Welcome back to the 6 week mini-course teaching you all about the basics of food science. You’re already in week 3! Last week I introduced you to the wonderful world of food chemistry. We discussed chemical formulas, structural formulas, learned more about atoms and how they form the basis of all of our molecules. In the end we got the chance to apply some of our knowledge to fats, proteins and carbohydrates. However, we didn’t get to the fun part yet: chemical reactions! Yes, once you understand the structures of molecules, it’s time to look what kind of amazing things they can do when they interact.
Missed the first two course weeks? No worries, you can still start: week 1 – Introduction & week 2 – Food chemistry basics. If you’d like to take the full course, including quizzes and extra exercises, go to my course page!
Today it’s all about chemical reactions, we’ll have a look at the following:
- Chemical reaction basics
- Writing them down
- Starting a chemical reaction (activation energy)
- Speed of a chemical reaction (kinetics)
- Examples of chemical reactions (the fun bit!)
- Acids & bases
- Colouring reactions (Maillard, enzymatic, caramelization)
1. Chemical reactions
Today we’ll be reshuffling molecules since we’ll be looking at chemical reactions. During a chemical reaction molecular structures are broken and atoms are reorganized in new molecules. Studying these chemical reactions lies at the basis of chemistry.
Just a note: during a chemical reaction no new atoms are formed, instead, the same atoms will still be present, but they will be re-organized.
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!
1.1 Writing down a chemical reaction
Chemical reactions can be represented in a very simple equation. You start by mentioning all the molecules you start with on the left side. You then add an arrow after which you write down all the molecules which have been formed.
Below you can find an example of such a reaction. This reaction is the one that takes place in plants under the influence of light in which glucose is formed, the reaction of photosynthesis. But let’s not focus on that now, let’s have a look at the reaction itself:
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).
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).
1.2 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!
1.3 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.
1.4 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:
1.5 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.
2. Examples of chemical reactions
This is the part you’ll have to do some browsing through my website! We’ll be looking into various chemical reactions to which we can actually apply the knowledge we’ve just learned.
2.1 Acids and bases
In the text above I’ve already shortly touched upon this type of reaction, introducing it as a chemical equilibrium. In a previous post on my website I’ve also given quite some background information already.
Read that post now (it’s also about muffins!) and come back again after!
After this quick introduction, let’s zoom in on and recap some basics by taking the formula we showed before:
AH <–> H+ + A–
If we ‘digest’ this formula, we learn the following:
- AH is an acid, it can give away a proton (H+) to form a base
- A– is a base, it can receive a proton to form an acid – see the equilibrium coming in again?
- The acidity of a solution (e.g. lemon juice) is expressed by the pH-value (in mathematical terms: pH = -log[concentration protons]). The maximum pH is 14. The more protons present, the more sour a solution is. This will result in a very low pH value. The other way around goes up as well, if there are only very few protons the solution is alkaline and the pH value is high.
- If the pH is 7 a solution is called neutral.
- The pH value of a food varies per piece, recipe, etc. Here‘s a nice reference to get a feel for the numbers.
Acid/base reactions are very common in food. They take place in a lot of cases, too many to discuss now, but we’ll name a few:
- The behaviour of proteins depends on the pH of a solution. The protons may interact with side chains of proteins.
- Remember, enzymes are proteins! They also depend on the pH.
- Bacteria all have an optimal pH, this means that they grow best at that pH. A lot of micro organisms do not grow anymore when the pH is too high or low, learn more about that in week 5 of this course.
- In browning reactions the acidity can define which type of molecules are formed during browning.
2.2 Browning reactions
Browning reactions are very common in food and they are all chemical reactions. New molecules (brown colours) are formed! Luckily, I’ve already discussed the main mechanisms of browning reactions, so read those posts as your homework!
When making sugar candy you’ll often caramelize some sugar!
Combining catalysis by enzymes and browning: enzymatic browning (of bananas, or apples, or…).
After you’ve read through all the posts you’re finished with this week’s course material. I hope you’ve learned a lot again! You will see, that now that you know this stuff, you’ll see it coming back everywhere in your food and your cooking!
Don’t forget to take this week’s test to strengthen your skills even further. You only learn by doing, so watch videos, read posts, make notes and practice. You can take the test by going to my course page and signing up for the (free) course.
See you next week!