To make bananas ripen faster, place the bananas in a warm place such as the top of a refrigerator, or bake them in an oven at 250 degrees Fahrenheit for 15 to 20 minutes. Bananas omit ethylene, a compound which converts starch into sugar.
Bananas tend to ripen faster when left in a warm or closed environment. A sealed environment causes the ethylene to build up around the banana, speeding the ripening process. Seal bananas in a paper bag to encourage this reaction. Other fruits such as apples and tomatoes also emit ethylene. Storing bananas close to these fruits helps all of the fruit ripen faster.
Baking bananas in an oven generally results in fruits that are slightly over-ripe, and therefore best used in baked goods. Warming a banana in a microwave allows for slightly more control. Pierce the banana peel with a fork and heat the banana in the microwave for 30 seconds. If the banana is still too hard, continue to heat in 30-second increments until they reach the desired softness.
Bananas left on a countertop in a bunch usually take 24 to 48 hours to ripen, depending on how green they are at the beginning of the process. The more bananas in the bunch, the more quickly they will ripen, due to the higher concentration of ethylene molecules.