Fruits and vegetables produce ethylene gas as they ripen. Certain fruits such as bananas and apples give off greater amounts of ethylene gas. Exposing unripe fruit to ripe fruit increases the unripe fruit's exposure to ethylene gas to speed up the process. Combining ethylene gas with the heat from a microwave helps kickstart the process.
Place the piece of unripe fruit in the microwave. Heat it on medium power for 15 seconds.
Put the piece of fruit in a brown paper bag.
Place two ripe apples or a banana in the brown paper bag.
Close the brown paper bag loosely so the ethylene gas is contained in the bag, but the bag still can let in oxygen to speed ripening.
Set the bag in an area out of direct sunlight at room temperature. The fruit should ripen within a few days. Check the fruit daily to make sure it doesn't over-ripen. The fruit is sufficiently ripe when it yields to pressure.