The Science Behind Overcooked Hard-Boiled Eggs


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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

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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23 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!

  2. Abigail Kreiss says

    I’ve had a problem with exploding eggs before but I was probably putting them in water that was too hot. I’m eager to try the cooler water.

  3. TAUNYA TEEL says

    I over cooked my last eighteen eggs that I was going to use to make bologna salad with..some of the whites popped thru the shell..and they were boiled for almost 20 minutes..can I still use them sinc they are going in a grinder and will it affect the taste and how long it stays good?

    • Jessica Gavin says

      Hi Taunya- The bologna salad might have a slight green tinge and sulfur aroma. I would chop the eggs up and taste them, if they don’t have a strong noticeable off taste that it will be okay to use. If kept in and air tight container and refrigerated, it would be okay to consume for up to 3 days.

  4. JK says

    This JUST happened to me!! Professional chef and I over-boiled eggs. Wasn’t sure if I went past the point of use for them but looks like I can salvage the whites and I’ll improvise a filling for devilled eggs. Your post was spot on, thank you.

    • Jessica Gavin says

      Oh man, it happens to the best of us! I’m glad you were able to still use the whites, great idea to use as deviled eggs! I’m so happy to hear that the post was helpful for you chef 🙂

  5. Cynthia says

    I do have another problem. I have 42 hard boiled eggs in the fridge. At one point I had 53. Everybody’s getting egg salad for lunch. It was all worth it though. Even the eggs are more tender, especially the whites. Thanks again

  6. Cynthia says

    And there we have it…..4 eggs put in the pot at 200 degrees and 4 beautiful eggs came out of the pot. You have no idea how happy I am right now. When I tried before with the farm fresh eggs by taking it off the burner the water must have still been too hot. I also used a different pot this time instead of the one I have always used for eggs so I don’t know if that also played a role in my success or not. All I know is that it worked, thank you! I just love science. I’m not good at it but I love it. Thanks again

    • Jessica Gavin says

      Whooooo hoooo! Your feedback totally made my day 🙂 I love your relentless pursuit of the perfect hardboiled eggs. I know it can be frustrating, but the results are the best! You don’t have to be good at science, just be open to the knowledge to help you in the kitchen. Cheers!!!

      • Jessica Gavin says

        Hi Sandra- Great question! A quick and accurate check is using an instant-read thermometer. If you don’t have one, you can look for the water to show bubbles the look like “a string of pearls”. You will see larger bubbles at the bottom of the pot quickly rising to the surface, creating several continuous chains of rising bubbles. This is about 195-205°F, just before it becomes a full boil at 212°F.

  7. Cynthia says

    Thanks for the feedback. I will try 200 degrees and see what happens. I don’t want to give up on boiling just yet because my steamer is small and it would take multiple batches to cook for a crowd. All I know is I need to figure this out because my old way was sooooo hard to peel and I don’t want to go back to that. I now have a bunch of hard boiled eggs in my fridge right now trying to get this right but I have the water cooking and I’m going to try this again. Fingers crossed!

  8. Cynthia says

    I have a question. I have always put my eggs in cold water and turned on the heat so when I saw your post I decided to try it your way (stovetop) and every time I do it a few of the eggs explode the second I put it in the boiling water. I have tried reg store bought eggs, organic eggs and farm fresh eggs and I get the same result. I have also tried low boil, take the pot off the burner and vinegar and still get the same results. What gives. Have I just been unlucky with the eggs I’ve gotten. I look for cracks on the egg before putting them in and never see them. I also gently put them in the water and I always leave them in the fridge until I’m ready. I was even wondering if my fridge temp is off. I am just so baffled. Sorry for the long post but I wanted to give you the full picture.

    • Jessica Gavin says

      Hi Cynthia- Interesting! How hot is the water? I make sure it’s at 212 degrees, but it’s not a rapid excessive boil. I haven’t had the exploding happen before. Perhaps you could try to make it a lower temperature, like 200 degrees and then bring it up to a boil for the 30 seconds and see if that helps. Another thing you can try is steaming the eggs. That seems weird that your eggs still explode even when you do a low boil and take the eggs off the heat. Let’s keep chatting!

      • Jordan says

        Wouldn’t see abrupt temperature change from fridge to boiling also contribute to the risk of the egg exploding, much in the way that a glass/pyrex roasting pan can crack if you take it straight from the oven and try to wash or cool it with cold water (or pour boiling water in a chilled glass, although it’s less clear why you would do that)?

        • Jessica Gavin says

          I use a low boil (200ºF) when adding the eggs into the water to help prevent cracking or exploding.

    • Jo says

      If you pierce the egg shell with a safety pin (I do narrow end) and then pop them in water, I find they don’t explode. Good luck!

  9. MeiMei Silverman says

    Thank you very much for this, Jessica! I love anything with eggs and this info is much appreciated! Please keep them coming!