June 2009


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.

This is a small detour in the series of posts on phase transitions due to a technical problem (the thermometer I need for my next experiment run out of battery :().

Fortunately yesterday I spent the day in a cooking-related activity and have some pictures to show you.

The Edinburgh School of Food & Wine organizes one-day courses on Saturdays where you learn how to prepare a given menu and then enjoy it as a late lunch with matching wines. I received for my birthday a voucher to attend one of these courses and I chose to attend the Scottish cuisine one (they also have Spanish food, Italian food, etc).

The techniques we used were fairly basic but they gave us lots of little tricks and tips to use in the kitchen and especially the opportunity to see a real chef working life in front of you for a day, which you don’t have every day. This together with the fact that all the staff there were extremely nice and helpful made a very enjoyable day that I recommend to any amateur cook.

The menu we prepared yesterday was the following. The entrée was cured salmon salad where we cured the salmon ourselves with a mix of equal parts of salt and sugar. This usually takes over 18 hours if you want to cure the whole fish, but if you slice it think enough it takes under an hour. This was my salad:

Cured salmon salad

Cured salmon salad

The main course was Pan fried duo of pheasant and pigeon with rumbledethump potatoes (which is a fancy word for potato mash with cabbage). What I enjoyed most of this recipe was to learn how to make a whisky sauce which here in Scotland very often accompanies meat and the number-one Scottish dish: haggis. Here is my main:

Duo of pheasant and pigeon with rumbledethump potatoes.

Duo of pheasant and pigeon with rumbledethump potatoes.

For dessert we had a Scottish classic: Cranachan (aka Cream Crowdie). This consists of a mix of whisked cream with toasted oatmeal and Drambuie layered with raspberries (which have also been macerated with Drambuie). we accompanied it with shortbread and a not so traditional but lovely piece of dark chocolate. I was very surprised to see how easy it actually is to make shortbread and I will certainly be doing that again. Here is the result:

Cranachan with shortbread.

Cranachan with shortbread.

They told me they were thinking about doing a one-day course on tempering chocolate. If they do, I’ll be the first to come since this is a technique that I have been wanting to learn for a while. Hopefully I will have some more pictures to show you soon then :).

Note: the fun fact of the day was a group of cows peacefully walking around which came to me when I arrived and stared at me for a couple of minutes which I found really intriguing. The staring cows are now the new header of the blog.

UPDATE: The Edinburgh School of Food & Wine will organize their first Chocolate Master class on August 8th. If you happen to be in Edinburgh that day I am sure it would be an excellent way to spend a Saturday morning.

After a quite technical post the other day I’d like to share with you some more microwave entertaining. In that post we said that a rough classification for the states of matter is: gas, liquid and solid but there are more, for example plasma. A plasma is a gas where some of the electrons have been detached from their atoms. In that way in a plasma one has free negative charges moving (the electrons) and also free positive charges moving (the ionized atoms). For this reason, some fun electrical phenomena happen such as the filamentation that one can observe in a plasma lamp:

Filamentation in plasma lamp

Filamentation in plasma lamp

As it turns out, you can create the same effect (at a smaller scale) in your own microwave. All you need to do is cut some grapes in halves, but not all the way through (just leave a small piece of skin holding together the two halves). Then dry the surface of the grapes with some kitchen paper and put them in the microwave for a few seconds.

Plasma grapes

Plasma grapes

Please beware that performing this experiment longer than a few seconds can be as dangerous as functioning the microwave empty and may result in permanent damage of your microwave oven. Also pay attention when taking the grapes out of the microwave because they will be very hot. This is the result:

Essentially what is going on is that the grapes are as little antennas, just like the light bulb was the other day. Their shape, composition (they are full of electrolytes) and size all conspire to make the electric field very strong near the grapes raising the temperature so much that the skin between the two halves burns and the air nearby is ionized creating a small plasma that provokes sparks.

Plasma grapes

Plasma grapes

You can actually see in the video that each grape bursts into flames exactly at the same spot in the microwave. This is because microwaves are not distributed uniformly and, since you need a high field to provoke this effect, you need to find the “hot spots” inside your microwave in order to create your own plasma.

An object is more than the sum of its parts. A glacier, a cloud and a lake are made of the same molecule: water. However, they seem very different to us. Essentially, this is because of the way those molecules are arranged in each case. In a gas, molecules are free to move and occupy all the space available to them. In a liquid, they are mostly free to move but they are also attached to eachother so they stay together and, although the shape of a liquid changes, the volume it occupies stays constant. Finally, in a solid, molecules can’t move, so they usually have a fix shape.

To describe all the different ways in which molecules of water (or of any other kind) can arrange themselves we say that these are different phases or states of matter. A rough classification is to say that there are 3 states of matter: gas, liquid and solid, but in fact there are many more.

In most solids, the molecules are not only not free to move but they are also organized according to a pattern, in that case we call them crystals. For example, the molecules could be arranged in a cubic lattice or in an hexagonal one like oranges in a fruitshop.

If you look at the solid state of water (ice) under a microscope, you will see such crystal structure. However, depending on where you collected your ice cube you will see a different one. In fact, water can crystalize in many different ways, each of them has different macroscopic properties despite the fact that they are all solids made of the same parts: molecules of water.

Perhaps one of the best examples to illustrate how two crystaline structures made with the same kind of molecues can exhibit very different macroscopic properties is carbon. When carbon is arranged in the hexagonal lattice typical of fruits in a supermarket we call it graphite and it is a very soft material that we use to make pencils. However, it can be arranged in a more complicated lattice, based on the cubic one, and then we call it diamond, which is the hardest known mineral. To distinguish these two situations we say that they are different phases of carbon (even though both are solids).

There are basically two ways to transform matter from one phase to another: by heating/cooling or by applying pressure. Sometimes, to achieve a certain crystal structure, a combination of a given temperature/pressure is required. For example to form diamonds you need very high pressure but low temperature. Also, sometimes it is important how fast/slow the change in temperature/pressure is, or you might need to heat and cool an object several times to different temperatures to make it crystalize in a given way.

Why am I telling you all this? I hear you cry. The reason is that most of the trasformations that food undergoes in a kitchen are either due to chemical reactions or are changes of phase. In the following posts I would like to discuss a few of examples where cooking is just forcing a change of phase. But I don’t want to make this post too long, so for that you’ll have to wait until the next one.

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