Why Do Apples Turn Brown After You Cut Them?

Slice up a juicy apple. Then step away for a few minutes. When you come back, you might find an ugly brown mess! 

Why do apples turn brown?

Apples turn brown because of enzymatic browning. This process requires three things:

1. Oxygen.

2. A special enzyme called polyphenol oxidase (PPO).

3. Polyphenols, which are also called phenolic compounds. These are complex organic molecules which contain at least one hydroxyl group (-OH) bonded to a hydrocarbon ring (C₆H₅).

Did you know?

Enzymatic browning doesn’t just happen to apples. It also affects many other fruits and vegetables, like avocados and potatoes!

Under the right conditions, these three ingredients will combine to turn a delicious apple into something much less appetizing!

PPO and polyphenols are found inside an apple’s cells. PPO sits in small compartments called chloroplasts, which are surrounded by a membrane. Most polyphenols are contained in other cell tissues.

Normally, the PPO and polyphenols in an apple never touch each other. That’s why freshly cut apples aren’t brown. But when you cut or bite into the apple, you cause cell damage. And cell damage is what brings PPO and polyphenols together. 

Cutting or biting also exposes an apple’s cells to air, which contains oxygen. This triggers the oxidation reaction that causes enzymatic browning.

Did you know?

Some types of apples have more phenolic compounds than others. These apples will turn brown much faster! 

What are the chemical reactions involved in enzymatic browning?

What happens chemically when these compounds come together? And why does the apple turn brown?

When polyphenols mix with PPO and oxygen, they create a compound called 1,2-Benzoquinone. This can also be called ortho-quinone or o-quinone.

Next, the individual molecules of o-quinone connect together to make larger molecules. This process is called polymerization. It creates a compound called melanin, which causes the apple to look brown.

O-quinone only polymerizes under certain conditions. 

For example, polymerization only happens at certain temperatures. Studies show that PPO reacts best at around 20 degrees Celsius. At very high or very low temperatures, PPO becomes completely inactive and will not react. 

The pH of the apple and its surroundings is also important. PPO reacts best at a neutral pH of 7.

Finally, metal ions also play a role in enzymatic browning. PPO contains copper ions, which are copper atoms that have lost some of their electrons. 

Did you know?

The melanin that causes apples to turn brown is similar to the melanin in human hair. It’s responsible for people’s distinct skin colours!

How do I keep my apples from turning brown?

Don’t like the taste of brown apples? You can prevent enzymatic browning by changing the temperature, pH or metal ion conditions!

For example, adding lemon juice will change both the pH and metal ion conditions. Lemon juice is an acid. It will lower the apple’s pH to below 7, making it harder for the reaction to happen. Lemon juice also contains citric acid. Chelating agents like citric acid react with metal ions and change their chemical make-up. When it reacts with the copper ions in PPO, citric acid slows down the browning process.

You can also try soaking apple slices in water. The water will prevent oxygen from reaching the PPO and phenolic compounds. But remember that PPO reacts best at room temperature. So make sure the water is cold!

Did you know?

A genetically modified type of non-browning apple has been developed in B.C! These apples have less PPO, so enzymatic browning doesn’t happen. They’re on sale trademarked as Arctic® apples.

So there you have it! Next time you slice up an apple, you’ll understand why it turns brown. And you’ll even know how to stop it from browning. How do you like them apples?