Why does the skin of a banana, kept in refrigerator, soon turn black?

Bananas being a tropical fruit do not withstand cold climate. As long as they are not ripe they are not affected by ‘frost bite’, but when they become ripe, oxides are produced by a bio-chemical process called oxidation-reduction (redox). As the enzymes start oxidation, black patches are formed on the yellow banana skin. This bio-chemical process is fast enough to make bananas rotten in few days. The only way to keep bananas fresh and tasteful is to isolate them from oxygen to the extent possible. Therefore, the traders store them in godowns with abundant carbon dioxide in the air. Central Food Research Institute at Mysore has invented in March, 1998, technology for reducing oxygen in godowns and increasing shelf-life of bananas up to 45 days.