Apple Oxidation Experiment: Why Apples Turn Brown

What is Oxidation?

In simple terms, oxidation is a loss of electrons by an element or a compound, which most often happens in contact with the oxygen in the air. The opposite process is called reduction. Reduction is the process where a compound or an element gains electrons.

The most well-known example of oxidation is when iron combines with oxygen to form iron oxide or rust. In these situations, we say that iron has oxidized into rust.

After the discovery of electrons, scientists concluded that oxidation and reduction are happening at the same time. That process is called Redox reaction or Oxidation-reduction.

Some elements, such as Sodium, Magnesium, and Iron lose electrons more easily than the others - we say they are easily oxidized. On the other hand, there are elements that hold their electrons more tightly, like Nitrogen and Chlorine - they are hard to oxidize.

Interestingly, oxidation doesn’t necessarily involve oxygen! Oxygen was the first discovered oxidant so the name stayed until today. There is another oxidation definition that includes hydrogen. Here, oxidation is the loss of hydrogen, while reduction is the gain of hydrogen. The two definitions agree with each other: gaining oxygen and losing hydrogen are just two ways of describing the same underlying thing, a loss of electrons. That’s why it is good to be aware of this alternative definition of oxidation.

What are the Antioxidants?

Antioxidants are the molecules that prevent oxidation in other molecules. Why is that important? Most life on the Earth requires oxygen for its survival. However, oxidation in our body can lead to the production of free radicals which may damage our cells.

If your body has high levels of free radicals, it can cause harm. Free radicals are linked to a variety of diseases, including diabetes, heart disease, and cancer. But they are mostly kept in check by our body's antioxidant defenses.

Antioxidants are slowing down that process of oxidation - in theory, they could slow down the aging process and prevent the development of certain diseases. The best-known antioxidants in the human body are vitamins C and E and can be found in food, especially plant-based foods like fruit and vegetable.

Lemon gives us two different things that both matter in today's experiment, and it is worth keeping them apart. Citric acid is what makes a lemon taste sour, and it works by making things acidic. Vitamin C (its chemical name is ascorbic acid) is a true antioxidant that gives up its own electrons. They are two separate molecules that happen to live in the same fruit.

But next to being good for our bodies, antioxidants are used as food preservatives too! In the food preservation process, antioxidants are compounds that delay or prevent the deterioration of foods by oxidative mechanisms. They do it by handing their own electrons to the oxygen first, so the fats, colours and vitamins in the food are left untouched for longer.

Materials needed for an Apple Oxidation Experiment with Lemon Juice

• Apple. One apple will be enough. The fresher apple will make the experiment more clear but any apple will suffice. Just make sure you are able to make at least 2 apple slices.
• Lemon. One lemon or even one slice of lemon will be enough. We just need to apply a little juice to one slice of the apple, and for that, a small amount goes a long way.
• Cup or bowl. Cup will help us to squeeze lemon in it so we can apply the juice more easily to the apple. But it’s entirely optional.
• Brush. As with the cup, the brush can help us to apply lemon juice to the apple slice more precisely for better experiment effects. But we can even soak the apple in the juice or apply it in any other way so the brush is also optional.
• Kitchen knife. To slice the apple. It shouldn’t be a problem to get a kitchen knife so we can cut 2, same-sized apple slices for the best comparison of experiment results.

Instruction for Apple Oxidation Experiment

For video instruction on how to do this experiment, watch the “how-to” video at the beginning of an article or continue reading for step-by-step instructions.

1. Cut an apple in half, so you have 2 equal apple pieces.

2. Take the lemon you have prepared and squeeze it into a cup. Or you can directly squeeze the lemon juice on one apple slice if you want to speed up the process.

3. Take a brush, dip it into the lemon juice, and spread it over one apple slice. This will be our experiment slice where we try to prevent the oxidation process. Don’t apply anything to the other slice. We will see what happens to the apple if we don’t apply any food preservatives to it.

   

4. Leave your 2 apple slices at room temperature for a couple of hours. You can check your apple slices every hour to see if there is any difference. You should notice the differences between the 2 slices already after one hour.

5. Check the state of your apple halves after 1, 2, and 3 hours and talk with your child about the observed effects.

Before you leave the slices to sit, make a prediction!

The science behind the Apple Oxidation Activity

Why is the apple quickly turning brown when peeled off or sliced? Apple contains a peel or shell that protects the inside of an apple. When the inner part of the apple is exposed to air, browning in apples occurs. That browning process is called enzymatic browning or oxidation reaction.

Inside the apple's cells there are two things that normally keep to themselves: an enzyme called polyphenol oxidase, and a group of substances called phenols. An enzyme is a tiny molecular machine that makes a chemical reaction happen much faster. The phenols are what that machine works on.

Cutting or bruising the apple tears the cells open, and suddenly all three ingredients meet at once: the enzyme, the phenols, and the oxygen from the air. The enzyme uses the oxygen to turn the phenols into brown pigments called melanins. So the browning is not the apple slowly rusting on its own - it is an enzyme doing its job very efficiently. To stop it, we have to stop the enzyme, and that can be achieved by using heat or acid.

Lemon juice protects the apple in two ways at once. Its citric acid makes the cut surface too acidic for polyphenol oxidase to work properly, so the enzyme is effectively switched off. And the vitamin C in the juice is a true antioxidant: it gives up its own electrons and undoes the browning reaction just as it starts. Between them, the slice stays pale for hours. Eventually the vitamin C is used up and the acid dries or spreads out, the enzyme goes back to work, and the apple browns after all.

What Will You Learn And Develop By Doing this Apple Experiment

• Formulating scientific hypotheses,
• Planning and conducting a scientific experiment,
• Observing and tracking results,
• Enhancing the logical thinking skills,
• Learning about chemical processes, oxidation, antioxidants, and food preservatives.

Frequently Asked Questions

If you enjoyed this experiment and would like to do more experiments with simple ingredients, check out these great activities:

• If you enjoyed the Apple oxidation experiment, you will love the Gummy bear osmosis experiment where you will learn about osmosis using tasty gummy bears.
• Learn How to make sugar crystals, and learn about crystallization while making a great homemade treat.
• Learn How to make plastic from milk and vinegar, and create your own amazing plastic from plain milk. With some molds, you can be your own toymaker!
• And for another kitchen experiment, be sure to try a simple Orange density experiment and learn about buoyancy.

Happy science learning and don’t forget about the most important thing - having fun!