1. How Century Eggs Are Made and Turn into Jelly Without Cooking
Making century eggs is surprisingly simple. Duck eggs are coated with a mixture of clay, lime, and sometimes charcoal, then dusted with rice husks and placed in a cool, dark spot [3]. The curing time varies by method, ranging from about 20 days to several months [3]. During this period, the eggs are never exposed to fire or boiling water. Yet, when you take them out and crack them, the white has solidified into a brown jelly, and the yolk has turned dark green [3]. Since no heat was used, something else must have caused the egg white to solidify. That something is produced by the lime in the clay.
2. How Strong Alkali Links Century Egg White Proteins Together
Lime becomes an alkaline liquid when it touches water. This article explains that the alkali seeping through the shell is what solidifies the century egg white [3]. In 1975, Japanese researchers tested if egg whites would solidify when strong alkali (sodium hydroxide) was added. They found that the egg white formed a gel, meaning it became jelly-like while holding water [1]. Between 0 and 45°C, higher temperatures made the gel form faster and become firmer [1]. The same study showed that alkali causes egg white proteins to interact. At pH 12.5, a protein called conalbumin disappeared from its usual position in an electrophoresis test. At pH 13.0, ovalbumin, the main protein in egg white, also disappeared [1]. This suggests the proteins bonded with other proteins to form something new. In this experiment, strong alkali solidified egg whites without heat. However, this was done in a lab with concentrated alkali, so the same process might not happen at the same speed inside a century egg. This is not an experiment to try at home.
3. How Heat Solidifies Egg White in Boiled Eggs
Egg white is a mixture of several proteins, not just one. When heated, their shapes unravel (denature) and stick together to form a solid. The temperature at which they start to solidify varies by protein. A 2025 study found that ovalbumin denatures at about 80°C and lysozyme at about 77°C when a 5% solution is heated at 5°C per minute [5]. However, these numbers change depending on concentration, heating speed, pH, and salt, so they are not fixed values. Time also matters. Heating an egg in water at 65°C takes about 30 minutes for the center to reach that temperature, and the white remains runny even after 210 minutes. At 70°C, the white stops flowing after 30 minutes [6]. Heat solidification depends on exceeding a certain temperature and how long it is held there. Alkali solidification depends on pH and how long it seeps in. While the triggers differ, both processes involve proteins unraveling, linking, and forming a net.
4. Why Boiled Egg White Turns Opaque White but Century Egg White Stays Clear
Boiled egg whites turn white because heat denatures the proteins, changing the clear white to opaque [2]. Century eggs are different. Alkali seeps in slowly from the outside, solidifying the egg white while it is still raw, which results in a translucent, brownish jelly [2]. The level of alkali needed to solidify the egg varies by source. Some explanations say gelation happens at pH 12 or higher [2], while the 1975 study showed protein changes at pH 12.5 to 13 [1]. A study in *Rice Catalyst* noted that the final internal pH of the egg was about 10, and full gelation took about 30 days [4]. These differences are thought to depend on conditions like egg white concentration, temperature, and how the alkali enters. Therefore, numbers like "solidifies at pH 12" should be treated as rough guidelines.
5. How to Compare Raw and Boiled Egg Whites at Home
You can compare the before and after states using eggs at home. First, crack a raw egg into a small dish and observe that the white is clear. Next, cut a boiled egg in half (be careful of burns when boiling, and do this with an adult). The white should be white and firmly solidified [2]. This difference shows "heat-solidified white." If you find century eggs, try cracking one open with a family member to compare. Even though both are solid, the century egg white has a different appearance: a translucent brown jelly. Within the sources used for this article, the reason century egg whites do not turn white rests on one explanatory article alone [2]. If you want to know more, search for terms like "egg white alkali gelation" or "century egg" in Japanese academic databases like CiNii Research to find papers and reports.