Have you ever wondered why bananas turn brown when left on the counter for a while? The reason for this is oxidation, a chemical process that affects many fruits including oranges, apricots and apples. These fruits contain an enzyme called polyphenol oxidase which causes a chemical reaction when exposed to oxygen.
Bananas contain polyphenol oxidase and other iron-containing chemicals which react with the oxygen in the air when the cells are cut open. When exposed to the air, these chemicals react in a process known as oxidation, turning the fruit brown. Similar to rust forming on a piece of metal, oxidation is a rust that forms on the surface of the banana.
Bananas turn brown when the fruit is either cut or bruised as both these actions damage the cellular structure of the fruit, allowing the oxygen in the air to react with the polyphenol ozidase enzyme. This is why bananas that have not been cut and have been properly stored will stay fresh for many days at a time, while fruit that has been cut quickly turn brown in a matter of hours.
The chemical reaction of oxidation can be prevented or slowed down. Cooking inactivates the enzyme, preventing oxidation. Adding an acid or antioxidant such as lemon juice reduces the pH on the surface of the banana and slows the chemical reaction. Vacuum packing the fruit reduces the amount of oxygen available, therefore slowing down the reaction. Using a low-quality knife that has some corrosion has been shown to actually increase the rate of oxidation, as it may produce more iron-containing chemicals available for the process. Use high-quality knives when cutting a banana if you wish to save it for later and store in an air-tight container to slow down the oxidation.
Eating a banana which has some oxidation on it is not dangerous. If you wish, you may choose to cut off the brown section to expose the fresh fruit underneath. However, be sure to eat it shortly, before oxidation has a chance to occur again.