Bananas, like other tropical fruits, are prone to “chilling injury”; that is, damage from cold storage. The most obvious sign of it is when yellow banana skins turn black after refrigeration.
If the bananas were fully ripened before you refrigerated them, they’ll still be fine to eat, though they’ll be mushy and the flesh might also have turned brown.
Banana skins are made up of plant cells. In each of those cells there is a vacuole, a little fluid-filled compartment that’s contained by a membrane. When a banana gets too cold, the membranes weaken and leak. Adel Kader, a professor emeritus in the Department of Plant Sciences at UC Davis, explains that phenolic compounds that are in the vacuole along with the fluids mix with polyphenol oxidase, an enzyme found in the cell’s cytoplasm (that’s the jellylike stuff that parts of a cell hang out in). The phenolic compounds oxidize and form a brown compound called melanin.
If the bananas weren’t ripe to begin with, Kader says, they probably won’t ever ripen properly, another symptom of chilling injury. The best bet for banana storage is to keep the fruit at room temperature.