks through the shell and turns the white into a translucent jelly and the yolk dark green. This is a chemical cure, not a microbial fermentation. Made in 4 steps over 3.2 months. The longest step, roll in rice husk and seal in jars (coating method), takes at least 2 months (Chinese Wikipedia).
| Main step | at least 2 monthsReported[s2] | Roll in rice husk and seal in jars (coating method) |
|---|---|---|
| Acidity | pH 10Measured[gao2022] | Soak in alkaline pickle, or coat with alkaline paste |
Century egg is cured by alkali, not microbes. The pH inside rises to about 10, the opposite of a lactic ferment. Salted eggs such as khai-khem use salt alone and keep a normal white.
Lead oxide was once used to speed the cure and is now strictly prohibited because of lead poisoning. Copper and copper-zinc salts are used instead, but residual heavy metals remain a concern, and heavy-metal-free methods are being developed. The soak listed here comes from a lab study and is not a tested home method. It is a lye solution, and sodium hydroxide burns skin and eyes. Swallowing large amounts of copper sulfate can cause vomiting and damage the liver and kidneys.
Hazards the sources name: Bacillus cereus (reported among the predominant bacteria); Heavy metals from the pickling liquid, which pass through the shell into the egg (not a microbe).
Every number is labelled with how it is known. How this atlas is made
Chinese Wikipedia says the earliest written record is generally taken to be from 1504 (Song Xu, 竹嶼山房雜部), which calls it 混沌子. A 1633 text (Dai Xi, 養餘月令) gives a salt, chestnut-charcoal ash and lime method for 牛皮鴨子, an early form, turned three times and then sealed for a month.
