Enzymatic Browning

Ever wonder what happens when a fruit or vegetable discolors soon after being cut? Here is the answer from On Food and Cooking, The Science and Lore of the Kitchen, p. 269:

"[The] discoloration is caused by three chemical ingredients: 1- and 2-ring phenolic compounds, certain plant enzymes, and oxygen. In the intact fruit or vegetable, the phenolic compounds are kept in the storage vacuole, the enzymes in the surrounding cytoplasm. When the cell structure is damaged and phenolics are mixed with enzymes and oxygen, the enzymes oxidize the phenolics, forming molecules that eventually react with each other and bond together into light-absorbing clusters."

It is a similar principle to when people react to sun exposure and become tan. 

So why does lemon juice prevent browning?

Browning enzymes work very slowly in acidic conditions. Aside from its acidity, lemon juice also has ascorbic acid (Vitamin C) which has antioxidant properties.