Boiling an Egg

10/7/2020 Topics: Chemistry Eggs

What exactly happens when you boil an egg? How do you cook a perfect soft-boiled egg? And at which temperature does water boil?

Boiling an egg seems the simplest thing to do. Here is how to do it right. This article also focusses on what happens inside an egg while it is being cooked.

When boiling an egg, the egg whites thicken. This is because the protein molecules, which are initially wrapped up, unwrap. Warming up even more, they will entangle with the other unwrapped protein molecules and makes the liquid firm. This is illustrated by Randal Munroe in this article in the New York Times. Munroe is the creator of, a site for self-confessed nerds like me.

Factors involved in getting your eggs perfect are:

  • Size of the egg
  • Freshness of the egg
  • Temperature of the boiling water.

The temperature of (the center of) the yolk is what it is all about.

Therefore size matters when it comes to eggs. Obviously, larger eggs will take longer to cook than small eggs, because the heat has to 'travel' to the center of the yolk. Generally I add 15 - 30 seconds for a large egg and reduce cooking time by 15 - 30 seconds for a small egg.

Always start with water already boiling. The alternative, putting the eggs into cold water and then wait for it to boil, adds a factor of randomness that makes the outcome unpredictable.

Hopefully you have a hole puncher for your eggs. Using an egg hole puncher reduces the risk of the pressure increasing inside of the egg and cracking the shell.

My perfect soft-boiled medium sized egg takes 5:30. Take the eggs out and give them a rinse with cold water to stop them from continuing to cook. 

The freshness of the egg is important as well. This has to do with the protein structure of the whites. The structure, caused by the strains of protein, slowly degrades with time. 

The key to a perfect egg is consistency: know your ideal time for a medium fresh egg and increase for bigger and older eggs.

These numbers also depend on the temperature of your boiling water. The temperature of boiling water is always the same, you say? That is almost true, but not quite. Read more in this post.