Keep Bananas Out In The Open
Bananas hold on to their nutrients better when they are kept out of the fridge. Cold temperatures actually slow down the ripening process (as we saw with the avocados). But there is also a lot of moisture in most refrigerators which can cause bananas to turn brown or even black.
If you really want to keep bananas around for a long time, slice them into small pieces and store them in plastic bags in the freezer. Frozen bananas are perfect for smoothies.