A chemistry essay on the oxidation of an apple.
When an apple’s skin is pierced and then left out in the air, then quickly it will turn from its beautiful fleshy white colour to a soggy brown colour. The flesh of the apple is being oxidized in the air, and thus the process is called oxidation. The oxidation of an apple is a very similar reaction to the rusting of iron with the air. This form of oxidation actually uses oxygen as its oxidizing agent, but that is not always the case. Oxidation is defined as the loss of electrons. For the loss of electrons to occur, there must also be a subsequent gain of electrons, as the electrons cannot simply float off into nothing. That process is called reduction.
In apples, the specific enzyme that causes the brownish colour to appear is called polyphenol oxidase (PPO).1 This enzyme is found in many other things, including noodles, breads, and chapattis, and is has the same browning effect in the other things as well. The enzyme exists inside the cell. When the cell is broken it releases the enzyme, which then reacts with the oxygen in the air and forms a layer on the outside. For the cell to be broken, in the apple’s case, the apple’s skin must be pierced or broken by an outside force. The skin is what protects it from turning into mush while it is growing. 2
Shown below is a diagram of the chemical reaction that takes place. In this diagram it only uses a monophenol as opposed to a polyphenol as well. The monophenol reacts with the oxygen in the air, which creates diphenol. And so forth. When the complex reaction is complete, what is left are complex brown polymers. This is what is seen as the “brown stuff” when an apple skin is pierced. The diphenol is being reduced by the o-quinone, which in turn is the molecule that is being oxidized. 1
So when an apple is turning brown, what is actually happening is the small molecules in each of the individual cells are reacting with the oxygen in the air, and forming a layer of stuff on the top that is brown. It does not make it harmful to eat, just slightly soggy.
1. Pollick, Michael (2003). What is oxidation?. Retrieved January 12, 2009, from WiseGeek Web site: http://www.wisegeek.com/what-is-oxidation.htm
2. N/A, (2007, July 30). Why do apple slices turn brown after being cut?. Retrieved January 13, 2009, from Scientific American Web site: http://www.sciam.com/article.cfm?id=experts-why-cut-apples-turn-brown