I've noticed that a banana in the refrigerator will turn pitch black in just a few hours. Why is this?
Your fridge speeds production of polyphenols, which speeds the blackening process. Unrefrigerated bananas ripen by ethylene gas which will also eventually turn them black, but the cold short circuits that process.
It is because, the water that is present inside the veins of bananas get expanded when is cooled from 4 deg celsius to 0 deg. at refrigerator. As the expansion of veins reaches climax it can't be holded any more and finally bursts causing blackening of bananas. It is included in the topic ANOMALOUS EXPANSION OF WATER!!!!