When you slice into an apple to reveal its crisp and juicy white flesh, you do more than simply make the fruit easier to eat. You set into motion a complex series of chemical interactions that does something very noticeable--it turns the apple brown. This is not possible without a number of different reactions, and removing any of these from the equation may keep the apple white and fresh-looking, at least for a while.
Although most people notice the browning of an apple several minutes after they cut or bite into it, the apple can turn brown when any interior tissue is injured. This may be caused by cutting, either with a knife or with teeth, but could also be caused by a blunt force impact, such as the apple falling from an elevated position onto a hard surface. No matter how it is done, an apple will not turn brown until the tissue sustains an injury.
Chloroplast and Oxygen
After the initial injury takes place and the skin is broken, the apple is then exposed to oxygen. While oxygen is necessary for all living things, it can also have some very interesting effects on certain types of food, including apples. All apples have chloroplast, the main substance responsible for photosynthesis and the green color in most plants and trees. The chloroplast and oxygen begin to interact with one another.
Once the interaction of chloroplast and oxygen begins, an enzyme in the chloroplast known as polyphenol oxidase begins to form compounds known as o-quinones. These form additional compounds with amino acids. The result of these compounds is the distinctive brownish color that is often associated with apples that have been cut and left too long.
Not all species of apples brown in the same way. There can be a difference in the time it takes even one apple and another within the same species or from the same tree to turn brown. This is because the levels of the enzymes can vary based on local conditions or the species of fruit. Apples with more enzymes, or more active enzymes, will brown faster than others.
In some cases, it is possible to short circuit the browning process by using a substance that neutralizes the chemical reactions. Syrup, sugar and even citric acid can stop these reactions. Many mass producers of food, such as restaurants that package apples, use corn syrup. Boiling for a few minutes also works, but will change the texture of the apple, which may not be a good thing for picky eaters.