Why does a cut apple turn brown?
The cells of an apple contain both certain pigment precursors (polyphenols) and an enzyme called polyphenol oxidase. As long as the cells are intact, the two barely come into contact.
Polyphenols + oxygen --(enzyme)--> ortho-quinones --> melanins (brown)
When you cut the apple, many cells break open, the enzyme and polyphenols mix, and both now come into contact with oxygen from the air. The enzyme first converts the polyphenols into reactive intermediate products called ortho-quinones, which then combine to form the actual brown pigments, the melanins.
This is exactly why lemon juice helps prevent browning: the acid it contains slows down the enzyme, and the ascorbic acid (vitamin C) reduces the reactive intermediates again before they can combine into visible melanins.
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