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Browning apple

Browning apple (GDragan, iStockphoto)

STEM in Context

Why Do Apples Turn Brown After You Cut Them?

Rebecca Fox

Summary

Has your fresh juicy apple turned an unsightly brown? Learn about the redox reaction behind the transformation.

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!

Chemical structure of phenol, the simplest of the phenols
Chemical structure of phenol, the simplest of the phenols (Source: M1ss1ontomars2k4 [Public domain] via Wikimedia Commons).

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.

Oxidation of a phenol can produce 1,2-Benzoquinone
Oxidation of a phenol can produce 1,2-Benzoquinone (©2019 Let’s Talk Science).

 

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.

Part of the chemical structure of eumelanin, a pigment that makes things look brown
Part of the chemical structure of eumelanin, a pigment that makes things look brown (Source: Roland Mattern [Public domain] via Wikimedia Commons).

 

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.

Chemical structure of citric acid
Chemical structure of citric acid (Source: NEUROtiker [Public domain] via Wikimedia Commons).

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.

Why do apples turn brown? (2014) by SciShow (2:31 min.)

 

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?

Starting Points

Connecting and Relating
  • When have you observed cut apples turning brown? Which other fruits have you noticed turn brown after being cut? 
  • Would you pay extra to have an apple that did not turn brown? Why or why not? 
  • Would you be hesitant or afraid to eat genetically modified apples? Why or why not? 
  • Do you know any culinary tricks for keeping apple slices from turning brown?
  •  
Connecting and Relating
  • When have you observed cut apples turning brown? Which other fruits have you noticed turn brown after being cut? 
  • Would you pay extra to have an apple that did not turn brown? Why or why not? 
  • Would you be hesitant or afraid to eat genetically modified apples? Why or why not? 
  • Do you know any culinary tricks for keeping apple slices from turning brown?
  •  
Relating Science and Technology to Society and the Environment
  • With consumer wariness about eating genetically modified foods, do you think there is a market for non-browning apples? Who might be most interested in this feature in apples and other types of fruit?
Relating Science and Technology to Society and the Environment
  • With consumer wariness about eating genetically modified foods, do you think there is a market for non-browning apples? Who might be most interested in this feature in apples and other types of fruit?
Exploring Concepts
  • What is enzymatic browning? How is enzymatic browning initiated?
  • How does temperature impact on enzymatic browning?  
  • How does pH impact on enzymatic browning? How does citric acid affect the process of enzymatic browning?
  • What chemical process is responsible for the creation of melanin in a cut apple?
  •  
Exploring Concepts
  • What is enzymatic browning? How is enzymatic browning initiated?
  • How does temperature impact on enzymatic browning?  
  • How does pH impact on enzymatic browning? How does citric acid affect the process of enzymatic browning?
  • What chemical process is responsible for the creation of melanin in a cut apple?
  •  
Media Literacy
  • If you were the marketing team responsible for marketing a variety of non-browning apples, how would you market them? What would be your key messages? What features, advantages and benefits would your marketing campaign address?
Media Literacy
  • If you were the marketing team responsible for marketing a variety of non-browning apples, how would you market them? What would be your key messages? What features, advantages and benefits would your marketing campaign address?
Teaching Suggestions
  • This article can be used to support teaching and learning of Chemistry related to acids & bases, enzymes, organic chemistry, polymers and redox reactions. Concepts introduced include enzymatic browning, enzymes, polyphenols, chloroplasts, oxidation, o-quinone, polymerizes, melanin, polymerization and pH. 
  • After reading this article, teachers could have students complete a Concept Definition Web learning strategy to consolidate the concept of enzymatic browning. Ready-to-use Concept Definition Web reproducibles for this article are available in [Google doc] and [PDF] formats. 
  • In addition to reading the article, teachers could also have students watch the SciShow video, Why Do Apples Turn Brown?  Students could then use a Print-Video Venn Diagram learning strategy to organize and compare the information in the article with that of the video. Ready-to-use Print-Video Venn Diagram reproducibles for this article are available in [Google doc] and [PDF] formats. 
Teaching Suggestions
  • This article can be used to support teaching and learning of Chemistry related to acids & bases, enzymes, organic chemistry, polymers and redox reactions. Concepts introduced include enzymatic browning, enzymes, polyphenols, chloroplasts, oxidation, o-quinone, polymerizes, melanin, polymerization and pH. 
  • After reading this article, teachers could have students complete a Concept Definition Web learning strategy to consolidate the concept of enzymatic browning. Ready-to-use Concept Definition Web reproducibles for this article are available in [Google doc] and [PDF] formats. 
  • In addition to reading the article, teachers could also have students watch the SciShow video, Why Do Apples Turn Brown?  Students could then use a Print-Video Venn Diagram learning strategy to organize and compare the information in the article with that of the video. Ready-to-use Print-Video Venn Diagram reproducibles for this article are available in [Google doc] and [PDF] formats. 

Learn more

Are Brown Apple Slices Safe to Eat? (2016)

The short answer is, yes! This article by CookingLight also gives some ideas about how to keep them from going brown

Fruits Gone Bad? Discover Enzymatic Browning (2019)

Simple experiment by Svenja Lohner to show how enzymatic browning occurs. Note that this resource was also used as a reference.

Why Do Fruits and Vegetables Brown? (2016)

Brief article from Best Food Facts discussing enzymatic browning in apples, also includes an interesting infographic. Note that this resource was also used as a reference.

Why Do Avocados Turn Brown? – The Chemistry of Avocados (2014)

An article by Compound Interest discussing the science behind the browning of avocados, also includes an interesting infographic. 

Apples Turning Brown (Time Lapse) (2017) 

Time-lapse (0:34 min.) video from Kieran Cox of enzymatic browning occurring.

References

Almeida, M. E., & Nogueira, J. N. (1995). The control of polyphenol oxidase activity in fruits and vegetables. Plant Foods for Human Nutrition, 47, 245-256.

Araji, S., Grammer, T. A., Gertzen, R., Anderson, S. D., Mikulic-Petkovsek, M., Veberic, R. Escobar, M. A. (2014, March 01). Novel roles for the polyphenol oxidase enzyme in secondarymnetabolism and the regulation of cell death in walnut. Plant Physiology, 164(3), 1191-1203. DOI:10.1104/pp.113.228593

National Institutes of Health. (n.d.). Keeping it fresh.

Petruzzello, M. (n.d.). Why do sliced apples turn brown? Encyclopedia Britannica.

Scientific American. (2007, June 30). Why do apple slices turn brown after being cut?