Catechol oxidase carries out the oxidation of phenols such as catechol, using dioxygen (O2). In the presence of catechol, benzoquinone is formed (see reaction below). Hydrogens removed from catechol combine with oxygen to form water.
Fruits and vegetables can be kept fresh by storage at coldish temperatures, because the cold slows down all cellular mechanisms. This is similar to hibernation - the cells are still alive, but do not need a high turnover of oxygen and nutrients. Nevertheless, during the ripening of apples and bananas, browning occurs. The browning is caused by a reaction between catechol and oxygen, catalyzed by the enzyme "catechol oxidase," resulting in benzoquinone, a brownish compound toxic to bacteria. When the peel is damaged, oxygen can then react with the catechol, protecting the fruit. Nitrogen is sometimes used to insulate the fruit - extending shelf life by not allowing oxygen to come in contact with the fruit.