Why does red cabbage turn blue, purple or pink in different liquids? Why does a steak turn brown when heated? Why can an oven bake muffins? Why does a higher temperature oven burn your carrots more easily? All of this can be explained with chemical reactions. Chemical reactions are at the heart of food chemistry and can be used to explain a lot of interesting phenomena in food.
In this post we’re introducing the concept of chemical reactions, discussing the basics and focusing on the theory. This is what we do in these food chemistry basics series. On the blog you will then find a ton of articles applying this knowledge!
What are chemical reactions?
In chemical reactions molecules react with one another. As a result, some molecules will disappear and others will be formed. During a chemical reaction you will have the same in- and outgoing atoms, however, they will be reshuffled into new structures. (If the concept of atoms & molecules is not familiar to you, have a look at this post on atoms & molecules first!).
Food chemists investigate a lot of these chemical reactions. They will study both the structures of in- and outgoing molecules as well as try to figure out what exactly happens during such a reaction. Chemical reactions follow certain patterns. Some groups of atoms are very reactive, whereas others aren’t and this helps chemists to predict chemical reactions as well as explain them.
Chemical reactions can be very complicated, but in very simple words they are simply molecules bouncing into each other. The bouncing into each other triggers them to share/discard/release atoms, forming new molecules!
Writing down a chemical reaction
As we’ve discussed before, molecules consist of several atoms that are bound together. Using chemical formulas chemists can easily describe a molecule. In a chemical formula the number and type of atoms that a molecule exists of are represented.
These chemical formulas are used again to illustrate a chemical reaction. In order to represent a chemical reaction, chemists first write down all the chemical formulas of the molecules that are there at the start. They will then draw an arrow (which represents a reaction taking place) and on the right side of the arrow all the molecules that are formed during a chemical reaction are written down.
Molecules that are present at the start and end (and thus don’t react) are not written down in that formula.
Below you can find an example of such a reaction:
6 CO2 + 6 H2O –> C6H12O6 + 6 O2
One the left side you can see 6 carbon dioxide (CO2) molecules and 6 water (H2O) molecules. These react and form 1 molecule of glucose (C6H12O6) and 6 molecules of oxygen (O2). This reaction represents photosynthesis, a complex process that occurs in plants under the influence of sunlight. Plants make ‘energy’ (glucose) from water and carbon dioxide.
In a chemical reaction it is important to note that the numbers of atoms on the left side of the arrow is the same as on the right side. In the equation above you can see that there are 6 carbon atoms on the left and 6 on the right, number of hydrogen (H) and oxygen (O) atoms also remains the same (12 and 18).
Chemical reactions and energy – exothermic vs endothermic
Chemical reactions can be grouped into all sorts of different categories. One of the ways to group them is in exothermic vs endothermic. These terms refer to the fact whether the net energy of a reaction is positive or negative. In other words, do you have to put in a lot of energy (e.g. heat or stirring), or will energy be released during the reaction (e.g. the heat of a fire).
In food, quite a lot of reactions are endothermic, which means that more energy has to be put in that is released during the reaction. An example is baking a cake, you will have to keep heating the cake in order for the reaction to continue.
Exothermic and endothermic are very important terms for chemists. In food chemistry we won’t use them very often, but it’s good you’re familiar with the concept. Take a look at the video below if you’d like to learn more, it will give you some great examples!
Starting chemical reactions – activation energy
Even though some chemical reactions will release energy during the reaction (exothermic reactions), most reactions do not start immediately or spontaneously. Often the so called activation energy has to be overcome. You will have to give the molecules some extra energy, just to kick off the reaction. This could for instance be done by bringing a mixture of molecules to a certain temperature. During this reaction though, energy (heat) might be set free again.
There are various ways to lower the activation energy of a chemical reaction. One way is to use a catalyst. A catalyst is a component that can lower the activation energy, thus help the reaction along, but doesn’t participate in the reaction itself. In food enzymes are very common catalysts.
Direction of a chemical reaction – equilibria
In my example of a chemical reaction above you can see only one arrow, pointing towards the right. This means the reaction will only proceed from left to right. However, in a lot of cases, chemical reactions are an equilibrium. This means that the chemical reaction can go from left to right and vice versa.
A very common example of an equilibrium reaction is an acid/base reaction (see below). The acid (AH) can be present as an acid (AH), but if the situation is such, it might lose its proton (H+) and split into two. It can do this again, but then backwards.
AH <–> H+ + A–
Which way the reaction takes place depends on various factors. In the example mentioned above the concentration of the different components might play a role for instance.
I found a very nice YouTube video (from the TED Ed channel) that explains this phenomenon in simple words, it’s worthwhile a look:
Speed of a chemical reaction – kinetics
Before applying our knowledge to reactions in food there is one more topic we have to touch upon: the speed of a reaction. The speed of a reaction can be influenced by a lot of parameters. I’ll give you a few examples:
- Temperature: nearly all reaction rates are higher at a higher temperature. At a higher temperature molecules move more. This causes them to bounce more often, thus reacting more. In food: think of honeycomb.
- Concentration of molecules: not all reaction rates depend on the concentration of molecules, but a lot do. In those cases, the more molecules are present in a certain volume, the faster a reaction will go. Since there are more molecules, they are much more likely to bounce into another one.
- Pressure: a higher pressure, means less space for the molecules, thus a higher reaction rate. Again, they will bounce against each other more often. In cooking: think of a pressure cooker!
For each of the topics we just discussed we could have probably written a book. In fact, books have been written about all these topics! We’ve only touched upon the basics here, hoping it will provide you with some fundamental understanding which you can use in the next section.