You may think how your hard boiled eggs turn out is all in the cooking. But actually it's also largely about the cooling. Watch this episode of Hungry for Science to find out why. Hi, I'm Julie. And in this episode of Hungry for Science, we're gonna take a hard look at hard-boiled eggs. What's your least favorite part about cooking hard-boiled eggs? Is it when they're difficult to peel? Or maybe you don't like it when that greenish ring shows up around the yolk? Turns out how you cook your eggs is important, but equally important is how you cool them. Now everyone has their favorite way or hard-boiling eggs. My favorite way is to put the eggs in a pot, cover them with water, bring it to a boil and let it simmer for 10 to 12 minutes. I've done that with these two eggs, but when they were done I cooled them in two different ways. One of them I put in an ice water bath and the other one I left on the counter. Let's see how they turned out. I'm gonna start peeling. They're actually both pretty easy to peel. But I left a tiny bit of this one on because you can see that there's a little membrane that sometimes gets stuck between the egg and the shell. And this membrane is actually a two-layer membrane. And in between the two layers is a little pocket of air called an air cell. Now when you boil an egg, that air cell expands because gasses expand at higher temperatures. And when you cool the eggs, if you cool it quickly that air cell collapses really quickly and pulls the membrane out of the shell. And so you get a really smooth clean peel. When you cool it slowly, like you can see here, there are a couple of dimples in this egg because the shell didn't come off quite as cleanly. And sometimes when you have a really difficult to peel egg, it's because that little membrane is keeping the shell stuck to the surface of the egg. Older eggs have a larger air cell and so they're usually easier to peel in general. Let's see how they look inside. Inside it turns out there's a difference too. If you see the one that I cooled in ice has this nice bright yellow yolk and the one that I cooled on the counter has a little bit of this green/gray stuff. That stuff is iron sulfide. It actually comes because a reaction from a gas that's in the white called hydrogen sulfide. That hydrogen sulfide, which has a scary name but it's totally safe to eat. That hydrogen sulfide reacts with iron that's in the yolk to create this gray-green iron sulfide. The iron sulfide is also safe to eat but a lot of people don't like the way it looks. The reason there's less iron sulfide in the ice cooled egg is because gasses tend to dissolve better in colder things. You can think of kind of a warm flat soda that doesn't have as much carbon dioxide as a cold fizzy one. When you quick cool the egg, the hydrogen sulfide stays in the white and doesn't make it to the yolk in order to create the iron sulfide. Slowly cooled though, that hydrogen sulfide is trying to get out and you end up having that reaction on the surface of the yolk. So the final verdict on the science of cooling, is if you want an easy to peel egg with a bright yellow yolk, choose an older egg and cool it with an ice water bath. Enjoy.