So why do bananas turn brown? Here is some of the chemistry for those who are interested.
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.