Most chemists seem to think that they are quite decent cooks. If that is true or not is open for debate. In my experience, they do show a lot of confidence and creativity in the kitchen. I only came across one exception in the last years, but the night when A.R.B. tried to slice an onion without peeling it first is a story for another day. It kind of makes sense that chemists make good cooks, because preparing food is very similar to what we do in the lab. A common thing we say is: ‘Cooking is like lab work, only that you can eat what you make’. You also can be a bit less accurate, more creative and less thorough with taking notes in the kitchen, the overall idea however is pretty much spot on.
But did you ever wonder what exactly happens chemically, when we prepare food? I want to dive a bit into that question from two perspectives today. First, I want to show you how close chemistry in the kitchen and the laboratory actually are and second, I want to share some thoughts on why we use heat to make these reactions happen.
Chemistry on the stove
Chemistry, what is that exactly? Well in principle, chemistry is the science of changing matter. Matter is everything that is made of atoms, and usually they form molecules. So rocks, plants, air, oceans, cocaine, and Elijah Wood, but not light, WiFi, and the concept of sustainable development. Changing matter, that means rearranging the way that the atoms form molecules. Ethanol becomes acetic acid by replacing two hydrogen atoms with one oxygen atom. And all the sudden the stuff smells and tastes different.
In the chemistry lab, we mix certain compounds, heat them and wait for the reaction to happen. At least that’s what generally happens in organic chemistry. Heating and waiting sounds very much like frying a steak, caramelising sugar, or cooking pasta, right? In a way, that is correct. It is chemical processes that are happening on your kitchen stove.
We understand these processes quite well, in fact. They’re not quite the same, though. Take frying something in a pan for example – how is it different from boiling something? First, the temperate in boiling water is around 100°C (keyword for the curious: latent heat) and there is a solvent, water, which is an important component in the process of cooking pasta. The water molecules enter the dry noodles and replace the connections between starch molecules, so they become soft and edible. This is technically not a reaction because the molecules do not change within, but only reorganise in a different way between one another.
If we fry something the temperate is much higher, and there is not necessarily a solvent (like oil). Here, actual reactions happen. For example, sugars react with amino acids from proteins and form new and colourful molecules (resulting in a dirty brown/black). The number of different molecules in food is enormous, and so is the number of reactions that can happen when we expose them to high temperatures and make these reactions more likely. Therefore, the outcome of these reactions is an unpredictable mess of compounds, with a larger variety than iPhone cases on Amazon.
Thermodynamics and kinetics: If and when.
Let’s compare cooking and frying again, this time from a different aspect. In physical chemistry, there are two concepts called thermodynamics, and kinetics. They’re quite complex and not completely inseparable, but try to understand them as follows: thermodynamics tells you about the energy of reactions, and whether they occur, while kinetics tells us how fast things happen. A reaction might be possible thermodynamically, but could be so slow that it would practically never happen. Or, the other way around: a reaction could be fast, but still doesn’t occur, because the molecules don’t have enough energy for it. Now, when we increase the temperature, we make both these things more likely. Reactions occur faster, and they are more likely to even occur, because the energy of the molecules is higher (due to increased temperature).
Which of the two effects are we actually using when we fry or cook something? Are we changing thermodynamics or kinetics?
This is quite easy to test. If we want to test for a kinetic effect, we pose the following question: Does the reaction also occur, if I just wait long enough? If it does, the increased temperature makes the reaction just faster, but it would occur anyway. If it doesn’t, there is an energetic barrier that we need to overcome with temperature. This is thermodynamics.
Thought experiment (you’re welcome to actually do it in your kitchen. These things are safe to do by yourself)
- Put pasta in cold water and wait a few hours. Will be become soft and edible or not?
- Put a steak in a frying pan, don’t turn it on and wait a few hours. Will it become brown, soft and smell nice?
The answer to the second test is quite easy, right? Nothing would happen, and we all know that. I wasn’t so sure about the pasta, so I actually tried it a few years ago. Turns out they become soft after less than two hours already. This means, for steak, we make the reactions occur at all, while for pasta, we make the process just go faster. So when we fry steak, we change the thermodynamics of the system, while when we cook pasta, we tweak on the kinetics.
Next time when you cook something, try to figure out why you are doing something from that perspective. It won’t actually change anything, but I think it’s cool to look at cooking from that point of view.
Homework question: Why does the egg white (which is actually transparent) become white once we fry or boil it. What is the chemical process behind that? Leave me a comment on the blog, or in the corresponding social media post once you got to a conclusion.
My pasta favourite: German Käsespätzle. It’s a type of southern German egg noodles (from Swabia, not Bavaria, and certainly not Austria!) and are super easy to make from scratch.
Disclaimer: This is of course a strong simplification of all the chemistry that happens when we cook. There are so many more processes that are going on, and I just picked two that I can relate to. And this doesn’t even cover processes that happen during baking or anything involving microorganisms. At least the latter of those is coming up soon!