Bananas, like most fruits, produce and respond to an airborne hormone called ethylene, which triggers the ripening process. Unripe fruit tends to be firm, more acidic than sweet, and often green due to chlorophyll, the plant pigment crucial for photosynthesis. When a fruit is exposed to ethylene gas, its acids begin to break down, the texture softens, and the green chlorophyll is replaced—in bananas, by a yellow color. This transformation reduces acidity and firmness, resulting in a sweeter, softer, and more enjoyable fruit to eat.
Bananas are unique because they generate much higher amounts of ethylene compared to most fruits, which only produce small amounts during ripening. Early in the ripening stage, a banana becomes sweeter and turns yellow, but as ethylene continues to accumulate, the fruit eventually overripens. Excessive ethylene leads to the yellow pigments breaking down into brown spots through a process called enzymatic browning, the same mechanism that causes bruised fruits to darken. A damaged banana releases even more ethylene, accelerating ripening and browning.
This explains why placing a green banana in a brown paper bag speeds up ripening—the bag traps the ethylene around the fruit. However, if the banana is left in this ethylene-rich environment for too long, it can ripen completely and eventually spoil.
