Bananas shipped from the tropics are shipped green so they won't rot before they arrive at the grocery store. Green bananas are more resistant to bruising, because the fruit is mostly starch, which is sturdier than sugar. They do not, however, have that mellow yellow banana flavor. A fully ripe banana has a deep yellow skin speckled with brown flecks. As the banana continues to ripen the skin turns darker and eventually black. Get green bananas to go yellow in a few days.
Hang the bananas in an open mesh basket so the air can circulate around them. If you place banans in a bowl, the moisture is trapped between the bowl and the banana skin and the skin may start to mold. Another option is to leave bananas on a wood board or counter.
Wait a few days. The bananas will start to lose their green color and start to turn yellow. They'll soften and smell more like bananas as well.
Peel a test banana. The peel should easily come away from the banana inside.
Brown Bag It
Place the bananas in a brown bag in a single layer with a ripe apple, pear or other fruit. If you don't have any other ripe fruit this method will still work.
Fold the bag down as close to the bananas as possible. Bananas and other fruits give off a gas called ethylene. The presence of the gas encourages fruit to ripen. Putting the bananas in the bag concentrates the gas so the bananas ripen in a few days.
Wait. Check the bag every day by unrolling it and looking at the color of the banana skin. When it's golden yellow the banana is ripe.