A banana ripening bag works by trapping ethylene gas emitted by the bananas. Ethylene exposure causes the bananas to ripen, so the fruit ripens faster when it is in a tight space with a high ethylene concentration.
Ethylene is a compound produced by many different fruit varieties, such as tomatoes, apples, pears and bananas. Ethylene helps to break down starches, acids and pectin, improving the flavor and texture of the fruit over time. Most fruits produce increased amounts of ethylene as they ripen. Placing a banana in a paper bag traps the ethylene around the banana and speeds the ripening process.
A very ripe pear or apple emits more ethylene than an unripe banana. Placing a ripe fruit in a bag with the unripe banana helps to increase the amount of ethylene surrounding the banana and speeds the ripening process.
Paper bags are generally a better choice than plastic bags for ripening bananas. While plastic bags are more airtight and may trap more ethylene, they also trap more moisture. Leaving bananas in a warm, moist environment for too long may encourage the growth of mold and cause the fruit to spoil. For best results, ripen bananas in a paper bag that is loosely sealed at the top.