If your yellow bananas aren't quite ripe enough for your liking, there's a great way to speed up the process. The peels give off ethylene gas as they ripen and the more of this gas there is around the bananas the quicker they will ripen. Put whole bananas in a paper bag so the gas is trapped in there with the banana. Don't use a plastic bag because this will do the opposite and slow down the ripening. You can also add other fruits (maybe apple, avocado, pear, or apricot) to the paper bag with the bananas since they also give off ethylene gas.