Keep your bananas out of the fridge
The first—and maybe only—rule that you need to know when it comes to storing bananas is to avoid keeping bananas in the fridge, especially before they're ripe. Bananas are tropical fruits, and, in general, tropical fruits don't handle cold storage well. That's why the best way to store bananas is at room temperature, not in the fridge, especially when they're green and not yet ripened. Placing an unripe banana in the fridge will stall the fruit's ability to ripen, and, according to the experts at Chiquita Banana, "they may not be able to resume the ripening process even if they are returned to room temperature."
And you want to give your banana a chance to ripen, otherwise, you'll be stuck with a green, starchy fruit that's not pleasant to eat. As a banana ripens, it gets sweeter, softer, more aromatic—and also browner. That discoloration isn't a sign that your fruit's gone bad, though. It's a natural part of the ripening process, and the more discoloration, generally the riper and sweeter the fruit. If you do insist on refrigerating your bananas, the time to do it is when it's got some of those speckles, indicating the starch has turned to sugar. As Harold McGee explains in On Food and Cooking: The Science and Lore of the Kitchen, "Once a fruit is ripe, it can be refrigerated with relatively little discoloration of the flesh," though he notes, "the peel will still turn black."
Those black bananas are still usable though, even if they look a little terrifying. Christina Tosi of Milk Bar even swears by using these black-brown bananas to make banana cream pie. As she wrote for Lucky Peach, that's her secret "to getting the most banana flavor is using bananas that are ripe to the point of being completely black and mushy. You can’t be scared of a banana at this stage. I’m not going to lie; they’re a little bit funky, but they’re the ideal bananas for pie."
The best way to keep your bananas fresh for as long as possible, then, is to buy them when they're still a little green, and let them sit at room temperature until they're ready to eat. Don't put them in a bag or a closed space; that'll only hasten the ripening process. If you do insist on putting them in the fridge, only do it once the fruit gets a little brown and ripened, but even then, you only have a couple of days before they become too mushy to eat without a spoon and know that the skin will turn black. And if that freaks you out, just remember that there's nothing wrong with eating or using a blackened banana—and maybe next time you shouldn't put a banana in the fridge.