Why do apples turn brown?

Imagine you cut an apple in half.

The inside is pale, crisp, and almost white.

You leave it sitting on the table. You come back a few minutes later...

Get a small piece of apple. You can use:

• An apple slice.
• A small piece of apple.
• Another freshly cut fruit if an apple isn't available.

Place the apple somewhere you can observe it for the next several minutes. Don't eat it yet! We're going to see what happens when the inside of an apple is exposed to the environment.

Now leave your apple sitting out while we investigate what is happening inside it. Take a close look at your apple.

An apple may look like one solid object, but it is actually made of millions of cells.

When the apple is intact, many of the molecules inside those cells are separated from the outside environment.

When you cut the apple, you break open many cells. Suddenly, molecules that were separated inside the cells can come into contact with molecules in the air — especially one important molecule:

Oxygen (O₂)

This gives us our first major clue.

Inside living cells, thousands of different chemical reactions are happening.

But many of these reactions would happen too slowly on their own.

Cells use special proteins called enzymes to help chemical reactions happen faster. Think of an enzyme as a tiny molecular helper. An enzyme doesn't usually get used up in the reaction — instead, it helps the reaction happen more easily.

Different enzymes help with different reactions. Apples contain an enzyme that is important for browning:

Polyphenol oxidase (PPO)

When you cut an apple, cells are damaged and their contents become mixed together. The enzyme polyphenol oxidase (PPO) can now interact with molecules called polyphenols.

When oxygen is also present, a series of chemical reactions occurs. These reactions produce new compounds that eventually form brown-colored pigments.

While you were learning about enzymes, oxygen, and the browning reaction, your apple has been sitting out the whole time. Go take another look at it.

You may have heard the word oxidation before.

In everyday life, oxidation often involves a substance reacting with oxygen. For example, iron can react with oxygen and form rust. Apples can also undergo oxidation.

✅ Quick check

Think back to the beginning of the mission. The inside of a whole apple is usually not brown.

But after we cut it, browning begins. Why?

Before cutting, the apple's cells keep many of their molecules separated from one another. The enzyme and its substrates are kept in different parts of the cell.

When the cells are damaged, the barriers between them are broken. Now the enzyme can interact with molecules it couldn't easily reach before — and the cut surface is exposed to oxygen from the air.

Now let's become experimental scientists. Imagine you have three identical apple slices.

You may have noticed that lemon juice is often used to keep cut fruit from turning brown. Why?

Lemon juice contains citric acid, which makes it acidic. That changes the chemical environment around the enzyme.

Enzymes work best under particular conditions. Changes in things like:

• pH
• Temperature
• Salt concentration
• The molecules surrounding the enzyme

...can all change how well an enzyme works. If the conditions are unfavorable for an enzyme, the reaction it helps carry out can slow down. So lemon juice can help slow the reactions involved in browning.

You may have heard people describe lemons as acidic. But what does that actually mean?

When certain substances dissolve in water, they can affect the amount of hydrogen ions (H⁺) in the solution.

Acids increase the concentration of hydrogen ions, and are often what make foods taste sour — both lemon juice and vinegar have a low pH. Bases, on the other hand, have a low concentration of hydrogen ions, and are often associated with a bitter taste, like baking soda.

The pH scale describes how acidic or basic a solution is, usually ranging from about 0 to 14:

• Low pH → more acidic → more H⁺
• pH 7 → neutral
• High pH → more basic → less H⁺

Remember that enzymes are proteins that help chemical reactions happen faster.

But enzymes don't work equally well under every condition. An enzyme has a particular shape, and that shape helps it interact with the molecules involved in a reaction.

Changing the environment around an enzyme — including changing the pH — can affect the shape and behavior of the enzyme. If conditions become very different from what the enzyme normally prefers, the enzyme may work less effectively.

• Right conditions → enzyme works well.
• Different conditions → enzyme may work less effectively.
• Very different conditions → enzyme may lose its shape and stop working properly.

Let's put all of our clues together. Why does an apple turn brown?

• An apple is made of millions of cells.
• Inside those cells are many different molecules, including enzymes.
• One enzyme involved in browning is polyphenol oxidase (PPO).
• When we cut the apple, many cells are damaged.
• This allows PPO to interact with molecules it was previously separated from.
• The cut surface is also exposed to oxygen in the air.
• PPO helps chemical reactions involving these molecules happen faster.
• Oxidation reactions produce compounds that eventually form brown pigments.

We've just learned that the apple behaves differently depending on its environment. For example:

• Oxygen present → browning reaction can occur.
• Oxygen absent → browning reaction slows down.

Computers can also make decisions based on conditions. Before we run our model, let's learn one new coding idea.

Sometimes a computer needs to make a decision.

An if statement tells the computer: "If this condition is true, do something. Otherwise, do something else."

oxygen = True

if oxygen:
    print("The apple begins to brown!")
else:
    print("The apple stays fresh.")

The computer checks: is oxygen true? Since it is, the computer runs the if branch and prints the first message. If oxygen were False instead, it would run the else branch instead.

Now let's model all three of our apple slices at once — Apple A (air, no lemon juice), Apple B (no oxygen), and Apple C (lemon juice). Our code will check both conditions and decide which message to print.

oxygen = True
lemon_juice = False

if lemon_juice:
    print("The apple browns slowly.")
elif oxygen:
    print("The apple browns quickly!")
else:
    print("The apple stays fresh.")

Before you run it — let's read it line by line.

Suppose you changed the code so that lemon_juice = True, while oxygen stayed True.

Complete the sentence: An apple turns brown because...