Is there any way to keep bananas fresh for longer? We tried putting them in the fridge but their skins turn black very fast.
Store them in a well ventilated place. Cool (15 degrees Celsius) and dark will probably do them good too. And buy green bananas.
Your bananas are alive. Seriously, they continue to live after they have been picked. Breathing, cell metabolism, hormone production, etc. goes on. You cannot stop this process, you can slow it a bit.
In many plants, fridge temperatures trigger their own growth slowing processes, because they are evolved to stop growing in winter months. But in tropical fruit, this doesn't work, because there are no cold months where they grow. Instead, it exposes their sensitive cells to an environment they are not meant for, and they change a lot. Bananas go black, tomatoes change their taste radically, and so on.
Still, keeping them coolish and dark may do them good, because it still gives them less energy to spend on ripening.
One of the mechanisms which controls ripening in fruit, including bananas, is ethylene gas. It is a positive feedback mechanism. Ripe fruit produces ethylene, and unripe fruit ripens faster when exposed to ethylene. People tend to stick green bananas in bags to make them ripe faster. If you can keep them well ventilated, the ripening will slow down.
Still, don't expect any wonders. Bananas will ripe after being plucked, no matter if on the tree or not. And they will overripen in few days, even when held far from ethylene. Starting with green bananas gives you more headroom, but you still have a few days only.
The skins turn black in the fridge, but the fruit itself is fine. Cooks Illustrated tested if refrigerating bananas keeps them good longer, and they found it does—five days longer. Extract below:
Most people store bananas on the countertop, and we wondered if chilling the fruit could slow ripening. To find out, we left 12 pounds of bananas at room temperature for three days until they were perfectly ripe (signified by a firm but yielding texture). We then moved half of the bananas into the refrigerator, leaving the remainder at room temperature.
For the next few days, the bananas were nearly indistinguishable. After four days, however, the room-temperature fruit became markedly soft and mushy, while the refrigerated fruit remained firm, despite blackened skins. We continued to taste the refrigerated bananas after the room-temperature samples had been discarded and were delighted to discover that they lasted an additional five days (so, almost two weeks after purchase) before the flesh became overripe.
The explanation is simple: As a banana ripens, it emits a gas called ethylene and develops acids that aid in ripening. Cool temperatures slow down the production of ethylene and acids, thereby decelerating ripening. However, refrigeration also causes the cell walls of the peel to break down, releasing enzymes that cause the formation of black-brown pigments.
Jolenealaska makes a good point - bananas should not be kept with other fruit, which is why fruit cages often have a long extension with a hook above - this keeps the bananas suspended away from the other fruits. Even better is a banana hook with no fruit cage beneath. The amount of ethylene is critical, so keeping them away from other fruits which are also producing ethylene helps to slow down the ripening process.
I always buy two bunches of bananas. One Bunch that is perfectly ripe and ready to eat within a few days. The second bunch is as green as I can find. I then keep these two bunches separated in my kitchen. This allows me to have bananas for a period of about 7 to 9 days without problems. Additionally when I get them home, the first thing I do is submerge the stems in a bowl of water with soap to eliminate any fruit fly eggs. This works very well!
To keep them pleasant for just eating by themselves keep them at room temperature. Refrigerating or freezing them quickly turns them black. They'll develop bruising as they ripen if they're resting against anything (including each other), so keeping them suspended by their stems helps keep them a nice even yellow banana hooks. Once they ripen (darken and soften) beyond the point that they're pleasant for "out of hand" eating they're still a very useful ingredient. Overripe bananas have much more flavor and aroma than yellow ones. For baked goods ripened all the way to black is perfect. Let them ripen to nearly mushy at room temperature, then freeze. Once you have several bananas stored that way you're halfway to a very nice banana bread. I peel them and freeze in a Ziploc bag, but many people swear by leaving them unpeeled for freezing, and that certainly works too.