The Real Reason Bananas Turn Black
If you've ever had a bunch of bananas turn dark brown or even black way too fast, then you've probably been frustrated at having to use them quickly or freeze them to keep from wasting them. The real reason bananas ripen as they do comes down to temperature as well as a chemical reaction. So manipulating the conditions in which bananas are stored can help keep them at their delectable yellow color for a longer period of time.
The first reason bananas ripen more quickly is if the temperature that the bananas are stored in is too high. Any temperature higher than 57 degrees Fahrenheit will cause bananas to ripen more quickly, and they'll change to a darker color at the same time. This is why bananas can become overly ripe within a matter of days. So, keeping your bananas cool will help them stay perfectly ripe for longer without turning black (via Leaf).
The second reason bananas turn black is because of polyphenol oxidase, an enzyme that causes the fruit to ripen when it reacts with oxygen. This surrounds the entire banana (or bunch of bananas) and causes them to turn dark brown or black.
Slow the process of bananas turning black
While you cannot completely stop the chemical reaction from happening, which means you can't halt the process of the bananas turning black, you can certainly take steps to slow it. There are quite a few ways to slow the reaction for whole bananas as well as peeled bananas. Though storing bananas in an airtight container like a vacuum-sealed bag will certainly help, there are other easy solutions as well.
The most hands-off method is, once the fruit is ripened to your preference, simply place the whole bananas in your refrigerator where the temperature is well below 57 degrees. The peel might still turn brown or black, but it is simply a change in the peel's pigment. The actual fruit itself inside the peel should still taste fine and sport a good texture (via Today).
Another method to slow the enzyme's reaction for peeled bananas is to coat them in an acidic wash. One acid that works well is lemon or lime juice, and another is vinegar. You can either toss sliced bananas in your chosen acidic liquid or brush it onto whole or halved bananas that have been peeled. You can use water to dilute either the citrus or the vinegar. However, if it affects the flavor of the bananas, simply rinse the fruit off well before eating them.