When an apple is cut, it releases an enzyme called polyphenol oxydase. This copper-based compound breaks down in the presence of oxygen, acting both as an antibacterial agent and as a deterrent to animals. This is what forms the brown coating on the apple.
Ethylene gas, which is released by fruits and vegetables when they age, speeds this reaction. Bananas are particularly sensitive to this gas: If you place sliced apples and bananas together in a covered container, the ethylene from the apples will turn the bananas brown in minutes.
By dipping apple slices in lemon juice, you are limiting the breakdown of polyphenol oxydase in two ways. To reach the polyphenol, the oxygen first must break down the Vitamin C in the lemon juice. Once that's completed, the chemical reaction between polyphenol oxydase and oxygen is slowed by the high acidity of the remaining juice.
Polyphenol oxydase is released by a wide range of fruits and vegetables, and the same principles used on the apple slices can be applied to keep them from browning. If you've ever made guacamole, you know that most recipes use lime juice, while there will also be a recommendation that the dip be tightly covered in plastic wrap. The juice slows the oxygen reaction, while the plastic wrap acts as a barrier between the guacamole and the oxygen and ethylene gas in the air.
Posted 2997 day ago