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.

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 160ÂșC/320F to be caramelized) and also it burns fast giving a bitter taste (just a bit over caramelizing temperature: 177ÂșC/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 180ÂșC (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 177ÂșC(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 RĂŒchel)

Picture of a polyacrylamide gel (taken by Reinhard RĂŒchel)

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 40ÂșC (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 70ÂșC (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 15ÂșC, 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 40ÂșC 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.