Why do bananas turn brown?

Bananas turn brown because their cells change as they ripen, letting an enzyme called polyphenol oxidase react with fruit chemicals and oxygen. That reaction makes darker pigments, so spots spread across the peel and the fruit inside grows softer.

At the same time, other enzymes turn stored starch into sugar, which is why a spotted banana often tastes sweeter. The brown color is usually part of ripening, not instant spoilage, though a banana can eventually rot if it sits too long.