The Grey Chemistry Inside an Egg

Do you know, what is the most liked picture on Instagram? It’s of an egg, the one you can see as the feature image! The Instagram handle @world_record_egg posted this photo with the caption “Let’s set a world record together and get the most liked post on Instagram. Beating the current world record held by Kyle Jenner (18 million)!” And guess what, this picture went viral and it currently has 54.9 million likes. Apart from being a celebrity, the egg is one of the most eaten food items all over the world. Be it boiled egg or omelet or scrambled egg or sunny side up, you can have them in breakfast, lunch, or dinner, anytime. It has a great nutritional value as it is rich in proteins, vitamins, and minerals. So, why am I suddenly writing a blog about ‘egg’?

The other day, I was having boiled egg in my breakfast and by chance, it was boiled a bit too hard. I noticed a greyish covering on the outer surface of the yolk and it was significantly grey. I have noticed a grey-colored covering on the hard-boiled egg a number of times, but it might be the lockdown monotony that compelled me to find out the reason behind it. While searching for it, I came across several interesting facts and found out that there is complex chemistry inside this guy. The chemistry inside the egg intrigued me as a Chemistry student and I decided to write it down. 

Composition of the egg:

Fig 1: The inside of an egg

Egg white or albumen mostly contains water, almost about 90%. That’s why we see the egg white as a clear liquid in raw state. Other 10% is composed of amino acids (basically proteins) and negligible quantities of fats and minerals. The proteins remain folded up in the water.  On the other hand, the egg yolk contains 48% water and 17.5% protein, but what is important is that it contains a significant amount (34.5%) of fat, minerals and vitamins.

Unless we beat an egg we shall notice that the yolk is always different from the white portion. Yes their colors are different because the yolk is pigmented, but what is more important is that they are also physically different; the yolk is much more solid than the albumen, or conversely, the albumen white is runnier than the yolk. Why is it so?

The albumen contains mostly water. And the negative charge repulsion among the amino acids and their side chains is also prevalent. These factors make the albumen runny. The yolk is more solid due to the presence of fat molecules. They trap the proteins and lower the negative charge repulsion among the amino acids. The hydrophilic end of the fat molecules helps to hold the water molecules. Thus the yolk is much less runny than the albumen. The hydrophobic end of the fat molecules restricts the yolk from mixing up with the albumen.

What happens when we start boiling the egg?

Fig 2:The egg yolk after several different intervals of boiling

One thing that we have understood from the above discussion is that the yolk is chemically more stable than the white due to the hydrophilic interactions with water molecules and lowering of repulsion of the amino acids. So, as we start boiling the egg, the albumen gets cooked first. Due to heating, denaturation starts, i.e., the protein starts unfolding and the free protein molecules start interacting with themselves. This leads to the coagulation of the albumen and it turns opaque and white. The water content in the albumen decreases significantly and it gets a rubbery texture. Experiments have shown that the albumen solidifies at about 63 0C to 650C.

Fig 3: Denaturation of protein inside the albumen

On the other hand, the yolk needs more heat to get cooked.  The yolk solidifies at about 710C to 740C. As the hydrophilic part of the fats trap the water molecules, they can’t escape and thus unlike the albumen the texture of yolk is not rubbery.

Fig 4: A cartoon of trapping of water molecules inside the egg yolk

The surprising thing happens when we hard boil the egg. If we boil the egg at about 930C, we shall notice a greyish coloured covering on the outer surface of the egg yolk. This occurs due to unwanted chemical reactions at higher temperatures. We know some amino acids like methionine, cystine, cysteine contain sulphur. After denaturation of such proteins, the sulphur reacts with hydrogen to form hydrogen sulphide gas. And if we recall, the yolk contains minerals and that includes a decent amount of iron. At higher temperatures, the hydrogen sulphide gas reacts with the iron to form iron (II) sulphide, which is grey in colour. Thus, this iron sulphide imparts a grey colour on the outer surface of the yolk.

Fig 5 :Sulphur containing amino acids

Further discussions:

We do not see the phenomenon when we cook the egg in a pan. Because in this case the egg gets cooked way before reaching such high temperature and if we try to increase the temperature to 900C, it will get burnt. Actually, when we boil the egg, we heat the egg in a controlled manner. The egg remains inside the shell and is heated by convection of heat inside the water. Thus we can boil an egg at such a high temperature without burning it.

The grey covering of iron(II) sulphide is not harmful at all. But still, to prevent it, we should not boil the egg for too long. We should stop boiling once the temperature has reached 740C. We can also keep the egg in cool water just after boiling to ensure that the temperature comes down and iron sulphide doesn’t form.

So, there’s a lot of chemistry inside an egg. And what I have just discussed is just the tip of the iceberg. There are hydrophobic and hydrophilic interactions, hydrogen bonding, ionic interactions, protein-lipid interactions, chemical reactions inside that oval-shaped structure. But luckily, our taste buds won’t bother about the complex chemistry inside the eggs. Enjoy eating and keep sciencing!

By Soumyanil Adhikary, Department of Chemistry, IISER Bhopal.


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About the Author:

Soumyanil is a 4th-year BS-MS student at IISER Bhopal, pursuing Chemistry major. Other than studies he likes playing guitar and read novels. He loves travelling and has travelled a lot; recently he has done his first trek. Besides he takes interest in cooking and needless to say he is a big foodie too. He is a big fan of football and cricket as well. Also, He is associated with The Qrius Rhino!

Other articles by the author:

4 thoughts on “The Grey Chemistry Inside an Egg

  1. Soumya Dey says:

    Never thought about that so deeply.. informative indeed.. keep posting.

  2. Soumya Dey says:

    But if it’s harmless then why it’s important to get rid of that iron sulfide layer?

    1. theqriusrhino says:

      It is not at all important to get rid of the sulphide layer, that is why I used the phrase ‘but still, to prevent it,’. I have just discussed how to control the unwanted chemical reaction to some extent in the hard-boiled egg. Being a chemistry student it is important to know how to control unwanted reactions, from that point of view I have discussed it.


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