Why do bananas turn black after some time?
Banana skin consists of plant cells and cellular tissue. This tissue contains phenolic compounds. On getting old, the cell membranes of the banana get weak. When the membranes weaken, the phenolic compounds mix with polyphenol oxidase (an enzyme present in the cells cytoplasm). As a result of this interaction, the phelonic compounds oxidize and produce a brown compound called melanin which results in the colour of the peel turning black. Also, 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.