Is this apple bad?: Learn about enzymatic browning

1Do you know why apples turn brown when they've been cut?  Or why bananas get brown spots?  The answer is the same for both fruits: enzymatic browning triggered by an enzyme called polyphenal oxidase (PPO).  Try this simple activity to explore this phenomenon!

What you'll need:
Banana (yellow with no brown spots
Stove
Pot
Water
Timer
Apple
Cutting Board
Knife
Lemon Juice
Distilled Vinegar
Milk

Instructions
Part one
1. Fill a pot with water.
2. With the help of an adult, heat the water to boiling on the stove.
3. Take your banana and look closely at its peel.  Record observations about its color.
4. Carefully submerge the bottom third of the banana in the boiling water for 30 seconds.
5. Remove the banana from the water and observe it for three minutes.  Does the banana look different after awhile? Record your observations.
6. When the banana has cooled down, peel the banana and look at the fruit inside the peel.  Does it look like how you expected it to look?

What happened?
The boiling water caused heat stress on the cells of the banana.  As the cells broke open, they released PPO, which reacted with the air and created melanin.  Most likely, only the peel was affected by the hot water, because the fruit was protected by the peel.  Try more.  What happens if you put a banana in the fridge?  What if you tape up parts of the banana before changing its temperature?

Instructions
Part two
1. Cut two slices from the apple on the cutting board.  Place each slice on its side.
2. Poke one of the apple slices several times with a fork.  Observe both slices for 15 to 20 minutes.  Is there a difference between the two slices?  Do you know why?  Record your observations.
3. Cut five more slices from the apple, placing each slice on its side.  Immediately after slicing, sprinkle milk on the first slice, distilled vinegar on the second slice, lemon juice on the third slice, water on the fourth slice, and leave the fifth slice as is.  Make sure you keep track of which slice is which.  Poke each apple slice several times with a fork.
4. Observe all five slices for 15-20 minutes.  What did each liquid do to the apple slices?  Do you know why?  Record your observations.

What happened?
Slicing the apple damaged its cells due to mechanical stress. This triggers enzymatic browning, just like with the banana.  When you poked the apple, you released more PPO, so that slice was likely much browner than the other slice.  When you sprinkled the slices with liquids, you likely observed that the slices sprinkled with acidic liquids like lemon juice got less brown.  PPO oxidase doesn't work well in acidic environments.  Next time you're eating apple slices and don't want them to turn brown, try a little lemon juice!

Try these easy fruit experiments and let us know how it goes!

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