Banana peels can turn brown or black when refrigerated. The cells on the peels of tropical fruits like bananas get damaged when stored under cold conditions. This releases some phenolic compounds that get oxidised by enzymes like polyphenol oxidase leading to the darkening of the peel. However the quality of the edible part of the banana fruit is not affected by refrigeration. In fact, refrigeration may delay the ripening of banana fruit for a few days. Temperate fruits like apple and pears do not get their cells damaged due to cold conditions and hence they can be refrigerated.
The schematic diagram gives the sequence of reactions of enzymatic browning as detailed below.