The usual way to speed up the ripening of green bananas is to put them into a paper bag. This will speed up ripening a lot, and you will probably have a ripe banana by the next day or so. To speed it up even more, put a ripe banana in with the green ones, or an apple. I recommend the apple, since ripe bananas in a bag can get mushy and messy. Apples give off more ethylene gas than ripening green bananas, so this will ramp up the ripening process. However, if you only have unripe bananas and you want to make banana bread like right now, don’t worry, there’s a trick for that.
Soften Green Bananas Quickly for Banana Bread
Placed the peeled bananas on a baking sheet and heat in a 350°F oven for 8 to 10 minutes, until very soft. Then mash as usual for your banana bread. That’s it!
As explained here, applying heat to green or under-ripe bananas modifies the starches in a way that is similar to ripening, making the banana softer and the starches easier to digest.
You Have Ripe Bananas But You Don’t Want to Make Banana Bread Right Now?
You are a wise old cook and you know that once bananas become overripe, you don’t need to waste them, you can cook with them. And the favorite way to do that is to make banana bread. But dang it, you don’t want to make banana bread right away. What to do?
No problem. You can freeze ripe bananas and thaw them to use in bread later. You can freeze them in their skins and they should be usable for up to 7 months. OR, you can take them out of the skins and freeze them whole in a plastic bag. Or, you can mash up the ripe bananas and freeze them in air-tight plastic containers. The easiest way, of course, is just to throw them in a Ziploc freezer bag. As you get more ripe bananas, just add them to the bag until the bag is full or you have enough to make banana bread.
You can, of course, freeze perfectly ripe or under-ripe bananas as well, but the freezing process will render them mushy when thawed. So you’ll have to use them for cooking. They will not ripen after thawing (duh), but you can soften them further, if need be, by using the oven trick above.