When your peel, bruise or cut fruits or vegetables, the plant tissue releases some enzymes; there is one called Polyphenol oxidase (PPO, phenolase) which in the presence of oxygen from the air, goes into chemical reactions of plant compounds to give brown pigments known as melanins. This reaction is known as enzymatic browning.
The video below shows what happens when you use a blow drier in the low setting on a banana that is already brown. Amazing!
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