Bananas turn brown because of a simple oxidation process. Oxygen oxidizes enzymes, according to food science professor Lynne McLandsborough in a July 2007 article for "Scientific American." This means that as soon as you cut a banana open, the pulp starts reacting to the air and turning brown. While you can't avoid the chemical reaction, you can slow down the process with a few kitchen tricks.
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With the Peel On
Whole bananas should stay out of the fridge to prevent them from turning brown too soon. Take them out of the bag and put them in a bowl. If the countertop gets lots of sun during the day, cover the bowl with a towel to protect the bananas from it. They will start turning brown after three to five days, so don’t buy more than you can eat in that time.
Once You Slice Them Up
Once you peel and cut bananas, they will start turning brown. The air starts affecting their color immediately, even if it takes a while for you to notice the change in color. One way to stop that is to dip or brush the banana slices in lemon or lime juice. Pineapple or other acidic fruits also work. You might need to experiment to see which one works best without affecting the taste too much.
Acidic Water Method
If applying lemon juice directly affects the taste too much, you can try making “acidic water.” You do this by adding a small amount of lemon juice or vinegar to cool water. RecipeTips.com recommends about ¼ cup of juice for each cup of water. Dip the banana, in slices or whole, into the water for about three minutes.
If you don’t have fresh fruits or juice available, you can try a vitamin C tablet. Simply crush the tablet and dissolve it in water. Effervescent vitamin C tablets work well for this. Once the effervescent process stops, dip the bananas in the water for a few seconds or use a brush to “paint” the banana slices with the liquid.