eli5: Why when an egg has been cooked and chilled to room temperature. It is really hard to cook it further?


I bought a soft-boiled egg from 7eleven. Then I decided to make a hard-boiled egg out of them. Thinking that it would be faster to go from a soft-boiled to hard-boiled that from a raw to a hard-boiled. But when I boil it. It never wants to get cook further. Why is that?

In: 1

What happens to the egg when you boil it is that the protein coagulates. This is similar to how blood coagulate in air but with eggs this process is caused by temperature rather then exposure to air. When you soft boil an egg you heat up the egg so that the white coagulates but not the yoke. This is both because the white is on the outside and therefore heats up faster and also because the yoke coagulates at a higher temperature then the white. But you still heat up the yoke, just not far enough.

When you start with a cooled down soft boiled egg you still need to heat up the yoke to where it will coagulate. It does not help that it was heated part of the way before as there is no changes to the yoke. So you still need to boil the egg for the full duration needed to hard boil the egg. It may actually take longer because the now hard white does not conduct heat into the yoke as well as raw white.

I have not tried it but it should be possible to hard-boil a soft-boiled egg.

Your problem is likely time. The time required to cook an egg is the primary one of conducting heat into the egg.

So it will not be quick, for a cool-down soft-boiled egg, the time will be close to the time it takes to hard-boil an uncooked egg. The time it takes to heat up the internal will be similar if the heat travel through a boiled or raw egg.

The reason for the short time difference between when you make soft vs hard-boiled eggs is that the time from the outer part is heated to get is soft-boiled to the inner part is heated and you get hard-boiled eggs. If you start with a cold soft-boil egg the outer part is cool. You need to heat it up, the time it takes is the time you need to soft boil an egg.

So try to cook it the same time it takes for a raw edge and the center will likely be hard-boiled. It could be the case that you need a bit of extra or a bit shorter time because the thermal conductivity has changed

I am not sure if the result is a good-tasting egg because reheating will result in the outer part being at a high temperature longer time compared to if you hard boil directly. That could change the consistency or taste in a negative way.

So you can hard boil the inner part the time is just close to the same as soft-boiled as it is from a raw egg. The overall texture and taste might get wors but you can make the center hard.