Why do bananas turn brown?
For grown-ups: the science
Bananas turn brown when air helps their ripening peel make brown color.
Banana browning is mainly enzymatic browning: when cells are damaged or ripening makes tissues more permeable, oxygen contacts phenolic compounds and enzymes, producing brown pigments. Chilling can slow some reactions but may also cause peel browning in bananas, so refrigeration does not simply prevent browning.
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Transcript
A yellow banana sits on the counter. Why does its peel turn brown?
As a banana ripens, its peel changes and gets softer. Air can reach more of the peel.
When air touches parts inside the peel, they make brown color. A bump or a peel tear lets in extra air.
Cold slows this color change, but it does not stop it. A brown peel can still protect a yummy banana inside.
So bananas turn brown as they ripen and meet air. Brown spots are a normal change!