The Science Behind Overcooked Hard-Boiled Eggs

Perfect hard-boiled eggs have tender whites and opaque yellow yolks, but cooked too long and the middle turns green. Learn why this color change happens and why there’s a strong odor that follows.

Two overcooked hard-boiled eggs with green yolks

Cooking eggs are most likely one of the first things many cooks learn in the kitchen. Simple as it seems, there are many common mistakes that can happen. The most tragic blunder is when an olive green ring appears around the yolk and a strong sulfurous odor forms.

The dreaded rotten egg smell, yuck! Sure, this could be covered up by either scraping off the green area, hiding it in egg salad or pretending it never happened. However, if you’re not repulsed by the smell, I bet someone will take notice. So if you’d like to geek out for a moment, let’s talk about overcooked hard-boiled eggs.

Metal tongs picking up a hard-boiled egg out of a boiling pot of water

Why Do The Egg Yolks Turn Green

Let’s admit that we have all either turned eggs green or have been served them. I used to think this was normal, and even hated eating hard boiled eggs due to the expectation. The reason why the green ring forms around the yolk are due to a chemical reaction that occurs under too high of a temperature and/or prolonged cook time.

What happens is the sulfur that is naturally contained in the egg whites reacts with the iron in the yolks, resulting in a discoloration due to ferrous sulfide formation. The color change happens around the perimeter of the yolk, where the two elements make contact and interact. This can also be observed sometimes when making scrambled eggs in a cast iron pan.

Why Boiled Eggs Smell

When eggs cook, the proteins in the yolk and whites denature, turning translucent-to-opaque in appearance. Cooking the yolk releases iron, while the whites release hydrogen and sulfur. The dreaded rotten egg smell, especially when eggs are overcooked is from the hydrogen, sulfur, and iron reacting to create the foul odorous compound hydrogen sulfide.

Hard-Boiled Egg Recipes

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Jessica Gavin

I'm a culinary school graduate, cookbook author, and a mom who loves croissants! My passion is creating recipes and sharing the science behind cooking to help you gain confidence in the kitchen.

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21 Comments Leave a comment or review

  1. Janine says

    Hi, I want to make a traditional Greek bread which contains hard boiled eggs that are coloured and then placed in holes in the bread dough and baked for 30min. Will those eggs be edible? Or should I not chance eating the eggs? Thanks in advance!

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