Bananas ripen because they produce a natural plant hormone gas called ethylene (also called ethene), which triggers chemical changes that turn their starch into sugar and soften the fruit.

The basic science

  • Bananas are climacteric fruits, meaning they can continue to ripen after being picked because they keep producing ethylene.
  • Ethylene switches on enzymes that convert stored starch into sugars, so the fruit tastes sweeter and the texture becomes softer.
  • As ripening progresses, pigments in the peel break down and new ones form, changing the skin from green to yellow and then to brown-speckled.

What speeds up ripening

  • Trapping ethylene around the fruit, such as putting bananas in a paper bag, makes them ripen faster because the gas accumulates and intensifies the ripening signal.
  • Placing other ethylene-producing fruits (like apples or avocados) with bananas in that paper bag boosts ethylene levels even more, speeding the process up further.
  • Warmer environments, like the top of a fridge or a warm room, also accelerate ripening because higher temperature makes the ripening reactions run faster.

What slows ripening

  • Cooler temperatures slow down the banana’s metabolism and enzyme activity, so ripening happens more slowly.
  • Keeping bananas away from other fruits reduces the extra ethylene they’re exposed to, which helps them stay in the firmer, yellow stage longer.
  • Very low temperatures, such as in a fridge, can cause peel browning and texture changes, so the fruit may look overripe on the outside even if the inside is less ripe than it appears.

“Artificial” quick-ripening tricks

  • Heating bananas in an oven or air fryer softens them and makes them taste sweeter more quickly by speeding up the same basic breakdown of starch into sugar, which is handy for baking.
  • These heat-based methods can make bananas very soft and dark on the outside in under an hour, which is usually perfect for banana bread or muffins even if the flavor is not identical to naturally ripened fruit.

TL;DR: What makes bananas ripen is primarily their own ethylene gas, which, together with warmth, turns starch into sugar, softens the fruit, and changes the peel from green to yellow and brown.

Information gathered from public forums or data available on the internet and portrayed here.