Book 1, Question 131
Why Do Eggs Get Hard When Cooked? Proteins on the Move!
Eggs get hard when cooked because heat unfolds their proteins, which then tangle and lock together.
A runny egg turns firm in the pan, and proteins are the reason. Raw egg proteins are folded chains, heat unfolds them, and then the chains tangle and lock into a firm bite. A raw egg white is clear and glossy while a cooked one is opaque and firm. A hard yolk is simply cooked longer than a soft one. Compare a fried egg white with a raw one by just looking, and never eat raw egg for the test.
Watch the videoWatch on YouTubeBuy on Amazon
Quick answers
Do whites and yolks set at the same temperature?
No. One egg-white protein starts to set at about 142 degrees Fahrenheit, and yolk proteins at about 150 degrees.
Which eggs peel best?
Very fresh eggs are harder to peel after hard-boiling than slightly older eggs.
Sources: Smithsonian Magazine, Smithsonian Magazine
From the book
Page from A Million Whys Book 1
More fun facts
- Egg whites and yolks are made of different proteins, and each one firms up at a different temperature.
- One egg-white protein starts to set at about 142 degrees Fahrenheit, while another waits until about 184 degrees.
- Egg yolk proteins start to thicken at about 150 degrees Fahrenheit.
- Because of this, a few seconds or a single degree can change how a cooked egg turns out.
- Very fresh eggs are harder to peel after hard-boiling than eggs that are a bit older.
- Starting eggs in already boiling water can help the shells peel off more cleanly.
Sources: Smithsonian Magazine, Smithsonian Magazine
Questions kids ask
Why does the egg white set before the yolk?
The white and yolk have different proteins that firm up at different temperatures.
Why are fresh eggs hard to peel?
In very fresh eggs, the white tends to stick to the shell more.