Food Science 101: Why Do Apple Slices Turn Brown?

Everyone knows that if you slice up an apple and leave it sitting out, the slices will quickly turn brown.  What everyone doesn't know is why.  But you are about to find out!

Apples (and other fruits and vegetables such as pears, bananas, and eggplant) turn brown due to a process called enzymatic browning.  Apples contain an enzyme called polyphenolase and structures called phenols.  In an apple just sitting in your fruit bowl, these compounds are separated by cell walls.  But when you cut the apple open, you break the cell walls.  The phenols and enzymes are then exposed to each other, and oxygen from the air.  The enzyme reacts with the phenol and produces the brown pigment melanin (the same thing that determines the darkness of our skin!).

Now the practical question becomes, how do you prevent enzymatic browning?  Because brown fruit is not attractive.  Fortunately, now that we know what causes brown fruit, we can come up with several ways to prevent it:

• Add acid--This is the most commonly used way to prevent browning in cut up produce.  The chemical reaction that takes place in enzymatic browning is inhibited in acidic conditions.  So if you want to prevent brown apple slices, you can coat them in orange, lemon, or lime juice, or a cream of tartar solution, or vinegar.  Although I must say vinegar on apple slices doesn't sound very good!
• Coat with sugar or water--Enzymatic browning only occurs in the presence of oxygen.  So if you prevent the surface of the fruit from coming in contact with oxygen, you can prevent browning.  This is why a recipe for apple pie, for example, might have you  mix cinnamon and sugar, cut up apples, and immediately coat the apples with the cinnamon/sugar mixture.  
• Chill it--Cold temperatures generally slow most enzymes, including our friend polyphenolase.  So exposing cut fruit to cold temperatures will slow browning, but it doesn't totally stop it.  This can be a good short-term fix for you though.
• Blanching--Dipping fruit in boiling water briefly (that's what blanching is in case you were wondering) can prevent browning by destroying the enzyme that causes the reaction.  However, there are two reasons this isn't the go-to method of preventing browning.  First of all, blanching fruit causes it to lose its texture and flavor.  Second of all, that sounds like a real pain!  I think I'll stick with lemon juice!

Okay, I think that's enough science for today.  But now you can tell all your friends that you know why apples turn brown, and four ways to prevent that from happening.  

Yes, I took organic chemistry in college and I watch a lot of The Big Bang Theory reruns.  Why do you ask?