If you're not a baker, feeding friends or family means that sooner or later you'll need to come up with uncomplicated dessert ideas. Fresh fruit is your friend in those moments because it's bright, sweet and colorful and easy to turn into a decorative fruit plate, salad or DIY smoothie. Unfortunately some fruits, especially fresh bananas, turn brown almost immediately after you slice them. That's inconvenient, but you can work around it without much difficulty.
Why Cut Bananas Turn Brown
The brown gunk on your sliced bananas isn't dangerous, but it isn't very appealing, either. If you're going to keep that from happening it helps to understand why it happens, so let's take a quick look at the science involved. When you cut the fruit or bite into it, you're breaking through a large number of cell walls and letting air in. Oxygen in the air reacts with enzymes and chemical compounds in the fruit with catchy names like 3,4-dihydroxyphenylethylamine. (If you can throw that out casually after a couple of beers, your friends will be pretty impressed!) These are what darken and soften the cut surfaces.
A handful of things you can do will slow down that chemical reaction. Just keeping air away from the cut surfaces will help. The enzymes react faster at room temperature than they do when the fruit is cold, so refrigeration helps, too. So does acidity, which interferes with the chemical reaction. If you want to keep your bananas from turning brown, you can try all of these tips. You won't stop the browning process completely, but you'll slow it down a lot.
Keeping Air From Your Bananas
Fruit won't oxidize if you keep oxygen away, so that's a good starting point. If you're slicing bananas ahead of time as a snack, you can save yourself some trouble by simply leaving the skins on. They're the original airtight packaging, after all. Arrange your slices on a plate, which seals the bottom, then cover them with plastic film wrap to seal the top; your slices will stay fresh-looking longer. It helps to refrigerate the plate until you need it, which puts the cold to work for you as well.
If leaving the peel on isn't an option, there are other ways to keep oxygen away. If you have a vacuum sealer, sealing the bananas in an airtight bag works pretty well. So does a zipper-seal bag with as much air as possible squished out. Even refrigerating the slices in a bowl of cold water works, because the oxygen in cold water won't react with the bananas as quickly as the oxygen in room-temperature air.
Use Acidity with Sliced Bananas
The most versatile tip to keep fruit from turning brown is to use acid. Wine, cider and most fruit juices are acidic enough to do the job, and they'll also add flavor to your bananas. That's why they're mostly used for things like fruit salads and punches, where the extra flavor is a positive not a negative.
For most purposes, lemon juice is the better choice. It's got a strong and sour flavor, but is so acidic that you can dilute it quite a bit – just a tablespoon in 2 to 3 cups of water – and still have enough acidity to keep sliced bananas looking nice. Drop the slices right into a bowl of diluted lemon juice as you cut them. You can refrigerate them in the bowl, bag them airtight for later or just add them directly to a fruit plate or salad.
The anti-browning powders you see in the produce and canning sections of the supermarket use the same principle, but instead of lemon juice they give you dry crystals. They're usually citric acid or ascorbic acid, which is better known as vitamin C. If you have some vitamin C tablets around the house, you can just crush one of them and dissolve it in water to make your own anti-browning solution.