Many fruits and vegetables turn brown when cut or damaged surfaces are exposed to
the air, with the reaction showing most clearly on light-coloured flesh. This browning occurs
due to the oxidation and dehydrogenation of colourless polyphenols present in the plants.
The initial reaction catalysed by polyphenol oxidase and produces reddish-brown o-quinones.
These are highly reactive, and so they subsequently undergo a series of non-enzymic
reactions to yield insoluble black-brown melanin pigments.