When I buy bananas, I buy more than I need. In other words, if I think that I’m going to eat three bananas in a given week, I buy five or six. The leftover bananas will ripen to the point of being overripe, with dark brown mottled peels that make them look a bit unappetizing as an in-hand snack, but perfect for turning them into smoothies and banana baked goods. Overripe bananas can also be stored in the freezer so that they are always there when you need them, if you have extras. All that being said, there are plenty of times when I need ripe bananas and don’t have them on hand (I go through a lot of bananas). When bananas ripen, the starch in the fruit turns to sugar, which is what makes the fruit both softer and sweeter. Fortunately, there are some tricks you can use to ripen bananas quickly so that you can bake that banana bread without having to wait a week to do it.
If you have a day or two: Buy a big bunch of the least green bananas you can find. Place them in a paper bag and put the bag in a warm place, such as on top of your refrigerator or in a window. Bananas give off ethylene as they ripen and the presence of more gas encourages the fruit to ripen faster. Being in a warm place simply speeds up the process even further.
If you have a couple of minutes: Line a baking sheet with aluminum foil or parchment paper and place your bananas on it in a single layer. Bake the bananas at 300F for about 15 minutes, or until the skin is black. These bananas will be very soft and perfect for baking, despite their less-than-appetizing appearance. Let the bananas cool before using.
If you have no time to spare: You can ripen a banana by microwaving it for 60-90 seconds (approx, depending on the size of the banana) by microwaving it on high heat. The banana will blacken, just like the oven-baked bananas, and the fruit will become very soft. The oven bananas seem to have a bit more flavor, but a microwaved banana will be ready to bake with almost instantly.