Removing the protective peel from a banana, slicing it and leaving the fruit exposed to air produces a brown, unappetizing result. Cooks who add peeled and sliced bananas to recipes such as fruit salads can dip them in lemon juice to preserve their color and firm consistency. The citric acid in lemon juice slows down the natural oxidation process that occurs when peeled bananas are left exposed to air for an extended time.
Bananas Exposed to Air
The protective peel of a banana keeps the edible part of the fruit protected from the effects of air exposure. Peel and eat a banana immediately and you will likely notice no browning. Slice a peeled banana for addition to a recipe such as fruit salad or a banana pie, however, and the fruit will turn brown as it is exposed to oxygen in the air.
Why Bananas Turn Brown
A peeled and sliced banana turns brown because air oxidizes enzymes in the fruit, which is nature's way of protecting the fruit and making it unattractive to animals. When you peel and slice a banana, cells break open and the chemical substances known as phenols and enzymes react to each other and the air's oxygen to produce a biochemical reaction that results in the browning of the fruit. This reaction is called enzymatic browning.
Citric Acid in Lemon Juice Lowers pH
Lemon juice contains citric acid and has a low pH, which means it is acidic. The acidic lemon juice deforms the enzymes in the banana, known as polyphenol oxidases, and prevents the enzymatic browning from occurring. Other low pH acids will also stop the fruit from browning, but lemon juice has a pleasant flavor and is a staple in most home kitchens.
Lemon Juice's Vitamin C Prevents Browning
Another attribute of lemon juice is its high vitamin C content. Vitamin C is a natural antioxidant and will prevent oxygen in the air from reacting with the banana's phenols.