The reaction is called oxidation .
And the phenomenon is an enzymatic browning .
It takes place in warm temperatures. Besides, the plant pH should be between 4.8 and 7.
-Here; pH, enzymes, temperature and oxygen play a role.-
The cells of some produce contain tyrosinase or polyphenol oxidase. These enzymes catalyzes the conversion of the phenolic compounds to pigmented areas (melanins) when it reacts with oxygen.
This explains why fruit inside a sealed container does not turn brownish.