Bananas can be perfectly ripe when they have brown spots on the skin and can even still be used when the skin has turned black from being stored in the refrigerator. Brown spots on banana skin occur when membranes in the skin cells get weak and leak. The liquid that leaks out mixes with enzymes in the cell and form melanin, which is brown. This does not affect the quality of the banana.
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Ripe vs. Overripe
Bananas are often picked and shipped before they are ripe to lengthen their shelf life. They reach the store with skin that is still partially green. Unripe bananas are less sweet because the starches they contain have not yet turned to sugar. Ripe bananas have no green on their skin and may have brown spots. Many banana bread and other baking recipes call for overripe bananas. This means letting the banana skin get almost all black. Inside, they will be at their sweetest and softest, which makes them best for mashing into a recipe.
Getting your bananas ripe without letting them spoil can be a balancing act. Store unripe bananas uncovered at room temperature, preferably hanging from a hook or rack. This prevents the bananas from getting bruises where they touch the counter. If you want to ripen bananas more quickly, place them in a paper bag with an apple, which will give off ethylene gas and speed ripening. To slow the ripening process, put bananas in the refrigerator. Even though the skin will brown quickly, the banana inside will be preserved.
Using Overripe Bananas
Even if your bananas have passed the point of no return and their skins are mostly black, don't give up on them. Make banana bread or muffins using mashed overripe banana. Or try Food & Wine's recipe for banana layer cake with a mascarpone frosting for something more sophisticated. If it's too hot to bake, freeze sliced banana for a couple of hours and then puree it in a blender or food processor for an ice cream-like treat.
How Far Is Too Far?
Although they are versatile and can be eaten at many stages of ripeness, even bananas have their limits. Fruit flies and other insects love ripe fruit and will eat their fill and leave behind eggs. Before using your overripe bananas, check for mold or evidence of flies. These are signs that the bananas are too rotten to use. If you have fruit flies in the kitchen, put the bananas in the refrigerator or invest in food tents designed for use at picnics. Because they are made of mesh, they let air circulate around the bananas while preventing bugs from getting to them.