Fresh cut
apples turn
brown when
iron-containing
chemicals in the
apple react with
oxygen in the
air. We see this every day when
iron objects
rust. The chemical reaction is called '
oxidation'.
There are several ways to inhibit oxidation.
Chemical anti-oxidants can be added to food. Lemon juice, for example, will inhibit the browning of a freshly cut apple because lemons are high in citric acid, an anti-oxidant. Sulphur dioxide, used in the commercial processing of many foods, does the same thing.
Removing the air (and thus the oxygen) from food can also inhibit oxidation. This is one reason canned food stays fresh so long.
Because the enzymes involved in oxidation need water to work, the reaction can be also be slowed by de-hydrating (removing water from) food.
Finally, the browning of apples and other fruit can be avoided by using clean, high-quality cooking utensils. Iron salts, such as those found on a steel knife corroded by organic acids, can act as potent catalysts to oxidation, and produce per-oxides and super-oxides.