Strategies for speeding ripening in bananas abound, but as we worked our way through over eight cases of fruit while developing our Ultimate Banana Bread, we found most of them ineffective.
One theory, holds that freezing or roasting underripe bananas in their skins will quickly render them sweet and soft enough for baking. While these methods do turn the bananas black—giving them the appearance of their super-sweet, overripe brethren—they actually do little to encourage the necessary conversion of starch to sugar.
The best way to ripen bananas is to enclose them in a paper bag for a few days. The bag will trap the ethylene produced by fruit that hastens ripening, while still allowing some moisture to escape. Since fully ripe fruit emits the most ethylene, placing a ripe banana or other ripe fruit in the bag will speed the process along by a day or two.