Bananas have more sugar and starch than most fruit -- each features 110 calories and 19 grams of sugar -- but they're also loaded with vitamin C and potassium. Their thick peels help protect the fruit, but they're not indestructible. As bananas ripen, the peels naturally develop brown spots. The fruit inside can also develop brown areas, which are, in most cases, perfectly safe to eat.
Eating a banana with brown spots won't hurt you. The brown spots simply indicate that the banana has passed from unripe to ripe. Within a few days, however, the banana won't sport just a few spots -- it will turn entirely brown. At this point, it won't hurt you to eat the banana, but it probably won't be very appetizing. Once the banana starts to shrivel, ooze liquid or grow mold, it has spoiled and should not be eaten. In addition to visual signs of spoilage, rotting bananas have a sour odor.
In addition to brown spots on the skin caused by ripening, you might also notice bruises or brown spots on the fruit. These areas brown because the cell walls of the fruit have been damaged and are becoming oxidized. Brown spots on the fruit might be unappetizing, but they don't alter the fruit's nutritional value. Simply cut the bruised area out with a knife.
Whether to eat a banana with brown spots or not is mostly a matter of personal preference. Green bananas have an unappealing woody texture and little flavor. Don't bother eating them. As the banana matures, the texture softens and the banana gains its characteristic flavor. A banana that is yellow with no brown spots still has a slightly firm texture but good banana flavor. Once the brown spots appear, the banana has softened even more. If you prefer soft bananas, by all means, eat the banana with brown spots. If not, throw those bananas in a batch of smoothies or toss them in bread and muffins.
Bananas are harvested green for two reasons. They ripen better when harvested green and, in most cases, they must travel a long way to reach their destination, so harvesting them green ensures that they won't become overripe too soon. At room temperature, bananas last from two to seven days, depending on how green they were when you bought them. If you want to store them longer, put them in the refrigerator for as long as five days. They stop ripening in the fridge, so set them on the countertop a day or two before you want to eat them. Storing bananas in the fridge can cause the skins to darken but doesn't affect the quality of the fruit. For longer storage, peel the bananas and place them in plastic containers or bags. Freeze them for as long as three months. Freezing bananas changes their texture but makes them ideal in smoothies or banana bread.