One of the most common questions I get about cooking eggs is about the green ring around the yolk that sometimes happens when eggs are hard boiled.

It's certainly not pretty, and not something you want to see when you're making hard-boiled eggs, deviled eggs, or egg salad, but what exactly is it? 

What causes the green ring? 

And is it safe to eat?

Why Do my Hard-Boiled Eggs Have a Green Ring around the Yolk?

Let's start with the basics - which is to say the science of it all. Because I know you love deviled eggs and are frustrated with the unattractive green color!

What Causes the Green Ring around an Egg Yolk

That green (or sometimes grayish) ring is actually a chemical reaction that occurs when the hydrogen sulfide in the egg white combines with the iron (ferrous) sulfide in the egg yolk.

As an egg heats up, the egg proteins start to break down and release sulfur gasses. These gasses react to the iron that is naturally found in eggs. 

This turns into ferrous sulfide on the surface of the egg yolk which is greenish-gray in color.

Cooking water that has iron in it can also contribute to the green ring.

Is an Egg with a Green Ring Safe to Eat?

If you do end up with green rings on your egg yolks, it's nothing to worry about. They are perfectly safe to eat. 

But, they're not the best-looking eggs, so here are some tips for avoiding the green ring.

How Can I Prevent the Green Ring around an Egg Yolk

Since the chemical reaction is triggered by heat - and specifically prolonged exposure to heat - there are several ways to try and prevent the green ring from forming:

• Use filtered water with low iron levels

• Use fresh eggs

• Cook eggs more slowly at a lower temperature (i.e. simmer vs. rolling boil)

• Don't overcook your eggs (under 15 minutes preferably)

• Cool eggs quickly in an ice bath

• Refrigerate cooked eggs immediately 

*The longer an egg is cooked, the more hydrogen sulfide it produces. Hard-boiled eggs should be done right around the 12 minute mark, so set your timer and be sure you don't go over. 

Even better, try steaming your eggs. It's a gentler way of cooking them that can help to prevent the green ring.

*Older eggs also are more prone to the green ring. The white of a fresh egg is only mildly alkaline, but as the egg ages, the alkaline level rises, which assists the chemical reaction and can cause it to occur more rapidly.

*As soon as the eggs are cooked, drop them into a bowl of ice water. Cooling the eggs as quickly as possible can also prevent the green ring from forming. 

Putting the cooked eggs in a bowl of ice water also makes them easier to peel! 

As the egg cools, the pressure inside the egg is reduced, which forces the hydrogen sulfide away from the yolk and towards the shell, thereby preventing the green ring from forming and also allowing the membrane to pull away from the shell.

Note: scrambled eggs can sometimes turn greenish as well if you cook them in a cast iron skillet. A similar chemical reaction can take place which is accelerated by the iron in the pan. 

To prevent this, always cook scrambled eggs over low heat, just until they're barely set, and preferably in a stainless steel or enameled skillet. I love my cast iron as much as the next girl, but this might be one time when you want to reach for another skillet!

Added Bonus! 

As an added bonus to preventing the green ring from forming, by preventing the chemical reaction, you also alleviate the "rotten egg" smell that you often notice in hard-boiled eggs which is a result of elevated levels of hydrogen sulfide.

I hope these tips will help you cook the most beautiful eggs!