Above: Image © GDragan, iStockphoto

You slice up a juicy apple, then step away for a few minutes. When you return, your fresh apple has been replaced by an unsightly brown fruit! Why? Well, a special compound called polyphenol oxidase (PPO) is behind the transformation.

Did you know? The process that causes cut apples to turn brown also occurs in many other fruits and vegetables, including avocados and potatoes.

PPO, meet polyphenols. Polyphenols, meet PPO

But PPO doesn’t act alone. There are two other main culprits: oxygen and other special compounds called polyphenols. Under the right conditions, these three ingredients combine to turn your delicious apple into a much less attractive piece of produce!

PPO and polyphenols are in every apple cell. PPO sits in small, membrane-bound compartments called chloroplasts. Meanwhile, most polyphenols are contained in separate compartments called vacuoles.

In fact, PPO and polyphenols never come into contact with one another when the apple is whole. That’s why freshly cut apples aren’t brown. However, when you cut or bite into the apple, you cause cell damage. And cell damage brings PPO and polyphenols together. It also exposes the fruit cells to air, adding oxygen to the mix.

Did you know? The melanins that cause apples to turn brown are similar to the melanins in human skin. They’re what gives people distinct skin colours!

Chemical reactions

So what happens at a chemical level when these compounds come together to cause a colour change? And why brown? Well, when polyphenols mix with PPO and oxygen from the surrounding air, they create a compound called o-quinone. The o-quinone can then polymerize, meaning it connects together to make large molecules. This creates compounds called melanins, which cause the apple to turn brown.

Polymerization of o-quinone only occurs under certain conditions. For example, it only occurs at certain temperatures. Studies have shown that PPO reacts best in fairly warm temperatures, around 20 degrees Celsius. At really high temperatures, like above 60 degrees, PPO becomes completely inactive and no longer reacts! 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. PPO contains copper ions, which are copper atoms that have lost a couple of their electrons. PPO uses these copper ions in the browning process.

Did you know? If you cut an apple and put it in a copper bowl without any lemon juice, it would turn brown even faster than normal! The copper bowl adds extra copper ions that speed up the browning process.

Keeping your apples from turning brown

So that’s why apples turn brown. But what if you don’t like brown apples? How can you keep them from changing colour? Try altering the temperature, pH, or metal ion conditions around the apple.

For example, you can change both the pH and metal ion conditions by adding lemon juice to sliced apples. Lemon juice is an acid. It lowers the pH of the apple to below the optimal 7.

Lemon juice also contains citric acid. Citric acid acts as a chelating agent (pronounced kee-lay-ting). Chelating agents tie up metal ions and change the chemical makeup of the metal. Citric acid ties up the copper ions in PPO. Because the pH and metal ion conditions aren’t ideal for PPO, the browning process slows down.

You can also try soaking apple slices in cold water. This prevents oxygen from encountering the PPO and polyphenols. Remember, though: the browning reaction likes warm conditions, so make sure that water is cold!

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

Learn More!

About apples turning brown:

Curiosities: Why Do Apple Slices Turn Brown? (2008)
University of Wisconsin-Madison

Why do apple slices turn brown after being cut? (2007)
L. McLandsborough, Scientific American.

Experiment 3: Enzymatic Browning of Apples
K. J. Campbell, B. Friend & J. Rupnow, Enzymes in Food Systems Experiments 3

About the role of polyphenol oxidase (PPO):

Effect of PH and Temperature on Peroxidase and Polyphenoloxidase Activities of Litchi Pericarp (2010)
G.P. Mizobutsi et al., Scientia Agricola 67

The Control of Polyphenol Oxidase Activity in Fruits and Vegetables (1995)
M.E.M. lmeida & J. N. Nogueira, Plant Foods for Human Nutrition 47

Rebecca Fox

I’m a second year undergraduate student studying Nutrition at the University of Alberta. I chose to study Nutrition, as it combines multiple interesting facets of STEM fields, including chemistry, biology- even math! There are so many things to discover and understand when it comes to this awesome subject; and I’m lucky enough to be able to practically apply all of the chemistry and physiology I’ve learned in my everyday life through nutrition! i.e. kale salad anyone?

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