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Food Science BasicsSession 3:

Food Chemistry – Chemical Reactions

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Ready, to immerse yourself in the world of food science?• Ever wondered why bananas turn brown?

• Never sure which microorganisms in food can be harmful?

• Ever wondered why your chocolate mousse stays light and fluffy?

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• Ever wondered why your pancake batter rises and forms air bubbles when baked?

• Never understood why your newly developed product changes color?

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Week 3

• You’re lucky! Because that is exactly what we’ll be discussing in this course of which this is the 3rd session.

• It’s a course for (amongst others): Food Professionals, Food Bloggers, Chefs, (High school) students

• Besides food chemistry (this week’s topic) we’ll be discussing: food physics, food microbiology and some food packaging!

• Want to take the complete course? Sign up (at no cost to you) at: https://foodcrumbles.teachable.com/courses/food-science-basics/

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Let’s get going!

In session 2 we discussed the basics of food chemistry (molecules & atoms), in this session, we’ll focus on chemical reactions:

1. The basics of chemical reactions• Writing them down• Reaction types (endotherm & exotherm)• Starting a chemical reaction (activation energy)• Reactions going back and forth (equilibrium)• Speed of a chemical reaction (kinetics)

2. Examples of chemical reactions in food (the fun bit!)• Acids & bases• Browning reactions (Maillard, enzymatic, caramelization)

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Chemical reactions

• Let’s first introduce chemical reactions. They are nothing more than re-shuffling of atoms. During chemical reactions bonds between atoms are broken and new bonds are formed.

• Here’s a simple example to explain. The black balls are hydrogen atoms, the red balls are oxygen, combined they form hydrogen (H2) and oxygen (O2) molecules. When they react, they reshuffle into 2 water (H2O) molecules.

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Reshuffling atoms

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Chemical reactions

• In other words, chemical reactions are:• Reactions in which atoms reshuffle and re-attach because of them bouncing

into each in some way.

• In this course session we won’t dive into the fundamental details of chemical reactions, you don’t need all of that to understand chemical reactions in food!

• We will start with writing down chemical reactions. This helps to understand what happens.

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Reshuffling atoms

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Chemical reactions

• Before writing down chemical reactions, we have to take one step back.

• As you’ve learned in the previous session, molecules are made out of atoms. Each atom has its own unique abbreviation (for example: hydrogen = H, oxygen = O and nitrogen N).

• By writing down the number of each type of atoms present, a molecule can be represented. Some examples:• Water molecules are made up of two hydrogen and one oxygen atom: H2O.

• Glucose is made of 6 carbon, 12 hydrogen and 6 oxygen atoms: C6H12O6

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Writing them down

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Chemical reactions

• Since chemical reactions are all about exchanging atoms, writing them down is nothing more than good bookkeeping. The same atoms should be there at the start as there are at the end.

• The way chemists to do this by using an arrow to represent the reaction itself. On the left side the starting molecules stand, on the right side the molecules formed during the reaction are written down.

• Let’s look at this reaction:

6 CO2 + 6 H2O --> C6H12O6 + 6 O2

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Writing them down

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Chemical reactions

6 CO2 + 6 H2O --> C6H12O6 + 6 O2

• This reaction describes photosynthesis which occurs in plants. They convert water and carbon dioxide into glucose and oxygen. Note how there are the same amount of C’s, H’s and O’s on the left and right side of the arrow?

• The arrow itself indicates that this is the chemical reaction. Everything on the left is what was there at the start, everything on the right is what is there after the reaction took place.

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Writing them down

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Chemical Reactions

• There are loads of different types of chemical reactions.

• They all work in a slightly different way. We won’t discuss all, but we will discuss some basic terms that can be used to distinguish them. If you’re less into this chemical part, just skip on to the examples of chemical reactions further on.

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Basic Terms

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Endotherm & Exotherm

• Whether a chemical reaction uses net energy or releases net energy is an important characteristic. This is called:• Endothermic: a net energy input is required, for example: baking muffins in

an oven, when you turn of the oven (stop putting in energy) the cooking process stops

• Exothermic: net energy is released, for example, a fire, it keeps on going by itself once it’s lighted

• 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.

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Activation Energy

• Most reactions do not start spontaneously, also exothermic reactions (think of the fire). A minimum energy input has to be overcome, the so-called activation energy.

• You will have to give the molecules some extra energy, just to kick off the reaction.

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Ener

gy

Progress of reaction

Activation energy

This graph shows an example of the activation energy. Imagine a reaction starting on the left side, it first has to climb the hill of the activation energy. In

other words, energy has to be put in for the reaction to climb it.

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Activation Energy

• So how to overcome the activation energy?

• A common way to do this is by increasing the temperature of the molecules that will react (think of lighting a fire).

• Another way (which is really very common in food) 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. Imagine a catalyst as a molecule that places the reacting molecules in just the right way for them to react. In food enzymes are very common catalysts.

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Equilibria

6 CO2 + 6 H2O --> C6H12O6 + 6 O2

• 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 can go back and forth. They can be in equilibrium. This is represented with the double arrows going left and right.

AH H+ + A-

A little extra note for those interested. A chemical reaction should be balanced in the number of atoms but also in charges. Note the positive and negative charge on this reaction, together they are neutral, just like the starting molecule.

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Equilibria

AH H+ + A-

• Which way the reaction takes place depends on various factors, one of which is the concentration of the three different components

• The example given above is an example of an acid/base reaction. Acid/base reactions are always equilibria. Later we’ll zoom into acid/base reactions in more detail in this session.

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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 a few examples:

1. 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.

2. 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!

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Examples of chemical reactions• 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.

Section 217

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Chemical reactions in Food

• In the text above I've already shortly touched upon this type of reaction, introducing it as a chemical equilibrium. In a 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!

• Acid/base reactions always involve acids and the counterpole bases. This type of reaction always involves water somehow. An acid is characterized by giving of a proton (H+) in water, whereas a base tends to take up a proton (H+). In chemical reactions:

Acid: AH + H2O A- + H3O+

Base: A- + H2O OH- + H2O

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Acids & Bases

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Chemical reactions in Food

• There is a measure of the acidity of a solution, it’s the pH-value. The pH-value is linked to the concentration of protons. • In mathematical terms: pH = -log[concentration protons]

• 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.• The maximum pH is 14. & 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.

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Acids & Bases

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Chemical reactions in Food

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:

1. 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.

2. Bacteria all have an optimal pH, this means that they grow best at that pH. A lot of micro organisms do not grow when the pH is too high or low, learn more about that in week 5 of this course.

3. In browning reactions the acidity can define which type of molecules are formed during browning.

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Acids & Bases

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Chemical reaction in Food

Last but not least: browning reactions. They are very common in food and are all chemical reactions. Luckily, I've already discussed the main mechanisms of browning reactions, so read those posts as your homework!

• The Maillard reaction involves proteins and sugars, you use it when making gumbo, bread and lots of other foods.

• When making sugar candy you'll often caramelize some sugar!

• Combining catalysis by enzymes and browning: enzymatic browning(of bananas, or apples, or...).

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Browning reactions

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Chemical reaction in Food

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 pageand signing up for the (free) course.

See you in the next session!

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Browning reactions

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