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 :).

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 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 :).

This delicious Spanish dessert exemplifies the last 2 posts: the liquid to gel transition and the Bain Marie. The quantities here are for about 6 people.

Ingredients

6 eggs
1/2 litre of milk
sugar

Very hot sugar can crack a ceramic pot.

Very hot sugar can crack a ceramic pot.

Cover generously the bottom of a metallic oven mold with sugar and then caramelize it directly on the gas fire. If you don’t have gas, you can also melt the sugar in the microwave (do not use a metallic recipient in that case!). Be careful because sugar gets very hot (needs to reach at least 160ºC/320F to be caramelized) and also it burns fast giving a bitter taste (just a bit over caramelizing temperature: 177ºC/350F) so, especially if you do it in the microwave, be sure to open and stir every 10-15 seconds. As  you can see in the picture, the high temperature of the sugar in the microwave even managed to crack my ceramic pot. Once the sugar has a light brown colour, turn the fire off and let it rest for a moment.

Meanwhile, mix the milk, eggs and 6 table spoons of sugar in a jar. Then just pour the mix onto the oven tray with the caramelized sugar and put it in the oven in a water bath. Let it bake for about 30-40 minutes at 180ºC (356F). To check that it is properly cooked inside, insert a needle in the flan. If it s done, it will come out clean. Let it cool down and then put it in the fridge to cool even further. After a couple of hours it will be ready to eat. Use a knife to separate the flan from the mold, flip it upside down onto a plate and enjoy!

As you can see the flan has now a gel consistency; the egg proteins have denaturalized and formed a permanent network trapping the milk with sugar inside it: the mix has undergone a liquid to gel phase transition. I did an individual portion this time:

Cooking Flan is a liquid to gel phase transition.

Cooking Flan is a liquid to gel phase transition.

Why do we need the Bain Marie? For two reasons. The first is that, if we are not careful, the sugar in the bottom will reach 177ºC(350F )and burn. The second is that the liquid mixture will start to boil when heated above water’s boiling point creating bubbles that will get trapped into our gel and ruin the pudding texture.

To see this, I cooked two individual flans: one in a water bath and the other just directly in the oven. After a few minutes, I could see how the volume of the latter was augmenting due to the bubbles under the surface. Compare the flan that was cooked in a water bath (left) with the other one (right):

Flan cooked in a water bath (left) and without it (right).

Flan cooked in a water bath (left) and without it (right).

These bubles resulted in the gel structure being ruined inside and the caramelized sugar being burnt:

Flan cooked directly in the oven (left) and in a water bath (right).

Flan cooked directly in the oven (left) and in a water bath (right).

As you can see, the one cooked in the water bath looks much tastier (and indeed it was :)!).

One of the most interesting and omnipresent states of matter in the kitchen is that of a gel. They have typically the density of a liquid and yet they behave like a solid. That is because in a gel a liquid and a solid are indeed superposed: we get two states of matter for the prize of one.

On the one hand, there is a net formed of long molecules similar to ribbons that link to each other in certain places. On the other, superpose to this network there is a liquid that flows throw it:

Sketch of gel structure

Sketch of gel structure

In most gels that liquid is water-based and they can contain as much as 90% of water. Here is a picture (taken with a transmission-electron microscope) of a gel:

Picture of a polyacrylamide gel (taken by Reinhard Rüchel)

Picture of a polyacrylamide gel (taken by Reinhard Rüchel)

An interesting property that most gels have is “thixotropy”: their viscosity decreases the longer they undergo shear stress. For example, a gel is liquid when you agitate it inside the bottle but recovers its gel consistency while at rest.

This is similar to what happens to toothpaste and ketchup: when you squeeze the tube, toothpaste comes out of it but then retains its form on the toothbrush and the same with ketchup. It is not exactly the same property because, although they also decrease their viscosity when undergoing shear stress, this doesn’t depend on the duration of this applied stress but rather on its strength.

Denatured proteins

In the kitchen, the long molecules that form the net are proteins. However, usually proteins are curled up forming a ball, so one needs to stretch them up before they can form the links that give rise to the net. We say that the protein has to be denatured.

Denaturing proteins

Denaturing proteins

Most proteins are denatured at temperatures around 40ºC (104F), others can be unfolded by fast motion like when we whisk an egg, by adding salt (cured meats) or acid (pickling) and also by kneading.

As it turns out, denatured proteins are more digestible that in their initial form. This is because they are much more vulnerable to attack by protein-breaking enzymes and is why we say that cured meats and pickles are somehow “cooked”. Some proteins, such as the collagen in meat and fish, are so tough and stiff before unfolding that they are almost inedible. The reason for collagen to be specially stiff is because it is not only one ribbon but rather a triple helical structure of ribbons (similar to the structure of DNA). To untwist these ribbons collagen needs to be heated about 70ºC (158F).

Forming the net

To have a gel we need not only to have denatured molecules as our ribbons, but we also need them to link to each other forming a net. In the case of collagen such links are formed when cooled down under 15ºC, but if you heat it up again they will break down returning to the liquid phase. Such gels are called thermo-reversible. This is what happens when you put stew, or fish with a sauce in the fridge. The collagen ribbons that have denaturalized from the meat to the sauce form a net when cooled down giving rise to a gel. If you heat it up in the microwave, the sauce goes liquid again.

Egg proteins are an example of a gel which is not thermo-reversible. When the proteins in the egg unfold at temperatures above 40ºC they form chemical bonds between them giving rise to a gel (we say that the egg coagulates). Such links are permanent and stay after the mix is cooled down so that we can enjoy our pudding :).

So every time you are making an omelet, baking pudding, putting stew in the fridge or preparing jelly that you are witnessing a beautiful liquid to gel phase transition.

The nightmare of chocolate manufacturers is a process called “chocolate bloom”. This is when chocolate appears to be covered with a white layer making it unappealing and losing its right texture.

There are actually two kinds of chocolate bloom: sugar bloom and fat bloom. Sugar bloom has to do with the bad relationship between chocolate and water. We saw in a previous post that a few drops of water in melted chocolate dissolve the sugars in the chocolate but not the fats and the lecithin molecules trap this sugar lumps making the chocolate less fluid.

What happens here is similar: when a chocolate bar is stored in humid conditions the water dissolves the sugar in the surface of the chocolate forming sugar lumps in the surface.Then the water evaporates and what remains is a sugar layer covering the chocolate. In this kind of bloom, you can remove the white layer by rubbing the chocolate surface with your finger and you can also taste the sweetness when putting it in contact with your tongue.

Fat bloom is much more complicated and also interesting. The other day we were saying that diamond and graphite are actually made of the same material: carbon atoms. The only different is that the atoms are arranged in a different way in each of them: they are two phases of carbon (only that they are both solid phases). The same thing happens with chocolate. Solid chocolate is a a crystal whose building blocks are basically the fats in the chocolate. These fats can arrange themselves in six different ways resulting in six solid phases of chocolate. Like with carbon, each of these phases has different properties although the differences are not as big as between diamond and graphite.

They are classified according to their melting temperature. The first one (type I) melts at 17ºC (63F) which makes it too soft and crumbly and the last one (type VI) melts at 36ºC (97F), which is a bit too hard. The perfect phase is number V. This one has a glossy appearance and a nice snap when you bite it. It’s melting point is at 34ºC (94F), just a bit below body temperature so that it melts nicely in your mouth when you eat it.

Something to note here is that this classification is for cocoa butter and not other fats. Cocoa butter is however quite expensive, so some chocolate manufacturers use other vegetable fats instead in their chocolate. Such fats usually melt at temperatures higher than the body temperature. The effect of this is that when you eat bad quality chocolate (i.e. made with vegetable fats other than cocoa butter) you feel that it stays in your throat, rather than going down smoothly, leaving you with a nasty feeling in your mouth.

The goal for chocolate manufacturers is to force chocolate to crystallize in structure number V as much as possible. This is quite complicated and is achieved via a process of heating and cooling the  chocolate carefully several times. This process is called tempering.

However, after the chocolate is crystallized in form V, there are several factors that can change the internal structure of the chocolate making it go back to a lower quality crystal, provoking fat bloom. One of these factors is sudden changes in temperature which is why when you put chocolate in the fridge on a hot day you will find it covered in a white layer after a while.

They way this happens seems to be not yet properly understood. One theory is that the lower quality crystals present in the chocolate (it is never number V 100%) melt, then the fats migrate to the surface and crystallize there creating the white layer. If the chocolate is not properly tempered, the percentage of such crystals in the chocolate will be higher and the formation of fat bloom easier. You can distinguish fat bloom from sugar bloom because it does not go away when you rub the chocolate with your finger, it also repels a drop of water rather than absorbing it and it does not taste sweet when putting it in contact with your tongue.

It seems that in any case bloom is just re-ordering of the chocolate ingredients so there is nothing wrong with it and the taste should be the same. However the texture is different and you might want to use bloom chocolate for cooking rather than just eating it. I also think that when you buy chocolate and find that it has bloomed it indicates bad storage conditions (in humid places, exposed to sudden temperature changes, etc). Since chocolate absorbs very easily odours and flavors (especially in humid places) the blooming might come together with changes in the taste. For example, if you storage chocolate in the fridge on a hot day, not only will you ruin it’s snap and colour, but also it might absorb flavours from other food in the fridge changing the taste of it.

As you can see, chocolate is actually quite tricky to manipulate. When you melt it at home and then use it for cake couverture, after cooling it won’t go back to crystal V, which is why is very difficult to achieve at home the glossy finish of couvertures. As I was telling you the other day, I know that here in Edinburgh, the Edinburgh School of Food and Wine is having a tempering lesson on August 8th where they will teach you how to do this at home. They also do such courses at Coco Chocolate (also here in Edinburgh) the last Thursday and Sunday of every month in 2009.

I read in some places that another way to do this is to nest melted chocolate with little pieces of solid chocolate which you know is in the right crystal. this way you will induce the chocolate next to it to crystallize in the same form. I tried it but I found that one has to be careful with the size and timing of the pieces added because of several reasons:

  • If the melted chocolate is too hot the solid pieces will melt as well losing their crystal structure.
  • If the pieces are too big you won’t obtain a flat chocolate layer (although you will on the other side, all you have to do is flip it).
  • Finally, the big difference of temperature between the melted chocolate and the solid bits can also force chocolate bloom in the interface.

Here you can see the chocolate bloom I obtained:

Fat bloom

Fat bloom

And here small areas where the induced crystallization actually worked (the area in the right crystal has a darker and more glossy colour than the rest of the chocolate):

InducedCrystalV

I apologize for the small pictures this time, the sample was quite small so it was difficult to take proper pictures. In summary, I wasn’t very successful with this technique, I will have to give a try to chocolate tempering and see what happens.

I always thought the name of this cooking technique was a bit strange because I can’t possibly imagine Mary (or anyone else for that matter) bathing in boiling water. In English it is also called just water bath and it consists in heating up something by putting it in a recipient and that recipient in a bath of boiling water (rather than putting the first recipient directly on the fire). This is generally used for melting chocolate, cooking puddings (such us the Spanish Flan), cheese cake, dulce de leche, etc.

Have you ever wonder why you had to heat up those things in a bath of boiling water? Why not just apply heat to them directly? The reason is that you are exploiting one very useful characteristic of phase transition: the temperature is constant during a phase transition.

To illustrate what I am talking about I did the following experiment. I put a pot with one litre of water on the kitchen fire and measured the temperature every 30 seconds.

Experimental setup

Experimental setup

To do this experiment all you need is a food thermometer (mine cost me 5 pounds) because the usual thermometers that we have at home don’t reach such high temperatures. The temperature increased more or less linearly until the water started boiling (which here in Edinburgh is at 98.6ºC (209.1F). Then it stayed constant regardless the fact that I was still applying heat to it (it stayed constant for more than 10 minutes and then I got bored, in the table I only registered the first 3 minutes of boiling):

Temperature of water from ambient until boiling

Temperature of water from ambient until boiling

The reason why this happens is that while the water is boiling a phase transition is occurring: from liquid water to water vapor. During that time, all the heat applied to the water is used to free the molecules from the pot into water vapor and not to increase the temperature of the liquid.

Now, thermodynamics tells us that, when we put two things with different temperatures the hot one gets colder and the cold one gets hotter and not the other way round. So if we put a recipient inside the water bath it will never go beyond the water temperature (unless of course the temperature in our kitchen is higher than 98.6ºC, in which case we probably wouldn’t be alive to see it).

To prove this, I put a pot with olive oil inside the boiling water:

Heating oil in a water bath

Heating oil in a water bath

In less than 10 minutes the temperature was 95ºC, and so it stayed for the following 10 minutes. Then I put the pot with olive oil and it very quickly reached 200ºC (392F) which is the limit of my thermometer. Since the boiling point of olive oil is 300ºC (572F) it could have even gone higher up. However the smoke point of olive oil is quite low (190ºC/374F), which is where the oil smokes or burns giving food an unpleasant taste, so when you cook you want to avoid reaching such temperatures.

Heating olive oil. Careful: very hot!

Heating olive oil. Careful: very hot!

This illustrates how one control temperature while cooking using the fact that during a phase transition the temperature stays constant. Nowadays cooks have sophisticated techniques to do this but in former times this was a pretty useful thing. If you wanted a different temperature, then you needed to change the substance in the bath (such us oils) since different liquids have different boiling points.