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 Kreb’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 peeled, the enzymatic reactions ethylene hormone continues.