Why do bananas turn black?

Answer: Enzymes oxidize polyphenols

Sugar caramelizes naturally — Wrong. Caramelization requires heat above 160°C to break down sugars. Bananas blacken at room temperature through enzymatic oxidation. Though bananas do convert starch to sugar as they ripen, this doesn't cause the dark color.

Skin cells die over time — Wrong. While cells do change during ripening, the blackening is specifically from enzymatic browning. It's polyphenol oxidase enzyme reacting with phenolic compounds in the presence of oxygen, creating melanin pigments, not just cell death.

Enzymes oxidize polyphenols ✓ — Correct! As bananas ripen, cell walls break down releasing polyphenol oxidase enzyme. This enzyme reacts with phenolic compounds and oxygen, creating dark melanin pigments - the same process that browns apples! Ethylene gas produced by the banana accelerates ripening. Cold slows this enzyme activity, which is why refrigerated bananas stay yellow longer (though the skin still darkens).