July 2009


This week I found a few cool videos on You Tube about other fun things that can be done with cornflour and water that I think are really worth a second post on the topic.

Remember that the curious thing about this mix is that it is a liquid which becomes solid under pressure. For example when you grab it, punch it or try to walk on it (as you will see in these videos) it becomes solid but, as soon as you release the pressure, it flows again between your fingers.

In a Spanish television program (El Hormiguero) they had a pool full of this mixture and you can see how is it possible to actually walk on it if you walk fast enough:

I think language does not matter in this case, but if you want an English version they did something similar in the Ellen Show:

Now, pressing with your hands and feets is not the only way to apply pressure to this mix, you can also do it with sound! The reason is that sound waves propagating in the air are nothing else than variations in the air’s pressure: molecules of air get closer together or more spaced in different regions as sound is moving. So if you could apply a loud sound for a while to this liquid it would become solid too. One way to do it is using a speaker and what you obtain are cornflour monsters.

I know that this all looks very messy but it cleans really easy. Just grab the liquid very fast and put it into a plastic bag (better not to dispose of it in the sink, just in case it decides to go solid at some point). Then you can wipe the rest out with a damp cloth. So kids, please do try this at home :).

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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 ­čÖé ) 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.

I can’t believe I haven’t done this before because it is quite a cool experiment and all you need can be found in most kitchens: water and cornflour. (Cornflour is called cornstarch in some countries. In Spanish we call it maicena). I also added a bit of red colorant, but that is completely optional.

Ingredients for goo

Ingredients for goo

All you have to do is put some cornflour in a bowl and add water to it slowly while mixing it with your hands until it has a creamy consistency. The feeling is very weird: it seems like you won’t be able to mix them because when you touch the flour it feels quite hard, however they do mix very quickly. And then, very strange things happen.

If you put pressure on the mix by punching it or grabbing it with your fingers it becomes solid, but as soon as you release the pressure it goes back to liquid! For example, you can make a ball with the liquid by moving the mix fast between your hands or closing your hand around it strongly, but as soon as you stop moving them, the ball liquifies and slips through your fingers.

Making a ball of liquid

Making a ball of liquid

It is not easy to explain it with words and the best thing to understand the feeling I am trying to describe is to experience it yourself, but I uploaded a couple of videos so that you can get an idea of what is going on:

Why does cornflour behave like that? This mix is again a colloid. Like in the case of miso soup we have a solid (the corn starch) suspended in a liquid (water). The molecules of corn starch are quite big and, when moving slowly they are able to pass by each other and flow. However, when pressure is applied, they come together making movement more difficult and trapping water in between a few starch molecules. In a small scale, the structure in this case is similar to that of a gel: water trapped between entangled starch molecules. This is actually just another example of a phase transition, only that the transition is not provoked by a change in temperature but rather a change in pressure.

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 4┬║C (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 80┬║C (176F) for only 3 minutes and it was good as new :).

The other day we learnt that a gel is a mixture of two different phases: a solid network and a liquid phase. These two share the same volume but they don’t completely mix, in a microscope one can still tell them apart. Such systems where 2 phases are simultaneously present but not completely mixed are called colloids.

The interesting thing is that you can have colloids involving also other phases. For example, you can have a solid phase suspended in a gas like smoke in the air, or a gas dispersed in a liquid like in foams, or a liquid in a gas like in the case of mist and clouds. Or one can have a solid phase dispersed in another liquid phase and this is the case of Miso Soup.

Essentially Miso Soup consists on a stock and some paste (typically bean paste), which is the solid. There might be other ingredients like tofu cubes or sea weed, but lets focus in the two main ingredients. Why does the paste not just dissolve into the broth like sugar in water? The difference with that case is the size of the solid “particles” which in the case of Miso are much bigger than sugar molecules and therefore they don’t dissolve.

IMG_3454

The paste "particles" in Miso Soup are too big to dissolve in the broth.

One curios effect of this big size of the paste particles is that the fluid motion can be traced by the motion of the particles so that one can see the convention movements that the soup undergoes as it cools down. This motion changes with temperature. When the soup is very hot, the immiscible part of miso convects with the broth. At intermediate temperature, the paste forms a sediment layer at the bottom. This layered structure is destroyed regularly by the instability caused by accumulated heat in the miso layer as a bursting and one can then see a “eruptions” in the miso soup. One can also provoke such eruption by hitting slightly the soup bowl after the cloud has settled on the bottom and before it gets too cold (this is when there is still accumulated heat in some points).

The convention patterns are also very interesting, although more technical to explain. If you are interested you can for example consult this paper where they explain the arrangement of holes on the broth, usually arranged in an hexagonal pattern.

In the next series of pictures you can see the first convention pattern in the first row (in this case the hexagonal structure appeared very briefly) and then 2 eruptions caused by instabilities before the “cloud” finally settles at the bottom of the soup bowl.

Miso soup