Our bodies do not digest underipe bananas as well as ripe ones and should be avoided. Most farmers pick their bananas while green then take them through a process which gasses them with ethylne.
If available, buy your bananas green because most likely they have not been put through the gassing process. It only takes a few days to ripen bananas at room temperature. Or you can place the bananas in a brown bag with an apple to ripen quicker. I love to cut up bananas and put them in the freezer to add to my protein shakes.
Click here to see one of my favorite protein shakes.