When you boil an egg you will see how often a greenish layer appears surrounding the hard boiled yolk. This is a sulfur and iron compound called ferrous sulfide. It is formed when the iron present in the yolk and the sulfur contained in the white meet at the surface of the yolk at high temperatures reacting and forming this compound.

The sulfur in the white is attached to a protein and only released in the form of a gas at high temperature. Then this gas diffuses through the egg and it is when it meets the iron in the yolk that they form this green layer.

This sulfur gas is responsible, in small quantities, for the pleasant smell of cooked eggs but, in larger quantities, it is the odor we associate with rotten eggs.

The way to minimize this layer forming is to keep the sulfur gas from reaching the surface of the yolk. This can be done in two ways. First, we can avoid it forming at all by cooking the egg at lower temperatures. We need the water to be hot enough so that the egg will cook but not so hot that this gas will be released in big quantities. This means keeping the water at around 85º-90º (185 F-190 F), that is slightly below boiling point. The second thing we can do is avoiding the gas reaching the egg yolk. We can slow the gas down by cooling it, this is, putting the egg under cold water as soon as it is cooked.

I did the following experiment. I cooked one egg in boiling water for 15 minutes and then did not put it under cold water immediately but rather let it cool down slowly. Then I put another egg in boiling water but with the fire turned off so that the water quickly cooled down. I added regularly boiling water to it so that the mix would be hot but under boiling point. I kept doing this for 30 minutes and then cooled down the egg by quickly putting it under cold water. This was the result:

Yellow and green yolks

Yellow and green yolks

The left yolk is the one that was cooked at a temperature lower than the water’s boiling point and the right one was cooked in boiling water. As you can see the green-greyish layer is not present in the egg cooked at a lower temperature.

Depending on what you are cooking it might not be worth going through the hassle of boiling eggs like this, but if you want to present pretty shinny yellow yolks you know what to do :).

We talked the other day about how proteins are very long molecules that often come entangled together in a ball which is basically inedible and to be able to digest them we need to unfold (denature) them. Very often this process changes the appearance of food, for example in the case of fish and egg whites they become opaque, and we associate this change with the fact that they have been cooked.

While I was writing that post I read that, although the most common way to denature proteins is by heating them, there are other ways such us using vinegar or salt. This is why we think of food that has been preserved in salt or vinegar as cooked, even though they have never been exposed to high temperatures. Some examples are: anchovies in salt, pickles, Serrano ham, cured salmon, carpaccio, salted beef, etc. I did a couple of experiments to test in a small scale how this works.

In the first one I tested how vinegar can unfold proteins with an egg. I put a raw egg in a glass (this time without the shell 🙂 ) and covered it with vinegar. After a couple of hours I could already see how the white had become a bit opaque and also it stayed together rather than spreading and mixing with the vinegar:

Raw egg in vinegar.

Raw egg in vinegar.

Raw egg in vinegar after 2 hours.

Raw egg in vinegar after 2 hours.

Even though I changed the vinegar and waited until the next day, there was no further change in colour, probably because the vinegar cannot penetrate further in the egg. This explains the trick that I had heard of pouring a bit of vinegar in the water while boiling eggs to prevent the whites spreading everywhere in the pot in case an egg cracks. Of course one has to be careful with the amount of vinegar in this case not to alter the taste of the eggs.

The second experiment I did was on raw fish (in this case salmon). With just a thin layer of salt you can see (and feel when you try to tear it up with your hands) after an hours the change in colour and texture:

Raw salmon

Raw salmon

Salmon sprinkled with salt after 1 hour.

Salmon sprinkled with salt.

Difference in colour between raw salmon and salmon cured with salt.

Difference in colour between raw salmon and salmon cured with salt.

If you want to cure salmon at home with the purpose of eating it you should use a mixture of 50% salt and 50% sugar. The sugar is necessary because it feeds a type of bacteria which accelerates the process which breaks down the enzymes normally responsible to causing food to spoil. You can use this mixture to cure other kinds of fish or even meat. The length of time will depend on how thick is your slice. For a thin slice one hour is enough but if you want to do the whole salmon at once you will need 12-18 hours.

In last post Ino left the following comment:

“The yolk does sink if the egg is raw, if you shake it enough to break the membranes. Using that idea, you can balance a raw egg on its bottom, which looks quite cool, but requires a really flat, still table to do it on.”

I tried to do this, but I wasn’t skilled enough to achieve it. However, it got me thinking if it was really perhaps the membrane somehow pushing the egg yolks up, so I did the following experiment. I put a single egg yolk in a glass and then poured over it 3 egg whites. This was the result:

Egg yolk floating in egg whites.

Egg yolk floating in egg whites.

As you can see the egg yolk seems to always float, membranes or no membranes. I think that the obstacle for keeping an egg vertically still on a table (other than the curved surface) is then the air bubble it has at the bottom:

Air bubble in a row egg.

Air bubble in a row egg.

For a better picture of the anatomy of an egg click here. I tried shaking an egg until I could hear the yolk was loose but I still wasn’t able to keep the egg vertically still. When I cracked it open the air bubble was indeed broken, but I can’t be sure that that happened during the shaking . If you manage to balance an egg on its bottom it’d be great if you could take a picture of it and send it to us.

These are the results from the egg poll: 40% of you thought that the yolk would sink, 50% that it would float and 10% had no idea. These data are based in 10 answers, so of course it is nothing scientific. In this case the majority was right: the yolk does float inside the egg. I personally thought that it would sink and was very surprised to see that it didn’t.

There are some membranes that attach the egg yolk to the outer membrane, so it is not completely free to move inside the egg. However, it has enough freedom of movement to go near the upper shell as you rotate the egg and one can see it using the naked eggs we did last week:

Egg yolk floatting in naked egg.

Egg yolk floating in naked egg.

Egg yolk floatting in naked egg, turned 90º.

Egg yolk floating in naked egg, turned 90º.

Egg yolk floatting in naked egg, turned 180º.

Egg yolk floating in naked egg, turned 180º.

The movement as you turn the egg around is quite slow, thus allowing you to keep the egg yolk centered by gently rolling it inside the pan while you are boiling it. The time you should be rolling the egg around depends on how far from the shell you want your yolk to be. This is because the egg is heating from the outside to the inside. As the white heats up, it solidifies not allowing the egg yolk to get any closer to the shell.

I found that for a medium sized naked egg and starting with boiling water, After 2.30 minutes, the thickness of the coagulated layer is a few millimeters. If all you want is that the yolk is not touching the shell, 2-3 minutes of rolling should be enough. After 5 minutes the egg white had a jelly consistency all the way through, so that should be enough to keep the yolk perfectly centered.

Last week I did one more experiment with the naked eggs, but before I tell you the results I am curious to know what your intuition is about the answer.

Sometimes when you open an egg after boiling it you find that the egg yolk is not centered but has moved to the side, which is inconvenient if you are trying to cook filled eggs. This problem can be avoided by rolling the eggs gently during the first few minutes while you boil them.

Why do you think this happens? Is it because the egg yolk sinks to the bottom of the egg and stays there if you don’t move it around? Or is it because it floats? You have 2 days to vote in my very first blog poll.

Please don’t Google the answer, just reply whatever your intuition tells you (I can tell you that mine was wrong). Otherwise it takes away all the fun.

The science museums of San Francisco (California) Exploratorium have a wonderful webpage with plenty of activities to experience science at home. In their cooking section, I found an experiment with which one can explore the phenomenon of osmosis that we were discussing in the last post.

Remember that the reason why osmosis is very important in the kitchen is because the cells that constitute all living beings are subject to it: they consist on a semi-permeable membrane containing a water-based solution in a water-based medium. Where can we find a big single cell to experiment with this? An egg! But eggs come with a shell which does not allow water to go through. To play around osmosis we must first of al strip the eggs naked.

I followed the instructions on Exploratorium and put 10 eggs in a plastic container covered in white vinegar. After 24h, the shell had began to dissolve because of the action of the vinegar on the solid calcium carbonate crystals that make up the eggshell and one can easily rub the shell off the egg as shown:

Eggs after 24h in vinegar.

Eggs after 24h in vinegar.

What the vinegar does is brake them into their calcium and carbonate parts (the carbonate combines with the oxygen in the water to make carbon dioxide, which are the bubbles that you see forming around the egg shells). This process neutralizes the solution so after 24 hours it is necessary to replace the vinegar with new one.

One day and a half after that, most of the shell of the eggs was completely gone! I rinsed them with water one by one and change half of the vinegar by new one to finish the result. These are the eggs at that stage:

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Eggs after 2 days and a half in vinegar.

Eggs after 2 days and a half in vinegar.

After three days and a half swimming in vinegar, the eggs had completely lost their shells and I had 10 big isolated cells to experiment osmosis with:

Eggs after 3 days and a half in vinegar.

Naked egg after one night in syrup.

To do this, submerge one of the eggs in some kind of syrup (corn syrup, black syrup) which is very concentrated and has a low content in water (about 25%). The next morning your egg will look like this:

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Naked egg after one night in syrup.

Naked egg after one night in syrup.

And in the pot you will see how the syrup is now liquid because part of the water inside the egg has migrated through the membrane to the outside, leaving you with a  fluffy egg and some liquid syrup.

However, if you put your egg in water again, after a few hours it will look like this:

IMG_3121

All plumpy and full of water again. Here you can see it next to an egg who spent the night swimming in syrup so that you get an idea of the change in size due to osmosis:

The effects of osmosis in eggs.

The effects of osmosis in eggs.

There are other things you can do with these naked eggs. For example, you could boil them in a cubic container and make cubic boiled eggs. Last night I could not find such a container, so I just put the egg in a box and made a (not very succesful) flat boiled egg instead:

Flat boiled egg next to a raw naked egg.

Flat boiled egg next to a raw naked egg.

In the process of looking for an appropriate recipient to boil the egg in, I broke one of them and found my self with the membrane in my fingers. This is what it looks like:

IMG_3112

Also, you can put the eggs in water with food colorant. This is the result:

Naked eggs died with food colorant.

Naked eggs died with food colorant.

Do you have any other ideas of what one could do with these naked eggs? Share them with us in the comments section!