It may seem that when bread goes stale is a result of it losing water and getting more and more dry. If that were the case, it would make sense to think that putting the bread in a humid and cold environment will slow down water evaporation and therefore the staling process. This would indicate that putting bread in the fridge, we should be able to keep it fresh longer.

I conducted the following experiment. I cooked pre-baked bread in the oven, sliced 4 pieces of it and store them in 4 different places: in open air, wrapped in a cloth at room temperature, in the fridge and in the freezer. After 10 hours, which one do you think was the hardest? In the next picture I have arranged them from the hardest to the softest:

Bread going stale

Bread going stale

The hardest one was of course the one that was in the freezer but then, surprisingly, the next hardest one was the one that was in the fridge! The one that had been left in open air was also hard, although less than the other two and the one I had kept inside a cloth had acquired a chewy consistency.

Actually in the process of going stale bread does not lose water, but rather the opposite. What happens is a phase transition. The starch molecules that constitute the bread slowly crystallize to a more rigid form, making the bread harder and giving the impression that it is drying out. As it turns out, to crystallize they starch molecules need to associate with lots of water so what they are doing is taking  free water molecules from the bread and the surrounding air and trapping them in a crystal form.

This also explains why the piece of bread that I kept in a cloth did not get harder but rather chewy (the effect is bigger if you keep it in a plastic bag). In that case, the starch can not access as many water molecules in the air and therefore does not crystallize.

One last thing to notice is that this is also the reason why cakes stale at a much slower rate that bread. This is because cakes have sugar and sugar loves water, so the sugar absorbs some of the water around which are then not available for the bread to absorb and then crystallize slowing down the staling process.

The speed at which this crystallization takes place depends, among other things, on temperature and it has its peak at around 4ºC (39F) which is why it stales quicker in the fridge than it does in open air.

The good news is that this process is partially reversible. All you need to do is heat the bread a little bit and it will look like fresh for some minutes (when it cools down again it will be worse than before because in the process it will have lost moisture). I put my 4 slices of bread in a preheated oven with a very low temperature 80ºC (176F) for only 3 minutes and it was good as new :).


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.