grey egg yolks

Why Do Hard-Boiled Eggs Have Grey Yolks?

Just the other day we went out for brunch. That is, we decided to do our grocery shopping early on a Saturday morning. As a result, we didn’t have breakfast on forehand, and decided to check out the breakfast options our favorite local lunch cafe has.

Going out for breakfast isn’t a very common thing to do here in the Netherlands. Having lunch or dinner out of the house is pretty common, breakfast is less so. Long story short though, the breakfast was great and allowed us to continue our shopping and cycle back home without an empty stomach.

During that breakfast though, I just couldn’t prevent myself from coming up with new science questions. One of the things that was part of our breakfast was a hard boiled egg. Besides there being a whole science to a perfectly boiled eggs, I noticed that our egg yolks had a grey layer on the outside. And that got me thinking, what was the reason that egg yolk turned grey? So, I decided to write a blog post about just that!

Location of the greyness

Most of us probably have had or seen such a grey layer in our eggs. Without realizing you’ve already solved the first part of the puzzle by seeing where that layer occurs. It can only be seen on the outside of the egg yolk, just within the egg white.

This can be explained easily. For the grey colour to occur, both components from the egg white and from the egg yolk have to interact. This is a great example of food chemistry, a chemical reaction occurs here! Two molecules react, forming a new component which has this grey/green/blue colour (I’ve found people calling it all sorts of colours).

The chemical reaction

The two components involved in this reaction are sulfur in the egg white and iron in the egg yolk. An egg yolk not only contains most fats, but it also contains a lot of vitamins and minerals which aren’t present in the egg white or in far lower quantities. One of these minerals is actually the iron that plays a role in this reaction. When the proteins in the egg yolk heat and unfold, the iron gets free, free to react.

The sulphur component of this reaction is present in both the egg white and yolk. It’s not completely clear to me why it only reacts on the side of the egg white though. The best explanation I could find is that it’s more loosely bound on the side of the egg white. By heating the egg white it comes loose in the form of a gaseous component, hydrogen sulfide and is able to react with the iron. I’ve read several theories on why it reaches to the egg yolk, but I’m not completely sure.

This reaction only happens under the influence of heat, which is why it happens only when boiling an egg.

Preventing a grey egg yolk

Since it only happens under the influence of heat, it means that it can be stopped/prevented by cold. Indeed, cooling your egg properly after boiling it and not boiling it for too long will prevent it from occuring.

The discolouration of the egg yolk only starts when enough iron and sulfur have come free to react. For that to occur temperature has to be high enough and the heating has to be long enough, which is why it only happens after a certain time of boiling.

Another factor into play is the acidity of an egg. The higher the pH (thus the less acid the egg), the faster the sulphur is released and thus the more prone the egg is to colouring blue or grey. As a matter of fact, during storage the pH of an egg slowly increases, this is part of the ageing process. So, the older an egg, the more prone it is to getting this blue/grey egg yolk.

Chemistry is all around you

Never realized that chemistry is so much fun and can be found all around you? Isn’t it great that you can use you chemistry classes again is something as delicious as food (and your breakfast boiled egg)! This particular type of science is part of the food chemistry discipline, there’s a whole lot more food chemistry on my website as well as an explanation as to what food chemistry actually is!


Some other sources I’ve used that I might not have quoted directly: Exploratorium & Chest of books.

Newsletter Updates

Enter your email address below to subscribe to our weekly newsletter


  1. Thank you for that explanation. I cooked 14 eggs in my instantpot and 4 of them were gray and the rest were yellow. If it was overcooking only, then they all would have been colored. Some of the eggs were older than the others. Next time I will put a mark on the older eggs to see if this happens again.

    • Would love to hear whether indeed the older ones got gray! Else we’ll have some more digging to do. Thanks for sharing.

  2. Really enjoyed this article. Somthing I’ve actually wondered about. I’m not a H.B. egg lover, and some of that actually came from me not liking that color change. I felt like the eggs were bad. The discoloration combined w a sulfur odor made feel like the eggs were tainted. It all makes too much sense now. Crazy how stumbling onto an article can answer questions you had no idea you needed the answer to. I love the chemistry angle to it all as well. I still think the only times I will be consuming H.B. eggs is in or on a Cesar salad.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.