Why do apples and pears turn brown...

First, the peel protects the fruit from the oxidation that occurs when air meets polyphenol oxidase, an enzyme present to some degree in all fruit. Apples and pears have a lot of the enzyme, so you’ll see them turn color quickly when exposed to the air. Bananas are susceptible to even more browning not because of the enzyme, but because air can penetrate their peels easily.