A banana skin turn black after a few days, even when the banana is kept in the refrigerator.
The reason behind it is that the green skin of banana contains a gaseous plant hormone, ethylene, which diffuses into the fruit and hastens the process of ripening. It is yellow when the fruit is ripe and turns black with the production of more and more ethylene. The large number of enzymes of Krebâ€™s cycle convert citric acid, mallic acid and oxalic acid into glucose in a banana, to make it sweet during the process of ripening. When the banana is kept in the refrigerator, the enzymatic reactions are stopped by the cold but the production of ethylene hormone continues.
This turns its skin black.But food value of banana does not change unless it is decomposed.