bp's science: why do apples brown after being sliced?

When apples are sliced, the moisture in the apple causes oxygen in the air to attach itself to the apple molecules (if we are getting specific, the enzyme polyphenol oxidase) and cause browning.

Now we must briefly pause and talk about oxidation.  Here is a simple explanation pulled from my handy copy of 730 Easy Science Experiments with Everyday Materials: 

When one substance gives oxygen to another, chemists say it is "reduced," and the substance that receives the oxygen is said to be "oxidized." Confused? Think of it this way: You have ten balls that stand for oxygen and a friend takes seven of them.  Your friend would be oxidized, because he received extra oxygen from you, but you would be reduced because you lost some of your oxygen.

In the apple example, the air gives oxygen to the apple and so the apple has oxidized.  But wait!  You can stop or slow down this reaction with a reducing agent (defined by google as a substance that tends to bring about reduction by being oxidized...), or in other words a substance that would be willing to take the oxygen from the air so the apple would not react with the air and cause oxidation.  One excellent reducing agent is lemon juice.  It is very acidic (containing ascorbic acid/vitamin C) and will react with oxygen first so that it can't react with the enzyme (polyphenol oxidase) in the apple.  Thus, the lemon juice is oxidized and the apple (enzyme) is not.

Still confused?  I sure was back in my Chemistry class in high school.  I remember the test on redox reactions and it was my lowest test score ever.  It was because I answered everything exactly opposite of what it was supposed to be.  The term "reduced" made me think that a molecule is losing something when in fact it is when it is gaining something, more specifically, gaining electrons.  And a reducing agent was something that brought about reduction by being oxidized.  In fact, all of this is still counter intuitive to me and it takes my brain a while to calibrate. 

For a more detailed explanation of the molecules, reactions, and history of the topics discussed in the post see here.  It sure helped me broaden my understanding.

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