Why does a banana peel turn black within a short time after the fruit inside is consumed, while it stays yellow for a much longer time when it is covering the fruit?
Blackening of the banana peel is due to the presence of an enzyme called polyphenol oxidase, which is oxygen dependent. This enzyme, in short called PPO, polymerises the naturally occurring polyphenols present in the banana skin. This is a chemical reaction that takes place in the presence of oxygen and is similar to that of suntanned skin resulting in increased melanin production.
As soon as we peel the banana the inner portion, the inner skin, gets exposed to atmospheric oxygen much more than when it is still covering the fruit. Because the inner skin is thin compared to the outer thick layers the membrane damage caused to the cells of the inner skin while peeling liberates the phenolic amines such as dopamines present in the vacuoles of the banana cells.
This leaked out amines come into contact with polyphenoloxidase enzymes present closely in the inner skin. This results in immediate oxidation in the presence of atmospheric oxygen forming a brown polymer which blackens as time advances.
But this enzymatic reaction needs a little warmer conditions. That is why in summer and hot conditions even the outer skin turns brown, black faster compared to that in cooler environment.
Not only banana, fruits like apples, grapes and vegetables like potato, raw banana also turn brown, black on peeling due to the presence of polyphenoloxidases in the layers just below the outer skin and polymerization of the phenolic amines present in their cells.
This enzymatic reaction is more pronounced in the ripening stage because by that time there is less acidity and the starches have been converted to sugars resulting in more sweetness.
Since bananas are tropical fruits the cells get damaged due to chill injury when we refrigerate them and immediately the PPOs present in the skin come into contact with the cell phenolics and they turn black unlike apples which are temperate fruits which can be stored in the fridge safely.
PhD (Chemistry), Bangalore