Individual banana preferences vary; some people consider bananas to be overripe when they have brown spots on them, while others draw the line at bananas whose peels have turned entirely brown. As bananas ripen, and the peel changes from green to yellow to brown, the starches in these fruits convert to sugars. The browner the banana, the sweeter and softer it will be. Bananas that appear to be overripe are often still safe to eat, although you may prefer to incorporate them into recipes if they no longer appeal to you.
In most cases, even bananas that appear to be extremely overripe are perfectly edible. Bananas that have turned fully brown may be very soft - even somewhat liquefied - but as long as there are no other problems with the bananas, they are still safe to eat. They may be far too sweet and mushy for most tastes, though.
If your bananas smell funny, have evidence of mould, have split skins, or have been colonised by fruit flies or other pests, then they are not safe to eat and should be disposed of.
Check bananas that have turned black (without having been refrigerated or frozen) carefully, because they may have fermented. Fermented bananas will smell more like alcohol than sugar, and the taste may be surprising or even unpleasant. However, if your black banana does not contain any splits, evidence of mould, or insects, then it is still safe to eat.
Preventing bananas from becoming overripe
Refrigerating or freezing your bananas will stall the ripening process. The skin will turn black, but because the banana is not getting any riper, there is no need to worry about fermentation. Refrigerated bananas are still fine to eat as they are, while frozen bananas are best used in baked goods or smoothies. Use refrigerated bananas within two or three days, and frozen bananas within two months.
What to do with overripe bananas
If the appearance, texture or taste of your very ripe bananas is no longer to your liking, try baking with them. Soft and sweet bananas are perfect for making banana bread, banana pancakes, or any of a number of other recipes.