Why do bananas turn brown?

Bananas release ethylene, a plant hormone that triggers ripening, softening the flesh and converting starch into sugar. As ripening continues, the yellow pigments in the peel break down and an enzyme called polyphenol oxidase reacts with oxygen to form brown melanin-like compounds. This is why the familiar brown spots, sometimes called sugar spots, signal a sweeter fruit. Cold temperatures in a refrigerator damage the peel cells and make the skin turn brown or black quickly, even though the flesh inside ripens more slowly. Keeping bananas away from other fruit, or wrapping the stems, can slightly slow the effect of ethylene.