Bananas release a gas called ethylene which is what makes it ripen. That gas is released by the stems, so wrapping them up slows the release of ethylene.
Another way to keep them ripe is by submerging them in water.
In water, the bananas are not exposed to oxygen, which speeds up ethylene production.
Placing them in water also keeps them away from other fruits that produce ethylene, like apples or pears.
