I had an apple after dinner as dessert. The apple was sliced and pieces of it turns brown. Then immediately I remembered my parent used to apply some salt on the apple after slicing it. It does prevent the apple from turning brown. It kept me ponder awhile and got me searching for hint on google.
Found out that, the reason behind the discoloration is due to it producing enzyme (polyphenol oxidase or tyrosinase) which is a reaction towards oxygen and iron-containing phenols. A more scientific term, oxidation reaction, which basically forms a sort of rust on the surface of the fruit. This oxidation effect can be slowed down by inactivating the enzyme with heat (cooking), reducing the pH on the surface of the fruit (by adding lemon juice or another acid), reducing the amount of available oxygen (by putting cut fruit under water or vacuum packing it), or by adding certain preservative chemicals (like sulfur dioxide).
And that explains why applying salt on the apple could maintains the color of it. However, it is always advisable to consume fruits which have been sliced to keep the freshness.