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Oxidation Reactions

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Is that fruit rusting?

Is that fruit rusting?

                  

Credit: Emilian Robert Vicol
Source: http://www.flickr.com/photos/free-stock/4899674329/
License: CC BY-NC 3.0

Most of us know that if you slice an apple and wait it will eventually turn brown in color. A color change generally signifies a chemical reaction! What is going on here?

Amazing But True!

  • In brief, apples turn brown when exposed to the air because some of the compounds present in the apple undergo an oxidation reaction. This reaction is catalyzed by an enzyme called polyphenol oxidase (PPO).
  • One of the reagents in this reaction is the oxygen from the air; thus, no browning reaction can start until you expose the inside of the fruit to oxygen gas (from the air). 
  • Interestingly, the enzyme PPO can only function in a range of pH’s (approximately pH 5 to pH 8); thus, you can inhibit the browning reaction by changing the pH of the apple. This is frequently done in the kitchen by adding lemon juice (an acidic liquid which lowers the pH below 5) to the apple.
  • Credit: rick
    Source: http://www.flickr.com/photos/spine/400509655/
    License: CC BY-NC 3.0

    Avocados go through the same browning process as apples. That's why guacamole is often made with lime juice [Figure2]

     

  • For a video that shows how the oxidation in apples is similar to the oxidation in old cars, see: http://www.youtube.com/watch?v=FWNj-9vg4o0

Can You Apply It?

With the links below, learn more about oxidation in apples. Then answer the following questions.

  1. Why do some apples brown faster than others?
  2. Can you stop the browning reaction with something other than lemon juice? If yes, how would you do it?
  3. If the browning were happening via a combination of PPO, oxygen, phenols and iron, what effect, if any, would using a plastic knife instead of a steel knife have on the browning process?

Image Attributions

  1. [1]^ Credit: Emilian Robert Vicol; Source: http://www.flickr.com/photos/free-stock/4899674329/; License: CC BY-NC 3.0
  2. [2]^ Credit: rick; Source: http://www.flickr.com/photos/spine/400509655/; License: CC BY-NC 3.0

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