We've all seen bananas turn brown. In this project I am going to show you how to use this browning process to intentionally cause certain points on a banana peel to brown prematurely. This lets you make pictures on the banana peel using only a small needle. When done properly, it doesn't affect the edible part of the banana, so you can still eat it later. This is a great arts and crafts project to add a little fun to your packed lunches.
Step 1: Background: Why Bananas Turn Brown
It all has to do with a series of chemical reactions that take place as the banana ripens. This process is called enzymatic browning. As the banana ripens, the membranes in the cell start to weaken. This lets the phenolic compounds in the cell's vacuole come in contact with enzymes like polyphenol oxidase. The enzymes cause the phenolic compounds to quickly oxidize. This reaction produces melanin which gives the ripening banana its brown spots.
If we manually rupture the cell membranes, we can cause this process to take place quickly in specific locations. Puncturing the banana peel with a needle will create a well defined brown dot in a matter of minutes. By making a lot of dots we can draw a picture on the banana peel.
TL;DR Puncturing or bruising the skin releases chemicals that make it turn brown.