Apples and other fruits like potatoes, bananas, and pears turn brown after they have been cut, because they consist of an enzyme known as polyphenol oxidase or tyrosinase. When this kind of fruit is cut, that enzyme reacts with oxygen and iron- containing phenols that are also found in the apple.
The oxidation reaction basically forms a sort of rust on the surface of the fruit. when the fruit is cut we notice browning. Browning damages the cells found in the fruit, allowing oxygen in the air to react with the enzyme and other chemicals.
Here are some examples through which we can prevent the particular reactions: we can inactivate the enzyme by heating ,by adding lemon or by putting cut fruit under water.
Tanoide substances give taste and shelf life of apples. Apple slices, exposed to air immediately after cutting, are turning brown because oxidation of tannins (polyphenol oxidase).
This enzyme, polyphenol oxidase, is inside of apple's cells and it reacts with oxygen from the air.The answer to reaction with oxygen from the air is a layer of dark brown color,a layer that appears in other vegetables / fruits that contain this enzyme. This enzyme is released when apple cell destruction begins, which happens when apple is peeled or cut in slices.
So, the conclusion would be: For apple damaged cells begin the process of oxidation, they need light, water and oxygen, therefore, to slow down the oxidation process, apples without peel or cut, should be kept in a cool and dark place. Also, the ascorbic acid from the lemon juice, reacts with oxygen from air, thus preventing that the oxygen to reacts with polyphenol oxidase from the apple. So, the more apple surface has an acid nature, the less oxidation process will happen.
Apples like potatoes have a specialized enzyme in them as well as an iron containing phenols. They react with oxygen. When an apple is cut open the enzyme and phenols become exposed and react with the oxygen in the atmosphere. It basically ends up forming a kind of rust on the surface. The same reaction occurs when a fruit is damaged in transport and by being dropped. The cells are damaged and the enzyme and phenols get exposed resulting in the bruising.
This past summer I taught science camp and the children based some of their hypothesis on which substances put onto the apple would result in less damage caused by oxidation.
You would have to determine what substances you want to test. Some ideas could include sugar, salt, parmesan cheese, dirt, etc...